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Cohort-Specific Growth and Mortality of JuvenileAmerican Shad in the Pamunkey River, VirginiaJoel C. Hoffman a & John E. Olney aa Virginia Institute of Marine Science , Post Office Box 1346, Route 1208 Greate Road,Gloucester Point, Virginia, 23062, USAPublished online: 09 Jan 2011.
To cite this article: Joel C. Hoffman & John E. Olney (2005) Cohort-Specific Growth and Mortality of Juvenile American Shadin the Pamunkey River, Virginia, Transactions of the American Fisheries Society, 134:1, 1-18, DOI: 10.1577/FT03-219.1
To link to this article: http://dx.doi.org/10.1577/FT03-219.1
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Transactions of the American Fisheries Society 134:1–18, 2005 [Article]q Copyright by the American Fisheries Society 2005
Cohort-Specific Growth and Mortality of Juvenile AmericanShad in the Pamunkey River, Virginia
JOEL C. HOFFMAN* AND JOHN E. OLNEY
Virginia Institute of Marine Science, Post Office Box 1346,Route 1208 Greate Road, Gloucester Point, Virginia 23062, USA
Abstract.—We estimated the variation in instantaneous rates of growth (G) and mortality (M)between intraannual cohorts of juvenile American shad Alosa sapidissima in the Pamunkey River,Virginia. The ages of juveniles captured by push net during the juvenile abundance index surveysin 1998 and 1999 were estimated by counting daily rings on the sagittal otoliths. Weight-at-ageand abundance-at-age data were used to generate instantaneous daily rates of growth and mortalityfor 5-d cohorts. In 1998, the peak hatch date lagged behind the peak spawning periods that hadbeen inferred from collections of American shad broodstock and occurred after peak spring flowevents. In 1999, the correspondence between the hatch date distribution and the peak spawningperiods was greater than in 1998. The instantaneous daily growth rate was relatively constantbetween cohorts, ranging from 0.037 to 0.064 in 1998 and from 0.046 to 0.066 in 1999. Theinstantaneous daily mortality rate was more variable between cohorts, ranging from 0.047 to 0.084in 1998 and from 0.044 to 0.093 in 1999. The physiological mortality rate, or M/G, was calculatedfor all cohorts. Most cohorts in 1998 and 1999, including the largest cohorts in both years, hadan M/G value close to 1.0, indicating that these cohorts were barely maintaining or losing biomassduring the early juvenile stage. The results of this study indicate that in both 1998 and 1999 theyear-class declined in biomass during the early juvenile stage and underwent demographic struc-turing that affected the composition of the population that ultimately migrated to sea.
Variability in the growth and mortality rates ofyoung of year fish is usually high, and small dif-ferences among years will determine the strengthof a year-class (Houde 1987, 1989). Within a year-class, variability in growth (G) and mortality (M)between intraannual cohorts will determine the rel-ative strength of each cohort (Rutherford and Houde1995). The progeny of temperate zone fishes thathave protracted spawning periods may experiencea wide range of temperatures and hydrographic con-ditions, leading to potentially large differences inrecruitment between intraannual cohorts. Larval-stage biomass increases when M/G, the ‘‘physio-logical mortality rate’’ (Beyer 1989), is less than1.0 and decreases when M/G is greater than 1.0.The size or time of transition, when M/G 5 1.0, isan important stage-specific parameter for determin-ing year-class strength (Houde 1997). Cohorts thattransition earlier will contribute more biomass tothe year-class, assuming that the trend through suc-cessive stages of early life history is towards a lowerM/G.
Variability in recruitment between intraannualcohorts has been observed in American shad Alosasapidissima, an anadromous alosine clupeid nativeto the Atlantic coast of North America. This var-
* Corresponding author: [email protected]
Received December 15, 2003; accepted June 14, 2004
iability has been attributed to dynamics duringboth the larval and juvenile stages. In the Con-necticut River, variability in mortality during thelarval stage establishes the year-class strength ofAmerican shad (Crecco et al. 1983; Savoy andCrecco 1988). Crecco and Savoy (1984) found thatthe juvenile abundance index (JAI) of Americanshad in the Connecticut River correlated signifi-cantly with June river temperature and flow, evi-dence that survival depends on conditions that af-fect both the larval and transitional stages. Theseconditions are likely to favor survival of certaincohorts within the year-class. In the Hudson River,the 1990 year-class of juvenile American shad wascomposed mostly of fish hatched after June 1, eventhough spawning activity peaked in early to mid-May (Limburg 1996). Limburg concluded that thatthe low survivorship of larvae hatched early in theseason was due to high river flow and low watertemperature. In a retrospective analysis of the oto-liths of returning adults, Limburg (2001) foundthat survivorship was greatest among early andlate juvenile cohorts. This finding suggested thatthere had been two stages of low survivorship—the first affecting larvae that were hatched early,and the second affecting juveniles that werehatched late.
For age-0 American shad, instantaneous dailymortality during prejuvenile life stages decreases
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FIGURE 1.—Map of the Pamunkey, Mattaponi, and York rivers. Stations on the Pamunkey River were locatedfrom river km 74 to river km 130. The Pamunkey and Mattaponi rivers are tidally influenced throughout the regionsshown.
with increasing age or size (Crecco et al. 1983;Houde 1997). Estimates of weight-specific instan-taneous daily growth during early life history varybetween stages, but a general pattern of asymptoticgrowth is observed among tributaries. Americanshad from the Connecticut River have two rapidgrowth phases (Crecco et al. 1983)—the first fromhatch to day 20 and the second after juvenile meta-morphosis, from day 30 to day 70. Growth slowsat age 75–100 d, or when 87–106 mm long. Growthof Hudson River juveniles is similar to that ofConnecticut River juveniles; the juvenile growthrate slows after age 120–140 d, or at 80–100 mmlength (Limburg 1996). An analysis of stage-specific growth and mortality of larval Americanshad from the Connecticut River indicated thatthese fish reach transition (M/G 5 1.0) when about12 mm long, or at 4.1 d posthatch, shortly afterfirst feed (Houde 1997). In Houde’s (1997) anal-ysis, variability in M/G between year-classes partlyexplained the relative strength of recruitment: theweakest two year-classes (1982, 1984) were as-sociated with a later age at transition and higherinitial M/G, whereas strong year-classes (1979,1980) had an earlier age at transition and a lowerinitial M/G.
Investigating growth and mortality of youngAmerican shad is important because populationsof American shad have been in decline throughoutEast Coast tributaries since the 1800s (ASMFC1999). Historically, the most productive systems
were located in the mid-Atlantic, from New Yorkto North Carolina (Leim 1924). In Chesapeake Baytributaries, spawning American shad deposit eggsin tidal and nontidal freshwater from March toJune (Murdy et al. 1997). The York River, locatedin the southern end of the Chesapeake Bay wa-tershed, currently supports the largest stock ofAmerican shad in Virginia (Olney and Hoenig2001). Spawning American shad in the York Riverprovide the primary broodstock for hatchery-basedstock enhancement on the James and Potomac riv-ers (Olney et al. 2003). The York River, a coastalplain tributary, is formed at West Point, Virginia,by the convergence of the Pamunkey and Matta-poni rivers (Figure 1). The Pamunkey River has alarger watershed, 3,768 km2, and higher averagespring flows, 47.5 m3/s, than the Mattaponi River,2,274 km2 and 27.2 m3/s, respectively (Bilkovicet al. 2002). Both the Pamunkey and Mattaponirivers are free-flowing and tidally influencedthroughout the spawning and nursery grounds ofAmerican shad.
In the York River, the spawning grounds en-compass the tidal freshwater regions of the Pa-munkey and Mattaponi rivers. The spawning sea-son for York River American shad is protracted(late February through June) and individualsspawn in batches every 3–4 d (Olney et al. 2001).From 1997 to 1999, Bilkovic et al. (2002) collectedAmerican shad eggs from the Pamunkey River atriver kilometers 98–150 (measuring from the
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3JUVENILE AMERICAN SHAD GROWTH AND MORTALITY
mouth of the York River), the highest densitiesbeing found between river km 104 and 131, andcollected larvae between river km 76 and 128. Inthe same period, they also collected American shadeggs from the Mattaponi River at river km 81–124, the densities being highest between river km96 and 124; larvae were collected between riverkm 68 and 124. American shad eggs and larvaewere more abundant in the Mattaponi than the Pa-munkey River by factors of 5.5 and 4.4, respec-tively (Bilkovic et al. 2002). Together, these tem-poral patterns in reproduction and spatial patternsin egg and larval abundance suggest that multiplecohorts of juvenile American shad are producedduring the spawning season at different times andlocations. Detection of these cohorts and knowl-edge of their growth and mortality therefore re-quire study of the age composition, size distri-bution, and catch rates of juveniles on the spawn-ing grounds.
In this study, we analyzed the intraannualcohort-specific growth and mortality of juvenileAmerican shad collected in the JAI push-net sur-veys on the Pamunkey River in 1998 and 1999using weight-at-age and abundance-at-age data.We also characterized hatch date distributions inrelation to river flow, temperature, and spawningstock biomass. Our objectives were both to quan-tify intercohort variability in mortality and growthand to interpret how this variability during earlylife stages influences juvenile demographics.
Methods
The study used juveniles captured in the JAIsurveys on the Pamunkey River in 1998 and 1999.This tributary was chosen because otoliths of thesejuveniles were scanned for hatchery marks in ac-cordance with a mandated monitoring program toestimate the contribution of hatchery fish to thewild population (ASMFC 1999). At the time,hatchery-marked larvae were not released on theMattaponi River and no otoliths-monitoring pro-gram was in place there.
Pamunkey River American shad were capturedweekly by using a bow-mounted push net, whichis a 5.2-m-long (body, 3.0 m; and cod end, 2.2 m)3 1.5-m 3 1.5-m four-panel, modified Cobb trawlnet with 1.27-cm stretch mesh in the cod end (Krie-te and Loesch 1980). The push net was deployedfrom a 6.4-m-long skiff. We made 10 cruises in1998 and 13 cruises in 1999. During each cruise,at least three stations were randomly chosen withineach of four adjacent regions 9.3 river km long.Stations are designated at every 1.9 river km.
About 12 stations per week were sampled fromlate May or early June to mid-August. The surveybegan at river km 130 and proceeded downriver.Sampling commenced 45 min after sunset, whenjuvenile American shad are most vulnerable to thepush net (Kriete and Loesch 1980; Loesch et al.1982; Wilhite et al. 2003). Each sampling lasted5 min. The push net was deployed at the surfaceand pushed downriver along the central axis of theriver channel. Tidal conditions, as well as surfaceand air temperature, were recorded for each tow.All specimens were identified in the laboratory,weighed to the nearest 0.01 g wet weight, andmeasured to the nearest 0.1 mm fork length (FL).A power model was fit to weight and FL data for1998 and 1999 with a least sum of squares method,that is,
bW 5 aL , (1)
where W is the weight (g), L is FL (mm), a is theintercept, and b is the allometric scaling coeffi-cient.
Catch rates of gravid American shad were ob-tained from the Virginia Department of Game andInland Fisheries (VDGIF 1998, 1999). During thespawning season, VDGIF deploys 91.4-m (300-ft)drift gill nets nightly at river km 72.4 to obtainbroodstock. Each evening, approximately ninenets are set for 3 h each (various mesh sizes, 0.11–0.15 m or 4.5–5.75 in). Nets were set from 16March to 7 May in 1998 and from 17 March to 8May in 1999. Gravid and nonspawning femaleswere counted and these data were used to generatecatch-per-unit-effort data (CPUE; number of fe-males per net). In both years, collection ended afterseveral consecutive days in which no gravid fe-males had been taken.
Daily mean water temperature in the nurseryzone at river km 72.4 (Temp72.4) was estimated byregressing temperature data collected by VDGIFduring broodstock collection with continuous wa-ter temperature data collected by Virginia Instituteof Marine Science (VIMS) on the York River atriver km 8 (Temp8). The regression for 1998 wasTemp72.4 5 1.006 1 1.047Temp8 (R2 5 0.79); for1999 it was Temp72.4 5 3.42 1 0.70Temp8 (R2 50.87). Daily mean water flow rates for the Pa-munkey River from 1 March to 31 August wereobtained from the U.S. Geological Survey gaugingstation no. 01673000, located near the fall line inHanover County, Virginia.
Sagittal otoliths were removed from juvenileAmerican shad captured during the 1998 and 1999
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push-net surveys on the Pamunkey River. Otolithswere cleaned, mounted in epoxy, ground in thesagittal plane with use of increasingly fine com-mercial sandpaper, and polished to the core on bothsides (Secor et al. 1991). In age-0 American shad,increments are laid down daily, the first incrementbeing deposited on day 1 for larvae raised at 158Cand 188C (Savoy and Crecco 1987). Daily incre-ments were counted along the major posterior axisunder a light microscope at 1003 magnification.Ages were estimated by averaging two indepen-dent counts of otolith increments. Age estimateswere discarded if the difference between the rep-licate counts exceeded 10% of the average.
After hatch dates were back-calculated as theday of capture minus the age at capture, each ju-venile was assigned to a cohort, which was definedas the group of all fish hatched within a 5-d period.The 5-d period was chosen because Crecco andSavoy (1987) estimated that young American shadcould be assigned to 5-d cohorts with greater than85% accuracy. Cohorts were numbered from 1(earliest hatch), starting with the first 5-d periodin which a juvenile was hatched (e.g., day of year91–95). The hatch date distribution was adjustedfor mortality because by the time of capture, fishthat had hatched earlier in the season experiencedgreater mortality than younger fish. The abundanceof each cohort was adjusted by dividing the abun-dance by the cumulative mortality experiencedfrom the hatch date of the cohort to the hatch dateof the youngest cohort. Cohort abundances for1998 and 1999 were corrected for effort becausethe survey began when the oldest cohort was 51–55 d old and ceased when the youngest was 56–60 d old. When possible, we adjusted hatch datesfor cohort-specific daily mortality; when a cohort-specific mortality rate was unavailable, we usedthe average of the cohort-specific mortality rates.The adjustment applied only to the juvenile stage;the abundance of each cohort was adjusted to age30 d, which was an approximation for the age atmetamorphosis (26–29 mm FL).
Cohort-specific instantaneous growth rates wereestimated for only those cohorts that were caughtfor more than 1 month (five or more cruises) toensure that rates were representative of the juve-nile stage duration that was characterized (;50 d).Additionally, growth rates were estimated only forthose cohorts containing at least 10 individuals forwhich age could be estimated. This number hadbeen determined to be the minimum number offish required to obtain a regression power of atleast 0.8, given a 5 0.05. In addition to the cohort-
specific rate, a juvenile growth rate was calculatedby pooling data for all age-estimated fish. Thegrowth in weight trajectory was exponential. Thegrowth rate was calculated as the slope of the lin-ear regression according to the following expo-nential growth function:
log (W ) 5 log (W ) 1 G(t 2 t )e t e t 2 12 1(2)
where W is the wet weight (g), t1 denotes time athatch, t2 denotes some time after hatch, and G (d21)is the instantaneous growth rate. The model wasfit to weight-at-age data. Residuals from the re-gression were tested for normality (Kolmogorov–Smirnov) and homogeneity of variance (LeveneMedian).
Cohort-specific instantaneous mortality rateswere estimated for those cohorts for which agrowth rate was calculated. Similar to the growthrate analysis, a juvenile mortality rate was cal-culated by pooling data for all age-estimated fish.The mortality rate was calculated as the slope ofthe linear regression according to the exponentialpopulation model
log (N ) 5 log (N ) 1 M ,e t e 0 t (3)
where Nt is the population size at time t, N0 is thepopulation size when the juveniles first fully re-cruit to the gear, and M (d21) is the instantaneousmortality rate. This model was fit to abundance-at-age data. Age data were binned in 5-d incre-ments and the bin was assigned the value of theaverage age of fish within the bin. As with thegrowth rate analysis, residuals from the regressionwere tested for normality and homogeneity of var-iance. Three assumptions were made in the mor-tality analysis. First, juveniles were fully recruitedto the gear on the date of the greatest catch rate.Second, gear efficiency was constant for all co-horts. Third, availability of each cohort was con-stant after the fish had fully recruited to the gear.
The recruitment potential was analyzed for thosecohorts for which growth and mortality were avail-able by calculating the physiological mortality rate(M/G) and the change in biomass of those cohortsfrom metamorphosis (age 30 d) to age 80 d. Theerror in M/G was estimated assuming that the er-rors in M and G are distributed normally. The frac-tional error (coefficient of variation, CV) in M/Gcan be obtained from the fractional error in M andG, that is, CVM/G 5 [CVM
2 1 CVG2]1/2. Cohort
biomass was estimated by using the relationshipreported by Houde (1997),
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5JUVENILE AMERICAN SHAD GROWTH AND MORTALITY
(1 2 M/G)B 5 B [W /W ] ,s s21 s s21 (4)
where Bs is the biomass or relative biomass at theend of the stage (day 80), Bs21 is biomass at thebeginning of the stage (day 30), and Ws and Ws21
are individual body weights at the respectivetimes. The initial cohort biomass, Bs21, was cal-culated by multiplying the average weight of ajuvenile 29–31 d old (Ws21 5 0.33 g in 1998, 0.31g in 1999) by the number of juveniles at meta-morphosis estimated from the hatch-date analysis(adjusted for mortality). The weight of an indi-vidual at day 80 was estimated by using the cohort-specific growth rate.
We used ancillary data to evaluate the possibil-ity that migration from the nursery zone might biasmortality rate estimates. Seasonal changes in thedistribution of juvenile American shad were de-scribed from the VIMS Juvenile Trawl Survey(1956 to present) and Striped Bass Seine Survey(1968 to present). These surveys sample sites fromthe upper Mattaponi and Pamunkey rivers to thelower York River and Chesapeake Bay annually.The downstream limit (i.e., closest to ChesapeakeBay) of the juveniles was determined by identi-fying for each gear, from July to March, the far-thest downstream catch of American shad. TheTrawl Survey operates year-round and samplesmonthly the entire length of the York River. Thesurvey uses a 9.1-m (30-ft) semiballoon otter trawlwith a 38.1-mm (1.5-in) stretch mesh body and a6.35-mm (0.25-in) mesh cod end liner, two 71-cm3 48-cm steel china-v doors, and an attached tick-ler chain. The Striped Bass Seine Survey operatesfrom July to mid-September and samples the York,Pamunkey, and Mattaponi rivers twice per month.The seine survey uses a 30.5 m long 3 1.22 mdeep 3 0.64-cm mesh bag seine.
The analysis of trawl and seine collections in-cluded only data from the York and Mattaponi riv-ers because too few fish were captured by beachseine on the Pamunkey River. Although this pre-vents description of movement within the tidalfreshwater of the Pamunkey River, any movementinto the brackish York River would be detected bythis method. Additionally, only data from thoseyears in which juvenile American shad werecaught in both tidal freshwater and brackish waterswere included so that the movement of juvenilescould be followed through the entire year. Con-current seine and trawl data were available from1968 to 2001. In that period, 21 years had spatialdistribution data that were suitable for describingthe downstream limit of the juvenile population.
The years 1973–1979, 1981, 1983, 1984, 1987,and 1991 were excluded. Data from 1973 to 1979were excluded because the Seine Survey was notfunded during these years. In 1981, 1983, and 1984no juveniles were captured in the Seine Survey.In 1987 and 1991, the Trawl Survey captured nojuveniles in the York River.
Results
Length and Weight Data
In 1998, American shad were collected in 10cruises from 8 June (day of year [DOY] 159) to11 August (DOY 223). In 1999, American shadwere collected in 13 cruises, from 23 May (DOY143) to 16 August (DOY 228). The length andweight distributions from 1998 and 1999 were sig-nificantly different (Kolmogorov–Smirnov, P ,0.05). The mean FL of juveniles captured in 1998was 40.6 mm, whereas in 1999 it was 46.1 mm.Fork lengths of American shad caught in 1998ranged from 27.0 to 99.1 mm, and from 25.4 to96.1 mm in 1999. Mean wet weights in 1998 and1999 were 1.1 and 1.4 g, respectively. Wet weightsof juveniles caught in 1998 ranged from 0.16 to12.35 g; they ranged from 0.14 to 12.21 g in 1999.A similar length to wet weight relationship wasobserved in both years. In 1998, the intercept (a)was 9.60 3 1026 and the allometric scaling factor(b) was 3.0683 (R2 5 0.97). In 1999, the interceptwas 7.06 3 1026 and the allometric scaling factorwas 3.1277 (R2 5 0.97).
Daily Aging and Cohort Analysis
A total of 416 American shad were captured in1998 and 328 in 1999. Otoliths of 364 (88%) ofjuveniles collected in 1998 and 273 (83%) of ju-veniles collected in 1999 were used in this study(Figure 2). The remaining otoliths were removedfrom the data set because either the percent dif-ference in age estimates exceeded 10% or the oto-liths were damaged during preparation. A contin-gency test found no statistically significant bias inaging between the two trials (Evans and Hoenig1998; P , 0.05). The mean age at capture in 1998was 46 d; in 1999, it was 50 d. In 1998, capturedAmerican shad ranged in age from 23 to 89 d; in1999, ages ranged from 28 to 85 d.
Sixteen cohorts were identified in 1998 (Table1). Hatch dates spanned from 6 April to 24 June(DOY 96–175). Cohort 11 was the most abundant(n 5 78, Table 1). Fewer than 10 juveniles werecaptured from cohorts 1–6 and 15–16. Seventeencohorts were identified in 1999 (Table 2). Hatchdates were similar to those in 1998, spanning from
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6 HOFFMAN AND OLNEY
FIGURE 2.—Age distribution of juvenile Americanshad caught on the Pamunkey River in 1998 and 1999.
TABLE 1.—Characterization of 5-d cohorts of American shad from 1998, including hatch date ranges and capturedates (days of the year), number of juveniles captured and the cruises upon which they were captured, instantaneousgrowth (G; d21) and mortality rates (M; d21), M/G values, and associated standard errors (SE). Growth and mortalityestimates are significant at a 5 0.05 unless there is an asterisk, in which case the estimate is significant at a 5 0.10.Empty cells indicate that no regression was done because an insufficient number of fish were captured; ns indicates thata regression was done but the slope was not significant.
Cohort
Hatchdate
rangeFirst daycaptured
Last daycaptured
Numbercaptured
Number ofcruises
G(SE)
M(SE)
M/G(SE)
1 96–100 179 179 1 12 101–105 179 179 1 13 106–110 159 179 4 24 111–115 172 200 4 35 116–120 179 179 4 16 121–125 179 179 2 17 126–130 179 200 11 38 131–135 165 207 23 6 0.037 (0.014) ns ns9 136–140 165 207 46 7 0.054 (0.008) 0.047 (0.013) 0.86 (0.37)
10 141–145 172 223 70 7 0.064 (0.005) 0.067 (0.017) 1.04 (0.28)11 146–150 179 207 78 6 0.056 (0.008) 0.072* (0.031) 1.29 (0.59)12 151–155 179 223 55 7 0.046 (0.008) 0.084 (0.023) 1.81 (0.60)13 156–160 186 207 30 6 0.048 (0.011) 0.080 (0.019) 1.68 (0.54)14 161–165 193 214 24 415 166–170 207 214 9 216 171–175 214 214 3 1
1 April to 24 June (DOY 91–175). Cohort 6 wasthe most abundant (n 5 37, Table 2). Fewer than10 juveniles were captured from cohorts 1–2 and14–17.
The unadjusted hatch date distribution in 1998was dome-shaped, and few juveniles that hatchedbetween 6 April to 30 May were captured (DOY
97–150; Figure 3). The average mortality rate,0.070, was used to adjust the hatch date distri-bution for those cohorts for which no cohort-specific rate was available. The adjusted hatch datedistribution was bimodal, with peaks at 8–18 April(cohorts 1–3) and 28 May (cohort 11). Gravid fe-male CPUE fell to 0 on both 6 and 7 May. Afteradjusting for mortality, we determined that 67.5%of juveniles were hatched after 6 May (DOY 125,cohorts 7–16). The peak of the adjusted hatch-datedistribution lagged behind peak female CPUE.Catch rates of gravid females peaked between 29March and 6 April (DOY 88–96; VDGIF 1998),whereas only 13.2% of juveniles were hatched be-fore 15 April (DOY 105).
The unadjusted hatch dates in 1999 were moreevenly distributed among cohorts than in 1998.The average mortality rate used to adjust the hatchdate distribution was 0.065. A strong peak in theadjusted hatch-date distribution occurred on 18April (DOY 108, cohort 4) and the distributionoverall was skewed toward younger cohorts. Grav-id female CPUE fell to 0 on 8 May. Fewer juve-niles (17.2%, cohorts 8–17) were hatched after 6May in 1999 than in 1998. As in 1998, there wasa lag between peak spawning activity and peakhatch date of juveniles. Catch rates of spawningfemales peaked between 4 April and 8 April (DOY94–98; VDGIF 1999). Many more juveniles,28.2%, were hatched before April 15 in 1999 thanin 1998.
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7JUVENILE AMERICAN SHAD GROWTH AND MORTALITY
TABLE 2.—Characterization of 5-d cohorts of American shad from 1999, including hatch date ranges and capturedates (days of the year), number of juveniles captured and the cruises upon which they were captured, instantaneousgrowth (G; d21) and mortality rates (M; d21), M/G values, and associated standard errors (SE). Growth and mortalityestimates are significant at a 5 0.05 unless there is an asterisk, in which case the estimate is significant at a 5 0.10.Empty cells indicate that no regression was done because an insufficient number of fish were captured; ns indicates thata regression was done but the slope was not significant.
Cohort
Hatchdate
rangeFirst daycaptured
Last daycaptured
Numbercaptured
Numberof cruises
G(SE)
M(SE)
M/G(SE)
1 91–95 151 164 2 22 96–100 151 164 2 23 101–105 151 164 11 34 106–110 143 192 26 8 0.056 (0.009) ns ns5 111–115 151 186 29 5 0.052 (0.007) ns ns6 116–120 151 186 37 5 0.046 (0.008) 0.067 (0.019) 1.45 (0.48)7 121–125 151 192 32 6 0.051 (0.008) ns ns8 126–130 157 192 32 6 0.066 (0.010) ns ns9 131–135 164 214 21 8 0.064 (0.015) 0.056* (0.024) 0.88 (0.43)
10 136–140 171 207 20 6 0.053 (0.013) 0.093 (0.026) 1.75 (0.64)11 141–145 179 228 19 7 0.053 (0.020) 0.044 (0.014) 0.83 (0.41)12 146–150 186 221 14 5 0.046 (0.013) ns ns13 151–155 186 221 13 514 156–160 192 228 8 615 161–165 207 221 8 216 166–170 214 228 2 217 171–175 221 221 4 1
FIGURE 3.—Hatch date distribution of juvenile Amer-ican shad caught in 1998 and 1999 (white bars representcohorts that were unadjusted for mortality, gray barscohorts that were adjusted for the average mortality, andblack bars cohorts that were adjusted for cohort-specificmortality), along with daily catch per unit effort of ripefemale shad caught on the spawning grounds (CPUE;females per net), average daily river discharge (solidlines) and estimated average daily river temperature (T;dashed lines). See Methods for calculations.
Adjusted hatch dates were also evaluated in re-lation to available temperature and flow data forthe Pamunkey River (Figure 3). In 1998, numerouslarge flow events occurred during the spawningperiod and river temperature rapidly increased.Most juvenile American shad were hatched afterflow events equal to or greater than 200 m3/s hadceased and river temperature was greater than208C. In 1999, river flow slowly declined duringthe spawning period and the river was cooler thanin 1998. Only the last few cohorts were hatchedafter river temperature exceeded 208C.
In general, cohorts that were hatched progres-sively later in the season were captured on pro-gressively later cruises (Figures 4, 5). Addition-ally, cruises in the middle of the sampling seasontended to catch the largest number of cohorts. Theoldest cohorts (i.e., earliest hatched) were not col-lected on initial cruises and were captured on rel-atively few cruises. In 1998, cohorts 1–2 were ob-served only on cruise 4. In 1999, cohorts 1–2 wereobserved on cruises 2–4. The youngest cohortsalso were caught on relatively few cruises. Cohorts8–13 were captured in six or more cruises in 1998,as were cohorts 4 and 7–11 in 1999.
Growth Analysis
All regressions were statistically significant (P, 0.05). Most regressions met the assumptions ofnormality and homogeneity of variance, though
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FIGURE 4.—Hatch date distribution by cruises for juvenile American shad captured in 1998. Frequencies are notadjusted for mortality.
data for cohorts 9 and 12 from 1998 failed to meetthe former assumption (P , 0.05). Instantaneousgrowth rates were similar between cohorts in 1998(Table 1); cohort 10 had the highest growth rate(0.064) and cohort 8 the lowest (0.037). The datasuggest a pattern of increasing growth rate to co-hort 10 and decreasing thereafter (Figure 6). Thegrowth rate for all cohorts in 1998 was 0.039/d(SE 5 0.003, P , 0.0001, R2 5 0.33). Growthrates were also similar between cohorts in 1999(Table 2); cohort 8 had the highest rate (0.066) andcohort 6 the lowest (0.046). No trend in growthrates was apparent (Figure 6). The rate for all co-
horts in 1999 was 0.050/d (SE 5 0.003, P ,0.0001, R2 5 0.51). Additionally, the cohort-specific growth rates for 1998 and 1999 were cor-related with the average daily river temperatureexperienced by each cohort from metamorphosis(age 30 d) to age 80 d. No relationship was found(Pearson’s correlation, P 5 0.5). Cohort-specificgrowth rates were similar between the 2 years,even though the average temperature experiencedby each cohort during the 50 d period ranged from29.38C to 29.98C in 1998 and from 20.08C to23.18C in 1999.
The average size of juveniles was compared
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FIGURE 5.—Hatch date distribution by cruises for juvenile American shad captured in 1999. Frequencies are notadjusted for mortality.
among the cohorts to determine whether later co-horts were smaller than earlier cohorts (Tables 3,4). Systematic differences in size could biasgrowth estimates, but there was no evidence ofbias in either year. In 1998, there was not a sig-nificant difference among the average fork lengthsfor each of the cohorts (analysis of variance [AN-OVA], P 5 0.8). In 1999, however, there was asignificant difference between lengths (ANOVA,P 5 0.001). A Tukey’s comparison test indicated
that cohort 12, which hatched late in the year, wasunusually large and cohort 6, which hatched earlyin the year, was unusually small (P , 0.05). Thesetwo cohorts had the lowest growth rates (Table 2).
Mortality Analysis
For all cohorts analyzed, juveniles becameavailable to the gear at about age 30 d, were cap-tured at peak abundance at ages 35–45 d, and de-clined thereafter. In 1998, the mortality rates of
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FIGURE 6.—Cohort-specific instantaneous dailygrowth (G; filled circles), mortality (M; open circles),and physiological mortality rate (M/G; diamonds) forjuvenile American shad captured in the Pamunkey Riverduring 1998 and 1999. Error bars represent one standarderror. The dotted lines indicate the transition point forthe physiological mortality rate (i.e., where M/G 5 1.0).
TABLE 3.—Length and age of American shad cohorts from 1998 for which instantaneous growth and mortality rateswere calculated. The minimum (Min), maximum (Max), mean, and SD of both fork length and age are reported foreach cohort.
Cohort
Length (mm)
Min Max Mean SD
Age (d)
Min Max Mean SD
8 29.0 67.6 38.3 10.2 30 69 46.7 15.09 28.0 75.8 38.6 10.2 28 69 43.0 11.2
10 27.1 99.1 40.9 12.9 27 80 44.8 11.711 27.3 78.5 40.5 11.4 26 75 45.1 8.912 27.0 77.9 41.4 10.6 26 72 42.8 9.613 27.4 77.0 40.7 9.6 23 66 42.9 9.7
four cohorts were statistically significant (P ,0.05, Table 1). Cohorts 9 and 13 failed to meet thetest for homogeneity of variance. Instantaneousmortality rates increased throughout the season,from cohort 9 (0.047) to 12 (0.084), and then de-clined slightly (Figure 6). The standard errors forthe mortality estimates were greater than for thegrowth estimates. Mortality estimates were lessprecise than growth estimates because mortalitycurves were generated from data in four to eightage bins. The two cohorts with the largest number
of juveniles captured, cohorts 11 and 12, had rel-atively high mortality rates. Hydrographic condi-tions were stable during the juvenile stage and didnot appear to be important to mortality. As pre-viously noted, the river temperature experiencedby each cohort varied only slightly. Similarly, theaverage daily flow varied little from July to Au-gust, ranging from 2 to 4 m3/s. The mortality ratefor all cohorts was 0.096/d (SE 5 0.012, P ,0.0001, R2 5 0.90).
The log-transformed catch-at-age data for eachof cohorts 9–13 (1998) were fit to the followingexponential decay model to test the hypothesis thatcohorts 9 and 13 failed the test for homogeneitybecause mortality was declining with age(SigmaPlot 6.00):
2Mtlog (N ) 5 a·e .e t (5)
The regression was significant for cohorts 9, 10,12, and 13 (P , 0.05). The exponential decay mod-el had a smaller residual sum-of-squares than didthe exponential model (equation 3) for cohorts 10and 12. It is therefore likely that for at least somecohorts the mortality rate was not constant duringthe juvenile stage and declined with age. The pa-rameters of the exponential decay models, how-ever, had large standard errors; the CV of the decaycoefficient, a, ranged from 0.40 to 1.21. The CVof the mortality rate estimated in the exponentialdecay model was either equal to or greater thanthat estimated from the exponential model, exceptfor cohort 10. This was the only cohort for whichthe exponential decay model performed better thanthe exponential model in all aspects (residual sumof squares, R2, parameter errors). Because of thelarge errors of the parameters, we did not includethe exponential decay models in any further anal-yses.
In 1999, the mortality rates of three cohorts werestatistically significant (P , 0.05; Table 2). Cohort9 failed the test for normality. Instantaneous mor-D
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11JUVENILE AMERICAN SHAD GROWTH AND MORTALITY
TABLE 4.—Length and age of American shad cohorts from 1999 for which instantaneous growth and mortality rateswere calculated. The minimum (Min), maximum (Max), mean, and SD of both fork length and age are reported foreach cohort.
Cohort
Length (mm)
Min Max Mean SD
Age (d)
Min Max Mean SD
4 28.2 72.4 43.4 11.6 33 85 52.6 10.95 33.9 71.7 43.3 10.4 37 74 50.3 11.16 30.0 61.3 40.0 7.7 31 66 42.6 9.17 27.9 74.8 42.9 10.7 28 70 44.1 11.58 27.4 67.0 45.6 11.9 29 65 47.5 11.39 27.0 67.1 49.8 10.9 32 83 55.8 12.2
10 30.3 61.0 47.2 9.2 32 70 47.8 9.111 25.4 65.0 45.7 9.0 36 83 53.6 11.4
FIGURE 7.—Locations of the most downriver sites atwhich juvenile American shad were captured in the Mat-taponi and York rivers in each month of the 21 yearsduring 1968–2001 for which catch data were analyzed.See Methods for details regarding the fishing gear. Riverkm 0–48 run from the mouth of the York River to WestPoint, Virginia (brackish water), and river km 48–801are the Mattaponi River (freshwater).
tality rates were generally 0.05–0.07, though themortality for cohort 10 was higher (0.093). Notrend between cohorts was apparent (Figure 6). Asin 1998, hydrographic conditions did not appearto be important to mortality. The average temper-ature experienced by cohorts 4–12 increasedthroughout the juvenile stage (30–80 d) from20.08C to 23.18C. Average daily flow rates weresimilar to 1998 and ranged from 2 to 4 m3/s duringJuly and August 1999. The juvenile mortality ratefor all cohorts was 0.078/d (SE 5 0.013, P 50.0004, R2 5 0.85).
Similar to 1998, an exponential decay model(equation 5) was fit to cohorts 6, 9, 10, and 11 totest whether mortality declined with age. The onlysignificant regression was for cohort 6. The ex-ponential decay model, however, had a higher re-sidual sum of squares than that for the exponentialmodel (equation 3). Thus, as for 1998, the standard
errors of the parameters were large and the ex-ponential decay model was not included in anyfurther analyses.
Historic catch data from the VIMS Seine andTrawl Surveys suggest that migration from thesampling zone did not occur during the studymonths (June–August). In the 21 years analyzed,juvenile American shad caught from July to Oc-tober were exclusively distributed in tidal fresh-water; the farthest downstream that American shadwere captured was river km 48, which is the gen-eral location of the salt wedge of the York Riverestuary (Figure 7). The downstream limit of Amer-ican shad caught in November spanned both tidalfreshwater and brackish water. In December, thedownstream limit varied within brackish water,ranging from the upper to the lower York River(km 0–47). In January and February, the Americanshad that were caught the farthest downstreamwere concentrated downriver, with the largest pro-portions close to the river mouth (km 0). In March,the downstream limit was distributed throughoutthe York River, from river km 0 to 47.
M/G and Recruitment Potential
In 1998, the physiological mortality rate wasgreater than 1.0 for all cohorts except cohort 9,indicating that almost all the cohorts were losingbiomass (Table 1; Figure 6). Cohorts with M/Gestimates close to 1.0 may have been either in-creasing or decreasing in biomass, given the stan-dard errors about the estimate. M/G increased withsuccessive cohorts, corresponding to the succes-sive increase in mortality. Mortality and growthwere not correlated (Pearson’s correlation, P 50.4); the two cohorts with the most and leastgrowth both had intermediate mortality (cohorts10 and 12, respectively). The M/G for all cohortswas 2.46 (SE 5 0.36), indicating that the year-class was losing biomass.
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TABLE 5.—Change in relative biomass (%) of cohortsfrom metamorphosis at day 30 (B30) to day 80 (B80). Onlythose cohorts from 1998 and 1999 for which physiologicalmortality rate (M/G) values were available were analyzed.
Year Cohort B30 B80
1998 9 7.7 19.810 29.4 45.511 32.9 25.712 18.2 5.013 11.9 4.1
1999 6 19.7 9.79 25.4 51.8
10 40.8 7.811 14.1 30.7
In 1999, the physiological mortality rate wasmore than 1.0 for cohorts 6 and 10 and slightlyless than 1.0 for cohorts 9 and 11 (Table 2; Figure6). Again, the biomass trends for those cohortswith estimates close to 1.0 are difficult to interpretbecause of the standard error of M/G. As in 1998,we saw no relationship between mortality andgrowth in 1999 (Pearson’s correlation, P 5 0.8);the two cohorts with an intermediate growth rate,0.053, were associated with the highest and lowestmortality rates (cohorts 10 and 11). As opposedto 1998, no trend in M/G during the sampling sea-son was apparent. The M/G for all cohorts waslower than in 1998, 1.58 (SE 5 0.27), indicating,as in 1998, that the year-class was losing biomass.
The contribution of each cohort to the year-class, measured as percent biomass, changed dra-matically during the early juvenile period in both1998 and 1999 (ages 30–80 days; Table 5). Of thefive cohorts for which M/G values were availablein 1998, only cohorts 9 and 10 increased theircontribution to the year-class. Cohort 10 increasedits relative contribution even though its M/G wasslightly greater than 1.0. Cohort 11 decreasedslightly and cohorts 12 and 13 decreased greatly.In 1999, those cohorts with an M/G less than 1.0increased their contribution to the year-class (co-horts 9 and 11), whereas the contributions of co-horts 6 and 10 decreased.
Discussion
Temporal patterns of spawning inferred fromcollections of American shad broodstock weregenerally unrelated to production of cohorts of ju-veniles in 1998. The earliest juvenile hatch date(DOY 97) was 23 d after gravid females were firstcaptured. The time between peak gravid femaleCPUE and peak hatch date (adjusted for mortality)was 60 d. Additionally, few hatch dates of juve-
niles overlapped catches of hydrated females. Thistime lag between spawning activity and juvenilehatch dates could not be accounted for by the eggdevelopment period. Limburg (1996) reported thategg development time decreases linearly as a func-tion of temperature. Based on this relationship, eggdevelopment time was about 10 d for cohort 1, 7–8 d for cohorts 2–4, 4–6 d for cohorts 5–10, andonly 2.5–3.5 d for cohorts 11–16. It is thereforelikely that selective larval mortality influenced therelative abundance of juvenile cohorts. Mortalityamong the earliest spawned larvae was probablyextreme; few juveniles were hatched from this pe-riod (DOY 88–93), even when hatch-date distri-butions were adjusted for juvenile mortality. Therewas also a mismatch in the production of cohortshatched late in the season. Cohorts 7–16 hatchedafter the end of egg-taking in 1998. These cohortsaccounted for 62.0% of juveniles (adjusted formortality).
The correspondence between the temporal pat-terns of spawning and juvenile hatch dates wasgreater in 1999 than in 1998. The difference be-tween the date on which the first gravid femalewas captured and the first juvenile was hatchedwas 9 d. The peak catch in gravid females wasabout 14 d before the peak hatch date (DOY 106–110; cohort 4). This time lag is consistent withestimated egg development periods. The egg de-velopment time was estimated to be 18 d for cohort1, 11–15 d for cohorts 2–5, 8–10 d for cohorts 6–9, 5–7 d for cohorts 10–14, and 4 d for cohorts15–17. As in 1998, there was a mismatch in theproduction of cohorts hatched late in the season.Cohorts 8–17, which accounted for 17.2% of ju-veniles, hatched after the end of egg-taking in1999.
The mismatch between the catch of gravid fe-males and the production of cohorts spawned latein the season suggests the VDGIF egg-taking effortfailed to detect late-spawning American shad. Therun timing and spawning location of the late-spawning adults is not known, even though theseAmerican shad produced the greatest number ofjuveniles captured in the 1998 survey. Juvenilehatch dates in this study suggest that adults spawnuntil late June. During 1998–1999, maturing fe-males were observed to enter the York River untilmid-May (Olney et al. 2001). Adults were ob-served emigrating from the river as late as mid-June in 1999 (Olney and Hoenig 1999). Becausethe VDGIF samples at only one site in the down-stream end of the nursery habitat and their col-lection efforts are influenced by hatchery produc-
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tion requirements, VDGIF sampling may not befully representative of temporal trends in spawningactivity. An icthyoplankton survey conducted onthe Pamunkey River in 1998, however, concurredwith the VDGIF data, suggesting that their indexof gravid females is a reasonable proxy for spawn-ing activity. Eggs were collected from the Pamun-key River during a period that closely matched thespawning period observed in the VDGIF survey,from 2 April to 14 May (DOY 92–134; Bilkovicet al. 2002).
A similar time lag between peak egg productionand peak hatch date of juvenile American shad wasobserved by Limburg in the Hudson River (1996;lag 5 35 d), as well as for striped bass Moronesaxatilis in the Pamunkey and Patuxent rivers(Rutherford and Houde 1995; McGovern and Ol-ney 1996). Striped bass spawn concurrently withAmerican shad, and juveniles also rear in tidalfreshwater. Limburg (1996) concluded that themostly likely explanation was differential recruit-ment of cohorts, not delayed development time ofeggs or age underestimation. This explanation isconsistent with hydrographic and temperature datafrom the Pamunkey River for 1998; few hatcheswere observed during late spring when large flowevents were occurring (Figure 3). Fifty-two per-cent of the juveniles we collected were hatchedafter 20 May (DOY 140; cohorts 10–16), afterlarge flow events had ceased and water tempera-ture remained above 208C. Those juveniles thatwere hatched during periods of unstable flow dur-ing 1998 formed weak cohorts that were not per-sistent in the push-net collections. The exceptionsto this pattern are cohorts 1 and 3. A relativelyhigh rate of mortality (M 5 0.070) was assumedwhen adjusting hatch dates of those cohorts forwhich no cohort-specific rate was available, butwe do not know if this was the actual mortalityexperienced by these cohorts. This is probably anextreme scenario. For example, if mortality werehigh during the larval stage and moderate duringthe juvenile stage, few juveniles would be cap-tured. If we assume a juvenile mortality rate of0.019 (Crecco and Savoy 1987), then the hatchdistribution adjusted for mortality closely approx-imates the unadjusted distribution in both 1998 and1999.
Reduced variation in river flow and cool tem-peratures throughout the spawning period in 1999may explain the longer hatching period and closecorrespondence to the spawning index. Bilkovic(2000) explored the relationship between riverflow and the JAI for American shad in the Mat-
taponi and Pamunkey rivers for the period 1990–1999. Although mean, minimum, and maximumlow flow in May was positively correlated with theJAI in the Mattaponi River, no strong relationshipwas detected in the Pamunkey River. The com-bination of stable hydrographic conditions andhigh zooplankton density has been positively cor-related with survivorship of larval American shad(Crecco et al. 1983; Crecco and Savoy 1984, 1987;Limburg 1996) and larval striped bass (Rutherfordand Houde 1995).
Comparisons to other studies of growth in ju-venile American shad were limited because datafrom our study were fit to an exponential growthfunction, whereas other investigators have foundgrowth for age-0 American shad to be asymptotic(Chittenden 1969; Marcy 1976; Crecco et al. 1983;Limburg 1996). An asymptote in growth was notobserved in this study because the growth rates ofjuvenile American shad slow around 80–100 mmFL and these juveniles are generally able to avoidthe pushnet (Loesch et al. 1982). The juvenilescaptured in this study were about 30–80 d old andgenerally ranged from 27 to 70 mm FL, whichcorresponds to the second phase of rapid growthin American shad early life history identified byCrecco et al. (1983).
Instantaneous growth rates were less than thatreported for larval American shad and similar tothose reported for juveniles of other anadromousspecies. American shad approaching juvenile tran-sition in the Connecticut River had a reportedgrowth rate of 0.21/d (Houde 1997). Juvenilestriped bass in the Pamunkey and Mattaponi riverswere estimated to have an instantaneous growthrate of 0.124/d (95% CI 5 0.078–0.170/d) and0.054/d (95% CI 5 0.042–0.066/d), respectively(Kline 1990). Rates reported in the literature arefor the entire year-class and are not cohort-specific.Growth rates for all juvenile American shad, 0.039in 1998 and 0.050 in 1999, are similar to that re-ported for striped bass.
Different size among cohorts was not a sourceof bias in growth rate estimates. The averagelength did not differ among cohorts in 1998. Thedifference was significant in 1999, although thedifference was not systematic (i.e., length did notdecrease in successively younger cohorts) andthere was no evidence of bias in the growth rates.Additionally, both year-class and cohort-specificrates could be biased if large juveniles are able toavoid the net. Growth rates would be underesti-mated, and thus M/G overestimated, if the gearselected for the slowest growing individuals. The
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majority of fish used in the growth rate analysiswere less than 70 d old (95% in 1998, 94% in1999). At age 60–70 d, the average length of ju-veniles was 51 mm in 1998 and 57 mm in 1999.The largest fish younger than 70 d in 1998 was78.5 mm; this was 86.7 mm in 1999. Although theactual size distribution of the population is un-known, these results suggest growth rates are de-rived from juveniles that are, on average, relativelysmall and also the push net caught fast-growingindividuals.
Instantaneous mortality rates estimated in thisstudy were higher than those reported elsewherefor juvenile American shad. In an analysis of the1979–1987 year-classes, Savoy and Crecco (1988)reported a daily mortality rate for juveniles (ages30–100 d) of 0.018. Mortality rates among otherclupeids vary. Juvenile blueback herring Alosaaestivalis from the Rappahannock River were re-ported to have an instantaneous mortality rate of0.073 and 0.035 for 1991 and 1992, respectively(Dixon 1996). Age-0 gulf menhaden Brevoortiapatronus demonstrated variable mortality betweenconsecutive years and different habitats, rangingfrom 0.0075 to 0.0209 (Deegan 1990). As withgrowth rates, mortality rates from previous studiesapply to the entire year-class, whereas estimatesreported here are cohort-specific. Juvenile mortal-ity rates estimated in this study for all cohorts(0.096 in 1998 and 0.078 in 1999) were higherthan that reported by Crecco et al. (1983) and Sa-voy and Crecco (1988), but close to the highestrate reported by Dixon for blueback herring(1996). The juvenile mortality rate in both 1998and 1999 estimated from all cohorts was greaterthan the cohort-specific rate reported for most co-horts; only the rate for cohort 10 in 1999 exceededthe respective rate for the year-class.
For at least some of the cohorts, mortality prob-ably declined with age. The exponential decaymodels provided a better fit to the catch at age datafor two of five cohorts in 1998. The large standarderrors of the parameters in the exponential decaymodel were the result of generating the mortalitycurves from a small number of age bins. Binningwas necessary because only a small number of fishwere captured in these 2 years of low abundance.This limited the complexity of models that couldbe used.
The analysis of historic catch data from theVIMS trawl and seine surveys suggested that mi-gration from the nursery habitat did not bias mor-tality estimates. A clear migration signal was ob-served in the trawl and seine catch data; in all 21
years, the downstream limit of juvenile Americanshad was never located in brackish water until No-vember. This study targeted juveniles in tidalfreshwater from late May to mid-August. Both theTrawl Survey and the Seine Survey sample theentire river system monthly through Septemberand no juvenile American shad were captured inthe brackish York River. There was no geographicoverlap in catch in the years analyzed; only theseine survey captured juveniles in tidal freshwater.All juveniles caught in the brackish York Riverwere captured by bottom trawl, which may not beas efficient as the push net in capturing pelagicjuveniles. The trawl catch data, therefore, probablyrepresent migration behavior for a very large num-ber of juveniles, and at this time we can neitherprove nor disprove the hypothesis that a few ju-veniles enter brackish waters in summer or earlyfall. Juvenile American shad have been estimatedto enter brackish waters as early as late June, orat age 41 d, in the Hudson River (Limburg 1995).
Observed migration patterns from northern andsouthern rivers are consistent with juveniles re-maining on the nursery grounds through early fall.On the Connecticut River, migration from the riverbegins in mid-September, corresponding to a de-crease in water temperature to below 208C, andends in late October to early November (O’Learyand Kynard 1986). In rivers farther south, such asthe York River, migration does not begin until Oc-tober or November, as noted for the Potomac River,Virginia (Hildebrand and Schroeder 1928); theCape Fear River, North Carolina (Davis and Cheek1967); and the Altamaha River, Georgia (Godwinand Adams 1969). Davis and Cheek (1967) ob-served that migration on the Cape Fear River be-gan when river temperature decreased to about208C, similar to data from the Connecticut River.The Pamunkey River surface temperature had notdecreased to below 208C by the end of samplingin either 1998 or 1999. Surface water temperaturein the lower Pamunkey River was 298C at the endof August 1998 and ranged from 28.28C to 29.98C(river km 58–89) in mid-August 1999.
Growth rates varied little among either cohortsor years. Mortality rates varied among cohorts butthe range was similar in both years. Intercohort var-iation in M/G was largely determined by the inter-cohort variation in mortality, which was greaterthan the intercohort variation in growth. The errorsin both growth and mortality rates were inverselyrelated to cohort size. The error in the mortalityestimates is sufficient to generate relatively largedifferences in cohort success, particularly consid-
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FIGURE 8.—Juvenile abundance index for the Pamun-key River, 1979–2002. Index values are not availablefor 1988–1990 (marked by asterisks). The mean indexvalue is indicated by the black horizontal line.
ering the lengthy stage duration of about 50 d (Hou-de 1987). Houde (1987), simulating cohort survival,demonstrated that larval growth has a large contri-bution to cohort survivorship. Yet, larval growthalone is not sufficient to explain relative year-classsuccess, even though larval mortality is growth-dependent (Houde 1997). We found no trend be-tween growth and mortality in 1998 or 1999. Al-though mortality did not appear to be related togrowth, evidence suggested that mortality declinedwith age. If the decline in mortality is due to theincrease in size, then mortality should decline fast-est for the fastest growing cohorts. The analysis isproblematic, however, because of the low confi-dence in the estimates of the decay coefficients. Thisproblem highlights the difficulty of interpreting thepopulation dynamics of year-classes with poor re-cruitment.
Variation in hydrographic conditions (temper-ature and flow) did not explain the patterns in in-tercohort variation in growth and mortality or in-terannual differences in growth and mortality. Theriver temperature was, on average, about 6–98Chigher in 1998 than in 1999, but the cohort-specificgrowth rates were similar in the 2 years. River flowwas low and constant at about 2–4 m3/s during theearly juvenile period in both 1998 and 1999. Dif-ferences in growth and mortality between yearsmay be due to density-dependent factors; however,no data were available to evaluate this possibility.
The physiological mortality rate of two cohortsin 1998 (12, 13) and one cohort in 1999 (10) wasgreater than 1.0 by more than the standard error,suggesting these cohorts were losing biomass.Most cohorts had ratios that were close to 1.0, andthe error in M/G inhibits further interpretation.Larval American shad from the Connecticut Riverwere found to reach transition (M/G 5 1.0) at arelatively early stage, after which time their bio-mass began to proliferate (Houde 1997). In thislight, the M/G estimates in this study are greaterthan would be expected if M/G were assumed tocontinue to decrease throughout early life history.Although it is possible that the M/G values areoverestimated because of biased mortality rates,the lack of evidence for migration during the studyperiod and the observation that mortality declinedwith age support the observation that mortalitywas high and that demographic restructuring wasoccurring during these 2 years. The possibility ofdemographic restructuring, that is, the change ofthe relative biomass contributed to the year-classby each cohort, was indicated by the analysis ofrecruitment potential. The percent biomass each
cohort contributed to the year-class decreased byas much as 33.1% from metamorphosis to the endof the juvenile stage. The relative physiologicalmortality rate was important to demographic struc-turing; although cohort 10 in 1998 slightly de-clined in biomass during the juvenile stage (M/G5 1.04), the recruitment potential increased be-cause it declined at a slower rate than other cohortsin the year-class.
The calculation of physiological mortality ratesand the analysis of recruitment potential assumedthat growth and mortality were constant during theearly juvenile stage. The exponential model pro-vided a better fit to the data for all cohorts except10 and 12 in 1998. Additionally, the exponentialmodel had a smaller residual sum-of-squares thanthe exponential decay model for 1998 and 1999when all the cohort data were pooled. If mortalitydid decline with age, then assuming a constant ratewouls bias the analyses. The mortality rate wouldbe underestimated for young fish within the cohortand overestimated for old fish within the cohort.Cohorts 10 and 12 in 1998 may have declined inbiomass during the initial juvenile stage andreached transition by the end of the early juvenilestage. Further study on juvenile physiological mor-tality rates, particularly with larger sample sizes,is important to predicting the time at transition, animportant parameter for determining cohort suc-cess.
In 1998 and 1999, the JAI was similar (1.1 and0.8, respectively) and quite low compared withthose for years with strong year-classes (Figure 8).The juvenile growth and mortality rates estimatedfor the entire population were more precise than
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the cohort-specific rates and the M/G values in-dicate that, in both years, the biomass of the pop-ulation declined during the juvenile stage. Theweak year-class strength may be explained in partby physiological mortality rates unfavorable tobiomass proliferation in almost all cohorts duringthe juvenile stage. A growth and mortality analysisof Pamunkey River juvenile American shad duringyears with a high JAI value would be useful foraddressing this question. In addition, a tributarycomparison of vital rates between the MattaponiRiver, where production is geater, and the Pamun-key River, where production is less, would provideadditional insight.
It is not clear whether juvenile mortality andgrowth are as important to overall year-classstrength and demographic structuring within year-classes as larval mortality and growth. Althoughcohort-specific physiological mortality rates willdetermine the demographic structure of juveniles,specific life history characteristics (stage duration,variability in M and G) are important to determinewhether the physiological mortality rate experi-enced during the juvenile stage will affect year-classstrength. Various studies using life-tables have dem-onstrated that juvenile growth and mortality can beimportant to year-class strength (Smith 1985; Baileyet al. 1996; Barros and Toresen 1998; Quinlan andCrowder 1999). Houde’s (1997) analysis of Amer-ican shad dynamics did not include postmetamorph-ic stages. Limburg (2001) found that juvenile co-horts from the Hudson River may experience highmortality during migration to sea; the hatch datedistribution of adults from the 1990 year-class thatreturned to spawn in 1995 suggested selective sur-vival of early and late migrants. American shad,however, mature at ages 3–7 and the 1990 HudsonRiver year-class probably did not fully mature until1997 (Maki et al. 2001). In the Connecticut River,the mortality rate of juveniles rearing in freshwaterhabitat was low (;2%/d) and was similar betweenstrong and weak year-classes (Crecco et al. 1983),indicating that juvenile dynamics were not impor-tant to year-class strength. The comparison betweenstudies is problematic because Limburg (2001) in-vestigated cohort-specific differences, whereasCrecco et al. (1983) investigated population-leveldifferences.
The hypothesis that is most consistent with thisstudy, as well as previous research on the York,Connecticut, and Hudson rivers, is that larvalgrowth and mortality of American shad determineyear-class strength and the hatch date distributionat metamorphosis, whereas juvenile growth and
mortality determine the demographic structure ofthe population that eventually emigrates to sea buthave a weaker effect on year-class strength. Thestage-specific dynamics during early life historymay be complex, such that the cohort or year-classbiomass may be decreasing in multiple periods.Complex stage-specific dynamics are possible ifan organism undergoes an ontogenetic shift acrossniches that are separated by different vital rates(Werner and Gilliam 1984). Thus, the year-classbiomass may decline and the demographic struc-ture may shift several times during early life. Apositive relationship has been observed betweenabundance indices for juvenile American shad andreturning adult populations in the Connecticut Riv-er (Savoy and Crecco 1988). The correlation wasstrongest at the extremes of year-class strength andweakest at intermediate values. High physiologicalmortality rates (M/G . 1.0) over the course of thejuvenile stage can contribute to this lack of rela-tionship, as well as other life stages in whichAmerican shad are vulnerable to biomass decline,including emigrating juveniles and immature fishof ages 1–3, whose fate at sea remains unknown.
AcknowledgmentsThis research was funded by the Wallop-Breaux
program of the U.S. Fish and Wildlife Servicethrough the Marine Recreational Fishing AdvisoryBoard of the Virginia Marine Resources Commis-sion (grant numbers F-116-R-1, 2, 5, and 6) andby the Anadromous Fish Conservation Act, PublicLaw 89–304 (Grant-In-Aid Project AFC-28, grantnumber NA86FA0261, and Project AFC-30, grantnumber NA96FA0229) from the National MarineFisheries Service. We gratefully acknowledge thefollowing individuals who conducted field sam-pling: J. Goins, G. Holloman, S. Denny, C. Leigh,P. Sadler, K. Maki, and M. Wilhite. M. Wilhitemade the age estimates for the juvenile Americanshad and conducted a preliminary analysis onwhich our paper is partly based (Aiken 2000). Wethank Rob Latour (Virginia Institute of Marine Sci-ence, Glocuester Point, Virginia) for his thoughtfulinput on the growth and mortality analyses. Themanuscript was improved by comments from EdHoude (Chesapeake Biological Laboratory, Solo-mons, Maryland), as well as from two anonymousreviewers. This is contribution number 2635 of theVirginia Institute of Marine Science, College ofWilliam and Mary.
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