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Vertebral, scale and otolith characteristics ofsenile kahawai, Arripis trutta: implications forage estimation

R.W. Gauldie

76 Inglis Street, Seatoun, Wellington, New Zealand

Specimens of New Zealand kahawai (Arripis trutta; Bloch and Schneider) reared in theNational Aquarium of New Zealand (NANZ) from 1±2 year old ®sh in 1975 to the presentshowed outward signs of senility and were large (.75 cm fork length (FL)) compared withA. trutta in the wild, which usually have a maximum size of .60 cm FL. One of the NANZ®sh died on 12 April 1997 and a second ®sh was killed and both were then examined bycomputerized axial tomography (CAT) for vertebral anomalies and otolith location. Twoyounger NANZ specimens in the .48 cm FL size range were also killed and CAT scannedfor comparison, as were three specimens of orange roughy (Hoplostethus atlanticus;Collette). The patterns of incremental scale growth of the aquarium ®sh were comparedwith that of scales from an unusually large (75.4 cm FL) wild specimen of A. trutta from theMuseum of New Zealand collection.

KEYWORDS: Arripis trutta, CAT scan, Hoplostethus atlanticus, otoliths, scales, senility,vertebrae

INTRODUCTION

The physiological consequences of age in ®sh that involve deterioration offunction in one or more organs result in an increased risk of predation. For somelarger species, size alone may deter predation so that survival with signi®cantphysiological senility may occur. However, for small or moderate-sized ®sh (,1 m)the physiological dysfunction associated with senility would be expected to leadto increased predation mortalities. Thus, physiologically senile ®sh can beexpected to be hard to ®nd in the wild. Aquaria have an important role inproviding predator-free healthy environments within which ®sh can live to theirbiologically maximum age and die from natural causes. Such ®sh provide valuablebenchmarks in assessing techniques of age estimation in ®sh. The NationalAquarium of New Zealand (NANZ) has been particularly successful in maintainingits ®sh in peak health resulting in many specimens living far beyond the age (andsize) that they might be expected to reach in the wild.

This paper describes the vertebral and scale characteristics as well as the insitu positioning of otoliths of two senile specimens of kahawai, Arripis trutta(Bloch and Schneider), from the NANZ. The teleost family Arripidae (Percom-orpha) comprises a single genus with three species (Paulin, 1993) in temperateAustralian and New Zealand waters. Arripis trutta occurs in the coastal waters of

Aquarium Sciences and Conservation, 2, 53±66 (1998)

1357±5325 # 1998 Chapman & Hall

New Zealand, Chatham Island (Roberts, 1991), the Kermadec Islands, Norfolk andLord Howe Islands (Francis, 1991), New South Wales and Tasmania. There isconsiderable movement of tagged A. trutta around New Zealand (Wood et al.,1990) and the New Zealand population is regarded as a single stock of ®sh(Kilner, 1988). Three specimens of orange roughy Hoplostethus atlanticus: Colette(Trachichthyidae) were available and were also examined for vertebral charac-teristics. Hoplostethus atlanticus has been described as a very long-lived ®sh of.150 years maximum age (Smith et al., 1995). Although those extreme ages arenow in doubt (West and Gauldie, 1994; Romanek and Gauldie, 1996; Whiteheadand Ditchburn, 1996) it was nonetheless of interest to compare their vertebralcharacteristics with ®sh known to be physiologically senile at an apparentlymuch younger age.

Arripis trutta is a hardly aquarium ®sh. Twenty-two small A. trutta ofapproximately 25 cm total length (TL), 1±2 years old by conventional annualageing (Nicholls, 1973; Eggleston, 1975; Wood et al., 1990), were placed in theNANZ in 1975. The mortalities in this group of ®sh have been low. By 1987, 12specimens remained, one of which died of old age in that year and wasrecovered from the tank with little damage and frozen. Similarly, a second ®shdied of similar symptoms in 1989 and was also recovered intact and frozen. Thedetails of the otoliths and scales of these two specimens were described inGauldie et al. (1993).

Mortalities occur at night and by morning the dead specimens are usually sobadly damaged by other ®sh in the tanks that the specimens are not worthfurther study. However, by 1997 four of the original A. trutta remained alive, oneof which died on 12 April 1997 and was recovered undamaged and frozen. It wasdecided to examine this specimen in more detail including visualization of theinternal hard parts using computerized axial tomography (CAT) scans. Aprevious study using magnetic resonance imaging (MRI) of whole live ®shproved a valuable source of information about the internal soft and hard parts of®sh (Seri et al., 1995). The remaining three specimens showed a number ofabnormalities particularly in swimming and it was decided to kill one of these®sh to allow examination of a fresh specimen. This paper describes the vertebraland scale characteristics of the two large NANZ specimens of A. trutta. Twoyounger specimens of A. trutta and three specimens of H. atlanticus from theNANZ were examined for comparison. In addition, the scales from an unusuallylarge wild specimen of A. trutta from the collection of the Museum of NewZealand were also examined.

MATERIALS AND METHODS

Captive ®sh

The fork lengths (FLs) of the two large NANZ specimens of A. trutta were 75.4 and76 cm, respectively. They were con®rmed as A. trutta from the key provided inPaulin (1993). The frozen dead specimen was allowed to thaw and placed in 12%buffered formalin. The second specimen was recovered from the aquarium using anet and killed and placed in 12% buffered formaldehyde. At the same time, two

54 R. W. Gauldie

younger specimens of A. trutta (48.3 and 49.4 cm FL, respectively) from aquariumtanks were also killed and placed in formalin.

A sample of ten scales was taken from each ®sh posterior to the tip of thepectoral ®n in the area of the lateral line canal. Scales from this region have beenshown to have the least errors associated with scale reading (Dannevig and Host,1931). The scale annuli were counted using the method of Eggleston (1975).

Wild ®sh

On 14 July 1997 a 75.4 cm FL specimen of A. trutta was caught in the commercial®shery in the Bay of Plenty and donated to the National Museum of New Zealand(MONZ) collection (NMNZ p. 34778). A sample of ®ve scales from the post-pectoral®n region was made available for this study by the keeper of the NMNZ ®shcollection.

CAT scanning

The formalin container was a cylinder made of low-density polyethylene, 79 cmhigh and 48 cm wide, holding 120 l with a tightly ®tting lid. Once sealed thecontainer was passed through a General Electric computerized axial tomography(CAT) scanner at the Wake®eld Hospital in Wellington, New Zealand. Thespecimens were CAT scanned less than 12 h after being placed in formalin. Scanswere made at 5 mm intervals. However, a shorter series of ten scans at 1 mmintervals was made to provide a detailed image of a small section of the spine ofthe 76 cm FL, NANZ A. trutta. The CAT scan data were analysed using the GeneralElectric Three Dimensional Reconstruction software package on a Sun work-station. Images were printed on a Color Transfer E7014LA photographic printer.

Scales

The scales were inserted into a photographic enlarger and the subsequent imageexposed on photographic paper as if the scale were a negative. Data analysis wascarried out using Data Desk software.

Otoliths

The otoliths were cut in half in the vertical plane (i.e. the mediolateral plane insitu) and sections mounted with epoxy resin on stainless steel pin mounts. Half ofthe otolith was mounted on a glass slide and ground down to provide a thin(50 ìm) optical section using 4000 grit wet and dry paper on a petrographicgrinder.

Water temperature records

Temperature and salinity records from December 1978 to June 1997 were obtainedfrom the daily records maintained by the NANZ (R. Yarrell, personal communica-tion).

Vertebral, scale and otolith characteristics of A. trutta 55

RESULTS

Senility

Similarly to previously examined specimens (Gauldie et al., 1993), the two large A.trutta specimens from the NANZ had advanced locomotor dysfunction involvingexcessive rolling in the water and swimming head downwards or with a pitchingmotion, as well as whitening of the body colour and both exophthalmia and mildcataract in the eyes. These are all indications of senility in ®sh (Gerking, 1959;Comfort, 1961; Liu and Walford, 1969).

On the morning of 12 April 1997 the ®rst large specimen from the NANZ wasfound dead at the bottom of the main tank. The ®sh was measured as 75.4 cm FL.An autopsy showed that it was a male in good condition but with atrophiedgonads and with no obvious signs of internal disease, parasites or internallesions. The second NANZ ®sh was killed on 6 June 1997. The ®sh was measuredas 76 cm FL. An autopsy showed it to be a female in good condition but withreduced gonads and with no obvious signs of internal disease, parasites orinternal lesions. In both ®sh the spinal vertebrae were fused together into a rigidmass with the characteristic `bamboo' appearance (Fig. 1a, and b) of vertebralankylosing osteoarthritis in humans. It was concluded that both ®sh had died ofold age.

A short series of ten CAT scans at 1 mm intervals revealed the detailedstructure of the spinal column of the 76 cm FL NANZ ®sh which is shown in Fig.2a. The base of the neural spine at the top of the vertebrae arches backwards (asin Fig. 1b) and is lost in this section. The transverse processes of the vertebraepass backwards and are fairly short. The haemal spine passes downwards andbackwards. Although the foramen of the spinal nerve and the spinal cord areplainly visible, the gap in the two neural processes that form the neural spines isnot visible. The whole vertebrae have become so ossi®ed that the normallyvisible surface sculpturing of the vertebrae is lost obscuring the fusion of thehaemal and neural processes into the haemal and neural spines. In addition, theossi®ed surface shows the lumpy surface typical of osteophytes associated withosteoarthritis.

The CAT scans of both of the large NANZ specimens of A. trutta were ofsections taken at 19 and 20.5 cm from the anterior of the ®sh, respectively (Fig.2b). Both ®sh showed the small dark gas-®lled vesicles within the spinal ¯uidthat are indicators of changes typical of senile degeneration of the spinal columnin humans. Gas-®lled vesicles were not observed in the vertebrae of either of thesmaller specimens of A. trutta or the specimens of H. atlanticus.

Water temperature records

The mean annual water temperatures are shown in Fig. 3a plotted against time.Water is drawn directly from Hawkes Bay into the NANZ and ®ltered beforecirculation. The temperatures track the local environment temperatures. Themean monthly temperatures are shown as a histogram inserted into Fig. 3a. Themean summer±winter differential is approximately 5.6 8C.

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Fig. 1. Three-dimensional reconstructions of the spines of (a) the 75.4 cm FL NANZ and (b)the 76 cm FL NANZ A. trutta showing the characteristic bamboo appearance of ankylosingosteoarthritis. The scale bars are 5 cm.


Age estimationScales

Both of the NANZ specimens of A. trutta showed clear checks in their scales of thekind conventionally used in ®sh ageing (Kelley, 1988), including those ®gured by

Fig. 2. (a) Three-dimensional reconstruction of a vertebra from the spine of the 76 cmFL NANZ A. trutta. The neural (N) and haemal (H) spines are indicated with arrows.The transverse process (T) of the vertebra passes backwards and out of the scan.The foramen of the spinal cord (SC) and the spinal nerve (SN) are indicated witharrows. Irregular osteophytes (O) on the surface are indicated with arrows. The bar is5 mm.

58 R. W. Gauldie

both Eggleston (1975) and Gauldie et al. (1993). Approximately 70% of the scalesdid not show a clear sequence of checks and appear to correspond to the type ofscales described as `erosion' or `replacement' scales, but were in signi®cantlygreater proportions than those found in old ®sh of other species (Paget, 1920) aswell as A. trutta (Gauldie et al., 1993).

The year in which the 76 cm FL NANZ ®sh was spawned, as projected byageing one of the scales, was 1973. Both the otolith and scale ages predict that a25 cm ®sh would be between 1 and 2 years old (Eggleston, 1975). As the ®sh wascaught in 1975, the scale-based date agrees well with a 1973 projected date ofspawning. However, the other two readable scales from the same ®sh gave agesof 17 and 19 years, respectively. The true birth date of this group of senile A.trutta was likely to be 1973 making these ®sh 24 years old at death. Only twoscales from the sample of ten scales taken from the second ®sh showed clearchecks. One scale was counted as having 24 checks and the other 18 checks.

Fig. 2. (b) The sections of the 75.4 cm FL NANZ ®sh (left-hand side) and the 76 cmFL NANZ ®sh (right-hand side) show gas-®lled vesicles within the spinal cord (shortarrows). Other gas-®lled inclusions as well as the gas bladder can be seen in theabdominal cavity including that of the 49.4 cm FL NANZ A. trutta at the bottom of the®gure.


Counting the scale annuli using the method of Eggleston (1975) yielded an ageof .9 years for the 48.3 cm FL NANZ specimen and .10 years for the 49.4 cm FLNANZ specimen.

The scales from the 76 cm SL specimen from the NMNZ were aged using themethod of Eggleston (1975), giving ages of .14 years from two of the scales and.13 years from the third.

The cumulative incremental growth of the scales from the 48.3 cm NANZ, 76 cmNANZ and 75.4 cm MONZ specimens, respectively, are shown in Fig. 3b. Thescales of the 76 cm NANZ ®sh grew more slowly than those of the 48.3 cm NANZand 75.4 cm MONZ ®sh. The scales of the 48.3 cm NANZ and 75.4 cm MONZ ®shgrew at similar rates over the ®rst nine annual increments. The regression of the

Fig. 3. (a) Annual scale increment deviation (d) back calculated from the year ofdeath and mean water temperature deviation (j) plotted against year with an insertedhistogram of the mean monthly (January±December, 1±12) temperatures (8C). (b)The cumulative scale increment growth is plotted against years for the 48.3 cm NANZ®sh (+), the 75.4 cm MONZ ®sh (3) and the 76 cm NANZ ®sh (s).

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60 R. W. Gauldie

cumulative scale growth of the 48.3 cm NANZ ®sh on the equivalent growth ofthe 75.4 cm MONZ ®sh showed a strong correlation (r2 � 98:2%).

The previous study of Gauldie et al. (1993) showed similarities between thepattern of scale growth measured as annual growth increment widths comparedwith the mean annual temperature recorded in the aquarium. The annual scalegrowth increment width generally decreases with increasing age so it is notuseful to examine the simple correlation of annual scale growth incrementswidths with the mean annual temperature. Instead the cumulative scale growthover age was ®tted with a power function in the form: cumulative scalegrowth� A 3 ageb where A and b are constants. The values of the constants Aand b were similar (Table 1) for the 76 cm FL NANZ ®sh (scale age .24 years),the 75.4 cm SL NMNZ ®sh (scale age .14 years) and the 48.3 cm FL NANZ ®sh(scale age .9 years).

The power curve ®tted to the 76 cm FL NANZ cumulative growth curve wassubtracted from the measured cumulative scale increments to provide measuresof divergence from the expected underlying curve. These measures were plottedagainst the mean annual temperature. There was no signi®cant regression( p , 0:23), as is evident from the deviations from the mean temperature and themeasures of divergence from the expected growth, both of which are plottedagainst the year in Fig. 3a.

Otolith location

The CAT scans proved useful in revealing the in situ position of the otoliths. Thereare three otoliths in each of the paired inner ears of most teleost ®sh. The sagittais generally the largest with a smaller astericus located anteriorly to the sagitta.The lapillus is the smallest otolith and is usually located in the dorsal part of thevestibule of the semicircular canals of the inner ear. The sagitta and astericus areusually enclosed in the otic capsule which is also the articulating surface of thehyomandibular bone of the teleost jaw.

The otolith location of the 76 cm FL NANZ specimen of A. trutta is shown inFig. 4. The astericus of A. trutta is relatively small, approximately 4% of theweight of the sagitta and cannot be detected with a 5 mm section scan becauseof its close proximity to the sagitta. The lapilli of A. trutta are slightly smallerthan the astericus and cannot be detected above the sagitta in Fig. 4. Thesagittae of A. trutta can be rendered in three-dimensional in various aspects usingthe CAT software. The general positioning and three views of the sagittae in situ(from above, from the anterior and from the left aspect) show the otoliths sitingcanted at an angle such that they are closest together at their posterior ends anddiverge both anteriorly and in the horizontal plane.

Table 1. Values of the constants of the power curves ®tted to cumulative scalegrowth

Fish Constant A Constant b

76 cm FL NANZ 2.687 0.55675.4 cm SL NMNZ 2.362 0.69048.3 cm FL NANZ 2.348 0.789


Fig. 4. (a) A CAT scan through the head of A. trutta shows the sagittae as two bright whitespots sitting inside the otic capsule. The weakly resolved hyomandibular bones (white arrows)can be seen either side of the otic capsule. (b)±(d) Three-dimensional reconstruction of thepositioning of the otoliths of A. trutta.

62 R. W. Gauldie

The otolith location of the 43 cm FL specimen of H. atlanticus is shown in Fig.5. Again, the sagittal otoliths diverge anteriorly but less so in the horizontalplane. The lapilli can be seen lying higher in the lower brain case, out of the oticcapsule. The articulation of the hyomandibular bone against the otic capsule isvisible in Fig. 5d.

Discussion

The specimens of A. trutta described in this paper and previously in Gauldie et al.(1993) showed symptoms of physiological senility. However, these symptoms werealso present in ®sh that died in 1987 and 1989. This implies that A. trutta can,under favourable circumstances, live for a long time in spite of obviousphysiological impairment. In the wild, the obviously unusual swimming motionthat these ®sh exhibit would probably alert predators, leading to death.Nonetheless, A. trutta show that senility can persist for a long time, more thanone-third of the life of the ®sh to date. Unfortunately, the large 75.4 cm FLspecimen from the NMNZ could not be dissected to allow examination of thevertebrae. The specimen is preserved in formalin and it may be possible toconduct CAT scans at a future date.

A model animal that can live for a substantial fraction of its life with thephysiological consequences of vertebral senility and probably other tissue-speci®c forms of senile degeneration is of interest to the study of the biology ofhuman ageing. Arripis trutta has a maximum lifespan in excess of 24 years whichwould probably disqualify it from being a practical experimental model of humanageing (Masoro, 1990). However, the vertebral senility in A. trutta raises issues inage estimation in ®sh. Generally, among vertebrates longevity is a function of size

Fig. 5. (a)±(d) CAT sections through the head of H. atlanticus at intervals of 1, 1.5 and 1 cmshow the location of the sagittae (short arrow in (c)) and lapilli (long arrow in (c)). Thehyomandibular bone fo the jaw is indicated by a long arrow in (d).


(Blueweiss et al., 1978; Peters, 1983), with the largest members of any taxausually the oldest. This general observation may re¯ect the opposing effects ofcell damage caused by a high metabolic rate in smaller vertebrates, on the onehand and the protection that size confers on the other. Thus, the onset of bonydisorders may be determined by both time and genetics (Craig, 1985), butsurvival after the onset of bony disorders is probably determined mostly by size.The A. trutta specimens held at the NANZ showed signs of physiological senilityin 1989 (Gauldie et al., 1993), implying that A. trutta can survive with bonydisorders of the spine that are similar to those seen in other senile vertebratesfor at least 8 years, 33% of their life. Given the lack of any evidence of bonydisorders in the specimens of H. atlanticus, for which ages of .75 years can beimputed from the length at age curve of Smith et al. (1995), it would beinteresting to know more about the genetic differences in the onset of age-relatedbony disorders in teleosts which is only possible in aquaria.

Otolith location and its angular positioning in situ has not attracted muchattention in ®sh studies although it occupies an important place in Schuijf's(1981) theory of binaural hearing in ®sh. The CAT scans showed that the sagittaewere quite close together at their posterior ends, but diverged anteriorly. Thisarrangement would bene®t the ef®ciency of ®sh binaural hearing according toSchuijf's (1981) theory. However, the articulation of the hyomandibular againstthe otic capsule raises the possibility of sound conduction through the jaws. Ifthis is so, then binaural hearing may be further enhanced.

Surprisingly, the normalized annual scale increments of A. trutta in this studywere not correlated with the water temperature. The pattern of imbrication ofthe scales of A. trutta is not known (e.g. Burdak, 1985, pp. 42±86. If theconstraints on scale growth related to imbrication were known, then therelationship between the scale growth increments and temperature might beclearer. The scales from the senile aquarium ®sh, the large wild ®sh and thesmaller aquarium ®sh all showed similar growth patterns. This suggests that thescales of senile ®sh have continued to grow in a normal way. In addition, thesimilarity between scale growth in both the aquarium and wild ®sh means thatthe growth of scales in aquarium ®sh of known age can, at least for A. trutta, beused to benchmark age marks in wild ®sh. This is an important issue for A. truttabecause age estimation using otoliths has not been satisfactorily validated. Theotolith marks in a previous study overestimated age (Gauldie et al., 1993). Scaleshave a tendency to underestimate age but the difference between the mean scaleage and the true age is less than 10%, compared to the 1.5±3 times difference inage estimates from otolith checks. If scale ages that have been calibrated againstknown age in aquarium specimens are useful in wild ®sh, then their scale growthpatterns ought to be similar. This study shows that they are indeed similar for A.trutta, paving the way for validation of scale-based age estimation.

Acknowledgements

This paper would not have been possible without the enthusiastic support of RobYarrell the Director of the National Aquarium New Zealand and his staff,particularly Kerry Hewitt. I am grateful to Dr Richard Feltham of Wake®eld

64 R. W. Gauldie

Hospital for his generosity in allowing access to the CAT scanner and to DeliaMorrison, radiologist, for her patience in demonstrating the CAT scan software.The Orange Roughy Management Company supported this project and thespecimens of orange roughy examined in this paper were generously donatedby Terry Gittings of Moana Paci®c Fisheries, Napier, New Zealand.

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Accepted 19 March 1998

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