Sea-ranching of brown trout, Salmo trutta L

Fisheries .Wtrtiu,qemetir litid E C O I O ~ I W4. I . h7 - 76

Sea-ranching of brown trout, Salmo trutta L. N. JONSSON, B. JONSSON & L.P. HANSEN Ncirwegiutr Itirrirure for Nuture Reseurch. Trondheim. Norwuv

Abstract In total, 8211 l-year-old and 14 839 2-year-old hatchery-reared brown trout, Sufmo rruttu L., from 11 stocks were released at the mouth of the River Imsa, south- western Norway. The recapture rates and total estimated yield were higher for 2- than l-year-old trout, a:lthough recapture rates varied between years of release and stocks. The recapture rate increased with mean individual weight at time of release. Total estimated yield from the individual groups of 1+ trout ranged from 2 to 20kg per lo00 trout released and for 2+ trout between 11 and 250kg per 10o0 fish released. In all cases, yields were lower than the economic break-even yield. Most fish were recaptured the year of release (89.2% of 2+ and 76.2% of 1+ trout). Almost 31% of the recaptures were caught at sea and 69% in fresh water; 95.8% of the latter were taken in the River Imsa trap.

K E Y W O R D S : ranching, trout, yield.

Introduction

Recently. fish farniing in the sea has increased substantially. There is also a growing interest in ranching, where artificially reared juveniles are released into natural waters with a view to harvesting them at a larger size. Ranching is applicable to anadromous and resident species, which can be harvested at or near the point of release. Fish species used in sea ranching include Oncorhynchus species, sturgeon, Acipenser sturio L., and Atlantic salmon, Sufmo sufur L. (e.g. Isaksson 1988). However, little is known about the suitabilily of brown trout, Sufmo rruttu L., for ranching.

Brown trout consists of permanently resident freshwater and anadromous stocks, (Jonsson 1985, 198'9). Anadromous brown trout remain in fresh water until smolting, at a length between 10 and 25cm. Downstream migration tends to occur in the spring when the fish move to the fjords and coastal waters to feed during the summer (L'AMe- Lund, Jonsson, Jensen, Saettem, Heggberget, Johnsen & Nmje 1989; Berg & Jonsson 1yyO). In autumn, immature and mature fish return to their home river to spend the winter in fresh water. The migratory fish repeat the movement to the sea each summer, returning to fresh water in late summer and autumn.

To test if brown trout are suitable for fjord ranching in Norway, young hatchery- reared brown trout from 11 Scandinavian stocks were released at the mouth of the River Imsa, south-western Norway. The economic viability of ranching was determined from recapture rates and estimated yields of 1- and 2-year-olds released.

Correspondence: Dr N Jonsson, Norwegian Institute of Nature Research, Tungasletta 2, N-7005 Trondheim, Noway.

67

68 N. JONSSON ET AL.

Materials and methods

Between 1983 and 1989 inclusive, a total of 8211, 1-year-old and 14839, 2-year-old hatchery-reared brown trout from 11 Scandinavian stocks were released at the mouth of the River Imsa, south-western Norway (Table 1). The brood stocks were caught in 11 different rivers (Fig. 1) and eggs and milt were brought to the Ims hatchery, fertilized and the progeny reared to the smolt stage.

More than 2 weeks before release, the fish were anaesthetized with chlorobutanol and individually tagged with numbered Carlin tags (Carlin 1955), and body weight (nearest g) and total length (nearest mm) were measured. Recaptures were made at sea, in fjords and fresh water by commercial fishermen and anglers. They reported body length, weight, fishing gear used, and time and place of recapture. All fish ascending the River Imsa were caught in a box trap, situated lOOm above the river mouth. The trap was monitored twice a day throughout the study period, and length and weight of

NORWAY SWEDEN

J I

Figure 1. Sites where brood stocks were collected: ( 1 ) Emin. (2) Vanern, ( 3 ) Hunder, (4) Sandvik. (5) Randselv, (6) Vikersund, (7) Tunhovd, (8) Imsa. (Y) Granvin, (10) k r d a l and ( 1 1 ) Gjengedal. The hatchery- reared fish were released at the mouth of the River lmsa (8).

SEA-RANCHING BROWN TROUT 69

Table 1. Number rclca:icd and recaptured. and total yicld (kg pcr I(JfMj rclcased tish) of individual groups rclcascd as I- and 2-ycar-old brown trout at the mouth of the River Imsa.

Agc 1 Age 2

Rccapturcd Recapturcd Stock Ycar of Rclcascd Rclcascd Total

Stock type rclcasc ( n ) n YO ( n ! n 46 yield

Emin

Randsclv

Sandvik Tunhovd

I rnSJ

Granvin Llcrdal

Hundcr

Vikcrsund

Vancrn

Gjengedal

Total

Sea trout

Rcsidcr t

Sea trotif Rcsidcrit

Sea trout

Sca tro dt

Sca trout

Residcn t

Rcsidcnt

Rcsidcn t

Sea trout

1983 1985 1986 1987 I988 1989 1 083 1986 1983 I983 I985 1986 I987 lY8X I wo I983 1085 I986 I987 1Y83 1985 1986 1 Y85 1986 1987 1986 lY8Y 1987 1988 1 98Y 1987 1988

396 4YY

2362 5(W) YY3

497

479

402

995 W8

8211

5 12 34 6

19

31

13

I6

11 80

227

I .3 2.4 1.4 1.2 I .9

6.2

2.7

3.3

1.1 8.0

Y95

Y50 4Y 7 686

lJY4

498 453 uh hY I 478 M9 4YY 40 1 583

w 482 493 457 452 499 Y61 747

499 I4839

163

87 47

146 2Jo

w 24 31

363 I29 1 I7 89 41 39

32 I 47 63 17

173 185 35 26

20 2502

16.4

Y.2 9.5

21.3 16.1

19.9 5.3 7.0

52.5 27.0 23.9 17.8 8.4 6.7

32.1 9.8

12.8 3.7

37.1 3.6 3.5

38.3

4.0

I26 4

10 4 2 I 1 42 45 76

I01 10 53 21 39

60 10141 35 17 26 9

125 18 14 25 71

128 9

15 2

14 14

238

all fish caught were recorded. Both juvenile and adult fish returned to the River Imsa. Irrespective of stock, returning immature fish often exhibited a silvery coloration whereas adults were generally darker brown.

Carlin tagging reduced survival of young fish; untagged wild Atlantic salmon smolts in the River Imsa had a sea survival rate more than double (7.7%) Carlin-tagged smolts (3.1%) (Hansen 1988).


From salmon stocking experiments in Norway, it has been suggested that approximately 50% of recaptures are reported (Hansen & Jonsson 1989). Thus, to account for non-reported tags of fish caught in sea water in estimating biomass and yield, the number of recaptures was adjusted by multiplying by a factor of two.

At Ims, production expenses for I+ and 2+ trout are estimated at N.kr. 7 and 10, respectively (1993 price). At an average first sale value of brown trout of N.kr. 30/kg, the cost benefit ratio of 1+ and 2+ achieves economic break-even at yields of 233 and 333 kg per lo00 trout released, respectively.

Results

Recapture rates were higher for 2-year-old trout, l6.!l% of the total number released, compared with 2.8% for I-year-olds (x’= 1006.24, df = 1, P<O.OOI) (Table 1). The proportion of fish recaptured in the River Imsa, relative to those at sea, was higher for brown trout released as 2-year-olds (67.2%) than 1-year-old trout (55.1%) (x’ = 13.74, df = 1, P<O.OOI). Recaptures of individual groups released as 1-year-olds range from 1.1 to 8.0%, and for 2+ trout from 3.5 to 52.5%. Most recaptures were reported the year of release, particularly for 2+, 89.2% of the total number recaptured compared with 76.2% for 1+ trout (x1=33.28, df = 1. P<0.001).

For all groups, the recapture rate ( R ) increased with mean weight (W) at release (Fig. 2):

R = 0.084 W + 7.63, r = 0.708, n = 17, P < 0.01.

Mean recapture rate for 50g trout was 4.1% and for 250g trout 23.2%. Mean weights of 1-year-olds ranged between 33 and 83g, and of 2-year-olds between 71 and 272g (Table 2).

The weight at recapture was higher for 2+ than 1+ trout, except for the Imsa and k r d a l stocks (Table 2) where the opposite was found. The mean recapture weights among stocks of 2+ fish varied between 199 and 766g and of I + fish between 142 and 380g. Total estimated yield of trout released as I-year-olds was lower than that of

.$ 2a - 4 - 24- c 2 20- f 16-

c

c 8 12-

a a -

20 40 60 80 1W 120 140 160 180 200 220 240 260 280 Weight (g)

Figure 2. Relationship between rccapturc-rate and mean body wcight at rclcase of I + and 2+ brown trout.


Table 2. Mean body weights ( W) at releasc and recapture of different stocks of brown trout, released as I - and 2-year-old fish. t shows significant differences in weight of recapture between fish released as 1- and 2-year-olds.

Age 1 Age 2

Releas: Recapture Release Recapture Stock W2SL) n W 2SD n W 2SD n WLSD n 1

EmPn Randsclv Sandvik Tunhovd lmsa Granvin h r d a l Hunder Vikersund VInern Gjengedal

~ ~ ~~ ~

62.4 2 i ~ . 7 500 379.6 2 368.x 67 241.1 2 98.7 163.2 2 74.3 163.4 2 55.4

56.2 2 14.6 50 166.1 2 324.5 29 220.2 ? 111.6 32.82 irl.1 50 3 s 9 . 3 - ~ 2 1 i . n 12 1 ~ ~ 2 5 6 . 1

YY.5 2 37.1 39.1 2 8.8 50 2xh.5 2 536.9 12 271.7 2 112.4

71.1 241.0 219.42 135.9

83.7 L 36'2 1 0 0 141.8 2 61.5 10 218.0 2 89.9 36.3 2 9.3 ioo 170.8 2 210.3 78 105.7 2 51.1

4YY

288 1400 WY 314 W ) 744 435 874 273

498 766.8 2 775.1 457.8 2 449.1 358.0 2 289.5 479.4 2 440.2 1YY.3 2 124.7 388.42 411.4 387.6 2 186.3 213.1 2 429.0 281.9 2 161.1 317.4 2 223.5 347.1 2 300.4

148 P <0.001 114 140 6% P-=0.001 355 P<0.01 35

2Y2 P>0.05 122 354 54 P<O.(W)l 17 P < 0.05

2+ trout, and in all releases yields were lower than the economic break-even yield (Fig. 3). The variation was high between stocks and years. One-year-old trout from the Emin stock yielded ca. 20 kg per lo00 fish released, and 2-year-olds from the Emin and Tunhovd stoc:ks gave the highest yields of 240 and 250 kg per loo0 trout released, respectively.

Brown trout were recovered from both the rivers and the sea along the Norwegian coast. Of the fish, 30.7% (n = 838) were recaptured at sea and 69.3% (n = 1891) in fresh water, 95.8% of the latter being caught in the River Imsa trap. The high recovery in fresh water at Ims is, at least partly, due to the 100% efficient trap fishery in the River Imsa. The proportion caught in coastal fisheries differed between stocks (Fig. 4a). recapture rates rainged between 1.7% and 14.9%. The recapture rates of the various stocks in the River Imsa were similar, except for the Vikersund stock, where a high percentage (32%) was recaptured (Fig. 4b).

Of the total number of fish recaptured, 17.2% were taken by rod and line in the sea, 12.7% in gill nets in marine waters, 66.2% were caught in the trap in the River Imsa and 2.6% by rod and line in fresh water. The remaining 1.3% were caught by other gears.

Discussion

The recapture rat,es of brown trout released as 2-year-olds (3-52%) were higher than for l-year-olds (l-8%). Similar results have been found in hatchery-reared Atlantic salmon smolts (Hansen & Lea 1982; Saunders, Henderson, Glebe & Loudenslager 1983; Hansen & Jmonsson 1989). This was due to the 2-year-olds being larger in sue and maturing earlier than l-year-olds. Larger and older trout generally have better ionic


20 - 18- 16- 14- 12- 10- 8- 6- 4-

-I 2- m

89

I 88 !?

250 7

200 -

150-

100 -

89 83

83 86

89 83

88 86 PC

tock

Figure 3. Estimated total yicld (kg per IONN) tish rclcascd) of brown trout rclcascd ;is (a) I-ycnr-old mid (b) 2-ycar-old fish at Ims. Thc figures bcsidcs thc dots arc the year o f rclcasc.

regulation in sea water than smaller fish due to their relatively larger volume to surface area (Parry 1960; Wagner, Conte & Fessler 1969). Moreover, the larger 2-year-old trout may be less vulnerable to predation than the smaller and younger conspecifics (Dill 1983). Also, larger fish can feed on a wider range of food items (Wootton 1W) and may be better competitors for food, as competitive ability is intrinsically linked to body size in an aggressive species like brown trout (cf. Wilson 1975).

The recapture rate of the 2-year-old Atlantic salmon smolts was of the same magnitude as 2-year-old hatchery-reared brown trout released into different Baltic Sea rivers (ranging from 3 to 52%) (Svardson & Fagerstrom 1982). Furthermore, in the Oslofjord, eastern Norway, recapture rates of the five releases with the highest returns ranged from 15.5 to 22.1% for sea trout and from 26.0 to 40.3% for non-migratory trout (Aass 1990). The present recapture rates for 2+ brown trout were higher than those of 2+ Atlantic salmon released at Irns (2.1-10.6%) (Jonsson, Jonsson & Hansen 1991), and 1+ rainbow trout, Oncorhynchus mykiss (Walbaum) (l.l-6.8%) released at Ims (Jonsson, Jonsson, Hansen & Aass 1993). The latter recapture rates were similar to those for 1+ brown trout.

At Irns, total estimated yield was highest for 2-year-old trout. The yields varied


2 25- r, 2 20-

15-

10 -

5 -

.-

.- il Q b

f - a) .g 1 5 , . .

. . . .

. - I I I r I I r I I r ( S t o c k

Figure 4. Proportional total yield (number rccapturcdlnumbcr rclcascd) of brown trout from different stocks harvestcd (a) in fjords and coastal waters and (b) in the fish trap in the Rivcr Imsa.

between 12 and 250 kg per lo00 fish released. In the Oslofjord, yield of released 2+ brown trout was of the same magnitude as at Ims (Aass 1990). There, the five releases with highest estiniated yield vaned between 176 and 356kg per lo00 non-migratory trout released and between 86 and 190 kg for sea trout. By contrast, yields of 2+ brown trout released in different rivers draining into the Baltic Sea were higher than those at Ims (Larsson, Steffner, Larsson & Eriksson 1979), between 9 and 908 kg per lo00 fish released. In the Baltic, both wild and hatchery-reared fish of the Eman stock, gave the highest yields, 787 and 908 kg per lo00 fish released, respectively. This stock also gave among the highest yields at Ims.

At present, sea ranching of brown trout at Ims is not economically viable. Two- year-old trout were closer to becoming economically viable than 1-year-old fish. Similar analysis has been camed out for sea-ranched Atlantic salmon at Ims, showing that 37.5% of the released groups of 1+ smolts and 66.7% of 2+ smolts gave economically profitable results for Norway (Hansen & Jonsson 1989).


Total estimated yield varied between stocks. This may be due to several reasons, including inherited differences in survival and growth-rates (Ryman 1970: Kanis, Refstie & Gjedrem 1976) and different tendencies to become residents (Jonsson 1989). Moreover, there may be different adaptations relative to the environmental factors encountered at Ims. However, variable yields appear not to be linked to the anadromous or freshwater resident origin of the stock, as both anadromous and resident fish exhibited low and high recapture-rates at sea and in fresh water. Conversely, brown trout populations vary in their tendency to migrate (Jonsson 1982: SvYrdson & Fagerstrom 1982). This may result in different habitat selection and feeding opportunities when released together at Ims. Furthermore, there are annual differences in yields. Such variations are to be expected because fish quality and environmental factors will differ between years.

Most recaptures were made the year of release, and in larger numbers in the river of release than in the sea. The migratory route of brown trout is relatively short, most fish stay and feed in the fjord, and enter the river of release in late summer and autumn (Jonsson, Jonsson, Hansen & Aass 1994). This parallels wild. anadromous brown trout. They stay close to the home river, and feed in fjords during summer (e.g. Jensen 1968: Jonsson 1985: Berg & Berg 1987; Lund & Hansen 1992). Studies of anadromous brown trout from the River Gjengedal showed that 97% of the recaptures were from within 100 km of the home river (Lund & Hansen 1992), and in the River Vardnes, northern Norway, brown trout migrated even shorter distances (Berg & Berg 1987).

The released brown trout also have a recreational value. Approximately 20% of the recaptures were caught by rod and line and ca. 13% with gill nets in the coastal and river sport fisheries. In the River Imsa, all fish are caught in a trap and sport tishing is prohibited. This influences the recreational value of the releases. The value would be greater in rivers where angling is allowed. Lund & Hansen (1992) reported that 19% of the recaptures of anadromous brown trout in the River Gjengedal were caught by anglers in fresh water and 4Y% were taken by recreational spinning and trolling at sea. The recreational value may add considerably to the total benefit of ranching brown trout.

At present sea-ranching of brown trout is not economically viable, when considering cost-benefit based on fish production costs and first sale value of the trout. However, the benefit may exceed the costs when local recreational fisheries are also taken into account, especially when 2-year-olds are released.

Acknowledgements

We thank our colleagues at the NINA Research Station at Ims for help with fish rearing and daily collection of the data, and Berit Larsen for helpful cooperation during the treatment of the material. The present research was supported by the Directorate for Nature Management.


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Sea-ranching of brown trout, Salmo trutta L