The hypothesis that interbreeding with domesticated rainbow trout reduces seasonal variation in seawater adaptability in wild-type anadromous steelhead trout ...
Aquaculture Aquaculture
121 (1994) 73-77
Hybridization with domesticated rainbow trout reduces seasonal variation in seawater adaptability of steelhead trout (Oncorhynchus mykiss) Jiirgen I. Johnsson*, W. Craig Clarke, John Blackburn Biological Sciences Branch, Department of Fisheries and Oceans, Pacific Biological Station, Nanaimo, B.C. V9R X6, Canada
Abstract The hypothesis that interbreeding with domesticated rainbow trout reduces seasonal variation in seawater adaptability in wild-type anadromous steelhead trout was tested. For this purpose, the growth of 18 maternal half-sib families of wild-type steelhead trout and steelhead/domesticated rainbow trout hybrids along with a control family of domesticated rainbow trout was monitored in a laboratory experiment. The trout were reared under ambient temperature and simulated natural photoperiod conditions from June 1989 to the end of May 1990. The seasonality of seawater growth was examined by comparing the growth rates of early (December) and late (April) transfers of the experimental strains to seawater. Further, on eleven occasions from September to late May, steelhead and hybrid trout were subjected to 24-h seawater (SW) challenge tests in 30%0 seawater. Significant male parent type by environment interaction for the growth rates showed that seawater transfer during winter was relatively more favourable for the rainbow and hybrid trout than for the wild-type steelhead trout. These results were supported by the results of the SW challenge tests. Winter transfers to seawater revealed significantly lower plasma sodium levels in hybrid trout compared to steelhead trout, while when SW-transferred in May, the wild-type steelhead trout had a higher hypo-osmoregulatory capacity than the hybrid trout. Our results support a genetic basis for the seasonality of seawater adaptability, and the hypothesis that hybridization with domesticated rainbow trout reduces seasonal variation in the seawater adaptability of steelhead trout.
*Corresponding author. Present address: Department of Zoology, University naregatan 18, S-4 13 90 GiSteborg, Sweden. Fax ( + 46-3 1) 4 16729. 0044-8486/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI0044-8486(93)E0281-D
of GGteborg, Medici-
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1. Introduction Natural rainbow trout (Oncorhynchus mykiss) populations display a high level of phenotypic variation, including anadromous and freshwater-resident life history patterns (Hershberger, 1992). Since the domestication of rainbow trout started in the 187Os,hatchery selection has been aimed at improving commercial traits such as growth and food conversion efficiency (Gjerde and Schaeffer, 1989). Recently, seawater rearing of domesticated rainbow trout has expanded considerably (Madsen and Naamansen, 1989). Several studies have indicated that the seasonal variation in seawater adaptability is less pronounced in domesticated rainbow trout than in anadromous steelhead trout (e.g. Boeuf and Harache, 1982; Johnsson and Clarke, 1988). However, studies of the genetic control of these differences have not been reported. In this study, we tested the hypothesis that hybridization with domesticated rainbow trout reduces the seasonal variation in seawater adaptability in wild steelhead trout.
2. Materials and methods On 6 April, half of the eggs from each of nine wild steelhead females were fertilized by a wild steelhead male and the other half by a domesticated male. In the laboratory, the growth of the resulting 18 maternal half-sib families of wild-type steelhead trout and steelhead/domesticated rainbow trout hybrids along with a family of domesticated rainbow trout was monitored. The trout were reared under ambient temperature and simulated natural photoperiod conditions from June 1989 (first feeding) to late May 1990. The seasonality of seawater growth was examined by comparing growth rates of early (8 December) and late (23 April) transfers of the experimental groups to seawater. Analysis of variance on specific growth rates was conducted only on steelhead and hybrid trout, because the single domesticated rainbow trout family could not be included in the genetic model (see Johnsson et al., 1993 for details). Furthermore, on eleven occasions from September to late May, steelhead and hybrid trout were subjected to 24-h seawater challenge tests (Blackbum and Clarke, 1987) in 30%0 seawater. The seawater challenge data were subjected to analysis of variance.
3. Results Fig. 1a presents growth in fresh water and seawater from 8 December to 15 April. There was a significant effect of male parental type ( F1,9= 16.7, P< 0.01)) indicating that the hybrids grew faster than the steelhead trout. Furthermore, a significant effect of environment ( F1,9= 20.0, P-c0.00 1) showed that growth rates were higher in seawater than in fresh water. Finally, a significant male parental 1) indicated a relatively type by environment interaction (F 1,15= 25.6, PC 0.00 faster seawater growth of hybrid trout compared to steelhead trout. Fig. 1b shows
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Fig. 1. Mean ( + s.e. ) specific growth rates (% body weight/day) for five pooled replicates (n = 27/ tank) of steelhead, hybrid and two pooled replicates (n = 1S/tank) of domesticated rainbow trout (a) in fresh water (FW ) and seawater (SW ) from 8 December to 15 April, (b ) in groups transferred early (ESW = 8 December) or late (LSW = 23 April) to seawater from 15 April to 24 May. Mean temperatures were: FW 6.3”C, SW 8.9”C, ESW 9.3”C, and LSW 9.4”C.
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Fig. 2. Mean ( * s.e.) plasma sodium levels in steelhead and hybrid trout subjected to 24-h seawater challenge tests in 30°/ooseawater on eleven occasions from 6 September to 28 May; n = 20 for each mean. “a” indicates significant difference between plasma sodium levels (t test, P< 0.05).
the growth of groups transferred to seawater either early (ESW, 8 December) or late (LSW, 23 April) from 15 April to the termination of the experiment on 24 May. There was no effect of the time of seawater entry on growth in the rainbow trout. In steelhead and hybrids, the LSW groups grew significantly faster than the ESW groups (Fl,lo= 17.4, PC 0.01). The relatively better growth of LSW steelhead compared to LSW hybrid trout was reflected in a significant male type by ),while there was no effect of male environment interaction (F,, , 5= 5.3, P-c0.05 type on growth. The results of the eleven seawater challenge tests are summarized in Fig. 2. A significant male type by time-of-test interaction ( Flo 407=3.6, P-c0,001) reflected differences between steelhead and hybrid trout ‘in the seasonal development of seawater adaptability. In both steelhead and hybrids, plasma sodium levels decreased only moderately between September and mid-February, despite an increase in mean weight from 8 to 47 g (steelhead) and 10 to 102 g (hybrids). During spring, plasma sodium levels dropped rapidly both in steelhead and hy-
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brids, but the latter reached a plateau at about 166 mM in early April, while the osmoregulatory capacity of the steelhead trout continued to improve until May. The results of the seawater challenge tests were concordant with the growth experiment. Between December and mid-April, seawater growth was faster and plasma sodium levels were generally lower (significant difference on four of eight occasions; t test, P-zO.05, see Fig. 2) in hybrids compared to steelhead trout. In contrast, the anadromous steelhead trout grew faster and had lower (significant difference on two of three occasions; t test, P-c 0.05, see Fig. 2) plasma sodium levels than the hybrids during their natural migration period in May.
4. Discussion The results of this study support the hypothesis that hybridization with domesticated rainbow trout reduces seasonal variation in seawater adaptability in the anadromous steelhead trout. Our findings are consistent with a previous study of two other populations of rainbow and steelhead trout (Johnsson and Clarke, 1988) which indicated that exposure to simulated spring daylength during winter accelerated a decrease in condition factor and an increase in silvering and seawater adaptability in steelhead trout but not in rainbow trout. In the present study, the ESW steelhead initially grew well in seawater and after 3 months the ESW group weighed 20% more than the LSW group. Thereafter, the growth of the ESW steelhead started to slow down. This suggests that premature seawater transfer may have disrupted the Parr-smolt transformation (Clarke and Nagahama, 1977; Nishioka et al., 1982) in the steelhead trout.
5. Acknowledgements Excellent technical assistance was provided by the staff at the Fish Culture Research Section, Pacific Biological Station in Nanaimo.
6. References Blackbum, J. and Clarke, W.C., 1987. Revised procedure for the 24 hour seawater challenge test to measure seawater adaptability ofjuvenile salmonids. Can. Tech. Rep. Fish. Aquat. Sci., No. 15 15: 35 PP. Boeuf, G. and Harache, Y., 1982. Criteria for adaptation of salmonids to high salinity seawater in France. Aquaculture, 28: 163-l 76. Clarke, W.C. and Nagahama, Y., 1977. Effect of premature transfer to sea water on growth and morphology of the pituitary, thyroid, pancreas, and interrenal in juvenile coho salmon (Oncorhynchus kisutch). Can. J. Zool., 55: 1620-1630. Gjerde, B. and Schaeffer, L.R., 1989. Body traits in rainbow trout. II. Estimates of he&abilities and of phenotypic and genetic correlations. Aquaculture, 80: 25-44. Hershberger, WK., 1992. Genetic variability in rainbow trout populations. Aquaculture, 100: 5 1-7 1. Johnsson, J. and Clarke, WC., 1988. Development of seawater adaptation in juvenile steelhead trout
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(Salmo gairdneri) and domesticated rainbow trout (Salmo gairdneri) - effects of size, temperature and photoperiod. Aquaculture, 7 1: 247-263. Johnsson, J.I., Clarke, WC. and Withler, R.E., 1993. Hybridization with domesticated rainbow trout reduces seasonal variation in growth of steelhead trout (Oncorhynchus mykiss). Can. J. Fish. Aquat. Sci., SO:480-487. Madsen, S.S. and Naamansen, E.T., 1989. Plasma ionic regulation and gill Na+/K+-ATPase changes during rapid transfer to sea water of yearling rainbow trout, Salmo gairdneri: time course and seasonal variation. J. Fish Biol., 34: 829-840. Nishioka, R.S., Bern, H.A., Lai, K.V., Nagahama, Y. and Grau, E.G., 1982. Changes in the endocrine organs of coho salmon during normal and abnormal smoltitication - an electron-microscope study. Aquaculture, 28: 21-38.
