PHENOTYPIC DIFFERENCES BETWEEN HATCHERY AND

WILD STEELHEAD FROM THE SAME GENE POOL AND THE

EFFECT ON FITNESS

Kostow, Kathryn. 2004. Differences in juvenile phenotypes and survival between hatchery stocks and a natural population provide evidence for modified selection due to captive breeding. Can. J. Fish. Aquat. Sci. 61: 577–589

Abstract: Juvenile phenotypes and fitness as indicated by survival were compared for naturally produced steelhead (Oncorhynchus mykiss), a new local hatchery stock, and an old nonlocal hatchery stock on the Hood River, Oregon, U.S.A. Although the new hatchery stock and the naturally produced fish came from the same parent gene pool, they differed significantly at every phenotype measured except saltwater age. The characteristics of the new hatchery stock were similar to those of the old hatchery stock. Most of the phenotypic differences were probably environmentally caused. Although such character changes would not be inherited, they may influence the relative fitness of the hatchery and natural fish when they are in the same environment, as selection responds to phenotypic distributions. A difference in fitness between the new hatchery stock and naturally produced fish was indicated by significant survival differences. Acclimation of the new hatchery stock in a “seminatural” pond before release was associated with a further decrease in relative smolt-to-adult survival with little increase in phenotypic similarity between the natural and hatchery fish. These results suggest that modified selection begins immediately in the first generation of a new hatchery stock and may provide

a mechanism for genetic change.

Quotes for the text: “The primary question for this paper was how hatchery juveniles from the new hatchery stock compared with naturally produced juveniles that had never been in captivity but came from the same parent gene pool as the hatchery fish. Fish from the new hatchery stock were also compared with fish from the old hatchery stock.

“Comparisons of age at capture and dates of capture in the mainstem and tributary smolt traps and comparisons of age at capture in smolt traps and age at smolting determined from adult scales revealed life history complexity among the naturally produced fish that was not present among any hatchery treatments. The only comparison that could be made was that the life history complexity was present in the naturally produced fish but absent in the hatchery fish.

“The hatchery fish were also nearly uniform in age, whereas the naturally produced fish were highly variable. Another consequence of a younger total age was that the average generation time of the hatchery fish was shortened by 1 year, from 4 years for the naturally produced fish to three years for both hatchery stocks. The new and old hatchery stocks had nearly identical age distributions. Both hatchery stocks showed similar characteristics and diversity in saltwater age and were similar to wild fish in this trait.

“Juveniles from both hatchery stocks out-migrated significantly earlier than naturally produced juveniles based on mean date of out-migration. However, the primary difference between the groups was that the out-migration time distribution for natural fish was highly variable, whereas the out-migration time for hatchery fish was very narrow. A large number of naturally produced fish out-migrated during the spring and the new hatchery stock matched this peak time reasonably well. But less than 1% of either hatchery stock was captured in any smolt trap after the end of June. In contrast, naturally produced fish were captured throughout the season.

“Among the naturally produced fish, small age-0 and age-1 fish were common in the tributary traps. These results indicate that young presmolting fish were moving for several years among different rearing environments within the Hood River Basin as they grew.

“Very young fish were also seen migrating past the mainstem trap, indicating that some of them were leaving the Hood Basin. However, age-0 smolts were absent, whereas age-1 smolts were rare among naturally produced fish based on the age at saltwater entry mark identified on adult scales. This result indicates that about 25% of the naturally produced fish out-migrated from the Hood Basin as presmolts.

“…all ages of naturally produced fish, with highly variable sizes and both smolts and presmolts, were moving within the Hood Basin and out of the Hood Basin at all times that sampling occurred. These behavior patterns were absent among the hatchery fish. They entered the mainstem trap from a few days to a few weeks after release and were large and relatively uniform sized.

“Both hatchery stocks had average egg-to-smolt survival rates over 60% compared with less than 2% for the naturally produced winter steelhead and less than 1% for the naturally produced summer steelhead. Both hatchery stocks therefore produced significantly more smolt offspring per parent than did the naturally spawning parents.

“The survival superiority was reversed between the natural and hatchery groups for smolt-to-adult survivals, which occurred while all of the fish were in the same environment. Naturally produced smolts had average smolt-to-adult survivals of 5-6% whereas both hatchery stocks had average survivals over the 5 years of about 1%. However, the survival superiority given the hatchery fish while they were captive continued through to return adults, as they had higher overall egg-adult survivals and produced many more adult offspring per parent than the naturally produced fish.

“The naturally produced winter steelhead had significantly higher egg-to-smolt survivals, smolt offspring produced per parent, egg-to-adult survivals, and adult offspring produced per parent than the naturally produced summer steelhead. For example, egg-to-smolt survivals were 10 times higher among the winter steelhead than among the summer steelhead. This difference in survivals was probably related to a difference in the proportion of hatchery parents and the origin of hatchery parents between the two naturally spawning populations. The winter steelhead populations included 2-51% hatchery fish, primarily first generation returns from the local broodstock included among the natural spawners in the last 2 years of the study, whereas the summer steelhead population included 78-88% hatchery fish each year of the study, all from the old hatchery stock. The two natural populations had similar smolt-to-adult survivals.

“The new hatchery stock (native brood stock from the same gene pool as the natural population) over the 5 years of the study had significantly fewer smolts and adult offspring per parent and lower smolt-to-adult survivals than the old hatchery stock. For example, over the 5 years, the new hatchery stock produced 35% fewer smolts and 60% fewer adult offspring per parent compared with the old hatchery stock. Comparisons of the direct-released new hatchery stock, the acclimated-releases new hatchery stock, and the old hatchery stock demonstrated that the acclimated new hatchery stock had significantly lower smolt-to-adult, egg-to-adult, and adult offspring per parent than either of the other hatchery treatments.

“…the average smolt-to-adult survival for the acclimated fish was only about half of what was observed for the direct-released fish and for the old hatchery stock. The direct-released fish and old hatchery stock had similar smolt-to-adult survivals.

“The new hatchery stock juveniles differed significantly from the naturally produced juveniles in every phenotypic trait that was measured in this study, except salt water age. The fish from the new hatchery stock more closely resembled the fish from the old hatchery stock, particularly by the time they reached the mainstem trap. The phenotypic differences were probably environmentally caused, since both the hatchery and naturally produced fish were from the same gene pool. However, large phenotypic responses by fish from the same parent gene pool to the differences between the captive and natural environments are consistent with the process of domestication.

“…over the 5 years, the elevated egg-to-smolt survivals of the (native brood stock) hatchery fish in captivity ranged from only 45% to 80%, which was lower than the 90% to 95% that is often reported for hatchery programs. Average smolt-to-adult survivals for the naturally produced winter and summer steelhead were five to six times higher than for the new hatchery stock.

“…the new hatchery stock had very low smolt-to-adult survival relative to the naturally produced fish while the two groups were in the same (natural) environment. The new hatchery stock had very similar phenotypes and survivals when compared with the old hatchery stock, particularly when it was released directly into the Hood River without acclimation.

“It appears that the most successful fish in the new hatchery stock generally resembled the old hatchery stock, following the homogenous life history strategy of being large yearling smolts that promptly entered the ocean during a narrow window after release.

“…as demonstrated by the apparent consequences for the smaller acclimated hatchery fish released in this study, hatchery fish may be made to ‘look more like’ wild fish without behaving like them or surviving like them.

“In conclusion, this study demonstrated large average phenotype and survival differences between hatchery-produced and naturally produced fish from the same parent gene pool. These results indicate that a different selection regime was affecting each of the groups. The processes indicated by these results can be expected to lead to eventual genetic divergence between the new hatchery stock and its wild source population, thus limiting the usefulness of the stock for conservation purposes to only the first few generations.