Coho salmon Oncorhynchus kisutch from the Strait of Georgia were used to test the hypothesis that slower growing fish in their first ocean year had lower survival over the late fall and winter than faster growing fish. The Strait of Georgia provided a suitable area for this study because it is a semi-enclosed rearing area for juvenile Pacific salmon that is distinct from the open marine rearing areas off the west coast. Coho salmon that survived the winter had significantly larger spacing between circuli on scales, indicating that brood year strength is related to growth in the first marine year. Other studies have shown that smaller fish of a cohort are less able to survive periods of energy deficit than larger fish. Thus, size-related mortality in the first marine fall and winter may be an important determinant of brood year strength of some coho salmon stocks and stocks of other species of Pacific salmon.
page 361 Introduction 361 Material and methods 364 Results 364 Discussion 366 References 368Abstract Otoliths, or 'ear stones', are calcium carbonate structures found in all vertebrates. In teleosts, they have a number of sensory functions, including hearing. Daily growth increments of these structures have permitted advanced age and population studies of teleosts. Whereas 'normal' otoliths are composed of crystals imbedded within a protein matrix as aragonite, a 'crystalline' form of calcium carbonate termed vaterite is also found. A review of the otolith literature demonstrates a significant level of understanding of the structure and function of otoliths, but the cause for crystalline otolith structure remains speculative. Pairs of otoliths from hatchery and wild juvenile and adult coho salmon (Oncorhynchus kisutch) were examined visually for determination of otolith microstructure type. The vateritic or crystalline otoliths were found in much higher percentages in juvenile hatchery-reared coho salmon than in juvenile wild coho salmon, supporting previous studies. There did not seem to be any negative impact on size or survival. There was also no correlation between crystalline otoliths and premature maturation in coho males. A preliminary study of adult coho salmon returning to Big Qualicum and Chilliwack hatcheries showed even higher ratios of vateritic otoliths than observed in juveniles.
An analysis of the results of a 10‐year study of the population ecology of juvenile hatchery and wild coho salmon Oncorhynchus kisutch in the Strait of Georgia produced new information about the interannual and interseasonal fluctuations in abundance and marine survival. A decline in the percentage of hatchery coho salmon was related to declines in hatchery fish abundance and marine survival; wild coho salmon abundance was more stable. The declining marine survival of hatchery coho salmon appeared to be related to a fixed average date of release from hatcheries and a possibility of earlier prey production. The relatively stable abundance of wild coho salmon may relate to a natural trend toward earlier ocean entry dates. Oscillations in hatchery coho salmon percentage and abundance were related to oscillations in abundance of juvenile pink salmon O. gorbuscha. The impact of oscillating density affected marine survival of hatchery coho salmon more than that of wild coho salmon. Marine survival and abundance of hatchery and wild coho salmon in July were positively related to average fork length, indicating that growth within the first few months after ocean entry affected marine survival. However, absolute size was not important, as wild coho salmon were consistently smaller than hatchery coho salmon. Wild coho salmon responded to conditions in the marine ecosystem differently than hatchery coho salmon, as relationships among growth, survival, and abundance were apparent for wild coho salmon earlier in the year than for hatchery fish. The length increase between July and September was inversely related to marine survival, suggesting that fish that were larger in July grew less and survived better because they were storing more lipids than smaller coho salmon. The study indicated that a more experimental management strategy is needed for both hatchery and wild coho salmon.
The percentage of hatchery-reared coho salmon Oncorhynchus kisutch in the Strait of Georgia, British Columbia, increased from nearly 0% in the early 1970s to more than 70% by 2001. These estimates were derived from fin clip and coded wire tag data collected from commercial and sport fisheries, research surveys conducted in the summer and fall of 1997 to 2000, and examination of the microstructure of otoliths extracted from juvenile coho salmon collected during our marine surveys. The increasing trend may be related to the proportions of hatchery and wild smolts entering saltwater, fishing rates, and changes in the ecological processes regulating coho salmon production in the ocean. The consequence for management is that the abundance of wild spawning salmon (escapement) depends on hatchery as well as wild production. The consequence for policy makers is that future enhancement activities need to have clear policies for assessing the effects of hatchery fish on the population dynamics of wild fish as well as for producing hatchery fish.
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