36 



Fishery Bulletin 94(1). 1996 



among-population variation in survival. This does not 

 invalidate the model as long as one assumes that 

 within a single species or group of closely related 

 species, the weight exponents of growth and mortal- 

 ity are stable parameters with similar ranges and 

 little variation. 



The second most important source of error was the 

 choice of an appropriate value of b for the growth 

 model. Most estimates of b were available from ex- 

 periments with young salmonids grown in enclosures 

 at constant temperatures and fed to satiation or ad 

 libitum. These are not conditions commonly faced by 

 wild salmonids, and thus making use of these esti- 

 mates in estimating marine mortality for wild salmo- 

 nids is suspect. There is clearly a need for more re- 

 search on the weight-, ration- and temperature-de- 

 pendence of growth of large, sea-going salmonids de- 

 rived from wild stocks and grown in marine enclosures. 



Effect of latitude on smolt-adult survival 



Koenings et al. ( 1993) demonstrated that smolt-adult 

 survival of sockeye salmon increased with latitude 

 of nursery systems as well as with smolt length, a 

 finding that was not confirmed by this study for sock- 

 eye salmon or for any other species o{ Oncorhynchus. 

 There are at least two possible reasons for the lack 

 of a latitude effect in this study. First, within-sys- 

 tem variation in smolt-adult survival was ignored 

 because adult weights were not available for each 

 brood year for each system. In contrast, Koenings et 

 al. (1993) were able to compare smolt-adult surviv- 

 als and smolt lengths for individual brood years 

 within each system. Second, this study regressed 

 smolt-adult survival on both adult weight and smolt 

 weight rather than on smolt weight alone. This al- 

 lowed variation in survival to be adjusted for varia- 

 tion in adult size between nursery systems as well 

 as for variation in smolt size. The latitude effect ob- 

 served by Koenings et al. (1993) may have resulted 

 from restricting analysis of survival-weight relations 

 to smolt size. 



Applications of allometry of mortality for 

 Pacific salmon 



The equations reported in this study may be useful 

 for making preliminary estimates of survival and 

 mortality for wild populations of Oncorhynchus spe- 

 cies for which information is limited. For example, 

 the results of this study imply that smolt-adult sur- 

 vival of wild chinook salmon should be similar to 

 other species of salmon with similar smolt and adult 

 weights. There are at present no published estimates 

 of wild chinook smolt-adult survival in the primary 



literature. The allometry of M reported in this study 

 may also be useful for modeling survival and mor- 

 tality of Pacific salmon in spatially explicit simula- 

 tion models (i.e. Walters et al., 1978). Not only may 

 it provide more realistic daily mortality rates than 

 could be calculated from fry-smolt and smolt-adult 

 survivals, but by relating mortality with body size it 

 links survival to trophic dynamics and to density- 

 dependent growth. 



Acknowledgments 



Michael Bradford provided a preliminary list of ref- 

 erences on survival of Pacific salmon. Gary Kyle pro- 

 vided adult sockeye weights for most of the Alaskan 

 nursery systems cited by Koenings et al. ( 1993 ). Criti- 

 cal reviews of early drafts of this paper were done by 

 Michael Bradford, Stephen Mathews, and several 

 anonymous referees. 



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