Abstract
Size-selective mortality owing to lack of energy reserves during the first marine winter has been suggested to be a result of juvenile salmon failing to reach a critical size or condition by the end of their first marine summer and not surviving the following winter due to this presumed energy deficit. This hypothesis implies strong size dependency of mortality, and is subject to empirical data support for acceptance. Scale circulus spacing has been interpreted as an index for body size, and we reviewed the effect of size-selective mortality with a knife-edge mortality function on descriptive statistics for a scale circulus spacing index (SCSI). In order to invoke size selection as an important driver of mortality during the first year of ocean rearing, it is necessary to demonstrate not only that size-selective mortality is directed towards the smaller members of the population, but that the selective nature of the mortality can account for a substantial portion of the observed mortality. If the assumption is made that a random sample of a single juvenile population has been obtained, then studies that employ a SCSI to infer size-selective mortality coupled with a critical size limit must demonstrate a shift toward larger values of the SCSI, but also a concomitant reduction in the variance and range of the SCSI and an increase in the skewness and kurtosis of the SCSI values. Through simulation we found that the percentage of adults that displayed a SCSI value greater than the maximum observed in the juvenile sample was highly dependent on the initial juvenile sample size and size-selective mortality rate. Geographical distributions of juvenile Pacific salmon can be stratified by size, with larger individuals migrating earlier from local ocean entry locations than smaller individuals, and thus differential timing migration of juveniles based upon body size prior to the collection of the marine juvenile sample may be a more plausible explanation of published trends in the SCSI, rather than invoking substantial size-selective mortality and a critical size limit.
Highlights
The role of juvenile body size in regulating mortality in marine fish has been an area of continuing interest [1], especially in Pacific salmon (Oncorhynchus)
A normally distributed juvenile population with the scale circulus spacing index (SCSI) displaying a mean of 30 and a standard deviation of 5 was subjected to simulated size-selective mortality with a critical size limit under the assumption that a critical size had to be obtained in order to survive to the adult stage
As it was assumed that the SCSI value for an individual was fixed during the life cycle, the percentage of adults that displayed a SCSI value greater than the maximum observed in the juvenile sample was highly dependent on the initial juvenile sample size and size-selective mortality rate (Table 1)
Summary
The role of juvenile body size in regulating mortality in marine fish has been an area of continuing interest [1], especially in Pacific salmon (Oncorhynchus). In a study based upon scale circuli spacing, Beamish et al [6] reported that coho salmon (O. kisutch) that survived their first winter at sea had significantly larger spacing between circuli on scales, indicating that brood year strength was related to growth in the first marine year As a result, they suggested that size-related mortality in the first marine fall and winter was an important determinant of brood year strength of some coho salmon stocks and stocks of other species of Pacific salmon, and that individual coho salmon needed to attain a critical minimum size by the fall of their first year at sea in order to survive the subsequent winter. If the adult sample was derived from two juvenile populations, the first population comprised of smaller sampled individuals, and the second population comprised of larger unsampled individuals, possibly unrepresentative sampling of the juvenile population may account for observed trends in descriptive statistics of the adult distribution These findings caused us to evaluate salmon studies in which marked size-selective mortality coupled with a critical size was reported over the first winter of marine rearing. We compare the results of previously published studies and evaluate whether the descriptive statistics support a sizeselective mortality coupled with a critical size limit hypothesis, or a second unsampled juvenile population of larger individuals hypothesis
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
