SIZE-SELECTIVE MORTALITY IN THE JUVENILE STAGE OF TELEOST FISHES - A REVIEW

Individual variability in body size provides a template for selective mortality processes during early life history stages of teleost fishes . This size variability has generated the logically intuitive hypothes is that larger or faster growing members of a cohort gain a survival a dvantage over smaller conspecifics via enhanced resistance to starvati on, decreased vulnerability to predators, and better tolerance of envi ronmental extremes. This review evaluates field and laboratory studies that have examined size-based differences in survival, with emphasis on the juvenile stage of teleost fishes. The results in general suppor t the ''bigger is better'' hypothesis, although a number of examples i ndicate non-selective mortality with no obvious size advantages. The r everse pattern, with enhanced survival of smaller individuals, is rare with the notable exception of bird predation. Major size-selective pr ocesses during the juvenile stage include overwinter mortality for tem perate species, associated with either starvation or intolerance of ph ysical extremes by smaller members of the young-of-the-year cohort, an d predation, with smaller fish more susceptible to successful capture by predators. Most studies examining these processes have used indirec t methods to evaluate size-selective mortality, with interpretation of results dependent on several critical assumptions. For methods that t rack size distributions over time, unbiased samples collected from the same population are critical, and changes in size distributions assoc iated with mortality must be distinguished from changes due to individ ual growth. The latter requirement can be met with the direct, ''chara cteristics of survivors'' method, but few studies have used this appro ach. Experimental methods isolating specific mechanisms of size-specif ic mortality must appropriately represent the natural context of envir onmental factors. Specific predator/prey combinations, for example, ca n elucidate size-based prey preferences bur may be irrelevant compared to the natural, multi-species predator field. The composition of the predator field and its correspondence to size-spectrum theory is cruci al to the probability of size-selective predation as a cohort progress es through the juvenile stage. Distinction of selection on body size v s. selection on growth rate has received little attention. However, a number of physiological constraints and ecological trade-offs can plac e restrictions on growth rates and apparently override the advantages of large body size. Identifying the major sources of mortality and how they operate in the juvenile stage has valuable applications in under standing population dynamics and recruitment variability.