Revisiting Size Selective Morality in Young Fish: Do Small Teleosts Really Pay a Cost?

Abstract

Small juvenile and larval teleosts are typically more susceptible to starvation and predation, so the largest individuals tend to survive. However, most support for this bigger is better hypothesis is experimental and does not account for the variation in food availability, competition, and other conditions that alter how starvation and predation affect small fish. To assess how natural populations experience size-selective mortality, we reviewed the past 30 years of literature on the subject while compiling 76 effect sizes of longitudinal survival data, which evaluates selection against size classes, to test for evidence that bigger is better. Our meta-analysis shows that the effect of body size on survival is consistently weak across species, populations, times, and locations. We discuss several reasons why larger young teleosts may not have a distinct survival advantage. We argue that they: 1) may be more profitable or noticeable to predators, 2) are unable to outgrow all their predators, and 3) require more resources, increasing their predator exposure out of necessity to forage. We recommend that mortality not be treated as constant across young teleost size classes, as different ecological conditions may favour smaller, larger, or neither size class. We suggest that the relationship between metabolic scope and mortality, geographic gradients in the importance of predation and starvation, and the interactive effects of alternative sources of mortality with predation and starvation in warming oceans are underexplored and could change how we think about the relationship between young teleost size and survival. Investigating these drivers will be important as temperatures rapidly rise, altering growth in wild fish.