A Length-Based Approach to Examining Predator-Prey Relationships in Marine Predators

Aquatic populations are generally size-structured. Predator-prey relationships are especially constrained by allometric patterns in morphology and behaviour of both predators and prey. Most previous analyses have used prey and predator weights, likely because metabolic and energy requirements are generally based on weight. However, from a behavioural and ecological perspective prey length is likely the more relevant metric. Here I take a length-based perspective on a large database of predator and prey size for marine predators (~35,000 prey items, 92 predators) and compare results to previous studies based on weights. I first assess the allometry of prey length and prey:predator length ratios, and then focus my analyses on the scaling of species-specific trophic niche breadth estimated using quantiles of relative prey sizes and comparing patterns for larval vs. adult fishes. I found a positive wedge-shaped relationship between prey and predator length and a negative converging relationship between relative prey sizes and predator size. For larvae, trophic niche breadths were mostly parallel and increased with predator size.  In contrast, for adults trophic niche breadths were converging and scaled negatively with predator length suggesting different feeding strategies. These results have implications for the effects of fishing and climate change on aquatic food webs.