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Predator–prey body size relationships of cod in a low diversity marine system

Susa Niiranen*, Alessandro Orio, Valerio Bartolino, Ulf Bergström, Meri Kallasvuo, Stefan Neuenfeldt, Didzis Ustups, Michele Casini

*Corresponding author:

ABSTRACT: How predators select their prey largely defines ecosystem trophic structure, function and dynamics. In aquatic systems, organism body size is an important trait explaining predator-prey interactions. Here, we use a unique cod (Gadus morhua) stomach content dataset with diet information from over 100.000 individuals collected from the Baltic Sea in 1963-2014, to explore prey size distribution and predator-prey mass ratios (PPMR) in the diet of Eastern Baltic cod. Maximum and average prey sizes increase with predator size, as for cod in other systems. However, the prey size spectra found in Eastern Baltic cod stomachs reflect the low species diversity in the Baltic Sea, and in general they feed on smaller prey in relation to their body size than other cod populations. Due to the truncated prey-size distribution in the Baltic Sea, cod cannibalism functions as a compensatory mechanism that allows Baltic cod to reach its prey size potential. The small- and intermediate-sized cod, on the other hand, prey mainly on a few invertebrate prey species, potentially making them vulnerable to changes in these prey populations. Our results encourage further studies disentangling the relative effects of prey preference and prey availability on cod trophodynamics in species-poor systems such as the Baltic Sea.