All Prey Are Not Created Equal

The energy density of prey should be a central component of any diet study as it is important for evaluating the trophic ecology of species and the energy dynamics of systems. However, quantitative measurements of stomach composition are often described by the prey consumed without any direct application of empirical measures of regional prey quality. In this study, energy density equivalents (kJ/g wet-weight) of consumed prey were applied to the stomach composition datasets from Atlantic salmon (Salmo salar) sampled at West Greenland (1965-1970 and 2006-2011) and 24 groundfish species sampled on the Northeast U.S. continental shelf (1973 – 2013). Patterns of energy consumption for Atlantic salmon declined over time and were strongly influenced by a significant reduction in the energy density of their primary prey, capelin (Mallotus villosus), mirroring patterns of reduced marine productivity over the past few decades. Groundfish energy consumption was highly variable, but was comparable seasonally with notable differences between piscivores and benthivores. We evaluate if prey quality driven by climate forcing factors and bottom-up processes is influencing survival and productivity of fish stocks in the North Atlantic. Quantitative evaluation of stomach composition using energy density equivalents can be a powerful technique to answer important ecological questions.