Identifying the Forage Fish Requirements of Marine Bird and Mammal Predators in An “Ecosystem Approach to Fisheries”

Marine predators such as seabirds and marine mammals feed heavily on small forage species, facilitating rapid transfer of energy to top trophic levels. Because of this, apex predators are useful indicators of the current state of pelagic ecosystems and informative to the “Ecosystem Approach to Fisheries” adopted by many fisheries organizations. Of course, marine birds and mammals are not just handy ecosystem indicators— we also want to maintain healthy wildlife populations and an adequate forage base to sustain them.  Whether their primary concern is for fish stocks or for wildlife, managers need quantitative data on the functional relationships between predators and prey to identify “minimum biologically acceptable limits” of forage biomass.  We are compiling and contrasting data from several marine ecosystems of the world to assess the form of these basic predator-prey relationships. Preliminary analyses suggest that response functions are usually non-linear and exhibit thresholds. The threshold biomass of prey at sea (bS) required by seabird predators for successful reproduction appears to be 1-3 orders of magnitude greater than the biomass needed to meet metabolic energy demands (bE) of populations during summer.  Different predators may have markedly different bS/bE ratios because of differing sensitivities to prey depletion, and sensitivity may be predicted from a few life history characteristics. Our initial findings show promise for discerning what different “indicator species” are actually indicating, and what levels of forage abundance may be required to sustain healthy apex predator communities.