Vertebrate exploitation of pulsed marine prey: a review and the example of spawning herring

Short-term bursts of prey availability occur in many ecosystems and have potential important consequences for both predator biology and ecosystem function. Examples of prey ‘pulses’ in marine ecosystems include spawning runs of several anadromous and marine fishes, horseshoe crab spawning, and salmonid juvenile outmigrations, which are exploited by numerous species of vertebrate predators. In a few cases, the fitness or demographic consequences of such predator–prey interactions are known or inferred, but too often that information remains unknown. We explored the extent of temporal and spatial variation in one example of a pulsed marine resource: the spawning of Pacific herring (Clupea pallasii). Spawning herring provide a rich, aggregated resource to which dozens of species of vertebrate predators often exhibit strong numerical responses. However, the spawning events are often variable in both time (annual differences of several to many weeks) and space (both regional and more local differences in size and timing of events). Such variability must affect more mobile predators less than area-restricted predators, and thus its effect would vary not only among species but also within species, depending on constraints of the predator life history. Unpredictability of the prey concentrations, whatever their proximate causes, may contribute to maintenance of metapopulations of prey such as herring, if unpredictability lessens the impact of predation.