Spatial relationships between marine birds and prey in the pelagic environment

The goal of this dissertation was to examine how prey fluctuations over both short and long time scales affect the structure of predator populations. In particular, I was interested in the nature of responses by top predators to changes in prey availability in marine ecosystems. In Chapter One, I examined the degree to which spatial associations between marine birds and prey at small measurement scales were dependent upon prey availability at larger measurement scales. I found that densities of four piscivorous marine bird species (Brandt's cormorants Phalacrocorax penicillatus, common murres Uria aalge, Pacific loons Gavia immer, and rhinoceros auklets, Cerorhinca monocerata) were spatially correlated with densities of prey biomass ("acoustic biomass") at small (<8 km) spatial scales only on days when regional (10 x 20 km) prey levels were relatively low. I also found that, in three species, birds tracked acoustic biomass at finer spatial scales on days when regional prey abundance was low than they did on days when regional prey abundance was high. These and other results suggest that fluctuations in regional prey availability may account for some of the observed variation in the strength of local spatial associations between birds and prey in marine ecosystems and other natural communities.;In Chapter Two, I investigated how oceanographic features that concentrate small prey into high-density patches affect prey-size use and the pelagic distribution of a top marine predator. I observed thousands of short-tailed shearwaters (Puffinus tenuirostris) feeding in long, well-defined lines in Akutan Pass, Alaska. I found that shearwaters in Akutan Pass foraged upon exceptionally small (11 mm) prey items, specifically, the euphausiids, Thysanoessa raschii and T. inermis. Small euphausiids appeared to be concentrated near the sea surface in Akutan Pass by a hydrographic tidal front. I found that shearwaters in pelagic settings, which lacked tidal fronts, took large (14--21 mm) euphausiids, even though small euphausiids were present. Birds that fed upon small near-surface prey also used a different foraging method, pursuit-diving, than did birds that foraged upon large euphausiids. This study provides empirical support for the hypothesis that oceanographic features influence the distribution of marine predators by enhancing the profitability of especially small-sized prey.;In Chapter Three, I examined how long-term (30+ yrs) trends in prey availability may have contributed to population declines in four species of piscivorous seabirds breeding at the Pribilof Islands, Alaska. I tested the hypothesis that changes over time in the abundance of juvenile (age-1) walleye pollock (Theragra chalcogramma) were responsible for changes in the reproductive success and consequent population size of black-legged kittiwakes (Rissa tridactyla), red-legged kittiwakes (R. brevirostris ), common murres, and thick-billed murres (U. lomvia). I found little correlation between the abundance of juvenile walleye pollock and annual reproductive success in seabirds on St. Paul and St. George Islands. I also found little temporal concordance between trends in seabird reproductive success and seabird population size. My results and the results of other studies suggest that food scarcity during the 1980s and 1990s led to increased adult mortality and reduced juvenile survivorship in seabirds at the Pribilof Islands. I further suggest that seabird mortality was highest during the post-breeding period, when food-related stress to seabirds was exacerbated by reproductive costs and when evidence of mortality in seabirds is especially difficult to observe.