Foraging Ecology of North Pacific Albacore in the California Current System (CCS)
The aims of this dissertation are to identify critical interactions between juvenile North Pacific albacore (Thunnus alalunga) and their prey in the California Current System (CCS), classify the foraging strategy of albacore, and examine the potential for top-down effects of albacore predation. Predator-prey interactions are the fundamental component of food webs, and quantifying variability in strengths of interactions is critical for predicting how ecosystems will respond to changing environments. Stomachs from 371 juvenile albacore were collected in the CCS during summer and fall 2005 and 2006. Interdecadal variability was examined by comparing these samples to four studies conducted during 1949-1983. I hypothe-sized that, given significant changes to communities in the CCS, albacore diet would reflect these changes. However, diet has been stable, and despite a recent resurgence of Pacific sardine, only Northern anchovy and Pacific saury consistently have been important prey. These results support theoretical predictions of optimal foraging models that albacore prefer cold, near-shore waters containing anchovy and saury while minimizing time in warmer, offshore habitat of sardine. Albacore derived 68-97% of nutrition from juvenile fishes, 3-30% from juvenile cephalopods, and the remainder from crustaceans. In the northern CCS, decapods, euphausiids, and anchovy dominated biomass of prey; in the central CCS, hake and saury dominated; and in the southern CCS, anchovy were the sole significant prey. Using graphical approaches examining foraging strategy, I demonstrated that albacore are not true generalists, as commonly thought, but rather employ a mixed-feeding strategy that includes specialization on anchovy, variable predation on localized prey such as hake, saury, or decapods, and generalized (but rare) predation on over 50 additional species. Given the importance of anchovy to albacore diet, I investigated the impact of predation mortality on young-of-year anchovy. Albacore removed between 0.1%-5% of anchovy recruitment biomass annually. Results demonstrated that albacore are attracted to regions of abundant anchovy and suggest that albacore predation exerts top-down influences. Based on historical importance, widespread patterns of consumption, specialization on anchovy, and potential top-down impacts of predation, I conclude that albacore and anchovy interact strongly and populations may be sensitive to changes in the other.