Trophic ecology of large pelagic fishes in the southern Gulf of California

An ecosystem-based approach to marine fisheries management requires an understanding of the interactions among large pelagic fishes. While trophic interactions have traditionally been described using stomach contents, stable isotope methods have recently emerged as a complement to these data in understanding ecological communities. Here, I analyze the trophic ecology of several large pelagic fishes in order to assess trophic interactions and their implications to fisheries management throughout the tropical eastern Pacific Ocean. In Chapter One, I describe the diets of twelve species of pelagic fishes in the southern Gulf of California. All of these species are specialized predators on epipelagic fishes and cephalopods, but prey proportions vary between species, or between years, locations, and seasons. I present a generalized model of trophic ecology of large pelagic fish in this region and define four main feeding behaviors: insular pelagic feeding on inshore species, open pelagic feeding on cephalopods, epipelagic feeding on near-surface fishes, and topography-associated feeding at shallow seamounts and offshore banks. In Chapter Two, I describe high variability found in nitrogen stable isotope ratios of large pelagic fishes and their prey in the southern Gulf of California. In many cases, mean delta 15N values of common prey items exceeded their predators by more than one full trophic level. I analyzed potential sources of this variability by examining diets, oceanographic conditions and fish migration. Diet shifts do not account for the variability, but the likely source is slow turnover rate of body tissues of migratory pelagic fish as they traverse both highly productive and oligotrophic water masses in the Eastern Pacific Ocean. In Chapter Three, stomach contents and stable isotope ratios were used to characterize spatial and temporal variability in the pelagic food chain of the southern Gulf by examining three large pelagic fishes, namely dolphinfish, striped marlin and yellowfin tuna. Dietary analyses were used to determine differential feeding between sites and seasons. Seamounts in the southern Gulf of California are important feeding grounds for pelagic fishes, and therefore must be considered in the design of marine management in this region.