Movement and Trophic Ecology of Large Pelagic Fishes Connecting Surface Waters with the Ocean's Twilight Zone

Abstract

The ocean’s twilight zone is a vast area of the global ocean that lies between the sunlit surface waters and perpetually dark midnight zones, covering depths from ~200 to 1000 meters. Recent work in the twilight (or mesopelagic) zone has revealed unexpected biomass and diversity that may not only challenge scientific understanding of ocean systems but also provide new and largely untapped resources for fisheries harvest. The extent to which commercially valuable, highly migratory top predators such as tuna and swordfish rely on mesopelagic biomass for forage has not previously been quantified but is thought to be substantial. Pressure from emerging industrial fisheries in the twilight zone makes determining the linkages between mesopelagic prey and migratory predators of pressing concern for sound management in keeping with the precautionary principle. Ocean predators are further hypothesized to dive into the deep ocean for a range of motives beyond forage, including for navigation on their long migrations. In this thesis, I begin by using compound-specific stable isotope analysis to trace the flow of carbon through pelagic ecosystems in the northwest Atlantic to three predators: bigeye tuna (Thunnus obesus), swordfish (Xiphias gladius), and yellowfin tuna (Thunnus albacares). I confirm the presumed high reliance of these predators on mesopelagic prey using a Bayesian mixing model approach that estimated 50-60% of their temperate carbon is sourced from mesopelagic food webs. Next, I take a larger view of epi- and mesopelagic food webs by sampling simultaneously across a pelagic food web from bottom to top at one point in time and space in the northwest Atlantic Ocean. I trace the movement of carbon and nitrogen from particulate organic matter, through mid-level consumers, up to top predators using compound-specific stable isotope analysis of amino acids. Nitrogen stable isotope analyses is also used to calculate trophic positions, providing a more detailed view of pelagic food web structure and function. To complement these trophic studies, I conduct a movement analysis of vertical habitat use by swordfish focused on their intermittent extreme dives. I explore possible motivations for these dives, including forage, predator avoidance, and navigation. Qualitative investigation of dive geometry, as well as quantitative logistic models of the physical and biological environment, indicate that navigation is the most likely motive. Finally, I consider the implications of predator reliance on mesopelagic forage in a fisheries economics context. Using my earlier diet sourcing results, I adapt a bioeconomic model with a new predator-prey dynamic to evaluate the effects of potential mesopelagic fisheries on their predators with bigeye tuna as the representative predator. Model results highlight the importance of recognizing predator-prey interactions in management of mesopelagic fisheries and demonstrate the sensitivity of equilibrium economic and ecological conditions for the tuna stock under different price and cost scenarios. Overall, these studies emphasize the importance of the deep ocean to marine predators and suggest that a new mesopelagic fishery could be economically viable in-and-of itself but may have significant negative impacts on existing tuna and swordfish fisheries due to reduced forage.