DOI: 10.25549/usctheses-c127-629435
title: Using electronic tagging data to estimate movement, identify habitat, and characterize behavior of marine pelagic predators

description: Electronic tagging has fundamentally changed our understanding of the lives of marine animals from tunas, turtles to squids in the past decade. For the very first time, we are able to collect detailed data from the animals as they conduct their lives in the ocean for months to, sometimes, years. An electronic tag is like a paparazzi’s camera, and our star celebrity is the animal. Unlike the vastly popular reality TV shows, lives of these animals are unscripted and challenge us to decipher the underlying mechanisms and motivations for doing what they do. In this thesis, I take a life history approach to present the aspects of lives from two species, great white shark (Carcharodon carcharias) and striped marlin (Kajikia audax) that we have learnt through electronic tagging. Akin to our actions as human in planning out our lifetime goals (e.g. get a Ph.D., build a career/ family) and daily plans (e.g. prepare a dinner menu), a pelagic animal’s life in the ocean has both aspects intertwined, but rarely revealed themselves to us scientists. A better understanding of this interplay will allow us to appreciate biology, and ultimately to provide potential solutions for management and conservation. ❧ Highlights of this thesis are outlined as follows: ❧ 1. Satellite sea-surface temperature products are effective in improving geoposition estimates of tracks derived from satellite tags. Low-resolution products (&gt; 1º in longitude and latitude) offer an optimal solution, balancing the trade-offs between the amount of details and computational performance. ❧ 2. White sharks were shown to perform frequent, continuous oscillatory dives between the surface and 400 meters throughout day and night in the offshore eastern Pacific Ocean (around 130-140 ºW and 25-30 ºN). Their bottom activities appeared to be confined by the concentration of dissolved oxygen at &lt; 2 ml/L. ❧ 3. When white sharks aggregated around Guadalupe Island, vertical movement of the sharks shows marked changes in depth and temperature distribution seasonally. Such seasonal changes corresponded to changes in the foraging dynamics around the island and in particular, related to the life history traits of seal populations that share the island habitat. ❧ 4. Striped marlin throughout the Pacific is an epipelagic species, spending at least 50% of time at the surface between 0-10 m. Despite this majority of time spent near the surface, striped marlin exhibited a variety of diving patterns, utilizing mid-water boundary conditions created by the mixed layer and the oxycline. Our results showed partitioning of maximum depths between western and eastern Pacific Ocean in accordance to dissolved oxygen levels, where the Eastern Pacific Ocean has a permanent oxygen minimum zone. Within a particular region, the swimming depths are likely to be limited by the interplay of oceanographic, biological and physiological conditions. ❧ 5. An individual-based limiting factor diving model is developed to relate the vertical movements recorded by a tag to potential biological functions, predator-prey interactions and water column features. Dynamics of vertical migration of prey (zooplankton) and predator (fish) are modeled as a function of efficiency of fish hunting by sight and provided predictions for observed data, such as peak predation activities by fish during sunrise and sunset. Enhanced diving by striped marlin during twilight, suggestive of hunting activities, were observed across various regions of the Pacific Ocean and support the model predictions. Using this model, I constructed a nested framework with key interacting layers in the water column, which can be used to understand striped marlin swimming patterns in the appropriate biological and environmental context.