Coupling GPS tracking with dive behavior to examine the relationship between foraging strategy and fine-scale movements of northern fur seals

Abstract

The foraging strategies of diving marine species are often categorized into 3 fundamen- tal groups (epipelagic, mesopelagic, and benthic foraging) based on diving, habitat use, and diet studies. Because these foraging strategies are influenced by the distribution and behavior of the prey being targeted, we would expect search behavior and space use to differ depending on the strategy employed. Since northern fur seals Callorhinus ursinus display both epipelagic and benthic foraging strategies, they were an ideal model to test the hypothesis that fine-scale movement and space-use patterns will vary when animals use markedly different foraging strategies. Dive bouts were charac- terized into foraging strategies based on numerous dive parameters (depth, duration, etc.). For each strategy, we compared movement patterns (e.g. transit rate and path straightness) and space use (area-restricted search (ARS) zones) around St. Paul Island, Alaska, USA. Nearly all dive parameters were significantly different between foraging strategies (epipelagic vs. benthic). In addition, epipelagic bouts were more sinuous and covered a greater total distance than benthic bouts. How- ever, the greater distances traveled in epipelagic bouts were due to longer bout durations, as transit rates were not different between the 2 strategies. On average, <2 ARS zones were identified per trip, and the characteristics of epipelagic and benthic ARS zones were not different. By combining dive behavior with precise at-sea locations, this study has provided a greater understanding of the fine- scale foraging behavior of northern fur seals. Monitoring changes in foraging behavior over time and comparing behavior among populations with differing population trajectories may provide more clues as to why northern fur seal numbers on St. Paul Island continue to decline.