Fine-scale Vertical Movements and Behavior of Immature Skipjack Tuna (Katsuwonus pelamis) off Eastern Taiwan

Skipjack tuna are globally distributed in tropical and temperate seas but little is known about their fine-scale diving patterns and how fish size and (or) maturity influences patterns off eastern Taiwan. A total of 211 immature skipjack (36 - 47 FL cm) in the study were caught by trolling near subsurface fish aggregating devices (FADs) near Green Island (southeastern Taiwan) and were tagged with Lotek LAT-2910 archival tags. The low rate of recovery (2.4%) suggests a high post-release mortality but this low rate could also be consistent with large population sizes, dispersal, tag loss or uncooperative fishers. Five individuals were recovered near the release locations and tags provided data archiving 11-31 days-at-liberty. Tagged skipjack tuna dove deeper during daytime than at nighttime (p < 0.001) and displayed repetitive bounce-diving behavior to depths between 50 - 310 m during daytime. During the deepest dives, ambient temperatures reached a low of 13.1 degrees C with peritoneal temperatures at 17.0 degrees C. The vertical movements and habitat preferences reported herein this study are the first fishery-independent observations on immature skipjack and provide evidence of the degree of ecological niche and fisheries interactions of skipjack tuna in eastern Taiwan.