Movements and distribution of juvenile common thresher sharks Alopias vulpinus in Pacific coast waters of the USA and Mexico

ABSTRACT: The common thresher shark Alopias vulpinus constitutes an important commercial fishery on the Pacific coasts of both the USA and Mexico. However, little is known about the juvenile phase of this species. This study used a combination of pop-up archival satellite tagging, tag-recapture, and fishery catch data to investigate the movement patterns, habitat preferences, ecology, and geographic distribution of juvenile common thresher sharks along the Pacific coast of the USA and Mexico. Juvenile threshers primarily utilized continental shelf waters, with a geographic range extending from Punta Eugenia in Baja California, Mexico (27.8°N) north to Morro Bay, California (35.3°N). Within this range, sharks were found at significantly lower latitudes in March and April. Satellite-tagged juvenile threshers exhibited diel patterns of vertical distribution, primarily inhabiting the upper 20 m of the water column by night, and significantly greater depths by day. In addition, juvenile threshers made frequent daytime dives to depths exceeding 50 m, with a maximum recorded dive depth of 192 m. Tracked sharks were most commonly associated with ambient water temperatures between 14 and 17°C, and inhabited significantly warmer temperatures at night than during the day. No tidal or lunar influence on vertical distribution was found, and vertical habitat utilization did not increase concomitantly with shark size. This study is the first to document movements of juvenile threshers between US and Mexican waters, highlighting the need for bi-national management strategies for this shared fishery resource.