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Assessing residency and movement dynamics of swordfish Xiphias gladius in the Eastern North Pacific Ocean using stable isotope analysis

Tatiana A. Acosta-Pachón*, Sofía Ortega-García, Brittany Graham

*Corresponding author:

ABSTRACT: Swordfish are migratory fish that inhabit tropical to temperate waters, but little is known about movement patterns or mixing between subpopulations of this species. This study examines carbon (δ13C) and nitrogen (δ15N) stable isotope ratios in growth rings of the anal fin spine and muscle tissue of swordfish from three different areas of the Eastern North Pacific Ocean (ENPO) — off Baja California Sur (Mexico), and California (USA), and the North Pacific Subtropical Gyre (NPSG) — to assess swordfish movements between these areas. The analysis of δ13C and δ15N revealed no relationship was observed between size and δ15N, suggesting a consistent consumption of prey items by this species over time. Differences between the anal fin and muscle were found, but interpreting these isotopic differences requires information on tissue-specific isotope turnover rates. The spatial pattern suggests that the swordfish populations from these areas in the ENPO exhibit a relatively unexpected homing behavior at the temporal scale, according to anal spine and muscle turnover rates. Our findings support that swordfish may show site-fidelity, which has important implications for their management and stock assessments.