Fishery-independent and -dependent movement data aid in defining the stock structure of a data-deficient billfish

The shortbill spearfish (Tetrapturus angustirostris) is a data-deficient billfish frequently encountered near the Main Hawaiian Islands where it is taken as non-target bycatch in commercial fisheries. The lack of information on the species' movement and stock structure is a primary management concern given uncertainty in its population dynamics and the lack of a formal stock assessment. Here, we combine fishery-independent satellite telemetry with fishery-dependent conventional tagging to describe the movement ecology of shortbill spearfish in the central North Pacific and contextualize it with respect to stock structure and the management considerations implicit with that structure. We show that shortbill spearfish are highly migratory like other billfishes and large pelagics, exhibiting multiple scales of movement with general fidelity to the region of the Hawaiian Islands and surrounding high seas but no discernible seasonality to their movements. The species' displacements from the island group into areas beyond national jurisdiction result in exposure to multiple distinct commercial fishing fleets, suggesting the need for multi-national cooperation in quantifying harvest. By comparing the limited sources of information on shortbill spearfish with the far more expansive knowledge base of other highly migratory fishes, we suggest a first-order division of stocks between the North and South Pacific. Continued interdisciplinary efforts are needed to confirm and further understand the proposed stock structure.