Movement Ecology of Female Shortfin Makos in the Gulf

Highly migratory species (HMS), like the mako shark, are often apex predators that serve critical ecological functions within vast marine ecosystems. Managing entities face serious and complex challenges as HMS frequently cross multiple jurisdictional boundaries during their long-distance movements, which also expose individuals to varying natural and man-made pressures (e.g., dynamic environmental conditions, prey resources, levels of fishing effort around man-made [artificial] habitat vs. open ocean, and illegal/unreported/unregulated fishing). As a result, knowledge of species movement patterns is essential for identifying the spatial and temporal scales at which a fishery can be best managed. Failure to recognize or accurately identify the stock structure of an exploited species can lead to changes in biological attributes and productivity, loss of genetic diversity, and overfishing and depletion of less productive stocks. Unfortunately, management of many HMS fisheries continues to be hindered by large data gaps regarding seasonal movement patterns and stock structure.

Prior to current regulations, mako sharks were highly prized in recreational fisheries and were considered high-value bycatch in directed commercial pelagic longline fisheries. Like other shark species, mako sharks have low resilience to fishing mortality due to their inherent life history characteristics (e.g., slow growth, late age-at-maturity). In the Atlantic Ocean (including the Gulf), the status of mako sharks is assessed by the ICCAT. The updated stock assessment, conducted in 2019, suggested the North Atlantic shortfin mako stock was still overfished and experiencing overfishing, therefore, a retention ban was implemented. Even with these regulations, recently updated projections suggest that there is only an 8% probability of rebuilding the stock and ending overfishing by 2070. Despite their imperiled status, little research has been conducted on this species regarding their movements, habitat use, and reproductive stock structure.

Specifically, the few previous studies on movement of this species have largely focused on males and juveniles, including few adult females. The locations of mating grounds are not yet known for mako sharks; however, two of the four females previously tagged by PIs in the western Gulf had fresh bite marks present at time of tagging (late March). Although these bites could be the result of fighting behavior, the bites may also be the result of mating behaviors, suggesting the Gulf may serve as a mating ground. Additional tag data from the PIs, indicated that male mako sharks were transiting during summer (leaving the Gulf) and early winter (returning to the Gulf) months, suggesting that mako sharks in the Gulf might be mating slightly later in the year than in other regions or that some males may leave the Gulf to reproduce elsewhere. This supports the possibility of multiple reproductive stocks and/or that seasonality of mating may be shifted by male sharks to accommodate the availability of fertile females. Sex-biased dispersal has been previously reported in mako sharks in the Pacific Ocean and Indian Ocean, with males making large movements while females were philopatric. Additionally, differences in movement patterns may be a result of sexual segregation, as has been reported in the White Shark, a species also in the family Lamnidae that has seasonal site fidelity to potential breeding and pupping areas.

Although little is known about exact pupping grounds, young-of-the-year mako sharks have been observed offshore in the Gulf and it is hypothesized that mako sharks in the western North Atlantic are born far offshore, likely to protect the pups from predation. One female mature mako tagged by PIs was tracked >2.5 years during which time she remained on the continental shelf and slope for >2 years until early summer when she moved further offshore for 30 days before returning to the continental shelf. Although pregnancy cannot be confirmed, this change in behavior is consistent with the hypothesis that mako sharks pup offshore. Additional tagging, particularly of mature females, would bolster sample size, aid in the identification of any differences in sex specific habitat utilization and further investigate the existence of distinct reproductive stocks for this species.

Because information regarding stock structure for mako sharks in the North Atlantic is limited, and especially lacking in the GOM, this project will provide much improved data for use in stock assessments and the development of spatial management plans. This has significant implications, especially if the degree of international exchange between the U.S. Gulf and other nearby countries (e.g., Mexican waters) is high. Collectively, these data can be used to aid the development of spatially explicit stock assessment models, which commonly produce more precise estimates than models that do not consider movement. Subsequently, the information obtained in this study will be essential in the development of future federal and international management plans promoting the sustainability of this resource. This directly fulfills both the team goals under the Wildlife and Fisheries Priority issues in the Governor’s Action Plan Number IV for Healthy and Resilient Coasts (2021-2026).