- Investigate the physiological consequences of fishing capture stress in elasmobranchs (sharks, rays and skates).
- Examine potential factors explaining differences observed in the stress response between elasmobranch species.
- Establish more accurate estimates of capture-induced immediate and delayed mortality rates of elasmobranchs by species and type of capture method.
- Reveal the more potentially deleterious aspects of different capture methods on a given species.
Due to factors such as slow growth, a long life span and low reproductive productivity, elasmobranch (sharks, rays and skates) populations are very susceptible to rapid depletion from human activities. For example, high levels of directed commercial (i.e. for shark fins) and recreational fishing activities have caused major (reported) declines in the populations of many elasmobranchs species around the globe in recent years. However, those harvested are not the only animals impacted by fishing activities. Many elasmobranchs are caught incidentally as bycatch, and although released, are still subjected to the cumulative physical and physiological rigors of capture, handling and deck-time (air exposure). Such treatments can have deleterious consequences on the animals, even after they have been returned to the ocean.
Because factors such as anatomy, behavior (when faced with a threat), and the physiologic threshold to cope with stress vary by species, the relative impacts of capture are diverse across elasmobranchs. However, relatively few studies have investigated the physiological consequences of capture stress in elasmobranchs, or have sought to compare responses to given stressors by species. Such data could reveal which species are more apt to succumb under duress, and which aspects of capture are most detrimental to a given species. It is vital to management to better understand the nature and magnitude of physiological disturbance and physical trauma induced by capture stress, and whether the ability to cope with these stressors varies between elasmobranch species, the populations of many of which are reportedly in peril.
Spiny Dogfish Bycatch Mortality
The spiny dogfish, Squalus acanthias, is routinely captured both directly and unintentionally as bycatch in recreational and commercial fisheries in the western North Atlantic. Due to highly dichotomous views on its population status and its infamy as a nuisance to fishing operations targeting other species, the spiny dogfish has generated vast attention in recent years. Because countless dogfish are discarded as bycatch following capture, estimated rates of post-release mortality hold great bearing on dogfish population models. Yet until recently, post-release (delayed) mortality had not been studied in dogfish. Our (NOAA) Saltonstall-Kennedy funded study investigated post-release mortality and physiological stress following otter trawl and hook-and-line (control) capture. A collaboration with commercial fishers/vessels ensured authentic fishing practices. Delayed mortality rates were estimated by retaining spiny dogfish in experimental pens for 72 hours after initial capture.
The study’s results showed a low post-release mortality rate for dogfish caught by trawl, and the species as resilient to this mode of capture. Since blood physiology was markedly altered by the capture, it appears that although disturbed, dogfish possess a high threshold for stress and are capable of recovering from capture assuming physical trauma is not outright lethal. Such findings bode well for dogfish populations, which again, experience high rates of incidental capture and release.
Immediate and Delayed Mortality of Northwest Atlantic Skates
The populations of several of the seven skate species indigenous to the Gulf of Maine have been designated as overfished and/or on the brink of respective biomass threshold levels in recent years. Despite regulatory and market factors dictating the release of the majority of these skates following commercial fishing capture, there remains a paucity of empirical (immediate and delayed) mortality data related to these species. Such findings are of critical importance to management when accounting for the overall fishing mortality of these species. In collaboration with the University of New England (Dr. James Sulikowski), we have been funded by the Northeast region of NOAA Fisheries (Saltonstall-Kennedy grant) to investigate the estimated immediate and delayed mortality rates of four of these skates—thorny (Amblyraja radiata), winter (Leucoraja ocellata), smooth (Malacoraja senta), and little (Leucoraja erinacea) skates—captured by commercial fishing operations (otter trawl and sink gillnet). Although it is presumed that skates differ with respect to their tolerance to capture stress, this is the first study directly attempting to explore species differences and the potential factors influencing them. The resulting species-specific mortality data, which will account for additional variables such as season, gradients between seawater and air temperature, and deck (air-exposure) time, is expected to contribute significantly to stock assessments and help inform new policy regarding these species.
Physiological Status of Longline-Caught Coastal Sharks
This work examines the physiological status of different carcharhinid sharks caught as bycatch in bottom longline fisheries. Many of these species are very active and attain large sizes, and as such are difficult to maintain in captivity. Thus, the subject of physiological status following stress events is understudied in this group. Through our results, it is very clear that the ability to modulate physiological functioning and survive this type of capture varies substantially across taxonomically similar species. Some species are very resilient, while others are particularly sensitive to capture. Our study is also preliminarily investigating potential causes into the mechanisms regarding differences observed.
Bycatch Assessment and Reduction Strategies
This study area is two-fold: 1) We have collaborated with scientists from several countries to publish a review paper on the nature and extent, as well as potential strategies to reduce, shark bycatch and depredation in pelagic longline fisheries primarily targeting tuna and swordfish; 2) We are investigating methods to reduce shark bycatch. Specifically, we are conducting a variety of laboratory trials assessing whether rare-earth metals can effectively repel two species of dogfish from attacking baited fishing gears.