The Effects Of Capture On Sharks And Rays

Sharks and rays, as well as their relatives, are slow growing species that take a long time to mature and have a long gestation period. Taken together, these traits make them sensitive to overfishing; however, in addition to the dangers presented by direct mortality from fishing, these species may also be sensitive to capture-induced abortion / birth. Whether an event is a “premature birth” or an “abortion” depends on the stage of gestation, but in either case, there is potential evidence that these can be caused by interaction with fishing gear.

Indeed, some anecdotal observations from scientific literature date back more than 200 years. According to the authors of this study, they know of no studies at all looking specifically at capture-induced birth, beyond noting it incidentally. The authors of this study wanted to remedy that, by looking at the phenomenon intentionally through a wide-ranging literature review. After analysis, they ended up with nearly over 600 articles, and 139 instances where capture-induced abortion / birth was directly observed, and 92 instances where it was inferred but not mentioned directly.

The review found that across 26 different studies covering 24 species, the average rate of capture-induced birth was 24%. That being said, the rate varies considerably by species, anywhere from 2-85%, and the authors note that there may also be considerable variation based on fishing methods; capture-induced birth was the highest overall with trawling, but there wasn’t enough evidence to make conclusions on what method of capture may have what effect on different species.  The study notes that, as fare as reasons why capture-induced birth is happening, it may be “self-sacrifice” (where a “stranded female” gives birth to increase the chances of pup survival), or “self-preservation” (where a stranded female gives birth to facilitate escape and increase her own survival chances).

The study is a first step towards understanding what appears to be a complex and highly variable phenomenon. For animal advocates, the study is both another strike against the practice of trawling, and a reminder that fishing and capture can have serious consequences, even when there is not direct mortality. The study shows that there are many facets of the phenomenon to explore further, from better understanding the causes and risk factors of capture-induced birth to examining the ways it may be prevented.