The estimated short-term discard mortality of a trawled elasmobranch, the spiny dogfish (Squalus acanthias)

To estimate the short-term bycatch discard mortality of otter trawl captured spiny dogfish (Squalus acanthias), individuals caught by a Northwest Atlantic commercial bottom-trawl vessel using 45–60 min tows were held in held in pens for 72-h trials in lieu of being released. Mortality rates were compared to those in minimally stressed hook-and-line (control) dogfish subjected to the same protocols. To interpret physiological consequences of trawl capture, blood acid–base parameters were also assessed in dogfish following capture by both trawling and the hook-and-line control. Whole-blood pH (significantly depressed), pCO2 (significantly elevated) and pO2 (significantly depressed) were all markedly disturbed in trawled dogfish, indicating that trawl capture elicited a far greater disruption to vascular acid–base balance. However, 72 h mortality was not negligible for control dogfish (24%) but similar to trawling (29%), indicating that pens likely caused the observed (72 h) mortality associated with both capture methods. Still, in evaluating the range of 72 h trawling mortalities found across pen trials, estimated tow-weight was a significant predictor of mortality, explaining 67% of the variation. This suggests that penning stress was merely an additive factor compounding initial capture stress that dictated the 72 h mortality of trawled individuals. When normalized by subtracting out hook-and-line pen mortality, 72 h trawling mortality generally remained well below the 50.0% spiny dogfish discard mortality rate currently estimated for trawling in the Northwest Atlantic and was even superceded by penning stress in lighter tows; a testament to the resiliency of this species. However, catch-weights exceeding 200 kg yielded rapid elevations in 72 h mortality that more closely approached current estimates. This intimates that as tows become more heavily packed, potentially fatal damage inflicted on this species can heighten quickly. Spiny dogfish discard mortality is thus postulated as more commensurate with current estimations for otter trawling when tows are heavily packed.