Spiny Dogfish Mortality Study
If there is one fish that aggravates commercial and recreational fishermen, it’s the spiny dogfish. During the 1990’s, spiny dogfish were a targeted fishery, but concern over stock structure, specifically a lack of large females, led to reductions in catch limits which effectively eliminated this fishery. Increasing population size caused an increase in by-catch and discards throughout all fishing fleets. Studies estimating mortality for gillnets and draggers had been conducted, but information for hook and line were lacking.
So, beginning in 2006, CCCHFA collaborated with the Gulf of Maine Research Institute to calculate spiny dogfish discard mortality for hook and line vessels in the Gulf of Maine and Southern New England waters using seafloor caging methods. Duplicating regular fishing practices, fishermen from CCCHFA and Gulf of Maine targeted spiny dogfish using rod and reel and longline. Caught dogfish were placed in a large cage and lowered down to the depth they were caught for three days. Prior to being placed in the cage, dogfish were measured, weighed and tagged using non-invasive methods. On the third day, cages were raised and dogfish were evaluated. Dogfish monitoring continued, with living fish transferred to a pen 25 feet in diameter for up to 27 days. Fish were fed every other day and dead fish were removed.
A total of 2,418 dogfish were sampled between the two regions; of these 682 were caged by GMRI (45% males and 55% females) and 1,234 were caged by CCCHFA (27% males and 73% females). Dogfish were subject to three gear/handling treatments: 1) snubbed, 2) unsnubbed and 3) control. An overall regional difference in short-term discard mortality was observed; GMRI recorded significantly lower total mortality (7%) than CCCHFA (22%). Regional, averaged findings showed a sex effect with males demonstrating higher mortality (26%) than females (14%). The largest dogfish of each sex demonstrated greater resilience to mortality. Treatment effects were observed with mortality being highest in snubbed fish (23%), then unsnubbed fish (16%) and finally control fish (13%). Gear effects were found with highest mortality resulting from long-line gear (22%) while the different hand gear-related mortality ranged from 8-17%. The relationship between hook removal treatment and hooking severity index (HSI) was significant; severe mouth and jaw injury was most frequent in snubbed fish but was rare in control fish and the overall trend was that mortality increased with increased HSI.
Considerable variation was observed between the GMRI and CCCHFA findings; this variation was likely caused by differences in: water temperature, levels of parasitic infestation (i.e. sand fleas) and possible differences in handling, tanking and caging procedures. This study’s long-term discard mortality assessments were aborted for logistical reasons and as such, these findings represent the range of likely short-term discard mortality from hook gears across the region.
This study, which ended in June 2007, was funded with a grant from the Northeast Consortium.
