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Commercial Fisheries News
Volume 35 Number 9
May 2008
Research leads to lower ‘assumed’ discard mortality rates
BIDDEFORD, ME – Recent research into dogfish discard mortality is backing up what most commercial and recreational fishermen have long suspected: Spiny dogfish are resilient – as long as they’re not severely wounded.
During a March 29 dogfish forum here, Gulf of Maine Research Institute (GMRI) scientist Shelly Tallack presented the collective findings of a joint discard mortality study for hook gear conducted from July through September 2006 by GMRI and the Cape Cod Commercial Hook Fishermen’s Association (CCCHFA).
Final results from the study are now undergoing technical review before being submitted to a scientific journal for publication. Following the review, principals in the project expect to present their findings to the National Marine Fisheries Service (NMFS).
The information is expected to help NMFS further enhance the “assumed” dogfish discard mortality rate factored into dogfish stock assessments. Recent discard studies by other researchers for gillnet and trawl gear led NMFS to adjust estimates in the 2006 stock assessment.
Two fishing vessels participated in the Gulf of Maine portion of the GMRI and CCCHFA hook-gear study and six participated off Cape Cod.
To cut to the chase, it turned out that carefully handled dogfish had high survival rates while severely wounded and badly snubbed fish didn’t fare as well.
Handling experiment
All together, 2,418 dogfish were sampled in the hook-gear study. Fish were classified by handling technique as follows:
Snubbed – assigned to longline-caught fish where the hook was ripped from the mouth;
Unsnubbed – assigned to longline-caught fish where the hook was carefully removed; and
Control – assigned to rod-and-reel, handline, and jig-caught fish where the hook was carefully removed.
Each fish also was given a “hooking severity index” (HSI) number, which ranged from zero, where no external mouth wound was visible – even though some internal injury might have occurred – to five, where the hooking wound resulted in a large, gaping hole and the jaw essentially was broken.
Besides being categorized by handling technique and HSI, the fish were given a “stamina index” of one-to-four, with “one” assigned to dogfish that were “alive, strong, with much resistance to being handled” and “four” being for fish that were dead.
Then, the live fish were placed in cages for approximately three days with roughly 17 dogfish per cage, explained Tallack.
After the three days, the condition of each fish once again was gauged using the stamina index, and survival rates were calculated.
According to Tallack, the results varied by region.
Of the sampled dogfish, 682 were caught in the Gulf of Maine in the GMRI portion of the study. Forty-five percent were males and 55% were females. Total mortality for these fish was 7%.
The other 1,234 dogfish were caught in the CCCHFA part of the project. Twenty-seven percent of the fish were males and 73% were females. Total mortality for these fish was 22%.
Tallack said environmental factors might have accounted for the regional mortality differences, including water temperature and the higher presence of sand fleas where the CCCHFA vessels were fishing. Variations in dogfish handling techniques and caging procedures also could have been a factor, she said, even though a standard protocol was developed and followed in both regions.
Overall results
Several interesting results surfaced from this cooperative research, which was funded by the Northeast Consortium.
For one, males had a higher overall mortality rate than females, averaging 26% vs. 14% respectively. However, the larger dogfish of each sex seemed to be hardier and more prone to recover from hooking injuries.
Coming as no surprise, snubbed fish had the highest mortality rate at 23%, while unsnubbed fish came in at 16%. The carefully handled hand-gear caught “control” fish faired best with a 13% mortality rate, said Tallack.
When the results were further broken down by gear type, the overall mortality rate for dogfish caught in longline gear was 22%. For hand-gear-caught fish, it ranged from 8% to 17% depending on the type of hand gear used – handline, rod-and-reel, or electronic jigging machines.
Furthermore, Tallack said the HSI factor became an important indicator in the study.
“Mortality overall increased for fish with more severe hooking injuries,” she said. “Internal injuries, ripped insides, swallowed hooks, these also affected the fish and appeared to cause mortality.”
Discard assumptions
The cooperative GMRI and CCCHFA project investigated discard mortality rates for commercially hook-caught dogfish only and is still in the final stages of technical review.
But other research by John Mandelman and Marianne Farrington of the New England Aquarium looked at discard mortality rates for trawl caught dogfish, and Roger Rulifson at East Carolina University conducted research on gillnet caught dogfish.
The results from these studies led NMFS to revise the dogfish discard mortality estimates used in the 2006 spiny dogfish stock assessment as follows:
Gillnets – 30%, down significantly from a previously assumed dogfish discard mortality rate of 75%;
Trawl gear – 50%, which remained unchanged from the previously assumed rate of 50%;
Commercial hook-and-line gear – 10%, down from the previously assumed rate of 25%; and
Recreationally caught dogfish – 20%, down markedly from the previously assumed discard mortality rate of 100%.
Fishermen towing trawl gear at times encounter extremely large schools of dogfish, which often get crushed in the net simply due to their sheer volume. This full-net/crushing factor is the primary reason the discard mortality rate for trawl caught dogfish remained so high, explained Tallack.
Janice M. Plante
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