Twin trawl nets deployed from the F/V Karen Elizabeth. Photo Credit: Chris Roebuck
Chris Roebuck, left, captain of the F/V Karen Elizabeth, talks with John Manderson (right) and John Hoey (far right) of the NEFSC prior to departure. Photo credit: NOAA Fisheries/Shelley Dawicki, NEFSC
Sweep Efficiency Study Targets Witch Flounder
Testing the efficiency of different sweep types was the focus of two weeks of twin trawling operations in August 2016 aboard the F/V Karen Elizabeth from Point Judith, RI. The target species: witch flounder, with American plaice and thorny skate secondary priorities. Witch flounder, also known as grey sole, is a flatfish that tends to stay on the bottom in deep waters.
The sweep efficiency experiment was recommended by the Northeast Trawl Advisory Panel, which is a joint advisory panel for the Mid-Atlantic and New England Fishery Management Councils. Both Northeast Fisheries Science Center (NEFSC) scientists and the fishing industry are interested in better understanding the catch efficiency of standard bottom trawl survey fishing gear to improve stock assessments. The results of this experiment were used to refine the research survey-based estimate of witch flounder population abundance, which was then used in the assessment.
The sweep efficiency study was conducted by Skipper Chris Roebuck and his four-person crew aboard the 78-foot Karen Elizabeth and five staff members from the NEFSC’s Ecosystems Surveys Branch and the Northeast Cooperative Research Program. Many of the data collection protocols used aboard the 209-foot NOAA Ship Henry B. Bigelow on its spring and fall bottom trawl surveys were used on the two one-week legs: 20 minutes towing time, constant towing speed between 2.8 and 3.2 knots, constant gear contact with the sea bottom, and net performance evaluation instruments indicating gear is operating within normal limits.
“The advantage of the twin trawl study is that you have two trawls as close as possible, one as a control and the other the experiment, being towed through the same body of fish at the same time,” said Roebuck. Both trawls are using the Bigelow survey nets, one with the rock hopper sweep as used on the Bigelow and the other an industry-designed chain sweep and roughly half the number of floats.
“We want to understand the efficiency of our survey gear,” said Michael Martin of the Ecosystems Surveys Branch, chief scientist on both legs of the study. “What fraction of the fish in the path of the net do we catch? We know no net can catch everything and we miss some fish. But how do different gear types compare for certain kinds of fish and habitat? Fishermen are interested in these questions as much as we are.”
On the first leg August 7-14, the Karen Elizabeth worked the area on the northern edge of Georges Bank, from Cultivator Shoals to the western edge of Closed Area II.
“In a few places it was hard to find a spot on the bottom that you didn’t catch rocks and mud and pull up a lot of brittle stars, but we were lucky and found some relatively hard areas free of rocks and with not much bycatch,” Roebuck said. “We’re happy with the results. We completed 65 tows, of which 61 were representative. On several tows we had five baskets, about 400 pounds, of the target species. It was a really good amount of fish.”
After a day in port to refuel, restock provisions and change a few members of the scientific and vessel crews, the vessel left Point Judith August 15 for another week at sea. They started working to the east side of Stellwagen Bank off Cape Cod and around Jeffrey’s Ledge. The team completed 60 tows on leg 2, with 57 representative, for a total of 125 tows, of which 118 were representative or good performance tows.
Overall, the chain sweep caught more fish than the rockhopper sweep did. Lengths of fish sampled during the two legs will indicate any variation in the efficiency of the two sweeps with respect to the size and age of fish. This data can be important in adjusting estimates of abundance for the different fish size and age classes.
“The Bigelow bottom trawl survey is effective as long as you understand how efficient it is and how that efficiency might be changing with changes in habitat distributions and animal behaviors as the climate changes,” said John Manderson, a fisheries scientist at the NEFSC’s Sandy Hook Laboratory who was aboard the Karen Elizabeth during leg 2. “Part of the reason for doing this experiment is to learn what that efficiency is and begin to take that into consideration as we move forward.”
The experiment will provide a ratio of the relative net efficiency of the rockhopper to the chain sweep, or a measure of relative net efficiency. In simple terms, net efficiency or detectability is the proportion of the fish occupying a sampling site that the boat catches in a bottom trawl. Net efficiency multiplied by availability, the proportion of the area occupied by the species that the survey covers, results in an estimate of catchability. Net efficiency and an estimate of availability could be used as a check against the catchability estimate from the model, as a value to fix catchability in the model or to estimate swept area population biomass from the survey.
“It was a good time for me to do this sweep study,” Roebuck said of the two-week experiment. “I am interested in helping figure out the science and what the information means.” He and his crew had just finished fishing for scallops, and starting September 1 will fish for squid for the rest of the fishing year, through April 2017.
No stranger to cooperative research, Roebuck has participated in a number of gear efficiency and other studies with the NEFSC since 2008 and says he likes the gear and bycatch reduction work.
“Chris thinks like a scientist,” Martin said of the skipper. “He is very focused and precise. He wants to understand the gear, how it is being used, and how effective it is with different species. There is a lot we can learn from fishermen like him.” Roebuck, in return, says he learns about the overall process and what the researchers are trying to understand.
For Martin and his colleagues, the trips meant standing 12-hour watches just as they do on the Bigelow, midnight to noon or noon to midnight. Operations ran 24 hours a day. The goal of leg 2 was to get 20-25 good tows in each area during day and night since fish often behave differently during the day than they do at night.
Once the catch was on deck, scientsts sorted flatfish, skates and scallops, then weighed and recorded the aggregate. The remaining catch was returned to sea as quickly as possible to avoid unnecessary mortality. Flatfish lengths were measured and recorded.
“This is a good example of what the process could look like,” Roebuck said of the effort. “We are out here now getting real-time information. There is a data meeting in mid-September, a model meeting in October, and the assessment in December.” The data from the sweep study was audited and analyzed by the NEFSC’s Ecosystems Surveys Branch, written up as a working paper and presented at the mid-September data workshop – all within three weeks of the at-sea experiment.
For John Hoey, who heads the Northeast Cooperative Research Program, collaborating with commercial fishermen on studies like this is the key to the future. “Fishermen are out there far more than we are, and they know the environment. Our methods are different, but our goal is the same - sustainable fisheries and efficient fishing communities. Putting scientists together with fishermen helps bring our respective knowledge and experience together and that can benefit everyone.”
The Northeast Trawl Advisory Panel (NTAP) is composed of Council members, fishing industry, academic, and government and non-government fisheries experts who will provide advice and direction on the conduct of trawl research. The NTAP was established to bring commercial fishing, fisheries science, and fishery management professionals in the northeastern US together to identify concerns about regional research survey performance and data, to identify methods to address or mitigate these concerns, and to promote mutual understanding and acceptance of the results of this work among their peers and in the broader community. Both Roebuck and Martin are members of the Northeast Trawl Advisory Panel