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Are large cod hiding in the Gulf of Maine's rocky bottom?

by NOAA Fisheries 6 May 16:14 UTC
Atlantic cod © NOAA Fisheries

In a new study, scientists have answered a nagging question about research survey data used in stock assessments for Gulf of Maine Atlantic cod.

The study suggests that lower abundance, rather than fish evading trawl gear, is the reason our bottom trawl survey seldom catches large cod in the area.

"There was a question of whether the large cod were hiding in the rocks where the trawl survey could miss them, and nobody knew the answer," said marine resources management specialist Jessica Blaylock, a co-author of the study, recently published in Fishery Bulletin.

Was survey gear missing cod or were the cod just not there?

Fishermen raised the question of whether large cod might be aggregating in rocky bottom habitat, thus avoiding detection by the Northeast Fisheries Science Center's bottom trawl survey.

The bottom trawl survey provides valuable measures of relative abundance, rates of population change, and size and sex composition for a wide range of species. However, it is not equally efficient at catching all species in all areas where it operates. This survey samples a variety of habitats between North Carolina and Nova Scotia, but towing the trawl net in rugged, rocky areas is difficult.

Scientists in the Cooperative Research Branch investigated the question by comparing bottom trawl survey data collected by a research vessel to that collected by a bottom longline survey, that is conducted from commercial vessels using hooks that can sample more easily in rocky habitats.

Catching a given species depends on whether a fish will be retained by the gear used, as well as the fish's population size and behavior. When comparing catch results from different gear types, scientists consider how probable it is that a fish will be captured by the gear. Scientists rely on this information to understand how well fisheries survey data represent the population.

A tale of two groundfish

In the study, scientists compared data on cod and white hake (another type of groundfish) in rough bottom habitat of the Gulf of Maine collected by both bottom trawl and bottom longline surveys between 2014 and 2018.

Atlantic Cod

If older, larger cod had indeed moved to hard-bottom locations, that could reduce their availability to trawl survey gear. It would also explain why the survey data reflected an increasingly smaller number of large, mature Atlantic cod. Atlantic cod more than 8 years old have become very rare over the past decade, though the natural lifespan of cod can be more than 20 years. For humans, this would be like not seeing anyone over the age of 30 on city streets.

Our scientists looked at the overlap between the length distributions of cod and white hake from the two surveys in the spring and fall seasons. There was no appreciable difference between catches of cod in the two surveys, with large cod extremely rare in both.

Large cod had a similar distribution between the two surveys, and were relatively consistent among sampling years. They were not more prevalent in rough-bottom habitats. This suggests a lower abundance, rather than fish hiding in the rocks, as the reason the bottom trawl survey seldom catches large cod.

Commercial landings data also support the absence of large cod in the Gulf of Maine.

White hake

In contrast to cod, scientists did find differences between the bottom longline and bottom-trawl survey data for large white hake. There was less overlap for white hake than for cod between the bottom trawl and bottom longline survey length distributions. The bottom longline survey detected large white hake on rough-bottom habitats, which shows that longline gear can catch large groundfish not often caught by trawl gear. This suggests that if large cod were present, the gear would capture them. This information also suggests that large white hake may prefer rough-bottom habitat.

Scientists already account for the fact that very young or very old white hake are less likely to be caught in the bottom trawl survey gear in the stock assessment models.

"We looked for white hake to get a comparison point with cod," said research fishery biologist Chris Legault, a study co-author. "We saw that the large white hake were indeed being caught on the hooks, so just because we don't see large white hake on the bottom trawl survey doesn't mean they're not there."

"It's not exactly a happy story from the cod perspective," said co-author and lead for the bottom longline survey Dave McElroy, "but it is positive from the white hake perspective. It reflects a better state of the stock for the white hake, and it's nice to see that there are still big fish in that population."

"We're always willing to explore hypotheses and admit that we might be wrong. Had we found a lot of large cod in the rocky habitats, I would have been really happy because it would be great news for the species and fishery," said Legault, "Part of science is being willing to question the assumptions you make and test them with data."

A tale of two surveys

The Northeast bottom trawl survey is the world's longest running standardized survey of its kind. It has been conducted in the fall since 1962, and in the spring since 1968. The survey samples the waters from Cape Hatteras, North Carolina, to the Canadian border. At each station, scientists collect a variety of data, including campelks used to estimate abundance, biomass, and size distribution of fish species.

The habitat-focused bottom longline survey is unique because it puts scientists on commercial fishing vessels using gear similar to that used by the industry. From the beginning, it has been a close collaboration between scientists and fishermen. The survey uses 1,000 baited hooks over one nautical mile, and is conducted on two commercial vessels in the spring and fall of each year. The timing of the survey is as close as possible to the bottom trawl survey, so we can compare the data from the different gear types.

Since 2014 the Cooperative Bottom Longline Survey has collected data at 45 stations each season in the Gulf of Maine. These include structured habitats that the bottom trawl survey may have difficulty sampling.

The Tenacious II

Captain Eric Hesse has been fishing since 1984 and has worked on the bottom longline survey from its beginning in 2014. His vessel, the 40-foot Tenacious II, was used for the study. With a master's degree in environmental engineering, Hesse credits his scientific background with his ability to connect scientists and the fishing community. He feels strongly about the sustainability of hook fishing and notes that it produces a high quality fish. "One of the best things about longline fishing is that everything comes up alive," he notes.

"Back in the 1990s, we would readily catch large cod east of Nantucket and George's Bank on hooks. So I think the conclusions of the study are correct. Cod are not shying away into rocky habitats because they're in decline. They're just hard to find," Hesse observed.

"The role I had in this study was gratifying. It was interesting to look at the difference in the way the gear types work. It's satisfying to know that the bottom longline survey can fill in the gaps in these hard-to-trawl areas. It's a further endorsement of the gear type for me."

Eighty-four percent of the bottom longline stations are rough-bottom habitat, compared with 20 percent to 35 percent for the bottom trawl survey. In particular, this survey provides information on data-poor species strongly associated with structured habitats, such as cusk, Atlantic wolffish, and thorny skates.

"The rocky bottom ends up being more of a blind spot for the bottom trawl survey because the rocks can tear up the trawl gear. The bottom longline survey fills this hole so that we can piece together a whole picture," explained Blaylock.

Value of cooperative research

This is the first major publication directly comparing the bottom trawl and bottom longline survey data. The study shows that the surveys help inform scientific advice and complement each other with different gear types. Both surveys investigate questions asked by the fishing industry.

"We work with the same captains consistently, and they provide a lot of input on the survey design: how we set the gear, soak the gear, control for different factors. They think about how the gear fishes a lot, so they have good insight into how to use the gear consistently," said McElroy.

This study highlights that cooperative research between the fishing industry and scientists can strengthen fisheries assessments and management.

What can the fishing community do to help rebuild the cod stock? Fishermen can stay involved in regional council meetings and accommodate fisheries observers to support sound science and management.

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