Yellowtail flounder study applies ‘best science’ to selectivity debate

Already losing landings to regulatory quotas and giving up fishing days to court orders, fishermen were clearly not inclined to give up legally caught fish to oversized net mesh.

As regulators contemplated increasing the required mesh size for groundfish trawls to reduce juvenile mortality, the fishing community pointedly questioned the need. Fishermen concerned that legal-sized fish were already escaping their nets wanted documentation that mesh size "does have the effect on populations that managers seem to think," says David Beutel, Rhode Island Sea Grant Fisheries Extension specialist.

"When you haul a net out of the water, fish fall out of the cod-end," he says matter-of-factly. "When these fish are larger than minimum size, fishermen know they’re losing legal fish."

Size and shape of the cod-end—the rear portion of a trawl net where fish collect—control retention of fish in the net and thus the makeup of the catch. Mesh size has increased by regulatory mandate over the years so that small fish, mostly juveniles, can escape. Presumably, says Laura Skrobe, also a Sea Grant Fisheries Extension specialist, these young fish will have a chance to spawn and contribute to their stocks.

Because stocks of many groundfish, including cod and southern New England yellowtail flounder, are designated overfished, regulators have imposed progressively stiffer restrictions on fishing effort for such species. Apart from outright area closures, larger meshes and larger minimum sizes are the primary regulatory tools for controlling fishing mortality.

But fishermen distrusted the numbers that were driving recommendations for a larger mesh. Before conceding to further restrictions on their fishing activity, they wanted to see data that reflected actual, rather than extrapolated, cod-end escape and retention rates. The R.I. Department of Environmental Management (RIDEM) followed up on their appeal and tapped Rhode Island Sea Grant to conduct a study to determine what size fish actually were retained in what size nets, as Beutel puts it.

That study took the form of an experimental fishery for southern New England yellowtail flounder to test the size selectivity of four cod-end mesh sizes: 6-inch and 6.5-inch diamond-shaped, 6.5-inch square, and the proposed 7-inch square, along with a 3-inch control. The study brought together Rhode Island Sea Grant, RIDEM Fish and Wildlife Division, and members of the Rhode Island Commercial Fishermen’s Association (RICFA) in a collaborative project that ultimately could influence new regulations.

Fishermen offered their vessels for the study, allowing Sea Grant researchers to conduct 19 to 20 pair-sampling tows, alternating experimental and control tows, for each mesh size. They brought 13,477 yellowtail flounder on deck to be measured—in a distribution fairly balanced among the different meshes. (Only the 3-inch control net, which "catches and retains pretty much whatever it runs into," strained the balance, accounting for nearly 80 percent of the catch, according to Beutel.)

Beutel and Skrobe, who ran the study for Sea Grant, explain that catch analysis generates a selection curve, whose midpoint is the point at which 50 percent of the fish of a particular length stay in the cod-end and 50 percent escape. What Beutel, Skrobe, and their collaborators set out to determine was the selection curve for yellowtail, to ascertain the average size fish caught in the various meshes.

Bottom line, the study "demonstrated, as suspected, that there’s a significant difference between what NMFS said fell out of a 6.5-inch mesh and what actually did," asserts Chris Brown, RICFA president. Brown is captain of the F/V Grandville Davis, one of the vessels participating in the study. "The findings are more in line with what fishermen thought."

The role this information could play in the future of the fishery is potentially significant. Beutel believes industry people, gear suppliers as well as fishermen, and managers will incorporate the results into fish harvest planning. Brown suggests the data will factor into impending regulations: Because the study results "are the most recent and best available science—which is the acid test for management—fisheries councils will probably use them in groundfish management decisions."

The findings could also exert a more far-reaching impact than their effect on minimum catch size and mesh size regulations. Days-at-sea is another tool used to regulate effort, Brown notes, and he and Beutel both acknowledge that new groundfish regulations will likely cut days at sea below even the 88 days currently allotted. "If we can demonstrate that effort is at a lower point than NMFS suspects it is, then the degree in the cut in days-at-sea can potentially be less," Brown offers hopefully. "If we incorporate data from this retention study, the cut could be, instead of 50 percent, maybe 30 percent."

—Tony Corey

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