Trawling’s troubles: bycatch and unobserved mortality

The prohibited species caps for Bering Sea trawlers fishing for pollock, cod and flatfish allows them to catch a preset amount of these species along with their target species, and until recently those caps had always had a “floor,” or a minimum level, no matter how dire the status of the prohibited species.

That changed when the North Pacific Fishery Management Council linked halibut bycatch PSC from factory trawlers fishing for flats in the Bering Sea to halibut abundance. That action might signal a greater willingness on the part of the Council to weigh the scales on the side of bycatch reduction, as opposed to optimum yield for productive trawl fisheries.

Large swaths of the Bering Sea are closed to hard-on-the-bottom nets in an effort to limit harm to crab stocks. But two new mysteries have cropped up that are challenging that management strategy.

The first is whether the areas closed to bottom trawling are still in the right spot. Non-pelagic trawling is prohibited in the Nearshore Bristol Bay Trawl Closure Area and the Bristol Bay Red Crab Savings Area, which together comprise most of Bristol Bay. But warming conditions have been shown to be pushing populations of marine life northward. It could be that the crab are walking out of their safe zones.

The other question is whether “pelagic” trawls should still be considered “mid-water,” and therefore not subject to area closures, because it is assumed they are not contacting crab on the bottom. It has come to light that some of the larger pollock nets being towed behind catcher-processors contact the bottom as much as 80% of the time they are fishing. We have very little direct information about the effects of that contact.

But if bycatch is the public face of trawling’s troubles, then its more mysterious brother — living up in the dark attic, a source of some speculation, but staying stubbornly in the shadows — is “unobserved mortality.” Unobserved mortality refers to marine life that is fatally contacted by fishing gear but not captured, and therefore not counted against any prohibited species cap.

Research has been conducted that estimates the degree of unseen fatal contact that occurs to crab during bottom trawling. We do know that a healthy crab with a hard shell has a pretty good chance of surviving being run over by a bottom trawl. We know this from research like “Quantification and reduction of unobserved mortality rates for snow, southern Tanner, and red king crabs (Chionoecetes opilio, C. bairdi, and Paralithodes camtschaticus) after encounters with trawls on the seafloor,” produced for NOAA by the Alaska Fisheries Science Center. For their research they designed a trawl with secondary nets that would capture crab not scooped up in the main net, but after they came into contact with the trawl.

“Estimates of the rates of mortality due to contact with the trawl gear, adjusted for capture and handling, were below 16%, with the exception of red king crab that encountered the wing section of the footrope, for which mortality was estimated at 31%.

“Overall, estimated mortality rates for all 3 species were significantly lower for crabs that encountered the sweeps than for those crabs that encountered the footrope and were higher for those crabs that encountered the wing portion of the footrope as opposed to the center footrope. 

“Although the mortality rates for the southern Tanner and snow crabs were similar, both had lower mortality rates than did the red king crab for all trawl components. Raising the sweeps with widely spaced disk clusters reduced red king crab mortality from 10% to 4%,” according to the Alaska Fisheries Science Center.

They found that most crab survived their injuries if they didn’t come up dead already. “Most southern Tanner and snow crabs captured behind the main trawl components had all reflexes present (76–93%), and the next most frequent category was dead crabs upon capture (2–17%). Similarly, a substantial majority (66–83%) of red king crab captured behind the trawl gear was uninjured and had all reflexes present. Very few of these animals died during holding. Of the red king crab, 6% were dead upon capture, making up 71% of mortalities. Therefore, nearly all of the observed crabs were either extremely likely to survive or moribund; relatively few crabs displayed an intermediate condition. …”

It was observed that the “sweeps,” or lines that herd fish into the net, did less damage to crab than the footropes at the mouth of the net. Since the sweeps have the effect of increasing the amount of the target catch, they also act to reduce harms to crab by decreasing the towing time. 

And footropes designed to roll over crab with rubber bobbins are less harmful than those rigged with chains, for instance. Regulation does not allow pelagic gear to use bobbins. Supposedly they are not on the bottom in the first place. Their footropes are commonly rigged with chain.

Some statistics jump out at you. “Large snow crab were approximately twice as likely to die as smaller snow crab or as any size of southern Tanner crab, and this difference persisted across all gear components and control catches.”

Hmmm.

Also: “Mortality of red king crab did not vary significantly between sexes or between new-hard-shell and old-shell crab, although the percentage of mortalities was high for soft-shell crab (4 of 5 died) and crab with very old shells (3 of 5 died) …” So red king crab are especially vulnerable when they are molting. They also mate when they molt, making them doubly at risk at that time. The problem is we aren’t sure where, or when, that is happening these days. The only survey presently conducted for red king crab happens in the summer, when their shells are hard.

NOAA Fisheries is taking public comment on a request to take emergency action to close the Red King Crab Savings Area and the Red King Crab Savings Subarea to all fishing gear that comes into contact with the ocean bottom. 

If implemented, the requested emergency closures would include pelagic trawl, pot gear and longline gear, and would be in effect for 180 days after the emergency rule is published. The petitioners requested that the closure occur for six months — from Jan. 1 to June 30, 2023. 

NOAA Fisheries is working with the North Pacific Fishery Management Council, which plans to provide feedback to NOAA at its December meeting. 

NOAA Fisheries will make a determination on the petition after considering public comments on this notice, and public comments received during the December Council meeting.

The deadline to submit comments on the Federal Register notice is Dec. 5. 

Terry Haines was a commercial fisherman in Kodiak for more than 30 years. He now produces the Alaska Fisheries Report for KMXT and is a member of the Kodiak City Council. He can be reached at thaines@city.kodiak.ak.us