July 23, 2012 — Crab pot loss creates an environmental and a resource problem. It may not tip the scales toward overfishing, but it is still something that fishers should be concerned about. Biodegradable twine and escape rings help, but do not solve the problem. To say that this is not an issue would be hiding your head in the seafloor sediment, so to speak.
I am a PhD Fishery Scientist, currently with the University of Maryland. I previously worked for NMFS/NOAA in Kodiak, Alaska, for 22 years, where I was responsible for Bering Sea crab stock assessment and research. I have many years of experience studying king, Tanner, snow, and Dungeness crabs, including the impacts of fishing and ghost fishing. I wish to address some points made by Mark Gleason, Executive Director of Alaska Bering Sea Crabbers, in his opinion piece about Ghost Fishing in the Bering Sea (Saving Seafood, July 11, 2012).
I have worked with both Wayne Donaldson and Doug Pengilly, and have great respect for their knowledge and experience. I am sure that their assessment of the cumulative mortality from ghost fishing is made using the best information available, and their conclusions about its impact are reasonable.
However, I take issue with some other statements made by Mr. Gleason. That there is “no need for such a (pot recovery) program” in the Bering Sea is questionable. In 1995, NMFS and ADF&G used sonar and submarines to assess the impact of ghost fishing by crab pots in Kodiak, Alaska (Stevens et al, 2000, Fishery Bulletin). We surveyed 2 square miles of sea floor, and found 200 derelict crab pots, i.e an average density of 100 pots per square mile, essentially one pot every 500 feet. By any measure, that is a lot of crab pots.
Second, although Alaska law requires devices that “render the pots incapable of holding fish and shellfish”, this is a theoretical definition. Crabs climb into and onto traps seeking shelter or food, whether or not bait is present, and commonly get tangled in torn webbing of old, degraded traps. Despite the fact that many pots we found were highly degraded, they contained an average of 1.5 Tanner crabs per pot. Some traps that had been lost during the previous season contained up to 125 crabs. Many other organisms were found in the ghost pots, including other crabs, snails, octopus, cod, rockfish, flounder, sea urchins, scallops, sea stars and anemones. Octopuses were present in every trap with more than 10 crabs, suggesting that they used such lost traps as “octopus restaurants”.
The presence of biodegradable twine helps but does not solve the problem. Up to a third of the pots we recovered had no evidence of degradable twine present nor holes left by its degradation (suggesting variable compliance with the law), and there was no relationship between the condition of the trap and the number of crabs it contained. In fact, the proportion of pots containing crabs was identical (16%) regardless of whether those pots were in good condition or highly degraded. This study was done before the implementation of escape rings, which can only work as long as the pot walls are intact and vertical, but will not work after the pot collapses.
Mr. Gleason also states that ”bait that is left in the pot for more than a few days breaks down completely and loses its utility as a crab attractant.”. However, every study of crab pot loss paints a different picture. Though the original bait may be consumed and degraded, crabs, fish, and other organisms that come into the pot also die, and then become bait for other organisms. In fact, lost pots continue to rebait themselves and ghost fish for many years after loss, as our study showed. Pots do degrade over time, but even highly degraded pots catch and trap crabs.
Although fishing activity may differ between Kodiak and the Bering Sea, if extrapolated to the most highly fished area of the Bering Sea, our estimates of ghost pot density gave an estimate of 1.7 million lost pots. Pot degradation takes a long time (5-10 years), and the degradation rate is probably not linear. Given the average rate of pot loss (10% according to published sources) and the estimated degradation rate of pots, we predicted a standing crop of 44,000 lost pots in the Bering Sea. The true number is probably between these two extremes.
By any accounting, pot loss creates an environmental and a resource problem. It may not tip the scales toward overfishing, but it is still something that fishers should be concerned about. Biodegradable twine and escape rings help, but do not solve the problem. To say that this is not an issue would be hiding your head in the seafloor sediment, so to speak.
Bradley G. Stevens, Ph.D. is Professor and Distinguished Research Scientist at the Living Marine Resources Cooperative Science Center at the University of Maryland Eastern Shore