Preliminary findings from the Gulf of Maine cod assessment revealed that the previous evaluation completed in 2008 had overestimated the stock's biomass by a factor of nearly 300 percent. The new assessment, performed by a group of independent scientists, now places the 2011 biomass levels at 26.5 million pounds, which is a stark contrast to the 74.9 million-pound number calculated in the 2008 report and only 20 percent of the mandated 2014 rebuilding deadline.
Why do these assessments report such conflicting results?
The new (2011) assessment includes an additional three years of trawl survey data (performed with a new vessel), uses a different mathematical model to calculate biomass and fishing mortality, a different estimate of recreational catch, and updated weight-at-age estimates. While much attention has been focused on the accuracy of the survey data and estimates of recreational catch, by far the most influential factor appears to be the new weight-at-age data.
The 2008 assessment (using 2007 survey data) based weight-at-age ratios solely on legally sized fish that were sampled from the commercial catch. Although the stock assessment team had weight-at-age data that included the entire size range of fish, they were not able to include it in the assessment (and the trawl survey did not yield enough fish for a representative sample). Gulf of Maine cod biomass was therefore overestimated, and the total allowable catch (TAC) based on this data resulted in the harvest of a large proportion of the spawning stock biomass during each of the last three years.
It is important to note that when NMFS scientists used the same data on both the old and new models, they got essentially the same results. Therefore it was the inclusion of new data, not a switch in models that led to the reduced estimates of biomass. In addition, 2008 biomass estimates were based on what appeared to be two strong year classes for 2003 and 2005, but neither year class has appeared in the survey data or the commercial catch since then.
Contraction or sub-stock depletion?
Changes in cod distribution and fishing patterns suggest a contraction of the Gulf of Maine cod population into the western area, which might account for the apparent increase in catch per unit effort industry has witnessed here. Cod distribution plots generated from trawl survey data and observed commercial catch seem to support this theory.
On the other hand, the data could also suggest local depletion of a central and/or eastern Gulf of Maine sub-stock(s), as opposed to a simple contraction of the Gulf of Maine-wide stock. Studies examining fishing history (Ames 2004), cod movement (Tallack 2009), body shape (Sherwood and Grabowski, unpublished), and genetic relatedness (Kovach et al. 2010), all point in the same direction. That is, the weight of this new evidence points towards distinct Gulf of Maine sub-stocks rather than one large stock.
The data identify a western Gulf of Maine stock, which extends down the western side of the Georges Bank stock area, and a distinct eastern Georges Bank stock. The observed depletion of cod in eastern Maine and the central Gulf of Maine may indicate another two sub-stocks or one larger sub-stock that extends from eastern Maine down to the western side of Jeffreys Ledge.
Sub-stock depletion implies that efforts to rebuild the western Gulf of Maine sub-stock might not result in rebuilding other sub-stocks as well. Scientists agree that these new theories about Atlantic cod sub-stocks in the Gulf of Maine and Georges Bank warrant further study, and may be critical components in the effort to accurately assess cod biomass and the impact of harvesting.
Before any of this new research can be incorporated into the management process, the New England Fishery Management Council's (Council) Science and Statistical Committee (SSC) in partnership with the Northeast Fisheries Science Center (NEFSC), was tasked to conduct a deeper examination of these sub-stock theories. Because of the high level of analysis involved, the SSC's recommendations on potential sub-stocks will likely not be ready to inform the Council's next catch limit decisions for 2013, but it could be incorporated into the 2014 stock assessment. Ultimately, a more refined understanding of cod stock structure will assist the Council in making biologically and economically sound management decisions.