A Simulation Study of the Effects of Spatially Complex Population Structure for Gulf of Maine Atlantic Cod

Abstract

There is growing evidence that numerous fish stocks consist of several smaller, reproductively isolated units that vary in their ability to produce new recruits each year because of a mismatch with environmental conditions. It has been suggested that spawning aggregations exist for the Gulf of Maine stock of Atlantic cod Gadus morhua and that approximately one‐half of the historic aggregations are locally depleted. We developed a spatially explicit, age‐based projection model to investigate the effects of complex population structure and spatially variable recruitment on Gulf of Maine Atlantic cod. The results of this study demonstrate that there are significant disparities in the estimates of spawning stock biomass, annual yield, and annual recruitment when a stock with a complex population structure is treated as a single stock. In addition, treating a stock with a complex population structure as a single stock can mask a steady decline in the values of these variables as the spawning aggregations gradually become depleted. To achieve rebuilding of the simulated Gulf of Maine Atlantic cod stock within 10 years, we determined that F = 0 would be required. Because continued bycatch mortality in other fisheries would prevent this even with the closure of the Gulf of Maine cod fishery, it may not be possible to rebuild this particular stock in a short time frame. When fishing mortality is removed, the stock only rebuilds to one‐half of the initial carrying capacity because about 50% of the reproductive capacity of the stock has been lost. Without recolonization, the simulated stock has lost its biological capacity to return to the original unfished condition. Based on the simulations in this study, the behavior of the Gulf of Maine Atlantic cod stock is more consistent with that of a stock with a complex population structure than with that of a truly single stock. In light of this, the management measures for this stock and other stocks with the potential for complex population structure should be carefully evaluated. Managing such stocks as single units may be inappropriate and lead to both inadequate protection of the resource and reduced fishery yields.

Red Snapper Distribution on Natural Habitats and Artificial Structures in the Northern Gulf of Mexico

“…Spatial mapping of the distribution of marine resources is a critical first step in many research and management applications, including ecosystem modeling (Cowen et al 2006), population connectivity studies (Cowen et al 2007;Reich and DeAlteris 2009), siting of marine protected areas (Hamilton et al 2010), and defining spatial management (Cadrin and Secor 2009;Gaines et al 2010). Unlike in the terrestrial environment, where habitats are relatively straightforward to identify and where organisms can be directly observed, the mapping of marine resources is more difficult because organisms and habitats are rarely directly observed and because many marine organisms exhibit a high degree of movement or dispersal.…”

Managing a Marine Stock Portfolio: Stock Identification, Structure, and Management of 25 Fishery Species along the Atlantic Coast of the United States

“…Incorrect stock boundaries or poorly known mixing rates confound estimates of immigration and emigration (Hammer and Zimmermann 2005). When multiple stocks are fished as a simple aggregate, not only is the measurement of stock dynamics confounded, smaller stocks are at risk of overexploitation or extirpation (Ricker 1958;Hilborn 1985;Smedbol and Stephenson 2001;Reich and DeAlteris 2009). Loss of genetic diversity is a concern, particularly with regard to small stocks for which there is typically insufficient data to assess their status (Slaney et al 1996;Olsen et al 2008;Hu and Wroblewski 2009).…”

Hog Daddy and the Walls of Steel: Catch Shares and Ecosystem Change in the New England Groundfishery

“…Other studies indicate that some fish populations, including groundfish species such as cod and haddock, comprise metapopulations (Reich and DeAlteris 2009;Skjaeraasen et al 2011), with some studies drawing explicitly on spatiotemporally specific industry observations (Bigelow 1924;Ames 2004). Subpopulations mix seasonally over large areas, then home to segregated spawning aggregations, demonstrating perennial site fidelity to shallow banks, rivermouths, or other features.…”