Multispecies interactions in the Georges Bank fish community

Dramatic shifts in the species composition of the Georges Bank fish community have occurred during the past 30 years. The bottomfish community, once dominated by cod, haddock, and flounder, shifted to lower-value species such as skates and dogfish. Herring and mackerel declined during the 1960s and 1970s, but have since recovered to record levels. These shifts are attributed primarily to high fishing pressure, but predation is the largest source of pre-recruit mortality, and is also considered important in controlling the dynamics of the fish community. Prior studies have identified the important predators as cod, silver hake, and spiny dogfish. Herring, mackerel, silver hake, and yellowtail flounder all experience high predation mortality, especially in the first and second years of life. To construct dynamic production models of the Georges Bank fish community, we aggregated the dominant species into four groups: gadoids, flatfishes, pelagics, and elasmobranchs. Inclusion of species interactions in the dynamic models was based on statistical fits to the biomass data and groundtruthed with known species interactions. The most important interactions are predation of gadoids and elasmobranchs on pelagics and apparent competition between gadoids and elasmobranchs. Equilibrium yields of each group depend on the abundances and hence the exploitation rates of the other species. Harvest strategies are simulated for both the multispecies model and the corresponding single-species equations, to illustrate the sensitivity of medium-term projections to species interactions.