Modeling and evaluating ecosystem in 1980s and 1990s for American lobster (Homarus americanus) in the Gulf of Maine

The Gulf of Maine (GOM) ecosystem has been observed to experience large changes over the last several decades, switching from a groundfish dominated system to a system dominated by crustacean species such as the American lobster (Homarus americanus) and crabs. The objective of this study is to develop a lobster ecosystem model and to evaluate the changes in the GOM ecosystem for the American lobster between 1980s and 1990s. A mass-balance ecosystem model was developed separately for the two time periods (1980s and 1990s) using Ecopath. The model includes 24 functional groups consisting of lobster, its key predator and prey species, and other ecologically important groups in the ecosystem such as zooplankton, phytoplankton, and detritus. Using the models developed we conducted a comparative analysis of trophic interactions and community structures of the GOM ecosystem for lobster in 1980s and 1990s. The balanced model suggests that the biomasses of shelled mollusk, crab, juvenile lobster, adult lobster, and Atlantic herring (Clupea harengus) were doubled, tripled or even more than tripled, but the groundfish biomass decreased substantially from 1980s to 1990s. The study shows that the trophic structures of the lobster ecosystem in the 1980s are different from those in the 1990s with a decrease in top predator biomass and an increase in the biomass of lower trophic level organisms. The ecosystem in 1990s was found to be more mature than in 1980s. This study suggests a regime switch in the GOM from high trophic groundfish dominated system to low trophic crustacean species dominated system during 1980s-1990s.