Author

Yuying Zhang

Date of Award

12-2005

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Oceanography

Advisor

Yong Chen

Second Committee Member

James A. Wilson

Third Committee Member

James D. McCleave

Abstract

The Gulf of Maine (GOM) ecosystem has experienced large changes over the last several decades, switching from a groundfish species dominated system to a system dominated by crustacean species such as the American lobster and crabs. Several hypotheses have been developed to explain such a switch, ranging from trophic interactions between groundfish and crustacean species to increased food availability to crustacean species due to discarded baits in the lobster fishery. The objective of my study is to develop a lobster ecosystem model to evaluate the dynamics of GOM lobster ecosystem. I developed a mass-balance ecosystem model separately for the two time periods (1980s and 1990s) using Ecopath with Ecosim (EwE). The model has twenty-four function-groups including lobster, its key predator and prey species, and other important groups in the ecosystem such as zooplankton, phytoplankton and detritus. The input data were obtained from published papers and reports. Using the models developed, I conducted a comparative analysis of trophic interactions and community structure of the GOM American lobster ecosystem for the mid-1980s and mid-1990s. I also simulated ecosystem dynamics in the GOM from 1985 to 1997 using Ecosim, evaluated the interactions of population dynamics of Atlantic cod and American lobster, and predicted the possible response of the lobster population with respect to the possible recovery of Atlantic cod stock in the GOM. The study shows that the Ecopath model, which snapshots the ecosystem in a given time, and Ecosim model, which tracks the long term dynamics of ecosystem, can well simulate the lobster ecosystem in the GOM. The trophic structures of the ecosystem in the mid-1980s are different from those in the mid-1990s with a decrease in top predator biomass and an increase in lower trophic-level organism biomass. The derived key ecosystem parameters suggest that the mid-1990s ecosystem tends to be more mature than the mid-1980s ecosystem. The results also suggest that there is a negative relationship between cod biomass and lobster biomass. A substantial increase in fishing mortality in the cod fishery is likely to lead to a large decrease in cod biomass, which in turn results in large increases in lobster biomass, and vice versa. Thus, the full-scale recovery of cod in the GOM may have some negative impact on the lobster stock. This study suggests that a reduced cod stock might contribute to the high lobster stock biomass in recent years. The ecosystem model developed in this study, although preliminary in nature, provides us with a new approach to evaluate the trophic interactions of lobsters and other organisms in the GOM, helps us better understand the ecosystem dynamics in the GOM, and yields the information critical to the development of an ecosystem-based management for the lobster fishery in the GOM. More studies are needed, however, to reduce possible uncertainty in input data and to evaluate the performance of the model.

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