Urchin Stock Structure in the Gulf of Maine: Preliminary Genetic Analysis
We will use highly polymorphic DNA markers to test for regional and local population linkages of green sea urchins in the Gulf of Maine. The specific geographic linkage we will investigate is the relationship between northern and southern populations potentially separated by the Penobscot spring freshwater. We will also sample juveniles associated with one of the adult populations to look for temporal variation in genetic composition. This data will be used to assess the utility of the proposed genetic markers for detecting variation within the Gulf of Maine. This project is timely because Gulf of Maine green urchin populations are currently subject to recruitment overharvesting, conservation management plans are currently under consideration in the state of Maine, and aquacultural development projects are proceeding in several New England states.
Green sea urchins are a lucrative and overexploited fishery in the Gulf of Maine. The urchin fishery rapidly expanded beginning in 1987 and peaked in 1993 in Maine, where the fishery was most active in New England. Declines in recruitment in the Gulf were observed in 1995 (Harris and Chester, 1996) and appear to have continued in 1996 and 1997 (L. Harris, personal communication, June 1998). Although a variety of management measures including size limits, dragging restrictions and closures were implemented, adult urchin populations are severely depleted. Further conservation efforts are underway in Maine, with the imminent designation of trial "no-harvest" areas following hearings between the Maine Department of Marine Resources and local fishers.
Prudent management of this important fishery requires identification and maintenance of source populations in the context of spatial and temporal heterogeneity in recruitment patterns, through regulated harvesting and perhaps through directed aquaculture efforts. Debate within harvest regions concerning the connection between various populations and urchin habitat areas is vigorous.
As harvesting effort in all locations has increased, particular areas of concern include populations with low gonad volume (e.g., many deepwater locations) and populations with infrequent recruitment. In addition, the degree of connection between larger regions within the Gulf of Maine is unknown. Order of magnitude differences in recruit number between northern and southern gulf urchin populations (Harris and Chester, 1996; Balch et al., 1998) suggest one possible explanation is a lack of connection between the two areas. Circulation patterns in the Gulf of Maine during the urchin larval period show strong variability in the degree of mixing between northern and southern Gulf waters on an annual and interannual basis (e.g., Lynch et al., 1997).
Currently, there is no information on the identity of source vs. sink populations of urchins, nor is there information on connections via larval dispersal and gene flow between various locations. Harvest quota should be based on a population model that incorporates patterns of supply between regional and local areas, including both spatial and temporal components of variability. Genetic studies provide a means to identify sets of connected populations, the degree of connection in the form of gene flow between them, and the temporal variation in that connection, by looking at patterns of shared distribution of highly variable genetic markers.