Population Genetics of Leucoraja ocelleta
As a group, skates face ever increasing exploitation pressures world-wide (Hoenig and Gruber, 1990; Martin and Zorzi, 1993). In the past, skates collected during commercial ground fishing operations were limited in number and discarded as by catch (Otwell and Lanier, 1979). The rapidly expanding market for skate wings, however, has made these species a commercially viable option as other ground fisheries have dwindled (Sosebee, 1998).
The increased fishing pressure and the failure to implement a proper fishery management plan has resulted in the near extinction of the common skate, Raja batis, in the Irish Sea (Brander, 1981) and the barndoor skate, Raja laevis, in the Northwest Atlantic (Casey and Myers, 1998). Data for northwest Atlantic species indicates that the abundance of winter skate, Leucoraja ocellata, and thorny skate, Amblyraja radiata, have dwindled in recent years (Sosebee, 1998) and could follow a similar destructive course to other New England fisheries such as cod, unless effective management policies are implemented (LeFevre, personal communication). However, the development of effective regulations governing the harvesting of skates has been stymied by the lack of information concerning their basic population biology (Sosebee, 1998).
Historically, skates of the northwest Atlantic, like many other commercially important species, have been managed as a single, genetically homogenous group. Skates species would consist of single panmixtic populations if individuals (or their gametes) experience random migrations away from their natal site at some point during their life history. However, several lines of evidence suggest that skate migration may be limited.
First, skate eggs are fertilized internally and are negatively buoyant when released (Hamlett and Koob, 1999). Thus, neither the gametes nor the zygotes are broadcast into the pelagic zone to be distributed by water currents.
Second, geographically distinct populations of skates can be distinguished on the basis of morphological (e.g., total length) and life history (e.g., timing of reproductive peaks) characteristics (Wourms and Demski, 1993; Hamlett and Koob, 1999). While many of these characteritstics may be influenced by environmental variables, the divergence of morphological and life history characteristics indicate that geographically distinct populations, which share a common environment, exist and are stable over many years.
Third, mark-recapture studies indicated that adult migration of skates is limited. A study by Templeman (1984), which tracked the recapture of 772 tagged thorny skates (Amblyraja radiata) for over 20 years, found that 80 of 94 recaptures (85%) occurred within 65 nautical miles of the tagging sites. Clearly, migration within the skates appears to be limited to within relatively narrow geographic regions. The absence of widely dispersed gametes or zygotes, the existence of geographic races, and the limited migration of adults all suggest that significant genetic structuring may exist within the North American skate fishery.
The presence of unrecognized genetic structuring within a heavily exploited species may be devastating to a fishery (Pratt and Otake, 1990). An overfished population may never recover and valuable genetic diversity within the species may be lost. The benthic community will also be damaged by the local loss of a skate population. Clearly, the extent of population structuring within the skate fishery needs to be quantified in order to determine the size of the management units and thus to create effective regulations governing their harvest.
We propose to evaluate the extent of population genetic subdivision in the skate Leucoraja ocellata by examining patterns of mitochondrial sequence variation (at the D loop sequence) in three geographic locations along the east coast of North America: the southern coast of Newfoundland, Georges Bank in the Gulf of Maine, and off the Virginia coast. Each of these locations are separated by a minimum of 700 nautical miles; based on the mark and recovery data, each geographic location is expected to support distinct populations distinguishable on the basis of mitochodrial sequence variation.
North American skates are threatened by over exploitation and effective management policies are needed to govern their harvest. A critical component to their successful management is a proper understanding of the geographic distribution of distinct populations. Such knowledge could then be used to identify specific management units.
The objectives of the proposed research are:
1) Examine three Leucoraja ocellata populations to determine if population structuring exists among populations along the eastern seaboard of North America
2) Identify which of the three populations are genetically distinct. Due to the distance separating the sampling sites and the expected migration distances of adults, we propose the following hypothesis: Each of the three populations represent genetically distinct units.
Tissue samples from Georges Bank will be collected from individuals that are being sampled for a separate, already funded, study on the age, growth and seasonal reproductive activity of many of the same populations. This piggy backing strategy will have two advantages. First, information from both studies can be combined and compared to produce a well-rounded picture of the population biology of this poorly understood species. Second, utilizing the same tissues for multiple studies is a cost effective strategy that allows us to propose a genetic survey with a limited budget. Tissues from our Virginia area will be provided by Linda Patanjo, the coordinator for trawl surveys from the NMFS Northeast Fisheries Science Center in Woods Hole. The Newfoundland samples will be provided by Jason Treberg at Dlahousie University.
The tissues will be sampled on board and stored in 100% ethanol. DNA will be extracted from the tissues in the lab through a standard phenol:chlorophorm organic extraction and ethanol precipitation. Primers designed from the recently sequenced Raja radiata mitochodrial genome (Genebank accession number AF 106038, Rasmussen and Amason, 1999) will be used to amplify and cycle the D loop region. Haplotypes of the D loop region will be identified through sequencing 25 individuals from each of the three populations.
The frequencies of these haplotypes in each separate geographic sample will be used to perform a hierarchical analyses of molecular variance. This test determines the probability that individuals from all three sampling locations are derived from a common population (i.e., that no population genetic structure exists). Chi-squared tests of allele frequency distributions will be used to determine which, if any, of the populations have significantly different allele frequency distributions. Those populations with distinct allele frequency distributions should be considered genetically distinct.