Doctor of Philosophy (Ph.D.)
Virginia Institute of Marine Science
The barndoor skate Dipturus laevis is one of seven species in the United States Northwest Atlantic skate complex. The species received little attention until a recent article published in Science reported that the barndoor skate might be on the brink of extinction. In this study, we address not only the virtual lack of information on the life history of the species, but also develop methodologies to assist in the assessment of the barndoor skate population. to investigate the life history of the species, data were collected from 2,310 specimens caught during commercial sea scallop dredging in the southern section of Georges Bank Closed Area II. Vertebral analysis was used to generate an age-growth relationship, and a visual inspection of reproductive tracts provided the timing of maturity. Our results suggest faster growth and younger female maturation than previously believed. Stomach samples were also collected from specimens to study food habits. Ontogenetic shifts in the utilization of prey items were observed, and above the size at maturity, food habits were significantly different between the sexes. In the second part of this work, we develop three methodologies for use in stock assessments. The first method is an extension of the Beverton-Holt mean length mortality estimator. This methodology is commonly used but has the restrictive assumption of equilibrium conditions. We derive an equation to describe the transitional behavior of the mean length statistic for use in non-equilibrium conditions and apply it to data from Goosefish ( Lophius americanus) in the northwest Atlantic. For the second method, we develop an extension of the Heincke method. This method requires only recruits and non-recruits to be identifiable and uses catch rate data from two consecutive years to estimate a mortality rate. We generalize the approach to include more than two years of data and compare results to those obtained from the Goosefish mean length analysis. For the third method we begin by discussing some common problems in the application of demographic models (i.e. Leslie matrices and life tables) to elasmobranchs. We illustrate these problems, and describe methods for estimating the maximum population growth rate, and present applications to two species of elasmobranch: barndoor skate Dipturus laevis and lemon shark Negaprion brevirostris. In the last section we use information from the NMFS annual groundfish surveys, our newly-derived life history parameters and our methodological developments to conduct a stock assessment of the barndoor skate. Mortality rates, estimated from both our mean length and catch rate methods, appear to have been very high in the 1960's and are currently at very low levels. There is no evidence of a high current fishing mortality rates or any threat to the population. In the final manuscript we develop both a stock-recruit and Leslie matrix model to gain insights on the population dynamics of the species. The results of both approaches were comparable suggesting that species may be more resilient to fishing pressure than previously believed and capable of growing at an annual rate in excess of 40%.
© The Author
Gedamke, Todd, "Developing a stock assessment for the barndoor skate (Dipturus laevis) in the Northeast United States" (2006). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1539616663.