Biology, Life History and Conservation of Elasmobranchs with an Emphasis on Western Atlantic Skates

Title of Dissertation: BIOLOGY, LIFE HISTORY AND CONSERVATION OF ELASMOBRANCHS WITH AN EMPHASIS ON WESTERN ATLANTIC SKATES Michael Guise Frisk, Doctor of Philosophy, 2004 Dissertation Directed By: Associate Professor Dr. Thomas J. Miller University of Maryland Center for Environmental Science Chesapeake Biological Laboratory In this dissertation two approaches were used to increase the knowledge of elasmobranch population dynamics and life history: (1), the comparative approach and (2), the species-specific approach. In the comparative approach I constructed standardized three-stage matrix models for 55 species of sharks and rays. Using these models I (1) conducted elasticity analyses to determine how the vital rates of mortality (M) and fertility (f) influence elasmobranch population growth rate r, (2) estimated sensitivity of elasticity to perturbation in vital rates, and (3) examined the taxonomic distribution of model inputs and species vital rates, such as size at maturity (Lmat), and total length (Lmax). I found positive relationships between the elasticity of λ (population growth rate) to changes in juvenile and adult stages to longevity and age at maturity; however, the age at maturity and the elasticity of λ to changes in the adult stage relationship appeared to be invariant. Combining vital rates and elasticities, I found similar suites of life histories and demographics within taxonomic groups at various levels. Further I examined where (or if) elasmobranchs fall in the evolved triangular ordination of life history strategies proposed by Winemiller and Rose (1992). My results indicate that when plotted using only the teleost ordination, elasmobranchs appear to be periodic strategists, outside the limits of the teleost ordination. However, when elasmobranch data is included in the ordination they form the extreme range of equilibrium strategists and are grouped by order. In the species-specific approach, I found evidence for a strong latitudinal trend in maximum size (l∞) and size at maturation (lmat) in little skate with individuals in northern regions reaching a larger size at maturity and maximum length and growing slower than little skate from more southern regions. No similar trend was found in winter skate. Little skate is smaller, reaches maturity at a younger age is faster growing and shorter lived then winter skate (Little skate: l∞ = 56.1 cm, k = 0.19/yr, Tmax = 12.5, Tmat = 7; Winter skate: l∞ = 122.1 cm, k = 0.07/yr, Tmax = 20.5, Tmat = 12.5). Winter skate has higher annual fecundity then little skate of 26-101 and 21-57 eggs per year respectively. Using estimated vital rates for winter skate and National Marine Fisheries Service’s survey data an age-structured model was constructed for winter skate from 1963-1998. The model indicated that the western Atlantic population of winter skate was rebuilding in the 1980’s following overfishing in the 1960’s and 1970’s. BIOLOGY, LIFE HISTORY AND CONSERVATION OF ELASMOBRANCHS WITH AN EMPHASIS ON WESTERN ATLANTIC SKATES