DEVELOPING A STOCK ASSESSMENT FOR THE BARNDOOR SKATE (Dipturus laevis) IN THE NORTHEAST UNITED STATES A dissertation presented to the faculty of the School of Marine Science The College of William and Mary In partial fulfillment of the requirements for the degree of Doctor of Philosophy ________________ Todd Gedamke 2006 APPROVAL SHEET This dissertation is submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Todd Gedamke Approved, December 2006 _____________________ John M. Hoenig, Ph.D. Major Co-Advisor _____________________ William DuPaul, Ph.D. Major Co-Advisor _____________________ John A. Musick, Ph.D. Major Co-Advisor _____________________ Courtney Harris, Ph.D. _____________________ David W. Kulka DFO-Canada St Johns, Newfoundland & Labrador _____________________ Dr. Kyle Shertzer, Ph.D. NOAA/NMFS SEFSC Beaufort, NC ii DEDICATION iii TABLE OF CONTENTS Page ACNOWLEDGEMENTS………………………………………………………..…..…..vi LIST OF TABLES……………………………………………………………..…….….vii LIST OF FIGURES………………………………………………………………..……viii ABSTRACT…………………………………………………………………………...…ix CHAPTER 1. General introduction……………………………………………………….1 LIFE HISTORY CHAPTER 2. Observations on the Life History of the Barndoor Skate, Dipturus laevis, on Georges Bank (Western North Atlantic)…………………….….……14 CHAPTER 3. Ontogenetic and Sex-specific Shifts in the Feeding Habits of the Barndoor Skate (Dipturus laevis) on Georges Bank ………………..…………...…48 STOCK ASSESSMENT METHODS CHAPTER 4. Estimating mortality from mean length data in non-equilibrium situations, with application to the assessment of goosefish (Lophius americanus)....77 CHAPTER 5. Simple Methods for Estimating Survival Rate from Catch Rates from Multiple Years ……………………………...…………………….……112 CHAPTER 6. Using demographic models to determine intrinsic rate of increase and sustainable fishing for elasmobranchs: pitfalls, advances and applications …………………………………...……………………………………...135 BARNDOOR SKATE STOCK ASSESSEMENT CHAPTER 7. Total Mortality Rates of the Barndoor Skate, Dipturus laevis, in the Northeast United States, 1963-2005…………………….……………...176 iv CHAPTER 8. Stock Recruitment Dynamics and the Maximum Population Growth Rate of the Barndoor Skate Dipturus laevis in the Northeast United States……………………………………………………….…………...208 CHAPTER 9. Conclusions and Future Work……………………………...…………...242 VITA……………………………………………………………………………………247 v ACKNOWLEDGEMENTS vi LIST OF TABLES vii LIST OF FIGURES viii ABSTRACT 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 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. The third method we develop is 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. 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 oft growing at an annual rate in excess of 40%. ix Chapter 1 GENERAL INTRODUCTION 1 The barndoor skate Dipturus laevis (Mitchill 1817), is one of many skate and ray species caught as bycatch in finfish trawl and scallop dredges in the Northwest Atlantic. Until recently, this species has received little attention, due primarily to its low market value (Musick et al., 1999; Dulvy et al., 2000; Kulka et. al., MS 2002), until a recent study suggested its potential extinction (Casey and Myers, 1998). Casey and Myers (1998) reported that the barndoor skate, once a common bycatch species off southern Newfoundland, had become locally extirpated in parts of its northern range due to overfishing. They also contended that “long-term research surveys on the continental shelf between the Grand Banks of Newfoundland and southern New England reveal that….the barndoor skate….is close to extinction (Casey and Myers, 1998).” Environmental groups quickly responded to the results presented in the Casey and Myers (1998) article. Greenworld Organization and the Center for Marine Conservation filed petitions with the National Marine Fisheries service to 1) list the barndoor skate as an endangered species 2) immediately designate Georges Bank as critical habitat and implement large area closures and 3) list other similarly appearing species of skates as a precautionary measure (NOAA 1999). The IUCN (Union for the Conservation of Nature and Natural Resources) followed suit and after initially listing the species as vulnerable, they upgraded the threat status to endangered. Thus, the barndoor skate became the focus of many conservation and extinction risk debates (Diaz-Soltera, 1999; Musick et. al., 1999; NEFSC, 2000). As the controversy brewed, I began my research in this arena while conducting field work for my Masters thesis on commercial scallop vessels on Georges Bank. Based on the data presented in the Casey and Myers (1998) article I had expected to encounter a 2 lone skate or two, and was only prepared to work up a few dozen animals. I was pleasantly surprised to document over 100 barndoor skates in the first two days of fishing. My dissertation was born. It began as a straightforward investigation of the life history of the species and has evolved to include the development and application of stock assessment methodologies to assess the status of the barndoor skate. Including this general introduction and a conclusion chapter, the dissertation consists of seven manuscripts, written in journal format, that fall under three general topics: 1) Life history 2) Stock assessment methods 3) Barndoor skate stock assessment. Life History Age-growth and Maturity Understanding the life history traits and quantifying the age and growth parameters for individual species has been recognized as crucial in the risk assessment and management of marine fishes (Parent and Schrimi, 1995; Musick, 1999a; Musick, 1999b). A recent stock assessment of the United States Northwest Atlantic skate complex (NEFSC, 2000) listed the three largest species; barndoor, winter (Leucoraja ocellata), and thorny skates (Amblyraja radiata), as overfished and identified a need for further studies of the age, growth, and reproduction. Our work has focused on defining these critical life history parameters for the barndoor skate. The manuscript in Chapter 2, entitled “Observations on the Life History of the Barndoor Skate, Dipturus laevis, on Georges Bank (Western North Atlantic)” was published in 2005 in the Journal of Northwest Atlantic Fishery Science (35:67-78). 3 Food Habits Understanding the feeding habits of the barndoor skate can bring valuable insight into predator-prey relationships and can contribute to future studies of trophic interactions (Caddy and Sharp 1986). Unfortunately, very little is known about their food habits and how they fit into the overall ecosystem. Limited information from descriptive studies of the Northwest Atlantic fish complex report that juvenile barndoor