Age, Growth, and Sexual Maturity of the Deepsea Skate, Bathyraja
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AGE, GROWTH, AND SEXUAL MATURITY OF THE DEEPSEA SKATE, BATHYRAJA ABYSSICOLA A Thesis Presented to the Faculty of Alaska Pacific University In Partial Fulfillment of the Requirements For the Degree of Master of Science in Environmental Science by Cameron Murray Provost April 2016 Pro Q u est Nu m b er: 10104548 All rig hts reserv e d INF O RM ATI O N T O ALL USERS Th e q u a lity of this re pro d u ctio n is d e p e n d e nt u p o n th e q u a lity of th e c o p y su b mitt e d. In th e unlik e ly e v e nt th a t th e a uth or did n ot se n d a c o m ple t e m a nuscript a n d th ere are missin g p a g es, th ese will b e n ot e d. Also, if m a t eria l h a d to b e re m o v e d, a n ot e will in dic a t e th e d e le tio n. Pro Q u est 10104548 Pu blish e d b y Pro Q u est LL C (2016). C o p yrig ht of th e Dissert a tio n is h e ld b y th e A uth or. All rig hts reserv e d. This w ork is prot e ct e d a g a inst un a uth orize d c o p yin g un d er Title 17, Unit e d St a t es C o d e Microform Editio n © Pro Q u est LL C . Pro Q u est LL C . 789 E ast Eise nh o w er P arkw a y P. O . Box 1346 A nn Arb or, MI 48106 - 1346 ACKNOWLEDGEMENTS This project would not have been possible without the cooperation and dedication of several individuals. I would like to thank my committee chair and advisor Bradley P. Harris for his tireless support and guidance throughout the entirety of this project. I would also like to thank my committee: David A. Ebert, Kenneth J. Goldman, and Cindy A. Tribuzio for their invaluable assistance and mentoring and Lisa J. Natanson for teaching and aiding me with the histological aspects of this project. Sarah Webster, Nathan Wolf, Aileen Nimick, T. Scott Smeltz, Kelsey James, and James Knuckey your help with this project was greatly appreciated. Last but not least, thank you At-Sea Processors Association for funding my project and education at Alaska Pacific University as part of the Fisheries, Aquatic Science & Technology Laboratory. ii ABSTRACT Research into the age, growth, and reproductive characteristics of chondrichthyan fishes has increased substantially over the past couple of decades. This study set out to estimate age deepsea skate, Bathyraja abyssicola using vertebral centra and caudal thorns, estimate length at age, and determine length at maturity. Sixty-three specimens of B. abyssicola (n=29 males; n=34 females) were taken on National Marine Fisheries Service bottom trawl surveys between 2001 and 2012. Information derived and structures collected from these samples included sex, maturity class, total length, caudal thorns and, vertebrae. Ageing methods attempted include histology and gross sectioning (vertebral centra) and surface staining (caudal thorns). Moderate success with centra sectioned using the histological method allowed some inference to be made into life history characteristics. Deepsea skates appear to have slow average growth (26 mm yr-1 ±5.41, 95% c.i.) and mature at a large size (males: TL50 = 1175.4 mm, females: TL50 = 1267.3 mm). Band pair counts were not validated as true ages. Males from which growth bands could be enumerated were smaller (� = 10, �̅ = 718 mm, SD = 209 mm) on average than that for females (n = 7, �̅ = 990 mm, SD = 319 mm). This study provides the first attempt to assess abyssicola age, growth rate, and sexual maturity traits; information needed for informed skate management. iii TABLE OF CONTENTS ACKNOWLEDGEMENTS...........................................................................................................ii ABSTRACT..................................................................................................................................iii TABLE OF CONTENTS..............................................................................................................iv LIST OF TABLES.........................................................................................................................v LIST OF FIGURES.......................................................................................................................vi GENERAL INTRODUCTION......................................................................................................1 ASSESSMENT OF AGE, GROWTH AND MATURITY OF THE DEEPSEA SKATE, BATHYRAJA ABYSSICOLA 1.1 INTRODUCTION....................................................................................................................6 1.2 METHODS…………………………………………………………………………………...8 1.3 RESULTS………………………………………………………………………………...…13 1.4 DISCUSSION………………………………………………………………………………16 REFERENCES.............................................................................................................................22 TABLES.......................................................................................................................................28 FIGURES.....................................................................................................................................31 APPENDICES..............................................................................................................................39 iv LIST OF TABLES Table 1…………………………………………………………………………………………...28 Procedure for histology embedding. Table 2……………………………………………………………………………………...……29 Procedure for histology staining. Table 3……………………………………………………………………………………...……30 Maturity classes and definitions. v LIST OF FIGURES Figure 1…………………………………………………………………………………………..31 Vertebral centra and caudal thorn sampling areas. Figure 2…………………………………………………………………………………………..31 Photographs of gross sectioned (A) and histological sectioned (B) vertebral centra and alizarin red stained caudal thorn (C) from Bathyraja abyssicola. Figure 3…………………………………………………………………………………………..32 Bathyraja abyssicola total length (mm) at band pair count as determined by histology with fit linear regression line. Figure 4…………………………………………………………………………………………..33 Female Bathyraja abyssicola total length (mm) at band pair count as determined by histology with fit linear regression line. Figure 5…………………………………………………………………………………………..34 Male Bathyraja abyssicola total length (mm) at band pair count as determined by histology with fit linear regression line. Figure 6…………………………………………………………………………………………..35 Bathyraja abyssicola total length (mm) at band pair count as determined by caudal thorns with fit linear regression line. Figure 7…………………………………………………………………………………………..36 Bathyraja abyssicola total length (mm) at band pair count as determined by histology and caudal thorns. vi Figure 8…………………………………………………………………………………………37 Bathyraja abyssicola total length (mm) at band pair count as determined by histology separated by maturity class for males and females. Figure 9…………………………………………………………………………………………38 Estimated probability of maturity at size based on an ogive for male and female samples of B. abyssicola. vii GENERAL INTRODUCTION The class Chondrichthyes (derived from the Greek prefix khondros meaning “cartilage” and suffix ikhthus meaning “fish”) is a species diverse group of fishes with simplified internal cartilaginous skeletons, lacking true bone (Ebert 2003, Klimley 2013). Other distinguishing characteristics of this class include fins without bony rays, true upper and lower jaws, nostrils on the underside of the head, and transverse rows of teeth or fused tooth plates that are constantly replaced from inside the mouth (Ebert 2003). Cartilaginous fishes also lack bony plates on their heads, have placoid scales (toothlike dermal denticles) and fertilization takes place internally (Ebert 2003). Two subclasses comprise the class Chondrichthyes, the small subclass Holocephali (chimaeras) and the large subclass Elasmobranchii (sharks and rays) (Ebert 2003, Klimley 2013). The Holocephali have a single gill opening on each side of their heads and are given the common name chimaeras because of their unique body shape, which vaguely resembles the mythic Greek monster of the same name (Klimley 2013). The subclass Elasmobranchii is a large taxonomic group comprised of the cylindrical-bodied sharks and flat-bodied rays (Klimley 2013). These species have multiple gill openings on each side of their heads numbering from five to seven (Klimley 2013). Over the past few decades, the sensitivity of global chondrichthyan populations to fishing mortality has been an area of focus because they typically possess life history characteristics such as slow growth rates, late maturation, and low fecundity which make them particularly vulnerable to exploitation (Holden 1974, Brander 1981, Dulvy et al. 2000, Stevens et al. 2000, Dulvy and Reynolds 2002). The order Rajiformes (skates) are a species-rich, but relatively undiverse group of batoids receiving increased attention as once common species are becoming 1 increasingly scarce or locally extinct (Brander 1981, Dulvey et al. 2000). In the northwest Atlantic, the winter skate, Leucoraja ocellata, barndoor skate, Dipturus laevis, and blue skate, Dipturus batis have all demonstrated population declines due in part to fishing mortality (Johnson 1979, Brander 1981, Casey and Myers 1998). Research efforts are relatively high for targeted or frequently encountered commercial fishery bycatch species. However, research efforts are generally lacking for non-target and rarely encountered skate species (Winton et al. 2013). Deeper-dwelling, cooler water species generally have slower growth rates, later maturation and lower productivity leading to increased sensitivity to fishing mortality and risk of overexploitation (Cailliet et al. 2001, Garcia