$25

Published by Alaska Sea Grant College ProgramUniversity of Alaska Fairbanks

Alaska

F i e l d G u i d e t o

Sharks, Skates, and Ratfish of AlaskaDuane E. StevensonJames W. OrrGerald R. Hoff John D. McEachran

Elmer E. Rasmuson Library Cataloging in Publication Data

Stevenson, Duane E.Field guide to sharks, skates, and ratfish of Alaska / Duane E. Stevenson … [et al.]. – Fairbanks, Alaska :

Alaska Sea Grant College Program, University of Alaska Fairbanks, 2007.

p. : ill. ; cm. (Alaska Sea Grant College Program, University of Alaska Fairbanks)

Includes bibliographic references and index.

1. Chondrichthyes—Alaska—Handbooks, manuals, etc. 2. Sharks—Alaska—Handbooks, manuals, etc. 3. Skates (Fishes)—Alaska—Handbooks, manuals, etc. 4. Chimaeridae—Alaska—Handbooks, manuals, etc. I. Title. II. Stevenson, Duane E. III. Series: Alaska Sea Grant College Program, University of Alaska Fairbanks.

QL638.6.S74 2007

ISBN 1-56612-113-2

CreditsThis book is published by the Alaska Sea Grant College Program, supported by the U.S. Department of Commerce, NOAA National Sea Grant Office, grant NA06OAR4170013, project A/161-01; and by the University of Alaska Fairbanks with state funds. The University of Alaska is an affirmative action/equal opportunity employer and educational institution.

Sea Grant is a unique partnership with public and private sectors combining research, education, and technology transfer for public service. This national network of universities meets changing environmental and economic needs of people in our coastal, ocean, and Great Lakes regions.

Book design and layout by Jen Gunderson, project coordination and graphics research by Kurt Byers, and production management and copyediting by Sue Keller, of Alaska Sea Grant. Cover art © Ray Troll. Cover design by Phil Raymond, Archetype Graphics, Fairbanks, Alaska.

University of Alaska FairbanksP.O. Box 755040 Fairbanks, Alaska 99775-5040Toll free (888) 789-0090(907) 474-6707 • fax (907) 474-6285www.alaskaseagrant.org Alaska

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iv Preface v About the Authors vi About the Cover vii Acknowledgments 1 Introduction 4 Using This Guide 6 General Anatomy of Sharks 7 General Anatomy of Skates 8 General Anatomy of Egg Cases 9 Key to the Sharks, Skates, and Ratfish of Alaska 14 Key to Egg Cases of Alaska Chondrichthyans 18 Ratfish Description 20 Shark Descriptions 38 Skate Descriptions 68 Photos of Alaska Sharks 69 Illustrations of Teeth of Alaska Sharks 70 Photos of Alaska Skates 71 Glossary 74 Selected References 76 Photo Sources 77 Index to Common and Scientific Names Inside back cover Map of North Pacific region

Contents

The primary purpose of this guide is to encourage and simplify the identification of chondrichthyans—sharks, skates, and ratfish—found in Alaska waters. It is intended for professional fishery biologists as well as nonprofessional audiences. The guide has an identification key, designed primarily for the identification of specimens in hand. In addition, color photographs, maps, and diagrams, as well as text features such as diagnoses and similar species synopses, are included to aid users interested in rapid identification of specimens that cannot be examined closely.

This guide also includes a key to the egg cases of chondrichthyans found in Alaska. Because this life stage is poorly known for some species, it may not always be possible to identify an egg case to the species level. We hope that including this preliminary key will stimulate further investigation into the interspecific differences in this important and interesting life-history stage.

Species are arranged in the guide in the order that they appear in the key to adult fishes, with the ratfish first, followed by sharks and finally skates. In most cases, this means that the most similar species are grouped together.

Most of the specimens, photos, and distribution data used in the production of this guide were obtained during bottom trawl surveys conducted by the Resource Assessment and Conservation Engineering (RACE) Division of the National Marine Fisheries Service, Alaska Fisheries Science Center (NMFS AFSC). These surveys provide a wealth of data for scientists and fisheries managers throughout the region, and represent an invaluable aid to the effective stewardship of Alaska’s marine resources.

We hope this guide will be useful to a broad audience, including professional fishery biologists and fishery observers, commercial and recreational fishers, divers, and all others interested in learning more about Alaska’s chondrichthyan fish resources. We also hope that this guide will encourage conservation by providing the basis for better management and future research, as well as a tool to promote public awareness and understanding of these vulnerable and poorly known fishes.

Duane E. StevensonJames W. OrrGerald R. HoffJohn D. McEachran

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Duane Stevenson and James Orr are research fisheries biologists at the National Marine Fisheries Service (NMFS), Alaska Fisheries Science Center’s Resource Assessment and Conservation Engineering (RACE) Division in Seattle, Washington. Their research interests include the taxonomy and systematics of many groups of fishes. In addition to published research, they have worked extensively within both the AFSC groundfish program and the North Pacific Groundfish Observer Program to provide training and produce field guides to several groups of Alaska fishes and invertebrates.

Gerald Hoff is also a research fisheries biologist at the NMFS Alaska Fisheries Science Center’s RACE Division in

Seattle. His research focuses on the biology and ecology of noncommercial deepwater fish species in the North Pacific. He has done extensive work on the ecology and reproduction of skates in Alaska with an emphasis on conservation of their marine habitats.

John McEachran is a professor in the Department of Wildlife and Fisheries Sciences at Texas A&M University in College Station, Texas. His research concerns the systematics and biogeography of skates and rays, and species diversity and biogeography of fishes of the central western Atlantic. He has also made several contributions to the series FAO Species Identification Guide for Fishery Purposes.

About the Authors

Ray Troll is an artist and naturalist based in Ketchikan, Alaska. Troll is widely known for his accurate and often “offbeat” renderings of sea life. His fish-centric art can be seen on T-shirts, in numerous books, and in nationwide museum displays. He is a lifelong “sharkophile” and promoter of

underappreciated fish. Troll even has a New Zealand species of ratfish named after him: Hydrolagus trolli. Troll’s accolades include the Alaska Governor’s Award for Individual Artist, and an award from the Academy of Natural Sciences for achievement in natural history arts. See www.trollart.com.

About the Cover

1. Thresher Shark, Alopias vulpinis, to 6.4 m (20 ft). 2. Basking Shark, Cetorhinus maximus, to 10 m (35 ft). 3. Okhotsk Skate, Bathyraja violacea, to 73 cm (2.5 ft). 4. Spiny Dogfish, Squalus acanthias, to 2 m (6.5 ft). 5. Sleeper Shark, Somniosus pacificus to 4.4 m (14.5 ft). 6. “Leopard Skate,” Bathyraja sp., to 139 cm (4.5 ft). 7. Whiteblotched Skate, Bathyraja maculata, to 147 cm (5 ft). 8. Big Skate, Raja binoculata, to 204 cm (7 ft). 9. Commander Skate, Bathyraja lindbergi, to 102 cm (3.5 ft). 10. Butterfly Skate, Bathyraja mariposa, to 76 cm (2.5 ft). 11. Salmon Shark, Lamna ditropis, to 3.1 m (10 ft). 12. Mud Skate, Bathyraja taranetzi, to 77 cm (2.5 ft). 13. Great White Shark, Carcharodon carcharias, to 6.4 m (21 ft). 14. Bluntnose Sixgill Shark, Hexanchus griseus, to 4.8 m (16 ft). 15. Spotted Ratfish, Hydrolagus colliei, to 100 cm (3 ft). 16. Brown Cat Shark, Apristurus brunneus, to 71 cm (2.5 ft). 17. Longnose Skate, Raja rhina, to 142 cm (4.5 ft). 18. Deepsea Skate, Bathyraja abyssicola, to 157 cm (5 ft). 19. Bering Skate, Bathyraja interrupta, to 86 cm (3 ft). 20. Blue Shark, Prionace glauca, to 3.8 m or more (12.5 ft).

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The work on which this guide is based is derived primarily from the unpublished research of J.D. McEachran, and was enhanced by the field observations of D.W. Kessler and R. Baxter. It also draws on the published works of Hart (1973), Eschmeyer et al. (1983), and Mecklenburg et al. (2002). The authors have received assistance from numerous scientists in the NMFS AFSC Resource Assessment and Conservation Engineering (RACE) Division, Resource Ecology and Fisheries Management (REFM) Division, and North Pacific Groundfish Observer Program (NPGOP); as well as the captains and crew of a number of chartered fishing vessels, particularly the F/V Aldebaran, F/V Arcturus, F/V Dominator, F/V Gladiator, F/V Morning Star, F/V Northwest Explorer, F/V Sea Storm, and F/V Vesteraalen; and several North Pacific groundfish observers. The majority of photographs were taken by NMFS personnel

on groundfish trawl surveys conducted by the Alaska Fisheries Science Center. Others were provided by Nancy Kohler (NMFS Northeast Fisheries Science Center), the North Pacific Groundfish Observer Program, Alexei Orlov (Russian Federal Research Institute of Fisheries and Oceanography [VNIRO]), and others. The photo sources are listed on page 76. Anatomical diagrams and tooth illustrations were prepared by Duane Stevenson, with many of the tooth illustrations relying on the observations of Bigelow and Schroeder (1948). Stippled line drawings of the sharks were prepared by Tim Gunther (www.gunthergraphics.biz). Theodore Pietsch (University of Washington), David Ebert (Pacific Shark Research Center), Ken Goldman (Alaska Department of Fish and Game), and Bob Lauth (NMFS, Alaska Fisheries Science Center) reviewed earlier versions of this manuscript.

Acknowledgments

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Sharks, skates, and ratfish constitute important components of Alaska’s marine ecosystems. Most of these fishes are relatively large compared with other marine fauna. They are thought to be long-lived and have low reproductive rates, and therefore may be some of the most vulnerable components of the ecosystem. Unfortunately, many aspects of the life history of these species, including longevity, fecundity, and food habits, are poorly known. Because cartilaginous fishes do not support large commercial fisheries, they have received comparatively little attention from fisheries managers and researchers. However, they are a large component of fisheries bycatch, and as the important biological characteristics of cartilaginous fishes are recognized, interest in their diversity and conservation is increasing.

The sharks, skates, and ratfish are members of the class Chondrichthyes, also known as the chondrichthyans, which includes two main evolutionary lineages: the holocephalans and the elasmobranchs. The holocephalan lineage is represented by several extinct forms, as well as the recent forms known as chimaeras, represented in Alaska by the spotted ratfish. Holocephalans are distinguished from other

chondrichthyans by the presence of a gill cover overlying the gill openings. Approximately 34 species of chimaeras are currently known from the world’s oceans. The elasmobranch lineage is much more diverse, with over 930 extant species worldwide. This lineage, distinguished from holocephalans by the presence of 5-7 gill openings on each side of the head, includes the sharks and rays (including skates, sawfishes, and guitarfishes).

DiversityThe marine waters of Alaska feature a broad diversity of chondrichthyans, including the relatively small and enigmatic spotted ratfish, representatives of at least four orders of sharks, and many species of skates. Alaska’s shark fauna includes open water predators, a large planktivore, and several bottom-dwelling species found from the intertidal zone to depths of over 2,000 meters. Some species are transient or seasonal visitors to Alaska, while others are constant residents that can be quite abundant in some areas. Distribution and abundance patterns are poorly known for pelagic shark species.

Introduction

The skates of this region have frustrated scientists and naturalists for decades. Due to their diversity and apparent external similarity, as well as gaps in the taxonomic knowledge of the group, skates have historically been considered very difficult to identify. Moreover, the terms “skate” and “ray” are often used interchangeably and are not clearly and universally defined. In general, “ray” is a more inclusive term that may be used to indicate any flattened elasmobranch, while “skate” denotes a specific order of flattened elasmobranchs (Rajiformes). We use the more precise term “skate” for all the flattened elasmobranchs included in this guide because they are all in the order Rajiformes.

Sexual DimorphismMany chondrichthyan species are sexually dimorphic. This can create identification difficulties as the morphological differences between males and females of a single species may appear more dramatic than the differences between two similar species. This phenomenon is most obvious in skates, in which males have paired claspers, one extending from the base of each pelvic fin, and in many species adult males have alar and malar hooks on the dorsal surface. Male skates

also tend to have a much more scalloped body shape, with a broad shallow indentation on the leading edge of each pectoral fin, while the females are much more rounded or diamond-shaped. Male chimaeras have an accessory clasping organ on the head in addition to the pair of claspers at the base of the pelvic fins, while females have neither. Shark species tend to show relatively little sexual dimorphism, with the exception of the presence of claspers in males (some species exhibit minor differences in tooth shape and relative fin placement). In most chondrichthyans, females reach a larger maximum size than males.

ReproductionThe chondrichthyans of Alaska exhibit a broad diversity of reproductive strategies. Most of Alaska’s chondrichthyans, including the spotted ratfish, the brown cat shark, and all skates, are oviparous, depositing thick leathery egg cases containing one to several embryos directly onto the seafloor. The embryo develops for an extended period (months to years) inside the egg case and the juvenile emerges fully developed. The remaining sharks of Alaska exhibit some form of viviparity. After an extended period of development inside the female, the young of these species are born looking much

2Introduction

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like miniature adults. The spiny dogfish, Pacific sleeper shark, and bluntnose sixgill shark exhibit aplacental viviparity, in which the developing embryos are completely dependent on the nutrition provided by a yolk sac. Basking, thresher, salmon, and great white shark embryos receive their nutrition from the consumption of additional nonviable ova that are produced by the female—a reproductive strategy known as oophagous viviparity. The blue shark exhibits placental viviparity, in which the embryo is nourished directly from the female through placental tissue.

SourcesThis guide combines morphological information from the authors’ extensive field collections and examination of museum specimens, with detailed distribution data based on resource-assessment surveys, observer collections, and information from the scientific literature. The result is the most comprehensive overview of Alaska’s cartilaginous fish fauna ever produced for use in the field. This guide includes all species of chondrichthyans (including chimaeras, sharks,

and skates) that have been reliably recorded from Alaska waters. It does not include some of the species that have been traditionally included in the faunal works of the region (e.g., Wilimovsky 1954, Mecklenburg et al. 2002), such as the manta ray (Manta birostris), shortfin mako shark (Isurus oxyrinchus), and Pacific angel shark (Squatina californica), because these species have not yet been confirmed from Alaska waters.

The book was designed for use in the field. As such, priority is given to characteristics useful in the field, as well as distribution information gleaned from the authors’ extensive recent field experience. The book is not an exhaustive review of the current state of knowledge for these fishes. For an introduction to the literature on these fishes, and for additional information on their taxonomic history and geographic ranges, the reader is encouraged to consult the references on p. 74. Some excellent sources of information on shark and skate life history, behavior, reproduction, and fisheries management are Hamlett (1999, 2005), Carrier et al. (2004), and Musick and Bonfil (2004).

Introduction

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This guide includes a dichotomous key to adult chondrichthyans of Alaska (pp. 9-13) as well as a key to the egg cases of the oviparous species. The portion of the key devoted to sharks primarily uses fin placement and morphology to distinguish the species, while the skate section relies on the presence or absence of various thorn series, as well as coloration pattern and, to a lesser extent, disc and fin shape. Although these suites of characters are generally quite reliable, and with practice easy to use, some of the included species exhibit a great deal of morphological variation. This is particularly true with respect to the relative size and strength of thorns in a few species of skates. Thus, users should remain open to the possibility that a particular specimen may have highly reduced thorns or, conversely, may even have a thorn in the “wrong place.” With experience, users will begin to recognize these variants as intraspecific anomalies and will not be discouraged by the occasional odd specimen.

Species KeyThe reader attempting to identify an adult is encouraged to start with the “Key to the Sharks, Skates, and Ratfish of Alaska” beginning on p. 9 and proceed to the individual species

descriptions to confirm the identification by comparison with similar species. The specialized terminology used in the key and species descriptions is defined on the anatomical diagrams (pp. 6-8) or in the glossary.

Diagnosis and PhotosAfter working through the key and proceeding to the species accounts, consult the diagnosis of the presumptive species. This diagnosis includes a suite of characteristics that will unambiguously identify that species to the exclusion of all others known from Alaska. In addition to the diagnosis, each species account includes photos as well as information on dentition, coloration, body shape, reproduction, maximum size, and geographic and bathymetric range. When possible, skate species descriptions include a photo of a male and a female to highlight shape differences. (Note that ventral views of the Okhotsk and deepsea skates show everted spiral valves.) A single-page composite of all Alaska sharks (p. 68), a composite of teeth for each shark species (p. 69), and a composite of dorsal views of all skates found in Alaska waters (p. 70) are included after the species descriptions for quick reference.

Using This Guide

IllustrationsEach skate species description also includes a thorn diagram, while each shark species description includes a tooth illustration. Thorn diagrams indicate which thorn series are typically present in that skate species. Series that may be present or absent are indicated in parentheses. The name of each thorn series and its respective abbreviation is on the skate diagram on p. 7. The total midline thorn count is the sum of the nuchal, mid-dorsal, tail, and interdorsal thorns. Tooth illustrations depict the lateral (external) view of one upper and one lower tooth, generally representing the fourth tooth from the center on the left side of the jaw. When possible, thorn or tooth characteristics, along with the range map and other characters, should be used to confirm the identification.

Similar SpeciesThe “similar species” section of the species account includes other chondrichthyans known from Alaska waters that may be confused with the species in question, as well as the characteristics useful in distinguishing them. This section should be consulted to provide final confirmation of the diagnosis.

Egg Case KeyThe reader attempting to identify an egg case is encouraged to start with the “Key to Egg Cases of Alaska Chondrichthyans” (pp. 14-16). The terminology used in this key is illustrated in the egg case diagrams on p. 8. The key is based on characters such as case length and width, surface texture, spine morphology, keel width, and byssal thread attachment. With the exception of the deepwater species Amblyraja badia, all egg-case characters were verified with specimens taken from inside adult females. Although egg-case size is dependent on maternal size, many species exhibit non-overlapping size ranges and therefore the length and width of an egg case can also be useful for identification. The egg cases of Bathyraja violacea and B. interrupta are presently indistinguishable, and these two species are grouped together in the egg-case key. An egg-case photo is provided with each species account.

MeasurementsAll adult lengths included in this book are total length. Egg case size refers to case length (i.e., without horns) or case width, including keels, and all egg case dimensions are given in millimeters (mm). All depths are given in meters (m).

5Using this Guide