Recent Contributions to the Knowledge of the Skates of Alaska by Duane E

FEATURE Recent Contributions to the Knowledge of the Skates of Alaska by Duane E. Stevenson and James W. Orr Figure 1. Raja binoculata (big skate). Photo by Duane Stevenson. he fish family Rajidae, commonly known imately 30 species worldwide and a North Pacific as the skates, includes about 280 species of assemblage of 6 species ranging from the Gulf of primarily benthic fishes found throughout California to the Bering Sea and into the western Tthe world’s oceans from tropical to cold temperate North Pacific. This North Pacific Assemblage is latitudes. The fishes of Alaska (Mecklenburg et al., thought to be a cluster of closely related species and 2002), the most recent inventory of Alaska’s fishes, will probably be recognized as a separate genus in lists 12 species of skates known to occur in the Gulf the future. The genus Bathyraja, also known as the of Alaska, Aleutian Islands, and Bering Sea from “soft-snout” skates due to their flexible rostral car- the intertidal zone to depths of over 1500 m, and tilage, includes over 40 species distributed through- at least 2 other species have been discovered in the out the world’s oceans. Bathyraja is the most broadly region since the publication of that volume as de- distributed as well as the most diverse of all the scribed in Bathyraja mariposa: a new species of skate skate genera, and the greatest diversity of Bathyraja (Rajidae: Arhynchobatinae) from the Aleutian Islands occurs in the North Pacific. The majority of Alaska’s (Stevenson et al., 2004a) and New records of two deep- skate species are included in this genus, although water skate species from the Bering Sea (Stevenson and some authors include one or two of the species in Orr, submitted). Many of these species are widely the genus Rhinoraja. In general, members of the ge- distributed, with ranges extending south along the nus Bathyraja tend to be smaller and inhabit deeper coast of North America as well as into the western waters than species of Raja. North Pacific, Sea of Okhotsk, and Sea of Japan. Many skate species attain very large sizes (Raja Alaska’s skate fauna includes representatives of binoculata reaches over 2 m in length and may weigh two genera: Raja and Bathyraja. The genus Raja, well over 70 kg) (Fig. 1) and can be locally abun- commonly known as the “stiff-snout” skates because dant. Therefore, skate populations represent a large they have a robust rostral cartilage, includes approx- proportion of the groundfish biomass in Alaskan AFSC Quarterly Report 1 waters, particularly on the Bering Sea shelf, and Field Identification are important components of the food web dynam- The Atlas and zoogeography of common fishes of ics and nutrient cycling in Alaska’s marine ecosys- the Bering Sea and Northeastern Pacific, (Allen and tems. However, due to their large size, relatively Smith, 1988) noted several problems with skate data long life expectancy, and low fecundity, skates may in the National Marine Fisheries Service (NMFS) be particularly vulnerable to fishing pressure. In the survey database and concluded that further study North Atlantic where fisheries target skate popula- was needed to clarify the distribution of these spe- tions, several species are in a state of decline and cies. The problems were largely a reflection of the at least two species have been extirpated from large poor state of skate systematics and the lack of use- portions of their native range. Furthermore, skate ful field identification tools. In fact, until recently, populations may be adversely affected by fishing ac- a comprehensive field guide to the skates of Alaska tivity, even if they are not directly targeted. was not available for use on the Center’s Resource Although they represent a large proportion of the Assessment and Conservation Engineering (RACE) worldwide diversity of cartilaginous fishes (roughly Division surveys. AFSC scientists began to con- 35% of elasmobranch species currently known are sistently identify skates to the species level begin- skates), skates are morphologically conservative and ning in 1997 during the Aleutian Islands triennial differences among species or even genera are often groundfish survey. For several years, identifications subtle. Moreover, the limits of morphological varia- were based on a dichotomous key provided by John tion in many species are poorly known, and as a re- McEachran of the Texas A&M University (later sult skates have been very challenging to identify incorporated into the key of Mecklenburg et al.). in the field, especially in the North Pacific. These Recently, McEachran’s key was refined, developed challenges are compounded by the fact that skates into a color field guide, and integrated into the on- are generally large fishes and are difficult to col- deck field guides used on groundfish surveys. This lect, preserve, and curate. Therefore, they are poorly field guide incorporates geographic and bathymetric represented in museum collections and difficult to distribution information for each species as well as study in the laboratory. As a result of identification high-resolution color photos taken on recent RACE difficulties and a relative lack of commercial im- surveys. Another version of this field guide, includ- portance, species-specific data on skate populations ed in Identification of skates, sculpins, and smelts by are often not available, and catch statistics are com- observers in North Pacific groundfish fisheries (2002- monly recorded only at the aggregate level. This is 2003) (Stevenson, 2004), has been developed for use a concern because apparent stability or increases in by groundfish observers and includes a dichotomous aggregate skate catches may mask declines in some key to the skates of Alaska. These tools have greatly components of those species aggregates, particularly facilitated species-level identification of skates in the larger species. the field. Recent emphasis on ecosystem-level manage- Confidence in field identifications has also been ment of federally regulated marine fisheries and the improved with the establishment of a voucher col- recent development of a commercial fishery target- lection process that allows for laboratory verifica- ing skates in the Gulf of Alaska have underscored tion or reidentification of significant specimens. the need for basic biological data on these fishes. As This process involves labeling and photographing these needs are recognized, researchers and fishery the specimen on deck, then preserving it and ship- managers will require a level of detailed informa- ping it back to the laboratory for identification and tion on the diversity and distribution of skates in further study. Many of these specimens collected as Alaskan waters that was previously not available. vouchers have provided the material to recognize In response to these needs, scientists at the Alaska previously undescribed species in the region. The Fisheries Science Center (AFSC) have been in- majority of specimens collected as part of the voucher volved in several projects that have advanced our process, including primary types of new species, are knowledge of the skates of the North Pacific and now archived at the University of Washington Fish Bering Sea. This research feature article describes Collection. This collection provides loans and access several of those efforts. to these and other specimens to researchers world- 2 January-February-March 2005 wide. Other type material, which serves as verified Although skate species diversity is particularly examples of new species, has been distributed to fish low on the Bering Sea shelf, total skate biomass ap- collections around the United States, including the pears to be greater in this area than in any other U.S. Natural History Museum at the Smithsonian region of Alaskan waters (Fig. 3). Skates are en- Institution, the California Academy of Sciences, and countered in nearly every haul on the Bering Sea the Los Angeles County Natural History Museum. shelf survey, and they often occur in large num- In addition, tissue samples from many specimens bers, particularly on the outer shelf in the northern have been collected for genetic studies and are Bering Sea. Skate biomass also appears to be high stored in the University of Washington and in the in parts of the central and western Aleutian Islands, University of Kansas Natural History Museum, where large species like the whiteblotched skate, B. Division of Ichthyology. From this vouchered tis- maculata, and the Aleutian skate, B. aleutica, are sue collection, genetic markers have been developed encountered in large numbers along with several by Mike Canino and Ingrid Spies of the RACE smaller species. In the Gulf of Alaska, big skates Division to identify all of the common skates of and longnose skates (R. binoculata and R. rhina, re- Alaska (see below). spectively), which are the largest species of skates in Alaskan waters, make up a higher proportion of the catch. These large species are most common in the Documenting Diversity Kodiak Island region. As survey personnel have been trained to rec- In addition to improvements in distribution ognize species-level differences among skates and data, new species of skates are being discovered field identifications have improved, a clearer pic- and others newly recorded from Alaskan waters. ture of skate diversity has begun to emerge. The Using morphological characteristics such as counts geographic and bathymetric ranges of each species of vertebrae, fin rays, and thorns, measurements of are now known with greater certainty than ever be- body shape, overall shape of clasper cartilages, and fore. Generally, in Alaskan waters, skate diversity body color, AFSC scientists James Orr and Duane is higher on the continental slope than on the shelf Stevenson have examined and identified hundreds as many species are found only near the shelf break of skate specimens collected in Alaska. In sum- and deeper. The area of greatest skate diversity is the mer 2003 they traveled to the Zoological Institute eastern Aleutian Islands and southeastern Bering of St.

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