Emerging patterns of species richness, diversity, population density, and distribution in the skates (Rajidae) of Alaska Item Type article Authors Stevenson , Duane E.; Orr, James W.; Hoff, Gerald R.; McEachran, John D. Download date 01/10/2021 18:27:47 Link to Item http://hdl.handle.net/1834/25497 24 Abstract — Six years of bottom- Emerging patterns of species richness, diversity, trawl survey data, including over 6000 trawls covering over 200 km2 population density, and distribution in of bottom area throughout Alaska’s subarctic marine waters, were ana- the skates (Rajidae) of Alaska lyzed for patterns in species richness, diversity, density, and distribution of Duane E. Stevenson (contact author)1 skates. The Bering Sea continental shelf and slope, Aleutian Islands, and James W. Orr1 Gulf of Alaska regions were stratified Gerald R. Hoff1 by geographic subregion and depth. 2 Species richness and relative density John D. McEachran of skates increased with depth to the E-mail address for D. E. Stevenson: [email protected] shelf break in all regions. The Bering 1 National Oceanic and Atmospheric Administration Sea shelf was dominated by the National Marine Fisheries Service, Alaska Fisheries Science Center Alaska skate (Bathyraja parmifera), Resource Assessment and Conservation Engineering Division but species richness and diversity 7600 Sand Point Way NE were low. On the Bering Sea slope, Seattle, Washington 98115 richness and diversity were higher 2 in the shallow stratum, and relative Department of Wildlife and Fisheries Sciences density appeared higher in subre- Texas A&M University gions dominated by canyons. In the College Station, Texas 77843 Aleutian Islands and Gulf of Alaska, species richness and relative density were generally highest in the deepest depth strata. The data and distribu- tion maps presented here are based on species-level data collected throughout Patterns of species richness, diversity, fishes, and although they represent the marine waters of Alaska, and this density, and distribution for the spe- a large proportion of the diversity of article represents the most compre- hensive summary of the skate fauna fcies o skates inhabiting the North elasmobranchs worldwide, external of the region published to date. Pacific Ocean and Bering Sea are morphological differences among spe- still largely unknown. Earlier stud- cies (or even genera) are often subtle. ies have been limited because of prob- Moreover, the extent of morphologi- lems with identification of skates in cal variation in many species is poor- the field and, to some degree, in the ly known despite earlier taxonomic laboratory. Summarizing trawl survey work (Ishiyama and Ishihara, 1977; data for commonly encountered spe- Ishihara and Ishiyama, 1985, 1986), cies, Allen and Smith (1988) reported and although molecular methods have serious problems in the fisheries and shown promise for species identifica- survey data reported for even the most ntion i the laboratory (Tinti et al., common skates throughout Alaska 2003; Bremer et al., 2005; Spies et and the eastern North Pacific Ocean al., 2006), skates are often difficult because of widespread problems with to identify in the field. In Alaska wa- field identification of skates. Simi- ters, namely the eastern Bering Sea, nlarly, i the only previously published Aleutian Islands, and Gulf of Alas- analyses of skate abundance and dis- ka, this has been particularly true tribution data for Alaska, Teshima as Allen and Smith (1988), Teshima and Wilderbuer (1990) and Raschi et and Wilderbuer (1990), and Raschi et al. (1994) treated their data in the al. (1994) have noted. More recently, aggregate at the family level because Mecklenburg et al. (2002) stated that of difficulty with identification of spe- the poorly understood taxonomic rela- ncies. I contrast, Japanese and Rus- tionships of the skates in this region sian authors have published several complicate the determination of spe- studies including general species-level cies distributions. These challenges information on the skate fauna of the are compounded by the fact that Manuscript submitted 16 May 2007. Manuscipt accepted 1 August 2007. western North Pacific Ocean and Sea skates are generally large fishes and Fish. Bull. 106:24–39 (2008). of Okhotsk (Dudnik and Dolganov, are, thus, difficult to collect, preserve, 1992; Nakaya and Shirai, 1992; Dol- and curate. Therefore, they are poorly The views and opinions expressed or ganov, 1999, 2001). represented in museum collections and implied in this article are those of the author and do not necessarily reflect Skates present difficulties for iden- difficult to study in the laboratory. the position of the National Marine tification because they are a mor- Becausef o identification difficulties Fisheries Service, NOAA. phologically conservative group of and a relative lack of commercial im- Stevenson et al.: Patterns of species richness, diversity, population density, and distribution in the skates of Alaska 25 portance, species-specific data on skate populations are abilityf o skates. The effects of these differences on often not available, and catch statistics are commonly estimates of skate species richness, diversity, density, recorded only at the aggregate (genus or family) level. and distribution remain largely unknown; therefore the This lack of data is a concern in that skates may be eastern Bering Sea shelf, Bering Sea slope, Aleutian particularly vulnerable to fishing pressure, even if they Islands, and Gulf of Alaska were treated separately in are only encountered as bycatch, because of their large this study. size, relatively long life expectancy, and low fecundity, The eastern Bering Sea shelf survey has been con- and are considered highly vulnerable to extinction or ducted annually in approximately its present form since extirpation due to overfishing or habitat disturbances 1982, and data from survey hauls during the years (Casey and Myers, 1998; Stevens et al., 2000; Dulvy 1999−2004 were used in this study. This survey covered and Reynolds, 2002). Moreover, apparent stability or the eastern Bering Sea shelf from the Alaska Penin- increases in aggregate skate catches may mask de- sula north beyond St. Matthew Island to approximately nclines i some components of those species aggregates, 62°40ʹN, from 20 m to 200 m depth, and was conducted particularly among larger species (Dulvy et al., 2000). with an 83-112 Eastern otter trawl. Hauls were made Therefore, species-level management, which can only be at previously established survey stations on a 20×20 achieved through accurate identification, reporting, and nautical mile grid, and bottom time for each haul was monitoring, is essential for maintaining viable skate approximately 30 minutes. For the purposes of this populations (Stevens et al., 2000). study, the region was divided into three subregions of Although skate populations in the eastern North Pa- approximately equal survey effort, each including three cific Ocean and Bering Sea have been inadequately depth ranges (<50 m, 51−100 m, and 101−200 m). Sub- studied and inaccurately represented because of ques- region 1 comprised the southeastern part of the eastern tionable field identifications, recent research efforts are Bering Sea shelf, extending from the Alaska Peninsula improving the resolution and consistency with which to the southeastern rim of Pribilof Canyon; subregion skates can be identified by field survey personnel. The 2 comprised the central part of the eastern Bering Sea taxonomic works of Ishihara and Ishiyama (1985, 1986), shelf, from the northwestern boundary of subregion 1 to Dolganov (1985), and Stevenson et al. (2004) have the southeastern rim of Zhemchug Canyon; and subre- helped to clarify the taxonomy of North Pacific skates. gion 3 comprised the northernmost portion of the survey In addition, range extensions of species previously un- area, bounded on the northwest by the U.S.-Russian known from Alaskan waters (Stevenson and Orr, 2005), border, with the northernmost hauls at approximately the ongoing development of a comprehensive field guide 62°40ʹN (Fig. 2A). For more information on the design to the skates of the region (Stevenson et al., 2007), and and methods of this survey, see Stauffer (2004) and the establishment of a voucher collection process that Lauth and Acuna (2007). allows for laboratory verification or reidentification of The eastern Bering Sea slope survey was conducted significant specimens, have greatly improved field iden- in 2000, 2002, and 2004. It covered the eastern Ber- tifications on Alaska Fisheries Science Center (AFSC) ing Sea upper continental slope (200 m to >1200 m resource assessment surveys. depth) from just north of Unalaska Island north to the fBecause o advances in the knowledge of the skates of U.S.-Russian border, and was conducted with a Poly the North Pacific Ocean and Bering Sea, species-level Nor’eastern bottom trawl equipped with mud-sweep skate identifications made by AFSC personnel begin- roller gear on the footrope. Haul locations were chosen ning with the 1999 survey year are reliable and consis- according to a stratified random sampling design, with tent. The purpose of this article is to describe species the region divided into six subregions and five depth richness, diversity, relative density, and distribution of strata, and bottom time for each haul was approximate- skates throughout Alaskan waters, based on data from ly0 3 minutes. For the purposes of this study the same six years of groundfish bottom-trawl surveys in the six sub-regions were used. Subregions 1 and 6 consisted Bering Sea, Aleutian Islands, and Gulf of Alaska from of a broad, low-angle slope, and form the southeastern 1999 through 2004. and northwestern edges of the survey area; subregions 2 and 4 consisted of Pribilof and Zhemchug Canyons, respectively; and subregions 3 and 5 were intercanyon Materials and methods faces characterized by a steep-angle slope (Fig. 2A). For this study, depth strata were combined into two depth Specimens were collected aboard a variety of commercial ranges, representing the upper slope (200−600 m) and fishing vessels chartered by the AFSC Resource Assess- the lower slope (>600 m).