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 
often not available, and catch statistics are commonly 
recorded only at the aggregate (genus or family) level. 
This lack of data is a concern in that skates may be 
particularly vulnerable to fishing pressure, even if they 
are only encountered as bycatch, because of their large 
size, relatively long life expectancy, and low fecundity, 
and are considered highly vulnerable to extinction or 
extirpation due to overfishing or habitat disturbances 
(Casey and Myers, 1998; Stevens et ah, 2000; Dulvy 
and Reynolds, 2002). Moreover, apparent stability or 
increases in aggregate skate catches may mask de- 
clines in some components of those species aggregates, 
particularly among larger species (Dulvy et al., 2000). 
Therefore, species-level management, which can only be 
achieved through accurate identification, reporting, and 
monitoring, is essential for maintaining viable skate 
populations (Stevens et al., 2000). 
Although skate populations in the eastern North Pa- 
cific Ocean and Bering Sea have been inadequately 
studied and inaccurately represented because of ques- 
tionable field identifications, recent research efforts are 
improving the resolution and consistency with which 
skates can be identified by field survey personnel. The 
taxonomic works of Ishihara and Ishiyama (1985, 1986), 
Dolganov (1985), and Stevenson et al. (2004) have 
helped to clarify the taxonomy of North Pacific skates. 
In addition, range extensions of species previously un- 
known from Alaskan waters (Stevenson and Orr, 2005), 
the ongoing development of a comprehensive field guide 
to the skates of the region (Stevenson et al., 2007), and 
the establishment of a voucher collection process that 
allows for laboratory verification or reidentification of 
significant specimens, have greatly improved field iden- 
tifications on Alaska Fisheries Science Center (AFSC) 
resource assessment surveys. 
Because of advances in the knowledge of the skates of 
the North Pacific Ocean and Bering Sea, species-level 
skate identifications made by AFSC personnel begin- 
ning with the 1999 survey year are reliable and consis- 
tent. The purpose of this article is to describe species 
richness, diversity, relative density, and distribution of 
skates throughout Alaskan waters, based on data from 
six years of groundfish bottom-trawl surveys in the 
Bering Sea, Aleutian Islands, and Gulf of Alaska from 
1999 through 2004. 
Materials and methods 
Specimens were collected aboard a variety of commercial 
fishing vessels chartered by the AFSC Resource Assess- 
ment and Conservation Engineering (RACE) Division 
from May through August in the eastern Bering Sea, 
Aleutian Islands, and Gulf of Alaska, between 1999 and 
2004 (Fig. 1). Survey gear and methods differed sub- 
stantially among the four resource assessment surveys 
(Bering Sea shelf, Bering Sea slope, Aleutian Islands, 
and Gulf of Alaska), and bottom habitat types differ 
among regions, resulting in differences in the catch- 
ability of skates. The effects of these differences on 
estimates of skate species richness, diversity, density, 
and distribution remain largely unknown; therefore the 
eastern Bering Sea shelf, Bering Sea slope, Aleutian 
Islands, and Gulf of Alaska were treated separately in 
this study. 
The eastern Bering Sea shelf survey has been con- 
ducted annually in approximately its present form since 
1982, and data from survey hauls during the years 
1999-2004 were used in this study. This survey covered 
the eastern Bering Sea shelf from the Alaska Penin- 
sula north beyond St. Matthew Island to approximately 
62°40'N, from 20 m to 200 m depth, and was conducted 
with an 83-112 Eastern otter trawl. Hauls were made 
at previously established survey stations on a 20x20 
nautical mile grid, and bottom time for each haul was 
approximately 30 minutes. For the purposes of this 
study, the region was divided into three subregions of 
approximately equal survey effort, each including three 
depth ranges (<50 m, 51-100 m, and 101-200 m). Sub- 
region 1 comprised the southeastern part of the eastern 
Bering Sea shelf, extending from the Alaska Peninsula 
to the southeastern rim of Pribilof Canyon; subregion 
2 comprised the central part of the eastern Bering Sea 
shelf, from the northwestern boundary of subregion 1 to 
the southeastern rim of Zhemchug Canyon; and subre- 
gion 3 comprised the northernmost portion of the survey 
area, bounded on the northwest by the U.S. -Russian 
border, with the northernmost hauls at approximately 
62°40'N (Fig. 2A). For more information on the design 
and methods of this survey, see Stauffer (2004) and 
Lauth and Acuna (2007). 
The eastern Bering Sea slope survey was conducted 
in 2000, 2002, and 2004. It covered the eastern Ber- 
ing Sea upper continental slope (200 m to >1200 m 
depth) from just north of Unalaska Island north to the 
U.S. -Russian border, and was conducted with a Poly 
Nor’eastern bottom trawl equipped with mud-sweep 
roller gear on the footrope. Haul locations were chosen 
according to a stratified random sampling design, with 
the region divided into six subregions and five depth 
strata, and bottom time for each haul was approximate- 
ly 30 minutes. For the purposes of this study the same 
six sub-regions were used. Subregions 1 and 6 consisted 
of a broad, low-angle slope, and form the southeastern 
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 
faces characterized by a steep-angle slope (Fig. 2A). For 
this study, depth strata were combined into two depth 
ranges, representing the upper slope (200-600 m) and 
the lower slope (>600 m). For more information on the 
design and methods of this survey, see Hoff and Britt 
(2005). 
The Aleutian Islands survey was conducted in 2000, 
2002, and 2004. These surveys covered the continental 
shelf and upper slope (to 500 m) of the entire Aleu- 
tian Archipelago from Unimak Pass (165°W) to Stale- 
mate Bank (170°30'E), and were conducted with a Poly 
Nor’eastern bottom trawl equipped with rubber bobbin 
