Stevenson et al Patterns of species richness, diversity, population density, and distribution in the skates of Alaska 
37 
However, specimens of both species collected from the 
Aleutian Islands are much more colorful, usually with 
vivid yellow or olive-green markings on the disc. Ad- 
ditional investigation of these regional differences may 
lead to the recognition of more undescribed species. 
The deepwater species A. badia and B. abyssicola have 
not been encountered in the Aleutian Islands in recent 
trawl surveys, although this may be due to the lack of 
deep trawling effort in the region because both species 
are known to inhabit the North Pacific and the eastern 
Bering Sea (see Stevenson and Orr, 2005), and B. ab- 
yssicola has been recorded from the Aleutian Islands 
(Zorzi and Anderson, 1990). 
Unlike the eastern Bering Sea and Aleutian Islands, 
the skate fauna of the Gulf of Alaska is dominated by 
the two Alaskan species of the genus Raja. Moreover, 
the encounter rate of skates in the Gulf of Alaska is 
even lower than that of the Aleutians. These two fac- 
tors combine to produce very low mean species richness 
values. The depth distributions of the two species of 
Raja appear to be somewhat complementary, in that 
R. binoculata generally inhabits shallower waters than 
R. rhina, but neither species is found frequently below 
400 m. Like these two species of Raja, Bathyraja inter- 
rupta and B. aleutica are both found throughout the 
Gulf of Alaska, and the ranges of both species extend 
well south of the Alaska border. Bathyraja parmifera 
is also found throughout the Gulf of Alaska, but is 
rare west of Kodiak (i.e., in subregions 3, 4, and 5). 
Like species richness values for the Aleutian Islands, 
species richness values are low in the Gulf of Alaska, 
but Shannon’s diversity indices are not particularly 
low. Some of the species common in the slope waters 
of the Bering Sea and Aleutian Islands, such as B. 
lindbergi (2005 survey data) and B. minispinosa (Love 
et al., 2005), have recently been recorded in the Gulf of 
Alaska, as well. Bathyraja trachura, a species relatively 
abundant off the U.S. west coast from Washington to 
California (Lauth, 2000) as well as on the eastern Ber- 
ing Sea slope, is also probably common throughout the 
deeper waters of the Gulf of Alaska, and it seems prob- 
able that the deepwater species B. abyssicola and A. 
badia (both known from around the Pacific Rim; Zorzi 
and Anderson, 1990; Stevenson and Orr, 2005) would be 
found in the Gulf of Alaska if additional deep sampling 
efforts were initiated. 
Interpretations of these data should be viewed within 
the context of the sampling limitations. Groundfish 
assessment surveys conducted in Alaska have been 
primarily restricted to the shelf and the shallowest por- 
tion of the continental slope. Although recent surveys 
on the eastern Bering Sea slope have improved our 
understanding of the distribution of skates and other 
deeper dwelling fishes, the lack of deeper samples from 
the Gulf of Alaska and Aleutian Islands is problem- 
atic because without these samples we are unable to 
make meaningful comparisons among regions. Another 
limitation of this data set concerns the gear used for 
AFSC bottom-trawl surveys. All surveys included in 
this study were performed with otter trawls. However, 
the sea floor substrate and topographic features differ 
markedly among the regions surveyed. Because of dif- 
ferences in the suitability of the sea bottom for trawl- 
ing, many areas within Alaskan waters (particularly in 
the Aleutians and Gulf of Alaska) have been considered 
“untrawlable,” and therefore remain unsampled. More- 
over, the relatively rough trawling conditions typically 
encountered in some regions necessitate the use of dif- 
ferent types of trawl gear. The effects that these differ- 
ences may have on the catchability of skates, and how 
these catchability differences may affect comparisons 
among surveys, are unknown (Kotwicki and Weinberg, 
2005). Finally, information on any seasonal changes 
that may affect skate distributions (Dolganov, 1998, 
1999) in Alaska is very limited. Virtually all NMFS 
groundfish assessment surveys in Alaska have been 
conducted during summer months, and we know little 
about the distributions of these species during the rest 
of the year. 
This study represents a major advance in our knowl- 
edge of the species richness, diversity, population den- 
sity, and distribution of the skate fauna of Alaska. It 
provides a reference strategy for the reliable assess- 
ment and monitoring of skate diversity and abundance 
with data from resource assessment surveys. It also 
provides an example of how advances in taxonomy and 
field identification tools can enable more detailed and 
robust assessments of species diversity than were pre- 
viously possible. These considerations are particularly 
critical for the skates of this region because of their 
high species diversity and their vulnerability to fishing 
pressure and habitat disturbance. We hope this study 
will serve for comparison with similar studies of other 
regions or other groups of fishes, as well as provide a 
baseline for monitoring future changes in the skate 
fauna of Alaska. 
Acknowledgments 
We thank the scientific staffs of the Alaska Fisheries Sci- 
ence Center groundfish bottom-trawl surveys conducted 
from 1999 through 2004, as well as the crews of the FV 
Aldebaran, FV Arcturus, FV Dominator, FV Gladiator, 
FV Morning Star, FV Northwest Explorer, FV Sea Storm, 
and FV Vesteraalen. We also thank T. Pietsch and K. 
Maslenikov (Univ. of Washington Fish Collection) for 
curation of specimens and access to collections. North 
Pacific groundfish observer R. Morse collected the only 
specimen of B. violacea. M. Martin, R. Lauth, and S. 
Kotwicki provided critical reviews of earlier drafts of 
this manuscript. 
Literature cited 
Allen, M. J., and G. B. Smith. 
1988. Atlas and zoogeography of common fishes in 
the Bering Sea and northeastern Pacific. U.S. Dep. 
Commer., NOAA Tech. Rep. NMFS 66, 151 p. 
