214 
Fishery Bulletin 108(2) 
are concentrated (Hoff and Britt, 2003, 2005, 2009; 
Stevenson et al., 2008). 
In the Aleutian Islands, over 50% of OSC consists 
of B. parmifera, B. maculata, and B. aleutica (Table 
2), which are the top three species in terms of recent 
biomass estimates for the region (Zenger, 2004; Rooper, 
2008; Rooper and Wilkins, 2008). However, the pro- 
portion of B. parmifera is higher (29.6% of observed 
skate catch) and that of B. maculata considerably lower 
(12.1%) in commercial catches in the Aleutian Islands 
than their biomass estimates in the region (20-25% 
and 48% of total skate biomass, respectively) would 
indicate. The reasons for these differences in relative 
catch weight are unclear, but may be due to geographi- 
cally and bathymetrically concentrated commercial fish- 
ing effort. Skate populations in Alaska are primarily 
segregated by depth, and B. maculata tends to be found 
in deeper waters than those inhabited by B. parmifera 
(Rooper, 2008; Stevenson et al., 2008). Therefore, shal- 
low-water fisheries are more likely to catch B. par- 
mifera, and although observers reported skates in the 
Aleutian Islands from depths to 2000 m, the majority of 
OSC came from 200 m or less. Thus, Aleutian popula- 
tions of B. parmifera may be disproportionately affected 
by fishing activity because of the shallow depth distri- 
bution of this species. 
The two species of Raja (and unidentified Raja — “ Raja 
sp.”) account for over 60% of OSC in the Gulf of Alaska. 
These results are also consistent with fishery-indepen- 
dent survey data, which indicate that Raja binoculata 
and R. rhina are the most abundant species in the 
Gulf of Alaska, making up about 37% and 33%, respec- 
tively, of the skate biomass in the region (Stevenson et 
al., 2008; von Szalay et al., 2009). Among species of 
Bathyraja in the Gulf of Alaska, survey-derived biomass 
estimates indicate that B. aleutica is the most common, 
and indeed B. aleutica accounts for a greater proportion 
of OSC in this region than all other species of Bathyraja 
combined. 
Deepwater skate species, such as B. lindbergi, B. 
minispinosa, and B. trachura, are rarely reported by 
observers in any of the three regions, probably due to 
the relatively small amount of fishing effort targeting 
deepwater species. Other species known to be rare in 
Alaska waters, such as B. abyssicola, B. mariposa, and 
Amblyraja badia, have been only rarely reported by 
observers, and only B. mariposa has been confirmed by 
photographs and collected specimens. 
Although the percentage of unidentified skates in 
observer species composition data has declined to a very 
low level, a large percentage of OSC is still identified 
only to genus. These less specific skate identifications 
are largely the result of uncertainty with identification 
in the field. Because observers encounter a relatively 
high diversity of skates, particularly of the genus Bathy- 
raja, and must often interpret subtle characteristics to 
identify skates to the species level, they are encour- 
aged to identify a skate only to the genus level if the 
specimen is not brought to hand for inspection or if 
the identification of the specimen is questionable. As a 
result, species composition of OSC is clearly affected by 
fish-handling practices and observer sampling methods 
on vessels with different gear types. Observers in trawl 
fisheries select their species composition samples at 
random from the catch after it is onboard the vessel. 
Therefore, the entire composition sample is weighed, 
and all specimens in the composition sample are iden- 
tified in hand. In contrast, on longline vessels species 
composition data are collected as the gear is being re- 
trieved, and not all of the specimens in the composi- 
tion sample are brought on board and weighed. Some 
specimens counted during the tally period, particularly 
larger species such as many of the skates common in 
Alaska waters, become “drop-offs.” These specimens are 
retrieved to the surface on the line but either fall off 
before they can be brought onboard or are intentionally 
released to save strain on the gear, the personnel, and 
the fishes. Therefore, many of the skates in the compo- 
sition sample from longline vessels are not brought to 
hand for identification, and are recorded at the genus 
level. Thus, the way the catch is handled and sampled 
in longline fisheries largely explain the influence of 
gear type on the species composition profiles reported 
here (Table 2). 
The influence of longline data is significant because 
the majority of OSC in Alaska waters comes from long- 
liners. In fact, the data presented here (Table 3) indi- 
cate that the longline fishery for Pacific cod in the Ber- 
ing Sea accounts for more skate bycatch than all other 
federally managed groundfish fisheries combined. This 
result must be interpreted with some caution because 
differences in observer coverage for different fisheries 
and regions may have influenced these figures, and 
a predominant species is not a precise indicator of a 
target fishery. But it is clear that longliners targeting 
Pacific cod catch a lot of skates. Moreover, longline gear 
is often fished deeper than trawl gear, and therefore 
may affect a greater diversity of skate species than gear 
fished in more shallow water because skate diversity in 
Alaska waters tends to be highest on the continental 
slope (Stevenson et al., 2008). Therefore, as long as a 
high proportion of skates encountered on longliners are 
identified only to genus, a potentially important seg- 
ment of species-specific catch data is still not available 
for analysis. 
The presence of skates in the catch of pelagic trawls 
may seem counterintuitive because skates are generally 
benthic, substrate-oriented fishes unlikely to be found in 
the path of midwater nets. Indeed, the amount of skate 
catch reported in pelagic trawls (about 6% of OSC) is 
much lower than in the other two gear types. There 
are two general explanations for the skates that are 
collected in pelagic nets: either the skates were swim- 
ming up in the water column or the net contacted the 
seafloor. The target of most pelagic trawling in Alaska 
is walleye pollock, a species that is often found very 
close to the bottom, and catch data from pelagic trawl- 
ers often include a variety of benthic species, such as 
flatfishes and sculpins, in addition to skates. Therefore, 
it is likely that at least a large proportion of the skate 
