Stevenson and Lewis: Skate bycatch in the commercial groundfish fisheries of Alaska 
209 
amining the species composition of observed skate catch 
(OSC) in Alaska’s groundfish fisheries and recent trends 
in skate retention by commercial fishermen. 
Materials and methods 
All data used for this study were extracted from the 
North Pacific Groundfish Observer Program (NPGOP) 
database maintained by the Fisheries Monitoring and 
Analysis (FMA) Division of the National Marine Fisher- 
ies Service’s (NMFS) Alaska Fisheries Science Center. 
This database houses all biological data collected by 
groundfish observers onboard commercial fishing vessels 
operating in the waters of Alaska’s federal Exclusive Eco- 
nomic Zone (EEZ). For an overview of the database, see 
the FMA Division website (National Marine Fisheries 
Service, http://www.afsc.noaa.gov/FMA/fma_database. 
htm, accessed November 2009). 
Federal law requires observers to be present at all 
times on commercial fishing vessels of 125 ft (38.1 m) 
or more in length overall (LOA) operating in the fed- 
eral EEZ. For vessels from 60 to 124 ft (18.3 to 37.8 
m) LOA, observer coverage is required for only 30% of 
fishing days and for vessels less than 60 ft (18.3 m) 
LOA no observer coverage is required. The catch data 
used for this study were taken from trawl hauls and 
longline sets during which an observer was present and 
was sampling, so that the catch statistics presented 
here do not represent the total catch of the fisheries in 
this region, nor do they represent biomass estimates. 
For some commercial fisheries in the area, pot gear is 
used, but observers rarely encounter skates in these 
fisheries, and therefore such data are not included in 
this study. 
The process used by observers to determine the spe- 
cies composition and catch weights of sampled hauls 
depends on gear type. Observers on trawlers may de- 
termine the species composition of a haul by identifying 
and weighing the entire catch, which is usually not 
possible, or by choosing a random sample (generally 300 
kg or more) of the catch and identifying and weighing 
all taxa within the sample. The proportion by weight 
of each taxon in the sample is then extrapolated to the 
total catch weight, which may be determined by a num- 
ber of methods, including flow scale readings, codend 
measurements, or bin volume estimates. On longline 
vessels, observers randomly select a “tally period” as 
the gear is being retrieved. During this tally period, 
the observer identifies and counts specimens, including 
specimens that drop off the line or are intentionally dis- 
carded. A subset of the specimens identified during the 
tally period (15 or more per species, when possible) is 
retained onboard the vessel and weighed to determine 
an average weight for each taxon. That average weight 
is then applied to all specimens identified during the 
tally period, and the resulting proportional species com- 
position is extrapolated to the total gear set to obtain 
a total catch weight for each species for each set. The 
basic data unit used for this study is the extrapolated 
catch weight for each taxon from each observed haul 
or set (hereafter trawl hauls and longline sets will be 
collectively referred to as “hauls”). The total observed 
skate catch (OSC) was calculated by summing extrapo- 
lated catch weights for all skate taxa (including the 
following unidentified skate groups: “skate unidenti- 
fied,” “Bathyraja sp.,” and “ Raja sp.”) across all hauls 
in which skates were identified. Scientific and common 
names for skate taxa follow Stevenson et al. (2007). 
From the inception of the NPGOP through the sam- 
pling year 2002, observers were not trained to identify 
skates and were therefore not required to identify them 
beyond the family level. During 2002 and 2003, a field 
identification key was developed (Stevenson, 2004) and 
experienced observers began receiving training in skate 
identification during annual briefings. Feedback from 
experienced observers was used to refine the identifica- 
tion materials and classroom training, and beginning 
with the 2004 sampling year, all new and returning 
observers were provided with skate identification train- 
ing and materials for identification of skate in the field. 
Since 1 January 2004 all observers have been required 
to identify skates to the species level when possible. 
Because of these changes in observer identification 
training and policies, two separate but overlapping 
time frames were used in this study. To investigate the 
trends in observed skate catch and the history of skate 
identification by observers an 11-year time frame was 
chosen and queries were restricted to data collected 
from 1 January 1998 through 31 December 2008. For 
investigations of species-level trends in observer data, 
queries were restricted to data collected from 1 Janu- 
ary 2004 through 31 December 2008 — a period that 
corresponds with the time period in which all new and 
returning observers have been trained to identify skates 
to the species level. Regions were defined on the basis 
of NMFS management areas: Bering Sea comprises the 
Bering Sea NMFS management areas 509-524; the 
Aleutian Islands region comprises NMFS management 
areas 541-543; and the Gulf of Alaska comprises NMFS 
management areas 610-650 (Fig. 1). All catch propor- 
tions are presented as a percentage of total observer 
reported catch weight. 
The targeted resource was not directly recorded in ob- 
server catch data, so that for the purposes of this study, 
the term “target species” is defined as the predominant 
species in the catch. “Predominant species” was defined 
as the species accounting for the highest percentage 
of the extrapolated weight in the species composition 
sample and was determined on a haul-by-haul basis. 
Percent retained data are subjective estimates made 
by observers using visual approximations, along with 
information provided by the vessel’s captain or factory 
manager. Mean retention rates used here are weighted 
averages calculated annually for each species with the 
following equation: 
xaa 
Xa ' 
