Conners and Munro: Effects of commercial fishing on local abundance of Gadus macrocephalus in the Bering Sea 
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Bootstrap methods (Manly, 1991) were used to 
evaluate the power of the experimental design to 
correctly identify the presence of a fishing effect. 
For each study year, daily catch data from all sta- 
tions and days were pooled for use as a sampling 
population. Random samples the same size as those 
used in each year’s experiment were drawn. Half of 
the simulated sample stations were randomly as- 
signed to the treatment group, and the mean catch 
for these stations in the March cruise was decreased 
by a fixed percentage to simulate a known fishing 
effect. Percentage changes were then calculated for 
each station, and we used the Wilcoxon rank sum 
test to compare stations in treatment and control 
groups. This algorithm was repeated 1000 times for 
each level of fishing effect. The P-values computed 
for each sample were used to compute probabilities 
of rejecting the null hypothesis for a = 0.05, 0.1, and 
0.2 for each of the levels of sampling effort. 
To check that substantial removals of Pacific cod 
took place during the study, we evaluated the har- 
vest in the study area using NMFS catch estimates 
and haul-specific data collected by the Alaska Fisheries 
Science Center observer program. Winter-season Pacific 
cod catch for trawl gear was summarized for each of 
the two federal reporting areas that intersect the study 
area. These totals included both catches in the study 
area and catches in other parts of the reporting area. 
Observer data did not cover all hauls on all vessels 
but were recorded with spatial precision to the nearest 
minute latitude and longitude. Records for all observed 
hauls within the two reporting areas were extracted, 
and hauls within a one-degree longitude by 30-minute 
latitude block around the study area were identified. 
The proportion of observed hauls and total fishing effort 
(duration of haul) in the study area as a proportion of 
observed hauls in each reporting area was calculated. 
The total catch of Pacific cod was determined from the 
smaller subset of observed hauls (for which there were 
catch composition data), and the proportion of Pacific 
cod catch coming from the study area was also estimat- 
ed. This fraction was then applied to the total catch for 
the reporting area to obtain a rough estimate of Pacific 
cod harvested from the study area by gear type. 
Tagging studies 
In conjunction with the localized depletion study, we 
conducted studies on the feasibility of determining on 
Pacific cod movements through tagging. Partial results 
of these studies are reported here to explain the results 
of the main experiment. Tagging work included develop- 
ment of methods 1) for capturing and handling Pacific 
cod with pot gear, 2) for tagging fish and determining 
data formats, 3) for releasing Pacific cod tagged with 
archival data-storage tags and standard spaghetti tags, 
and 4) for studying preliminary tag-induced mortal- 
ity. The goal of this effort was to collect information 
on both seasonal migration patterns and small-scale 
movements of Pacific cod during the spawning season. 
Figure 2 
Example of the frequency distribution of pot catch data 
for Pacific cod (Gadus macrocephalus) . Catch data are in 
numbers of fish per pot from all stations during the March 
2004 cruise. 
An understanding of these movements is important for 
the interpretation of the local abundance study. During 
a special tagging cruise in February 2003, tagged fish 
were released at a series of locations, including within 
the notrawl zone off Cape Sarichef. Tag recapture was 
conducted entirely by the fishing industry. Date and 
location of the recaptures were provided by fishermen 
and fishery observers. In total, 295 archival tags and 
over 6000 spaghetti tags were released between 2002 
and 2004. Approximately 35% of both types of tags 
were returned over the five years after tag release. Full 
results of these tagging studies will be presented in a 
separate publication. 
Results 
Field data confirmed that assumptions for the experi- 
mental design were met. The frequency distribution of 
the raw catch data (Fig. 2) illustrated the properties 
of pot catch. Similar results were obtained with either 
numbers or weight of Pacific cod per pot as the measured 
quantity. Zero catches (empty pots) were rare, and there 
was no evidence of gear saturation at high catches (the 
upper tail of Fig. 2 declines gradually without a sharp 
cutoff). Although the distribution of pot catch data was 
slightly skewed, the skewness and heteroscedasticity 
were much smaller than is typical for many types of 
fishery data. 
The level of Pacific cod catch varied strongly between 
study years and seasons but was fairly consistent within 
each study cruise. Average catch ranged from 8.8 Pacific 
cod per pot (27.7 kg) in March 2003 to 31.7 cod per pot 
(105.5 kg) in March 2004. Coefficients of variation for 
the raw catch data ranged from 64% for January 2004 
to 42% for March 2005. Coefficients of variation for 
cruise averages at individual stations ranged from 14% 
to 43%. The average catch rate changed substantially 
between January and March in each year of the study, 
