288 
Fishery Bulletin 106(3) 
Table 1 
Dates and summary statistics for cruises conducted during the Bering Sea localized depletion experiment for Pacific cod ( Gadus 
macrocephalus ). Each year’s experiment included a “before” cruise in early January (before the trawling season) and an “after” 
cruise in late March immediately after the main trawl harvest. 
Cruise dates 
Cruise purpose 
Number of 
days 
fishing 
Number of 
pots 
fished 
Average 
number of 
cod/pot 
Average 
weight (kg) 
of cod/pot 
30 Mar-25 Apr 2002 
Pilot study and tagging 
21 
703 
28.8 
103.4 
28 Dec 2002-8 Jan 2003 
Abundance experiment — before 
4 
160 
22.2 
104.4 
4-17 Feb 2003 
Tagging 
11 
336 
22.3 
78.3 
12-31 Mar 2003 
Abundance experiment — after 
14 
475 
8.8 
27.7 
2-10 Jan 2004 
Abundance experiment — before 
9 
360 
19.3 
93.3 
15-31Mar 2004 
Abundance experiment — after 
15 
604 
31.7 
105.5 
9-22 Mar 2005 
Abundance experiment — before 
14 
481 
21.2 
85.3 
16-29 Mar 2005 
Abundance experiment — after 
14 
500 
25.7 
95.4 
for the three study years were 0.70, 0.92, and 0.81, 
respectively. Although the range and mode of the ob- 
served d’s changed from year to year, in each year the 
distribution of 5’s over stations within the two zones 
was very similar (Fig. 4). 
Power simulations gave us confidence that the strong 
failure to reject the null hypothesis in 2004 and 2005 
reflects a true absence of a treatment effect in the study 
area. Power was poor in 2003 because of the low sample 
size and higher variability of the data; only imposed 
fishing effects of 50% or more would have given a high 
probability of correctly detecting a difference between 
treatment and control groups. In 2004 and 2005, how- 
ever, full replication of the experiment resulted in good 
power. We were able to detect differences at 25-30% 
fishing effects in the 2004 experiment and at as low as 
20% fishing effects in 2005. 
We verified that substantial fishing removals occurred 
within the trawled portion of the study area during our 
experiment. The reported January-March fish harvest 
data from the NMFS Alaska Regional Office indicated 
that the harvest of Pacific cod by bottom trawl gear in 
the two federal reporting areas around Cape Sarichef 
was on the order of 25,000 metric tons (t) per year. 
Based on observed hauls, approximately 45% of the 
total harvest in these two reporting areas came from 
the study area (Table 2). 
Auxiliary biological and tagging data were useful for 
interpreting our results. Sex ratio and length-frequency 
data collected during each cruise indicated that the 
population of fish sampled sometimes changed substan- 
tially, even within the two-week duration of a study 
cruise. Sex and maturity class data from systematic 
subsamples of Pacific cod collected during the March 
2004 cruise are shown in Figure 5. Although shifts 
from developing and prespawning stages to ripe and 
spent stages may reflect seasonal maturation of indi- 
vidual fish, differences in the proportion of mature to 
immature fish can only be explained by movement of 
Table 2 
Estimation of the Pacific cod ( Gadus macrocephalus) har- 
vest from the study area. Total Pacific cod catch during 
the winter trawl season in each year is shown for National 
Marine Fisheries Service reporting areas 509 and 517, 
which included the study area. Catch data from observed 
hauls were used to estimate the proportion of cod catch 
taken from a 1° latitude-longitude block around the study 
area; this proportion applied to the total harvest was used 
to show the approximate harvest (in metric tons) from the 
study area during the experiment. 
Estimated harvest (metric tons) 
taken with bottom trawl gear 
Year 
Reporting areas 
Study area 
Proportion 
2003 
20,971 
10,215 
48.7% 
2004 
25,158 
12,295 
48.9% 
2005 
29,870 
13,882 
46.5% 
fish into and out of the study area during the two-week 
period of the cruise. The overall sex ratio for the 15-22 
March samples was 0.93 (males/female), whereas the 
sex ratio for 23-20 March was 0.75. This difference 
represents a significant change in sex ratio (P=0.0004) 
over the two-week period. 
Qualitative tagging studies were conducted concur- 
rently with the localized depletion experiment. Although 
these studies are to be reported separately, some of 
their results help to explain the outcome of the localized 
depletion experiment. Partial results from the February 
2003 tagging studies indicated substantial movement 
of Pacific cod, both over spans of several months and 
over shorter time scales (Table 3). Over 70% of the 
fish released in the study area in February 2003 and 
recovered within two weeks of release were recaptured 
