Conners and Munro: Effects of commercial fishing on local abundance of Gadus macrocephalus in the Bering Sea 
289 
more than 18.5 km (10 nmi, the radius of 
the notrawl zone) from their release loca- 
tion. The majority of fish recaptures took 
place east and slightly north of the release 
site within the Cape Sarichef notrawl zone, 
and only a few recaptures were documented 
for the study area immediately outside the 
notrawl zone. 
Discussion 
The results of our experiment were ex- 
tremely clear. Although the direction and 
magnitude of the net seasonal change in 
abundance differed between study years, in 
each study year direction and magnitude of 
the percentage change (<5) were similar in 
the trawled and untrawled areas. Nonpara- 
metric tests used to compare <3’s from the 
two zones consistently had P-values over 
70%, indicating that there was no evidence 
that the distribution of the two groups dif- 
fered — a conclusion that is evident without 
any statistical testing simply by compar- 
ing the frequency distributions of the <5’s in 
the trawled and untrawled zones (Fig. 4). 
Analysis of the raw catch data with linear 
models leads to the same conclusion. Checks 
for more subtle indications of fishing effects, 
such as spatial patterns in catch within the 
study region or temporal trends within each 
two-week cruise, were also negative. These 
experimental results are inconsistent with 
the hypothesis of strong stationary localized 
depletion at the scale of the existing notrawl 
zones in Alaska. 
Power simulations indicated that the fail- 
ure to see a difference between trawled and 
untrawled areas was not simply due to a 
lack of resolution in the data. Although 
the sample size for 2003 was low, results 
for 2004 and 2005 showed that fishing re- 
movals that resulted in a 20-30% decline 
in catch rates in the trawled area would 
have been detected. There is little infor- 
mation on the size and duration of prey 
density decreases that would be necessary 
to impact Steller sea lion foraging success. 
It is possible that Pacific cod abundance in 
the study area was so high that even the 
substantial fishing removals resulted in 
a <20% change in local Pacific cod abun- 
dance. If this is the case, then the question 
becomes one of whether or not such small 
changes in prey availability would signifi- 
cantly affect Steller sea lion foraging. 
Data from the auxiliary tagging and biological 
ies clearly indicated that Pacific cod in our study 
were highly mobile over much shorter time scales 
A 2003 
Trawled 
Untrawled 
- 100 % 
-25% 
-50% -125% -200% -500% 
C 2005 
Figure 4 
Frequency distribution of seasonal percent change in abundance (d) 
for the Cape Sarichef localized depletion study. Dark bars are frequen- 
cies of 6 for untrawled stations inside the notrawl zone, and hatched 
bars represent trawled stations outside the notrawl zone boundary: 
(A) 2003, (B) 2004, and (C) 2005. P-values for the nonparametric 
rank-sum test are shown in each figure. 
previously assumed. The stationary localized depletion 
stud- scenario is based on the assumption of a closed local 
area pool of fish that is reduced by local removals. Both the 
than tagging data and the observed shifts in maturity and 
