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Fishery Bulletin 106(3) 
In order to test for the presence of localized depletion, 
the temporal and spatial scales of the presumed ef- 
fects must be specified. Localized depletion is a general 
term that could encompass several types of interactions. 
Short-term movement rates of the target species in re- 
lation to the scale of the fishery are a critical factor in 
determining fishing effects. To illustrate, we offer three 
conjectures on fishery interaction, where the dynamics 
of harvest and fish movement result in different effects 
on fish abundance. 
For the first conjecture, harvest results in a localized 
reduction in fish abundance in the immediate vicinity 
of fishing. This reduction remains geographically stable 
for some period of time. We refer to this effect as sta- 
tionary localized depletion. This form of depletion may 
be envisioned as the action of a dipper to remove cer- 
tain amount of mud from a bucket; a hole or depression 
would remain where the mud was removed, eventually 
filling in but persisting long enough to be observable. 
Implicit in such stationary localized depletion is the 
notion that movement by the fish is on a scale smaller 
than that of the fishery; therefore it does not obscure 
the geographic imprint of the removal. 
For the second conjecture, the movement of the fish 
interacts with a locally intense harvest. In this case, 
short-term fish movement occurs on a geographic scale 
greater than that of the fishery and the effect of the 
removal would quickly dissipate over the area occupied 
by the fish. This form of depletion would more closely 
resemble the action of ladle in dipping water out of a 
bucket, with no persistent depression left behind by 
the ladle. On the scale of the ladle, the effect is transi- 
tory and there is no apparent localized depletion, even 
though there is a measurable removal on the scale of 
the bucket. 
For the third conjecture, the short-term fish move- 
ment includes a net flow in one direction. In this sce- 
nario, fishing effects would show as a change to the 
flow of fish or as an area of reduced abundance that is 
displaced downstream. This effect would be similar to 
intercepting part of a flowing stream with a dip net. 
Combinations of random and directed short-term move- 
ment may also produce effects that are both dispersed 
and spatially displaced from the location of harvest. 
In the specific case of Bering Sea Pacific cod, the 
scale of fishery harvest is known from commercial fish- 
ing data, but the scale of Pacific cod movement is poorly 
understood. From an early tagging study (Shimada 
and Kimura, 1994), it was found that Pacific cod make 
large-scale movements over the eastern Bering Sea on 
a seasonal basis. Short-term movement dynamics of 
Pacific cod, on the other hand, are only beginning to be 
studied (Nichol and Chilton, 2006). Assumed scales of 
Steller sea lion foraging are based largely on satellite 
tagging studies of adult female and juvenile Steller sea 
lions (Merrick and Loughlin, 1997). Measures enacted 
to protect Steller sea lions include trawl exclusion zones 
18.5 or 37.0 km in diameter around Steller sea lion 
haulouts and rookeries. Use of trawl exclusion zones 
to reduce competition between Steller sea lions and 
trawlers implies an assumption of stationary localized 
depletion at this scale. Thus, under the current regula- 
tory framework, the general question of whether fishing 
causes some form of localized depletion becomes much 
more specific, namely that of whether or not trawling 
causes a stationary localized depletion on the geograph- 
ic scale of the trawl exclusion zones. Our experiment 
was therefore designed to address the specific mecha- 
nism that causes stationary localized depletion, and 
we assumed that this type of depletion would have the 
greatest potential for negatively affecting Steller sea 
lion abundance. 
Materials and methods 
Study area 
The study area was located in the southeast Bering Sea 
near the tip of the Alaska Peninsula (Fig. 1) — an area 
situated off Unimak Island at Cape Sarichef on the east- 
ern side of Unimak Pass, at depths of approximately 70 
to 110 m. This area is one of the most productive trawl- 
ing grounds in the Bering Sea, where fisheries focus a 
great deal of trawling effort for Pacific cod. The trawl 
exclusion zone around the Steller sea lion haulout at 
Cape Sarichef intersects this preferred trawling ground 
and the two areas provided us an opportunity to use a 
spatially adjacent treatment zone and control zone. The 
spatial scale of the experiment was determined by the 
18. 5 -km boundary of the Cape Sarichef trawl exclusion 
zone and the extent of preferred depths for trawl fishing. 
The temporal scale, the length of time an effect must 
persist to be observable, was taken to be approximately 
two weeks, which was the time required to conduct each 
phase of the research fishing. 
Research pot gear 
We used the catch of standardized pot gear as an index 
of local Pacific cod abundance. Pot gear is widely used 
in commercial fishing for both crab and Pacific cod in 
the eastern Bering Sea and Gulf of Alaska. Research 
catches were based on catch from pots, even though the 
fishery effect being tested for would have been due to 
trawling, because pots provide large sample sizes and 
can be deployed at a very high spatial resolution. Pot 
catches cannot be easily or reliably used to estimate 
absolute abundance of fish, but provide a consistent 
index of relative abundance that can be used to make 
statistical comparisons between different survey areas 
or times. 
Commercial pots were used during initial feasibil- 
ity and pilot studies for this project; a standardized 
research pot was then developed and used for the ex- 
periment. The research pots were slightly larger than 
most commercial pots, and had a smaller net mesh and 
modified tunnel openings: they were 2.3 m by 2.3 m by 
1.2 m, and had 5-cm stretched mesh and two entrance 
tunnels, each with 68-cm by 23-cm tunnel openings. 
