Conners and Munro: Effects of commercial fishing on local abundance of Gadus macrocephalus in the Bering Sea 
291 
Table 3 
Partial results from tagged Pacific cod ( Gadus macrocephalus ) released near Cape Sarichef in February 2003, showing net move- 
ment of tagged fish. Table columns show distance travelled (km) between release and recapture points of a tagged fish. Table rows 
show weeks at liberty (the time elapsed between release and recapture dates). Table values show the percentage of recovered tags 
for which the distance travelled is within the specified range. 
Distance (km) 
Number of 
Weeks at liberty 
<9.3 
9.3-18.5 
18.5-37.0 
37.0-74.1 
74.1-148.2 
>148.2 
tags recovered 
<2 
12.2% 
15.1% 
25.9% 
18.0% 
3.6% 
25.2% 
139 
2-4 
7.0% 
2.3% 
9.3% 
34.9% 
23.3% 
23.3% 
43 
OO 
1 
2.9% 
5.9% 
5.9% 
26.5% 
32.4% 
26.5% 
34 
8-16 
18.8% 
12.5% 
6.3% 
25.0% 
6.3% 
31.3% 
16 
13 February through 24 March 2001 as localized deple- 
tion due to fishing removals. The models used in the 
Fritz and Brown study (the models of Leslie and Davis, 
1939, and DeLury, 1947) are based on the assumptions 
of a closed population and constant catchability. These 
models are unable to distinguish between changes in 
abundance due to fishing removals and those due to 
fish dispersal or movement across the boundaries of 
the study area. If the assumption of a closed popula- 
tion is true, then declining CPUE would indicate a 
regional-scale effect (conjecture two). Given the high 
mobility indicated in Table 3, however, we doubt that 
the model assumptions are met for Pacific cod. Under- 
standing patterns in Pacific cod abundance must take 
into account both substantial short-term movement and 
seasonal processes of migration and aggregation related 
to spawning. 
The formal statistical inference presented in the pres- 
ent study applies only to the study area; extending this 
inference to other areas is reasonable but can only be 
considered a qualitative exercise. As with many com- 
parative environmental studies, this project included 
only two experimental units, in the sense that the treat- 
ment (trawling) was applied to one region and the other 
region (notrawl zone) was used as a control. Hurlbert 
(1984) pointed out that, in an observational study with 
only one treatment and one control area, a statistical 
test constitutes evidence only for a difference between 
the two observed areas. The observation of an effect 
(or lack of effect) must be combined with biological 
knowledge of the system to extend inference from the 
observed areas to other parts of the system. In the case 
of Cape Sarichef, the experimental area was selected 
not as a representative location for the entire Bering 
Sea, but as the location where trawl fishing was most 
intensive and most likely to produce measurable local 
effects. Qualitative inference to other areas will require 
consideration of similarities in fishing pressure, Pacific 
cod behavior, and Pacific cod movement. 
Localized depletion has not been widely discussed 
in the scientific literature. It has been proposed as a 
mechanism primarily in coral and sedentary benthic 
species (Gorfine et al., 2001; Jamieson, 2001; Harriott, 
2003; Smith et al., 2004). Our results demonstrate that 
the mobility of the target organism must be considered 
in looking for localized spatial effects on groundfish. 
If localized depletion is to occur, it will result from 
the interaction of fishing pressure, fish abundance, 
and fish movement. To evaluate the impacts of fishery 
removals on other predators, such as Steller sea lions, 
the relevant scales of fishing, fish movement, and preda- 
tor feeding must be clearly defined and understood. 
For Pacific cod, the very small spatial scale associated 
with the current regulatory notrawl zones appears to 
be smaller than the relevant scale of fish movement. 
Potential fishery effects at broader spatial and temporal 
scales may be more appropriately addressed by continu- 
ing to manage seasonal and spatial dispersal of the 
Pacific cod harvest. 
Acknowledgments 
Funding for this project was provided by the National 
Oceanic and Atmospheric Administration, National 
Marine Fisheries Service (NMFS). S. Neidetcher of the 
Alaska Fisheries Science Center (AFSC) and O. Ormseth 
of the University of Alaska were instrumental in organiz- 
ing and conducting field work for this project. We thank 
the captains and crews of the charter vessels who partic- 
ipated in this research and the many NMFS employees 
and associates who braved the Bering Sea in winter to 
help us perform the experiment. We thank L. Fritz, W. 
Stockhausen, and D. Somerton of the AFSC for prelimi- 
nary reviews of the manuscript. Three anonymous review- 
ers also contributed improvements to the manuscript. 
Literature cited 
Alverson, D. L. 
1992. A review of commercial fisheries and the Steller 
sea lion Eumetopias jubatus: The conflict arena. Rev. 
Aquat. Sci. 63(3-4)203-256. 
