281 
Abstract — Groundfish fisheries in 
the southeast Bering Sea in Alaska 
have been constrained in recent 
years by management measures to 
protect the endangered Steller sea 
lion ( Eumetopias jubatus). There is 
concern that the present commercial 
harvest may produce a localized deple- 
tion of groundfish that would affect 
the foraging success of Steller sea 
lions or other predators. A three- 
year field experiment was conducted 
to determine whether an intensive 
trawl fishery in the southeast Bering 
Sea created a localized depletion in 
the abundance of Pacific cod (Gadus 
macrocephalus). This experiment 
produced strongly negative results; 
no difference was found in the rate 
of seasonal change in Pacific cod 
abundance between stations within a 
regulatory no-trawl zone and stations 
in an immediately adjacent trawled 
area. Corollary studies showed that 
Pacific cod in the study area were 
highly mobile and indicated that the 
geographic scale of Pacific cod move- 
ment was larger than the spatial scale 
used as the basis for current no-trawl 
zones. The idea of localized depletion 
is strongly dependent on assumed spa- 
tial and temporal scales and contains 
an implicit assumption that there is 
a closed local population. The scale of 
movement of target organisms is criti- 
cal in determining regional effects of 
fishery removals. 
Manuscript submitted 6 June 2007. 
Manuscript accepted 28 March 2008. 
Fish. Bull. 106:281-292 (2008). 
The views and opinions expressed 
or implied in this article are those 
of the author and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
Effects of commercial fishing on local abundance 
of Pacific cod ( Gadus macrocephalus ) 
in the Bering Sea 
M. Elizabeth Conners (contact author) 
Peter Munro 
Email address for M. E. Conners: Liz.conners@noaa.gov 
National Marine Fisheries Service 
Alaska Fisheries Science Center 
7600 Sand Point Way NE 
Seattle, Washington 98115-6349 
As fishery management in the United 
States moves toward an ecosystem 
approach, both the direct and indi- 
rect effects of commercial harvests on 
marine food webs are being considered 
(Schoener, 1993; Murawski, 2000). 
Localized depletion is the hypothesis 
that intense fishing pressure may 
cause small-scale effects on local den- 
sities of the target fish — effects that 
are disproportionate to the managed 
overall harvest mortality rate. The 
possibility of localized depletion has 
been considered in developing and 
implementing regulations to reduce 
the jeopardy of mortality of Steller 
sea lions ( Eumetopias jubatus) due 
to fishing in the Bering Sea. In 1997, 
the western stock of Steller sea lions 
was listed as endangered under the 
U.S. Endangered Species Act. Because 
the period of decline in sea lion num- 
bers coincides with the period of rapid 
expansion of U.S. domestic fisheries in 
Alaska (Braham et al., 1980; Hiatt, 
2005), there has been concern that 
commercial fisheries may indirectly 
affect sea lion abundance through 
competition for available prey (Alver- 
son, 1992; Fritz et al., 1995). 
During their consultations regard- 
ing the U.S. Endangered Species 
Act, scientists of the National Ma- 
rine Fisheries Service (NMFS) cited 
the federally managed fishery for Pa- 
cific cod (Gadus macrocephalus) as 
one with a potential to adversely af- 
fect Steller sea lions. Pacific cod is 
the third-most valuable commercial 
fish species in the United States; the 
value of the 2006 harvest was esti- 
mated at $197 million. Approximately 
40% of Pacific cod catch is taken by 
trawling in the southeastern portion 
of the Bering Sea during the winter 
“A” season, from 20 January through 
31 March (Thompson and Dorn, 
2005). The trawl fishery has histori- 
cally been concentrated on an area of 
the continental shelf north of Unimak 
Island, where Pacific cod form dense 
spawning aggregations during the 
winter season. 
Although the Pacific cod population 
in the eastern Bering Sea is managed 
at sustainable overall harvest levels, 
intensive trawl fishing in a smaller 
area has been suspected of causing 
short-term, small-scale impacts on 
fish abundance (localized depletion) 
that are disproportionate to the over- 
all harvest rate. Particularly worri- 
some is the possibility that fishing 
mortality rates in the heavily trawled 
area may be much higher than those 
for the total region, and that this 
disproportionate mortality may cause 
reduced availability of Pacific cod as 
prey for Steller sea lions (Fritz et al., 
1995). Evidence indicating such a lo- 
cal effect has been seen in commer- 
cial catch data in the eastern Bering 
Sea (Fritz and Brown, 2005). Under 
the assumption of reduced prey avail- 
ability from localized fishing, trawl 
exclusion zones have been placed 
around Steller sea lion rookeries and 
haulouts in western Alaska, and sea- 
sonal and spatial allocation of catch 
quotas have been used to disperse 
fishing effort. Our goal was to test for 
effects of localized depletion of Pacific 
cod caused by the Pacific cod winter 
trawl fishery. 
