Kimura et al.: Stock structure and movement of Anoplopoma fimbria 
479 
migrating sablefish taking advantage of enhanced 
feeding opportunities presented by strong upwelling 
or are they displaced sablefish subject to irregular but 
recurring episodes of ocean-forcing events and passively 
entrained in strengthened currents southward? 
Dorn (1992) noted two oceanographic phenomena 
that make west coast waters a highly productive re- 
gion: coastal upwelling and the advection of cool, zoo- 
plankton-rich water from the Alaska Subarctic Gyre. 
These phenomena could be distinguished (Roesler and 
Chelton, 1987) and Hollowed and Wooster (1992) have 
pointed out that they are positively correlated. 
Hollowed and Wooster (1992) noted that climate 
and ocean circulation in the northeast Pacific oscil- 
lates between a type-A pattern (i.e. weak Aleutian 
Low and Gulf of Alaska circulation, intensified Cali- 
fornia Current, and strong upwelling off the west 
coast) and type-B ENSO-associated conditions (i.e. 
intensified winter Aleutian Low and Gulf of Alaska 
circulation, weak California Current, and weak 
coastal upwelling). It is the more prevalent type-A 
scenario, with a strong California Current, that pos- 
sibly facilitates the migration of sablefish south. 
Thus, the strength of migration from Alaska to the 
west coast would be negatively correlated with ENSO 
events. Dorn (1995) concluded that increased north- 
ward movements of Pacific whiting along the west 
coast during El Nino years were due to intensified 
northward currents. Whether or not a subset of 
Alaska sablefish follow enhanced feeding gradients, 
a strong California Current could deliver them to the 
west coast. 
Because the California Current is a surface cur- 
rent and because there exists a natural boundary 
between the California Current and the Alaska Gyre, 
it is somewhat speculative to claim that a strong 
California Current plays an important role in the 
migration of Alaska sablefish to the west coast. Nev- 
ertheless, Alaska sablefish do make this migration, 
and the migrations coincide with an enhanced Cali- 
fornia Current and increased upwelling. It would 
seem difficult to conclude that these phenomena are 
totally unrelated. 
One difficulty in discussing sablefish migration is 
that we cannot be sure that the larger fish found 
along the northern portion of the west coast are of 
Alaskan origin. A partial explanation could be that 
the strong upwelling zone from Cape Mendocino to 
Point Conception in central California, as measured 
by Ekman transport (Parrish et al. 1981), represents 
a physical barrier to larger (Alaska?) sablefish. This 
barrier seems plausible because large sablefish be- 
come relatively rare in the length frequencies for the 
areas where upwelling intensifies. The second part 
of this theory would tell us whether Alaska sablefish 
are able to reproduce in waters off Washington and 
Oregon. It is possible that the gross numbers of 
Alaska sablefish migrating to the west coast are not 
substantial enough to affect length frequencies di- 
rectly but that the numbers may be substantial 
enough to affect the population genetically over a 
much broader geographic range. Bakun (1996) ar- 
gued that the Southern California Bight and the 
Columbia River Plume offer two locations where fish 
larvae can survive the disruptions of a strong up- 
welling environment. These locations may corre- 
spond to those for northerly and southerly west coast 
sablefish populations. 
However, Moser et al. ( 1994) believed that although 
sexually mature and spawning sablefish are encoun- 
tered as far south as Baja California, virtually none 
of their reproductive potential survives below the 
Southern California Bight and they do not contrib- 
ute to the standing stock in any meaningful way. They 
concluded that it is the advection of eggs and larvae 
from the north that ensures the stability of the popu- 
lation south of Pt. Conception. This hypothesis, that 
sablefish abundance off the west coast (and particu- 
larly California) is dependent on a steady “leakage” 
of eggs, larvae, or juvenile fish from more northern 
center) s) of abundance, parallels the hypothesis that 
Asian stocks are dependent on the migration of adult 
fish from northern and eastern areas of the Pacific. 
Although the appearance of northern fish in the 
mixed-zone off the Washington-Oregon coast is well 
established, we cannot yet quantify the net contri- 
bution of immigrants. Also, we found little evidence 
of tagged adult fish migrating from areas north of 
central California to southern and Baja California. 
However, this does not preclude Moser et al.’s ( 1994) 
conjecture that the southern California population 
is dependent on eggs and larvae advected from the 
north. The dependence of sablefish stocks on spawn- 
ing populations appears not to be well understood. 
Acknowledgments 
We thank Heather A. Parker, LTJG/NOAA, Pacific 
Fisheries Environmental Group, for providing data 
on the Bakun upwelling indices, and Sandra A. Lowe 
for providing catch data and a careful review. Jef- 
frey T. Fujioka, Anne B. Hollowed, and William T. 
Peterson (Northwest Fisheries Science Center, New- 
port Laboratory) provided extremely helpful inter- 
nal reviews. We would like to give special thanks to 
three referees who pointed out numerous errors in 
our manuscript. One referee in particular shared a 
great deal of his or her knowledge of sablefish biol- 
ogy and bathymetry. Because of divergent views ex- 
