Nichol and Somerton: Seasonal migration of mature males of Chionoecetes opilio in the eastern Bering Sea 
323 
graphic differences, however, migration habits among 
large MM males in the 2 regions appear to be quite 
similar. In both locations, large MM males migrate to 
deeper water after their terminal molts (Ernst et ah, 
2005), make limited seasonal migrations back inshore, 
overlap the distribution of mature multiparous females 
during the spring mating season, and exhibit size de- 
pendence in regard to migration distance. 
Assuming that large MM males seasonally migrate 
for the purpose of mating, we believe the extent and 
timing of their migrations is likely to depend on both 
the distribution of mature females and the seasonal 
timing of their reproductive cycle. Snow crab distri- 
butions have been shown to contract northward after 
years of warmer bottom temperatures (e.g., 1975-1979; 
Orensanz et al., 2004), and to shift back to the south 
(Turnock and Rugolo 4 ) after a series of more recent 
colder-than-average years (2007-2010) and more exten- 
sive cold pools (Stabeno et ah, 2012). The tagged MM 
males in this study were, therefore, at liberty during a 
relatively cold period. As indicated in this study, mul- 
tiparous females were in close proximity to the tagged 
males, but perhaps, when waters are warmer, a shift in 
male and female distributions may necessitate longer 
migrations for mating. Colder temperatures also delay 
seasonal embryo hatching (Moriyasu and Lanteigne, 
1998; Webb et al., 2007) and consequently subsequent 
mating, and therefore may result in earlier occurrence 
of migrations during warmer years. 
Although the tagging data indicate timed migra- 
tions across depth gradients, and therefore inshore 
and offshore movements, we could not resolve move- 
ments that may have occurred along depth isobaths 
and therefore could not describe distinct migration 
routes for individuals. Another limitation of our study 
is that, although data from the NMFS bottom trawl 
surveys provided a clear pattern for the summer dis- 
tribution of mature females, we did not have similar 
information collected during the mating period. Simi- 
larly, the distributional data for small MM males was 
limited to summer samples, and their overlap (or lack 
of) with tagged males during other seasons could not 
be determined. Additionally, although broad-scale dis- 
tributional differences were found among adolescent 
and MM males of different size classes and with dif- 
ferent shell conditions, annual differences in their 
distributions could not be determined because of a 
lack of annual CH-CW data needed to differentiate 
between adolescent and MM males. This shortcoming 
highlights the need for more comprehensive annual 
CH-CW collections. 
Because of the lack of a substantial inshore mi- 
gration of large MM males, it is important to rec- 
ognize the component of the population targeted by 
the fishery and the impact of the fishery on female 
mating dynamics and reproductive success. Because 
large commercial-size MM males remain on the outer 
shelf, the existence of a summer-winter spatial dif- 
ference among large males (including adolescents) on 
the outer shelf must, in part, be due to the annual 
removal of MM males by the fishery. The highest con- 
centrations of commercial-size males observed during 
the NMFS summer bottom trawl survey in 2010 were 
found near the Pribilof Islands on the middle shelf 
(Turnock and Rugolo 4 ), but our tag recoveries do not 
indicate that this finding is a result of migration of 
commercial-size MM males. Instead, these males must 
have been recently terminally molted MM males (i.e., 
new shell males) that had yet to migrate offshore, as 
well as adolescents. Again, considering what appears 
to be low relative abundance of large MM males on 
the outer shelf during summer, there must be some 
level of large MM male depletion during winter, and 
the winter fishery must be largely dependent on the 
annual recruitment of these new-shell MM males to 
the outer shelf. 
Because of significant removals of commercial-size 
MM males by the fishery on the outer shelf, female 
spawners could become more reliant on either small 
MM males or adolescent males for mating (see En- 
nis et al., 1990). As stated earlier, because pubescent- 
primiparous females reside on the middle shelf, they 
are likely to mate with small resident MM males. 
For multiparous females, which reside on the out- 
er shelf, their likely mates are the large MM males 
targeted by the fishery. However, because the fish- 
ery in the eastern Bering Sea typically occurs from 
January through March, just before multiparous mat- 
ing ( -March-July), the capture of large MM males 
would exclude them from multiparous mating. Con- 
sidering that recently terminally molted males can- 
not mate for perhaps 3 months (Conan and Comeau, 
1986; Paul et al., 1995; Sainte-Marie et al., 1999), 
even those large recently molted males that survive 
the fishery may not contribute to multiparous mating 
that year. 
Of concern here, other than the reliance of the fish- 
ery on only one pseudocohort (individuals that termi- 
nally molted in the same year; see Ernst et al., 2005), 
is whether the male sperm contribution to spawning 
is sufficient in terms of both quantity and quality. 
Are the numbers of mature males that remain after 
fishery harvesting abundant enough to provide the 
sperm reserves necessary for healthy clutch fertiliza- 
tion and subsequent robust populations? Preliminary 
research on spermathecal loads among primiparous 
and multiparous females in the eastern Bering Sea 
has shown that they were significantly lower than 
those of females in the Gulf of St. Lawrence (Rugolo 
et al. 1 ; Slater et al., 2010). Although there was no evi- 
dence of sperm limitation within the eastern Bering 
Sea stock, there is still a concern that the stock could 
be vulnerable to recruitment overfishing that results 
from reliance on a male-only fishery. The question 
remains, do multiparous females then become more 
reliant on stored sperm from pubescent-primiparous 
mating that occurred before their own offshore migra- 
tion, or do they become more reliant on sperm from 
small, noncommercial-size MM males (e.g., Ennis et 
al., 1988; 1990) and potentially bias the genetic pool 
