Tribuzio et al.: Age and growth of Squalus acanthias in the Gulf of Alaska 
133 
The relatively large variability in size at age of spiny 
dogfish in the GOA could also reflect true underlying 
variability in growth rates. Individuals experiencing 
different thermal and feeding histories are expected to 
have different growth characteristics. It is also conceiv- 
able that our samples represent collections of dogfish 
from multiple, mixed populations. For instance, 4 of 
2940 recoveries (0.14%) of spiny dogfish tagged in Brit- 
ish Columbia were recovered in Alaska (McFarlane and 
King, 2003). Because the movements of spiny dogfish 
from other areas to and from Alaska are unknown, 
the degree of mixing is uncertain. However, there is 
no evidence of genetic differentiation in the Northeast 
Pacific based on analyses of eight microsatellite loci 
from dogfish sampled from the Bering Sea, the Gulf of 
Alaska, Strait of Georgia, Puget Sound, and the coasts 
of Washington, Oregon, and California (Hauser, 2009). 
Mixtures of spiny dogfish from other areas with growth 
characteristics that are different from those of Alaska 
resident dogfish could contribute to the variability in 
size at age that we observed in the GOA. Nevertheless, 
the existence of a statistically significant difference in 
growth rates from different areas of the Northeast Pa- 
cific (Vega, 2006; Table 4 this document) indicates that 
mixing is incomplete. 
Our findings have at least two important implications 
for management of the species. First, for estimation of 
stock productivity and biological reference points for 
spiny dogfish in the GOA, it is important to use growth 
curves that are fitted to size-at-age data from dogfish 
captured in the GOA. Although alternative growth mod- 
el parameters were not statistically significantly differ- 
ent from one another in our study, the variation among 
predicted length may be of biological significance. For 
instance, the worn-band estimation curves for the GOA 
and British Columbia resulted in very different esti- 
mates of ages (Fig. 5); use of growth curves for British 
Columbia would result in estimated numbers of worn 
bands from dogfish spines in the GOA that would be 
biased low. For example, for a spiny dogfish with a 
1.8-mm EBD, the GOA model would estimate an age 
of one year, whereas both of the British Columbia mod- 
els would estimate an age of four years. A fish with a 
6-mm EBD would be estimated to be 30 years old by the 
GOA model and 46 and 37 years old by the two British 
Columbia models. Such biases in growth estimates may 
lead to biases in estimates of biological reference points 
for fishery management. 
Second, as in other portions of their range, the largest 
spiny dogfish are the oldest females. Because commer- 
cial fisheries for spiny dogfish select for the largest in- 
dividuals, fishing mortality rates are disproportionately 
higher for this reproductive segment of the population. 
In the Northwest Atlantic Ocean, a sharp increase in 
landings during 1987-1993 led to a fivefold increase in 
fishing mortality rates on females from 0.016 to 0.26; 
and fishing mortality rates exceeding 0.10 on large 
(a80-cm) females resulted in negative pup replace- 
ment, subsequently leading to stock decline (Rago et 
al., 1998). Thus, to sustain spiny dogfish in the GOA, 
fishery management plans should consider not only 
slow growth rates, low fecundity, and late maturation 
of this species (King and McFarlane, 2003), but also 
the potentially disproportionate number of removals of 
mature females from the stock by commercial fishing by 
estimating size- and sex-specific fishing mortality rates 
and biological reference points. 
Future research should address the many uncer- 
tainties remaining about spiny dogfish biology and 
life history in Alaska. In particular, results from this 
study indicate several areas of research needed to 
improve our understanding of spiny dogfish age and 
growth. First, although demonstrated for fish captured 
off British Columbia (Beamish and McFarlane, 1985; 
McFarlane and Beamish, 1987; Campana et al., 2006), 
validation of annual band formation, as well as worn- 
band properties, for spiny dogfish collected from the 
GOA should be conducted to describe potential sources 
of bias in the age estimates for spiny dogfish at this 
northern portion of their range in the Pacific Ocean. 
Second, the collection of juvenile dogfish (<50 cm) is 
needed to provide more precise estimates of growth 
over their full life history, as well as to help identify 
statistically best-fit growth models. Third, tagging 
studies, such as those conducted in British Columbia 
(King and McFarlane, 2003), would help elucidate the 
degree to which dogfish in Alaska represent mixed 
stocks with different growth attributes; such tagging 
results would help to delineate stock boundaries essen- 
tial for fishery management. Fourth, controlled experi- 
ments are necessary to fully examine the selectivity of 
various fishing gears for spiny dogfish by size and sex. 
This would be an important preliminary step toward 
gear standardization, if long-term sampling programs 
are envisioned for spiny dogfish. Finally, continued 
sampling of spiny dogfish over small regional scales 
is necessary to fully evaluate potential geographic 
differences in growth and resultant parameters (i.e., 
natural mortality) within the GOA, as well as to more 
broadly understand the life history of this species in 
this portion of its range. Although our study would 
not have been possible without the diversity of low- 
cost sampling opportunities afforded to us, including 
the valuable assistance of state and federal agencies 
and sport and commercial fishermen, further progress 
will be accelerated by a full-scale, directed field pro- 
gram, which would be more successful at providing 
an unbiased sample set of spiny dogfish in the waters 
off Alaska, and which would aid in efforts to build a 
more detailed stock assessment, and thus models of 
population dynamics. 
Acknowledgments 
We are grateful for funding of this research by the 
North Pacific Research Board (NPRB publication no. 
227), the Rasmuson Fisheries Research Center, and the 
Alaska Fisheries Science Center’s Population Dynamics 
Fellowship through the Cooperative Institute for Arctic 
