Echave et al.: Interdecadal change in growth of Anoplopoma fimbria in the northeast Pacific Ocean 
371 
Figure 5 
Comparison of time series of spawning biomass, measured 
in kifotons (kt), of female sablefish ( Anoplopoma fimbria ) in 
Alaska from the sablefish reference model (Hanselman et al., 
2006; shown as a solid line) and from the same model with 
updated growth parameters from our study (shown as line 
with open boxes). 
show that recent estimates of female spawning 
biomass from our updated growth data are slightly 
lower but appear to be rising at a steeper trajectory 
than recent estimates determined with Sasaki’s 
growth data in the current model. 
Discussion 
Although a specific cause (changes in sampling 
method, environmental factors, or differences in 
fish abundance) and time of the changes observed 
in sablefish growth were not identified, these 
changes have occurred. The division of the sable- 
fish age-and-length data set into 2 growth regimes 
was not based on any detectable shift in growth but 
on a change in the sampling design of the longline 
survey. Separating the data into 2 time intervals 
might not completely capture the temporal pat- 
tern of changes in growth. Sablefish growth, for 
example, might have changed slowly, instead of 
in a stepwise fashion. However, we also analyzed 
growth data by individual years, and no obvious 
temporal patterns were noted. It is unlikely that 
other changes in the survey could explain appar- 
ent differences in growth over the time series because of 
the standardization in most other aspects of the survey 
design between the 2 periods (Hanselman et al., 2007). 
In addition, both the ages and the abundance indices for 
the sablefish stock assessment are treated as separate 
surveys (Japan-U.S. cooperative and domestic NMFS 
longline surveys) with different catchability values and 
sensitivities, and therefore updated growth estimates 
fitted to these 2 survey periods (which theoretically 
represent the 2 surveys) follow accordingly. 
Aging error could be a very plausible cause for see- 
ing changes in growth when in fact growth has not 
changed. Although Heifetz et al. (1999), among oth- 
ers, have validated the currently accepted aging prac- 
tices (Beamish and Chilton, 1982) and have examined 
sources of error in the aging of sablefish, there is still 
much disagreement on the possibility of obtaining reli- 
able ages from sablefish otoliths (Pearson and Shaw, 
2004). We feel aging error is an unlikely cause for the 
growth changes seen in this study. The NMFS stock 
assessment of sablefish in Alaska uses an aging error 
matrix, one with known ages that make it particularly 
realistic. Aging error should not have a major effect on 
growth estimation if the aging error is imprecise but 
not biased, and Heifetz et al. (1999) found bias to be at 
a minimum for younger ages when most growth is oc- 
curring. In addition, otoliths for Alaskan sablefish have 
been read consistently by the same age-reader using the 
same protocol during the timeline of this study and age- 
reader agreement tests have been in place throughout 
that entire time thus, removing the possibility of an 
age-reader effect. 
The best documented causes for change in growth 
of various fish species (juvenile sablefish; Pacific hali- 
but; yellowtail flounder [ Limanda ferruginea]\ haddock 
[Melanogrammus aeglefinus]; and Pacific chub mack- 
erel [Scomber japonicas ) have been density dependence 
and environmental conditions (Ross and Nelson, 1992; 
Clark et al., 1999; Wilson, 2000; Sogard and Olla, 2001; 
Watanabe and Yatsu, 2004). In our study, we did find 
evidence that changes in growth may be the result 
of intraspecific density-dependent mechanisms. It ap- 
pears that sablefish growth is most influenced by fish 
density that fish are exposed to while in the larval and 
juvenile stages. This response in turn is linked highly 
to favorable environmental conditions for recruitment 
and YOY survival (McFarlane and Beamish, 1992; Si- 
gler and Lunsford, 2001; Sogard and Olla, 2001). Re- 
sults of our growth analysis show that sablefish from 
the more recent time period of our study (1996-2004), 
when compared to sablefish from earlier time period of 
our study (1981-93), exhibited faster growth rates and 
reached larger sizes-at-age as biomass steadily declined 
(Hanselman et al., 2007). Although the Alaskan sable- 
fish population is considered to be at a sustainable, 
healthy level and is neither overfished nor approaching 
an overfished level, it is by no means close to its peak 
abundances of the late 1980s and early 1990s (Hansel- 
man et al., 2010). Across the time series, abundance 
of Alaskan sablefish was characterized by relatively 
consistent high values (e.g., age-4+ abundance of 489 
kt in 1986) during the early period of this study and 
consistently lower values (e.g., age-4+ abundance of 223 
kt in 2000) during the more recent period (Hanselman 
et al., 2010). Since 1988, abundance has decreased sub- 
stantially, whereas growth has increased significantly 
(Hanselman et al., 2007). 
Although no direct relationships were observed be- 
tween sablefish growth and any of the tested environ- 
mental factors, it is important to note that evaluating 
