Rooper and Martin: Comparison of indices of abundance with biomass estimates from trawl surveys 
33 
characteristics have been found to be related to rockfish 
growth and condition in other studies (e.g., Boldt and 
Rooper, 2009), a more direct measure of zooplankton 
abundance throughout the Gulf of Alaska would un- 
doubtedly be more useful in explaining rockfish catches 
than the proxy variable related to the depth of the 
thermocline. A measure of substrate type would also 
be useful in this type of modeling study, but this infor- 
mation is unavailable for most of the Alaskan seafloor. 
Knowing substrate type at each of the survey stations 
would improve the predictive ability of the models, es- 
pecially for those species whose presence or absence was 
not well determined (northern rockfish, dusky rockfish, 
and harlequin rockfish) because these are species that 
have a predilection for rocky, untrawlable habitats. 
Additional information on these two habitat variables 
(food availability and substrate type) are critical for 
improving future distribution modeling for rockfishes 
throughout their ranges and improving our ability to 
identify trends in rockfish population abundance from 
bottom trawl survey data. 
Conclusions 
In most age-structured models used for rockfish assess- 
ment in Alaska, the catchability parameter, q , can 
account for differences in the accessibility of a popula- 
tion to the bottom trawl survey gear. The fitted survey 
abundance trend in these stock assessments are usually 
a smooth time series of abundance, similar to what we 
observed in this modeling study. Thus, the habitat-based 
indices appear to result in the same smooth trends in 
the population as those determined with stock assess- 
