Rooper and Martin: Comparison of indices of abundance with biomass estimates from trawl surveys 
31 
Most of these variables were proxies for the actual 
habitat characteristics that could be measured, such as 
the thermocline depth-to-bottom-depth ratio as a proxy 
for water column production. Some of the other variables 
were not directly collected at the bottom trawl location, 
such as local slope (taken from depth contours). Direct 
measurement of the important variables would no doubt 
have improved the ability of the models to predict rock- 
fish abundance and presence or absence. Additionally, 
there were probably some habitat processes that were 
missing from the analyses because there was a lot of 
unexplained variability in all of the models. Some of 
the habitat variables did not perform as expected. In 
particular, shrimp abundance was negatively correlated 
to the abundance of two of the three shrimp-eating spe- 
cies. Thermocline depth-to-bottom-depth ratio was also 
negatively correlated with the abundance of three of the 
five species examined and insignificant for an additional 
species. These were disappointing results in that it was 
expected that the proxies for prey abundance would be 
positively correlated to abundance. There are two poten- 
tial explanations for this disagreement. It is likely that, 
for example, the habitat preference for shrimp may not 
be the preferred habitat for shortraker and rougheye 
and blackspotted rockfish. It is also true that trawl sur- 
vey stations where the water column was more evenly 
mixed occurred away from the continental shelf break 
(in more nearshore areas); whereas most of the species 
