Factors affecting the availability of walleye pollock to acoustic and bottom trawl survey gear
Abundances of semi-pelagic fish are often estimated using acoustic or bottom trawl surveys, both of which sample only a fraction of the water column. Acoustic instruments are effective at sampling the majority of the water column, but they have a near-surface blind zone and a near-bottom acoustic dead zone (ADZ), where fish remain undetected. Bottom trawls are effective near the seabed, but miss fish that are located above the effective fishing height of the trawl. Quantification of the extent of overlap between these gears is needed, particularly in cases where environmental factors play a role. We developed logistic regression models to predict the availability (qa) of walleye pollock (Gadus chalcogrammus) to both acoustic and bottom trawl gears using factors shown to affect qa (depth, light intensity, fish length) and introducing additional factors (tidal currents, surface and bottom temperature, sediment size). Results build on earlier studies and quantify the uncertainty associated with the estimation of the ADZ correction using Bayesian methods. Our findings indicate that on average during the day, walleye pollock are more available to the bottom trawl than to the acoustics. Availability to both gears depends mostly on bottom depth, light conditions, and fish size, and to a lesser extent sediment size. Availability to the acoustic gear is also related on surface temperature. Variability in availability to both gears also depends on environmental factors.