COMPARISON OF ACOUSTIC AND TRAWL METHODS FOR ESTIMATING DENSITY AND AGE COMPOSITION OF KOKANEE

Three acoustic and three trawl methods of estimating density and age c omposition of kokanee Oncorhynchus nerka were compared. Density estima tes from the three acoustic methods were significantly correlated with each other and with the trawl estimates. Age-classes 1-3 were not dis tinguishable from each other in acoustic records, but their combined p eak could be separated from that of age-0 kokanees on the target stren gth frequency distributions produced by both the dual-beam and the dec onvolution analyses. Dual-beam age composition estimates were signific antly correlated with deconvolution (r = 0.97) and trawl (r = 0.70) es timates. Deconvolution density estimates for age-1-3 fish were 34% hig her than the dual-beam estimates, but neither of the acoustic density estimates differed significantly from the trawl values due to higher v ariance associated with trawl estimates. Acoustic estimates of total d ensities were less consistent than those for age-1-3 fish because of t he problem of establishing a consistent lower threshold for age-0 fish . At the extreme, duration-in-beam, dual-beam, and deconvolution estim ates of total density were 3.3, 1.9, and 1.8 times the trawl estimates . Differences in towing speed (1.0 versus 1.5 m/s) for the slow and fa st otter trawls had little effect on the estimates of either density o r age composition, but the beam trawl was less efficient than the otte r trawls. Estimates of densities from the beam trawl averaged 54%, 35% , and 21% of otter trawl estimates for age-0, age-1, and age-2-3 kokan ees (which averaged 42, 130, and 198 mm in length, respectively), sugg esting serious size selectivity for one or both trawl designs.