W.E. Ricker and Eastward Ho cruise to study the effect of trawling on rockfish behaviour, October 15-27, 1990 / by R. Kieser, B.M. Leaman, P.K. Withler, and R.D. Stanley.: Fs97-4/2161E-PDF

"A previous cruise collected acoustic observations of undisturbed rockfish behaviour and school structure, over several 24 hour periods (Leaman et al. 1990). The objective of this cruise was to obtain complementary observations on the distribution of rockfish before, during, and after the passage of a trawl net. Additional work investigated the influence of towpoint location and ballast distribution on the stability of the towed body housing the acoustic transducer. The experiments were carried out near the edge of the continental shelf, off Quatsino Sound, British Columbia. The principle rockfish studied was Pacific ocean perch (Sebastes alutus). Direct observation of the trawl warps and the fishing gear by a moving acoustic vessel was unsuccessful. Comparisons of acoustic records from the fishing and acoustic vessels indicate that both the fishing vessel and the fishing gear cause avoidance reactions. Noise from the fishing vessel forces fish schools downward in the water column. This effect diminishes with increasing range from the fishing vessel. The bottom trawl appears to force fish in the lowest portions of the school upward, but fish in the middle and upper segments of the schools display only the downward movement generated by the fishing vessel. A method to determine the pitch of the towed body from echogram traces was developed. Investigation of towed body stability showed that the towed body can be balanced only for a narrow range of speeds. Several balance configurations may be required to accommodate typical trawling and acoustic survey speeds"--Abstract, page iii.

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