Silent ships sometimes do encounter more fish. 2. Concurrent echosounder observations from a free-drifting buoy and vessels

De Robertis, A., and Wilson, C. D. 2010. Silent ships sometimes do encounter more fish. 2. Concurrent echosounder observations from a free-drifting buoy and vessels. – ICES Journal of Marine Science, 67: 996–1003. The reactions of walleye pollock (Theragra chalcogramma) to the NOAA ships “Miller Freeman” (MF), a conventionally designed research vessel, and the “Oscar Dyson” (OD), a noise-reduced research vessel, were compared near the Shumagin Islands, Alaska. Observations with a buoy-mounted echosounder indicated a stronger decrease in pollock backscatter strength and a greater increase in pollock backscatter depth associated with the passage of the MF than the OD. The pollock began to respond at a distance of ∼270 m from the vessel and were disturbed for several minutes after vessel passage. The reaction to the OD was weak, suggesting that measurements of pollock made by the OD in this environment are not strongly biased by vessel avoidance. Comparison of echosounder observations collected on board each vessel indicated that the MF measured less pollock backscatter, a deeper pollock depth distribution, and weaker single-target echoes than the OD. Together, the results indicate that acoustic measurements from the two vessels are not equivalent because of a reduced avoidance response to the noise-reduced vessel.