Ian H. McQuinn

Tilt angle and target strength: target tracking of Atlantic cod (Gadus morhua) during trawling

Vertical orientation (tilt angle) is known to affect the target strength (TS) of ensonified fish and is a large component of the variability inherent in acoustic-biomass estimates. To measure the effects of changes in tilt angle on TS during diel vertical migrations, a concentration of migrating Atlantic cod (Gadus morhua) was observed acoustically from a research vessel over several days. Single-target data were collected from a split-beam echosounder and were subsequently tracked, corrected for vessel orientation and movement, and analysed for 3-dimensional displacement (speed and direction). The results revealed a large variability in TS and several patterns of swimming behaviour from random to directed orientation and movement, with changes in both vertical and horizontal displacements and inferred orientation. These behavioural patterns and their affects on TS were analysed as a function of “time-since-sunset”. Regular diel orientation patterns were observed as cod rose from the ocean bottom in the evening, increasing their tilt angle, and descended at sunrise to regain the ocean floor. Standardized TS (B 20 ) was found to be highly correlated with tilt angle. This relationship can be used to correct for the diel changes in the TS of these migrating cod as a function of the in situ-measured tilt angle and thus to improve the accuracy of acoustic-biomass estimation.

A threatened beluga (Delphinapterus leucas) population in the traffic lane: Vessel-generated noise characteristics of the Saguenay-St. Lawrence Marine Park, Canada

The threatened resident beluga population of the St. Lawrence Estuary shares the Saguenay-St. Lawrence Marine Park with significant anthropogenic noise sources, including marine commercial traffic and a well-established, vessel-based whale-watching industry. Frequency-dependent (FD) weighting was used to approximate beluga hearing sensitivity to determine how noise exposure varied in time and space at six sites of high beluga summer residency. The relative contribution of each source to acoustic habitat degradation was estimated by measuring noise levels throughout the summer and noise signatures of typical vessel classes with respect to traffic volume and sound propagation characteristics. Rigid-hulled inflatable boats were the dominant noise source with respect to estimated beluga hearing sensitivity in the studied habitats due to their high occurrence and proximity, high correlation with site-specific FD-weighted sound levels, and the dominance of mid-frequencies (0.3-23 kHz) in their noise signatures. Median C-weighted sound pressure level (SPL(RMS)) had a range of 19 dB re 1 μPa between the noisiest and quietest sites. Broadband SPL(RMS) exceeded 120 dB re 1 μPa 8-32% of the time depending on the site. Impacts of these noise levels on St. Lawrence beluga will depend on exposure recurrence and individual responsiveness.

Capelin TS: Effect of individual fish variability

Capelin (Mallotus villosus) is an important forage fish in northern latitudes. The effect of the individual biological variability on the target strength (TS) at various acoustic frequencies was investigated from a sample collected in the St. Lawrence estuary. The geometric properties of the fish and its swimbladder were measured from radiographs obtained from 45 anesthetized fish, from 12‐ to 16‐cm total length. The data were input to a backscattering model exploring the effect of fish shape on TS as a function of acoustic frequency, length, and tilt angle. The swimbladder had similar cross sections in both lateral and dorsal views. It represented 5.5% (s.d. 1.1%) of lateral body cross section and 8.2% (s.d. 1.8%) of the dorsal body cross section. The swimbladder cross section was related to the fish total length but the variation around the mean for a given length was ±40%. This large variability is equivalent to the change in cross section between tilt angles of 0 and 90 deg for an average fish. The va...