ACOUSTICS UNPACKED A General Guide for Deriving Abundance Estimates from Hydroacoustic Data

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### Vessel noise and avoidance

All vessels radiate underwater noise.  Fish species are able to detect this vessel noise over a range of frequencies from tens to at least several hundred Hz (Mitson 1995).  Whether the fish react to the vessel noise, thereby altering their behavior and detection probability, has been the subject of much research ( Mitson 1995, Handegard et al. 2003).  Avoidance reactions typically occur when fish are 100-200 m from the vessel, but particularly noisy vessels may elicit such a response at distances as great as 400 m (Mitson 1995).  Vessel lighting may cause avoidance reactions as well.  In addition to vessel avoidance, some fish (alewives) avoid broadband sound pulses at relatively high frequency (>100 kHz) and source levels of typical scientific echosounders (Ross et al. 1996).

Fish may react to a survey vessel by swimming away from the vessel or by diving.  Horizontal avoidance includes herding, which results when fish respond to the sound field of an approaching vessel by swimming ahead of the vessel on the vessel track.  This may occur as fish move into a null in the emitted vessel sound field that exists ahead of the vessel (Aglen 1994).  Fish remain in this null until the vessel is visible at which time they may swim perpendicular to the vessel track to avoid it (Soria et al. 1996; Volpatti et al. 2002) or they dive vertically (Soria et al. 1996; Vabø et al. 2002).  The degree of horizontal avoidance varies among and within species, age classes, time of day, season, and even within a single survey of the same aggregation of fish (Volpatti et al. 2002).  Vertical avoidance may be dependent on fish depth distribution, with no response occurring below a given depth (Vabø et al. 2002).  In addition to affecting estimates of depth distribution, vertical avoidance can also affect density/biomass estimates in two ways.  First, active head-down swimming will increase tilt angle and decrease scattering strength for individual fish.  Second, change in pressure resulting from rapid changes depth can reduce swimbladder volume, also leading to a decrease in scattering strength.  On the other hand, fish attraction to survey vessels has also been observed and would induce an opposite bias (Røstad et al. 2006).

Mitson (1995) provides guidelines for making noise measurements and gives recommendations for dealing with vessel noise and avoidance.  Noise-reducing innovations in new survey vessels have minimized avoidance-related biases to negligible levels (Fernandes et al. 2000).  Avoidance has received little attention in the Great Lakes.  Noise levels are also important for data collection and detection probability as both mechanical and electrical noise are added to the signal of interest by the echosounder.