Fish also appear to have the ability to localize sound. 

 Evidence has been presented that indicates fish can determine 

 the distance to the sound source as well as its position in 

 both the vertical and horizontal plane (Schuijf and Buwalda, 

 1980; Hawkins and Myrberg, 1983; Buwalda et al., 1983). 

 Evidence has also been presented that some species, such as 

 cod and salmon, may be capable of increasing frequency 

 sensitivity by the presence of a frequency selective audio 

 filter (Hawkins and Chapman, 1975; Hawkins and Johnstone, 

 1978). 



Sounds generated by fishing gear vary greatly in 

 intensity and frequency. Frequency ranges for vessels and 

 gear have been measured between 25 Hz - 15 kHz. Most of this 

 energy is generated in the range between 25 Hz - 5 kHz, 

 particularly below 1 kHz. The frequency ranges below 1 kHz 

 correspond well with the known hearing abilities of fish. 

 Intensity levels, taken as far as 300 m from vessel and gear, 

 have been measured as high as 39 dB above background noise at 

 250 Hz and 28 dB at frequencies below 150 Hz (Schwarz, 1985). 

 In addition to maximum intensity levels, there is evidence to 

 suggest that changes in intensity may be more important in 

 determining a fish's reaction than intensity levels (Schwarz 

 and Greer, 1984) . 



Initial orientation to the gear, prior to the 

 occurrence of herding, is probably the result of sound 

 stimuli. Trawl gear produces wide frequency audio stimuli 

 which are constantly changing in intensity. These signals 

 propagate through water to distances well beyond the visual 

 limits of fish. The hearing capabilities of fish indicate 

 these signals are detected and possibly undergo extensive and 

 complex analysis. From this analysis fish can probably 

 determine the relative position of the gear, possibly speed 

 of approach, and conceivably other factors. It can be 

 speculated that this would result in the fish orienting 

 themselves in the most advantageous position to react to the 

 disturbance. 



Distances at which flatfish were observed reacting 

 directly to the gear were relatively short and were probably 

 determined by visual limitations. Light intensity and visual 

 distance depend upon several factors, including: depth, 

 turbidity, refractive index, dissolved organic and inorganic 

 matter, and line of sight. Most wavelengths, and light 

 intensity, are attenuated rapidly with depth; consequently, 

 even at relatively shallow depths, light levels are low and 

 monochromatic. This results in objects being seen as shades 

 of grey and visibility being dependent upon the degree of 

 contrast between the object and its background Guthrie 

 (1986). 



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