The visual capabilities of fish are well adapted to 

 this environment. Wardle (1984) indicates that fish are 

 likely to react visually to fishing gear at light levels as 

 low as 10 - ' lux. Contrast discrimination in fish also 

 appears to be acute, with the ability to discriminate between 

 various contrasts mediated somewhat by object size (Anthony, 

 1981). Therefore, the distance at which an object is first 

 seen varies, and as Wardle (1986) indicates, can lead to a 

 variety of reactions depending upon object range, object 

 speed, and the fish's swimming ability. 



The trawl footrope, equipped with 14 inch black rubber 

 roller gear interspersed with 6 inch black rubber cookie 

 gear, undoubtedly presented a high contrast visual image 

 against the sediment cloud which it produced. Previous 

 observations have shown that trawl doors and other groundgear 

 produce high contrast images when viewed from directly in 

 front. This contrast is the result of the darker object 

 being surrounded by a sediment cloud which is illuminated by 

 downwelling light (Main and Sangster 1981a; Wardle 1986). 

 The high contrast image presented by the footrope provides an 

 excellent orientation point for the fish during herding. 



The overall herding behavior observed in front of the 

 footrope tended to be the same as described by previous 

 authors; that is, the flatfish would swim in the direction of 

 tow attempting to maintain station with the footrope. Unlike 

 roundfish, once having crossed the footrope most flatfish did 

 not continue to attempt maintaining station but swam directly 

 into the net. Wardle (1983, 1986) indicated that swimming 

 ability is the critical factor in determining when a fish 

 enters the net. With the exception of a few larger fish all 

 the fish observed were swimming in a burst pattern, implying 

 that the fish were swimming at or near their maximum swimming 

 speed. Burst swimming has been shown to produce high speeds 

 but can only be sustained for short periods (Wardle 1975, 

 1977). This factor undoubtedly accounts for the relatively 

 short herding times observed. 



The herding effects of the net mouth are well known. 

 In situ observations have shown that roundfish, depending on 

 size, maintain station with moving nets for extended periods. 

 Hemmings (1973) demonstrated that haddock swim within the 

 mouth of a net even when the netting behind the mouth is 

 completely removed. Herring schools have been observed 

 outside a midwater trawl approaching, orienting to, and 

 moving with the net (High and Lusz, 1966). The herding of 

 flatfish in close proximity to the footrope is probably 

 analogous to the herding of roundfish by the net mouth and 

 relies on the same visual stimuli. 



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