Behavioural Reactions of Demersal Fish to Bottom Trawls at Various Light Conditions

ICES mar. Sei. Symp., 196: 68-76. 1993

Behavioural reactions of demersal fish to bottom trawls at various light conditions

Stephen J. Walsh and William M. Hickey

Walsh, S. J., and Hickey, W. M. 1993. Behavioural reactions of demersal fish to bottom trawls at various light conditions. - ICES mar. Sei. Symp., 196: 68-76.

Comparisons of the behaviour of demersal roundfish and flatfish towards a bottom trawl were made during light and dark conditions, as well as in dark conditions with constant artificial light introduced. Observations of fish orientation and swimming behaviour were recorded from 123 still photographs and 8 h of video recording taken by a camera attached to an underwater remotely operated vehicle. The results of the analysis show that, at low light levels (<10 4 lux), the ordered pattern of reaction behaviour seen in roundfish during high light intensities ceases. The attachment of artificial light (> 10~4 lux) to the footgear, at night, was insufficient to stimulate fish to react in the ordered way. Flatfish generally do not orientate themselves to the general direction of the tow, and show little difference in day and night reactions to the trawl. At night, in the absence of light, there is no ordered orientation to the trawl and there is a lower activity level, in response either to lower light intensity or to endogenous diel rhythms.

Stephen J. Walsh: Department of Fisheries and Oceans, Northwest Atlantic Fisheries Centre, PO Box 5667, St John’s, Newfoundland, Canada A1C 5X1; William M. Hickey: Department o f Fisheries and Oceans, Industry Development Division, Conser vation Technology Section, PO Box5667, StJohn’s, Newfoundland, Canada A1C5X1.

Introduction intensities, below the threshold value of a fish’s vision (<10~6 lux), fish showed none of the recognizable The use of scuba diving, remotely operated underwater ordered patterns of reaction to or avoidance of ground- vehicles, and photographic techniques has contributed gear as seen during daylight until the fish were within significantly to our understanding of fish behaviour pat centimetres of collision with the gear (Wardle, 1986; terns in the fishing zone of trawls extending from the Glass and Wardle, 1989). These authors suggested that doors to the codend [see Ben-Tuvia and Dickson (1969) the “low” light in earlier experiments was not precisely and Wardle (1983, 1986, 1987) for excellent reviews]. measured and that dinoflagellate bioluminescence, acti Behaviour of fish is governed by endogenous factors and vated by turbulence near the net, was not considered. by external physical factors, such as illumination, water Several studies have discussed the potential of biolumi transparency and temperature, and by visual and tactile nescent light in allowing fish to detect and react to a responses to design and riggings of the trawl. Investi towed net (Parrish, 1969; Chestnoi, 1971; Kelly and gations in the 1960s showed that during daylight, fish Tett, 1978; Wardle, 1986; Glass et al., 1986; Glass and were orientated in front of the trawl in the direction of Wardle, 1989). Glass and Wardle (1989) concluded that the tow, whereas, at night, this orientation was less at light intensities below a fish’s threshold vision, fish marked (Parrish et a i, 1962; Blaxter et al., 1964). showed no ordered pattern of behaviour because of the Continuing research led to conflicting results about lack of optomotor reactions normally associated with orientation of some fish species at night (Beamish, 1969; visual images. They further concluded that non-visual Parrish et al., 1969), and non-visual senses such as senses in the absence of vision do not produce the detection of sound or hydrodynamic pressure disturb ordered behaviour reaction pattern and that fish reacted ances were often cited as being responsible for night to the gear only when it collided with them. time reaction patterns to the gear. The aim of this study was to test the no-ordered- Recently, research has demonstrated that at low light reaction hypothesis by comparing the orientation and

68 swimming behaviour of various fish species at low light ROV. We compensated for lack of light measurements levels (<1CT4 lux) and at levels (>10-4 lux) induced by by designing the experiments based on two a priori null artificial light during night tows. Behaviour of the differ hypotheses. The first null hypothesis was that there is no ent fish species was measured during daylight (>1 lux) to difference in the reaction behaviour of fish to the trawl serve as a baseline description of ordered patterns of between night and day. The alternate hypothesis stated reaction to the trawl. that absence or presence of the characteristic ordered daylight behaviour pattern indicates that light levels are below or above the threshold value of fish vision (10-6 Material and methods lux). The second null hypothesis was that at night, the introduction of a constant supply of artificial light Experiments were conducted on board the research (>10~4 lux) on the footgear should result in fish vessel “Lady Hammond” on the Scotian Shelf, Nova exhibiting the ordered pattern of behaviour. The alter Scotia, in October 1990. A total of seven fishing tows nate hypothesis stated that artificial light (>10-4 lux) (two day and five night), using a bottom trawl equipped introduced in an ambient environment of <10~4 lux with rockhopper footgear, were made during which 8 h would not result in fish exhibiting the ordered pattern of of video observations of net and fish were recorded and behaviour like that seen during daylight. 661 35 mm photographs were taken. A similar experi Artificial light was created during one night haul by ment carried out in March 1990 yielded preliminary tying 11 x 15.3 cm Cyalume plastic chemical lightsticks results but these were not included because of the small (American Cyanamid Co.) at 2 m intervals along the number of observations on roundfish, although flatfish entire fishing line of the rockhopper footgear. The were abundant. lightsticks were activated on deck just prior to shooting Video and still camera observations were recorded for the trawl. Each lightstick gives off a light intensity of 34 cod (Gadus morhua), haddock (Melanogrammus aegle- lux 15 min after activation, which gradually reduces to finus), American plaice (Hippoglossoides platessoides), 23 lux after 1 h (manufacturer’s fact sheet). yellowtail flounder (Limanda ferruginea), winter Observations on fish reactions were assessed from 661 flounder (Pleuronectes americanus), pollock (Pollachius still photographs, 123 of which showed fish and a section virens), silver hake (Merluccius bilinearis) and skate of the footgear. Fish were identified and direction of (Raja sp.) found in the mouth area of the trawl. The orientation to the mouth of the footgear was recorded as average towing speed was 3.5 knots over a 44 to 53-m an arbitrary compass direction (N, NE, NW, E, W, S, bottom depth range, with an average bottom tempera SE, SW), with north (N) representing the direction of ture of 8°C. Tows were made around mid-day and late in the tow. Swimming behaviour of each fish was categor the evening with an average tow duration of 1 h. An ized according to body configuration as: (1) body in a underwater remotely operated vehicle (RO V) equipped glide or flexed shape indicating regular or cruising be with an Osprey OE1321 SIT (video) camera (with a haviour; (2) body in U- or S-shape indicating fast burst measured sensitivity down to 10-4 lux) was moved into characteristic of startle behaviour; (3) resting on bot position ahead of the wing quarter, to look across the tom; and (4) unknown, i.e. body shape unidentifiable. mouth area of the footgear. A Photosea 2000-35 mm still The number of collisions of fish with the footgear was camera (400 frames) was mounted alongside the video also counted. The three flatfish species were treated as camera and flash photographs were taken by the opera tor on the bridge. one group because they exhibited similar behavioural reactions and in many cases were difficult to separate. A Observation techniques followed closely those out separate category for all unidentifiable roundfish was lined by Glass and Wardle’s (1989) experiments. At used in the analysis. The number of observations of night, the ROV’s single QL-3000 quartz halogen lamp skate in the photographs was too low for orientation and (250 W output) was switched on in order to move the swimming behaviour to be calculated. ROV into position. After positioning, the floodlight and shiplights were extinguished and all ROV’s LEDs were covered. Periodically, to check the ROV for position, the floodlight was turned on and then off. A 2min Results interval elapsed before the next series of photographs were taken; photos were spaced at 20-30 s intervals. Trawl catches (kg) The average catches of cod and haddock were higher Experimental protocol during the daylight fishing hauls than at night, whereas average catches of flatfish, pollock, silver hake, and Because no underwater light meter was available, no other species were higher at night (Table 1). During the measurements of light intensity or bioluminescence single night haul with the lightsticks attached, catches were taken. The only estimate of light intensity available were similar to the average catches of regular night was the sensitivity threshold of the SIT camera on the hauls. Table 1. Comparison of fishing catches (kg) of various species during daylight (3 hauls), dark (3 hauls), and artificial light (1 haul).

Day Night Artificial light

Total Total Total Species catch (kg) Mean s.d. catch (kg) Mean s.d. catch (kg)

Cod 583 194.3 90.7 357 119.0 146.4 121 Haddock 2288 762.7 501.4 878 292.7 293.1 253 Flatfish 97 32.3 20.5 195.5 65.2 83.7 92 Pollock 4 1.3 2.3 10.3 3.4 5.7 1 Hake 194 91.0 128.7 647 215.7 370.9 198 Other fish 152 50.7 26.6 209 69.7 88.1 46

Total 3318 1659 1171 2296.8 765.6 987.7 711

D ay - high light intensity ing back towards the net. No roundfish were seen resting Of the individual fish recorded on the still photographs, on the seabed. The number of photographic obser 96% were shown swimming ahead of the net in the same vations of hake and flatfish was too low (<5 fish) to make direction as the tow (N), with only 3% swimming at an any meaningful observations about swimming behav angle (NE) (Table 2). A pronounced ordered pattern of iour. However, video observations showed flatfish, behaviour was clearly identifiable (Fig. 1). Over 90% of ahead of the trawl, resting on the seabed and reacting to cod, haddock, pollock, and unknown roundfish were the footgear at distances of about 0.5-1 m. They gener showing cruising behaviour in front of the net; startle ally moved a short distance in a lateral direction towards behaviour (<10%) was minimal (Table 3). Cod gener the opposite side of the trawl, settled on the bottom, and ally were located close to the bottom, ahead of the net, repeated the behaviour again when the footgear was whereas haddock were generally higher off bottom by near by, until they were either caught or escaped under comparison and occasionally some were pictured turn neath the footgear.

Table 2. Orientation to direction of tow (North) and swimming behaviour of various fish species during fishing hauls conducted during daylight, dark, and artificial light.

Cruising Startled Resting on Behaviour Total swimming swimming bottom unknown Total Time Direction nos. (%) (%) (%) (%) (%)

Day North 288 94 5 1 0 96 Northeast 10 80 10 10 0 3 South 1 100 0 0 0 1

Total 299 94 5 1 0 100

Night North 87 70 21 5 5 59 Northeast 20 45 30 20 5 14 Northwest 6 100 0 0 0 4 East 6 66 17 17 0 4 West 8 75 13 12 0 6 Southwest 4 25 50 0 25 3 South 14 64 14 8 14 10

Total 145 66 21 7 6 100

Artificial light North 48 83 10 2 5 63 Northeast 6 83 17 0 0 8 Northwest 10 70 20 10 0 13 East 1 100 0 0 0 1 West 6 50 50 0 0 8 South 5 40 0 20 40 7

Total 76 76 15 4 5 100

70 Figure 1. Cod and haddock showing the ordered reaction pattern to the trawl (out of photo at far right) during daylight. Fish are swimming forward in the direction of the tow.

was activated, several round and flatfish species were Night - low light intensity seen resting on or close to the bottom and were slow to The number of fish seen in individual photographs was react to the approaching footgear. Reaction distances considerably lower than in daylight, with only 5 out of 67 appeared to be less than 2 m and reaction occurred in photographs showing 6 or more fish (the largest number random directions. was 7), in contrast to 22 out of 26 photographs during daylight. Fifty-nine per cent (59%) of individual fish Night - artificial light observations showed fish swimming in the direction of the tow (N), while 18% were orientated at an angle (NE Again, the number of fish observed in the photographs and NW) (Table 2). Considered collectively, 84% of the was less than in daylight observations, with only 1 out of 61 night photographs, depicting more than one fish, 30 photographs containing 6 fish. Sixty-three per cent of showed fish to be randomly orientated to the net. The the individual fish were orientated to the direction of tow recognizable ordered daylight pattern of behaviour was (N), while another 24% were orientated at an angle (NE absent (Figs. 2 and 3). Many fish, including cod, had and NW) (Table 2). However, 70% of the 30 photo dock, flatfish, and unidentified roundfish, were seen on graphs that showed more than one fish collectively or close to the bottom. Many were close to the footgear showed random orientation of fish to the net, and or swimming in random directions. Four out of the 67 several fish were very near to the footgear. The ordered photographs showed collisions of the fish with the foot reaction pattern was again absent (Figs. 5 and 6). One of gear (Fig. 4). The number of fish showing startle behav the 30 photographs showed a skate in collision with the iour was higher at night (21%) than during the daylight footgear. (5%) and included haddock, flatfish, and unidentified The number of fish showing startle behaviour (5% roundfish (Tables 2 and 3). The numbers of pollock and day; 21% night; 15% artificial light) was again higher hake were too low to reveal day and night differences in than in daylight but lower than at night-time (Table 2). swimming behaviour. When the ROV’s halogen lamp Several flatfish and unidentified roundfish were ob

71 Table 3. Comparison, by species, of orientation to the direction of the tow (north, northeast, and northwest combined) and swimming behaviour. Unidentifiable swimming behaviour and other orientations are not included.

Direction of tow Cruising Startled Resting on No. (N, NE, NW) swimming swimming bottom Species Time obs. (%) (%) (%) (%)

Cod Day 26 100 100 0 0 Night 18 67 39 12 6 Night/Light 5 20 20 0 0 Haddock Day 128 99 90 8 0 Night 75 87 65 20 3 Night/Light 13 92 69 15 0 Flatfish“ Day 5 20 0 20 80 Night 19 32 21 5 16 Night/Light 6 17 17 17 17 Pollock Dayb 27 100 93 7 0 Hake Day 2 100 50 50 0 Night 13 100 92 8 0 Night/Light 25 96 92 4 0 Unknown roundfish Day 111 100 100 0 0 Night 23 39 4 17 17 Night/Light 27 78 59 7 8

a Only north was considered for orientation to direction of tow because flatfish generally moved in a lateral direction in response to the approaching footgear. b Pollock were not identified at other times.

served resting on the sea bottom (Table 3). Although the At night, with and without artificial light, variability in number of photographic observations of cod and had orientation of individual fish to the direction of the tow dock was lower and the numbers of hake and unident was greater than that by day. Fish were orientated ified roundfish higher than during the two other sam randomly and appeared to be less active, with many pling periods, there was a general trend to observe less resting on the bottom or in close proximity to the startle behaviour and more cruising behaviour. How footgear, and some collisions were evident. During ever, the percentage of startle behaviour, with the daylight video observations, only skate collided with the exception of cod, was higher than during daylight obser footgear. Skate showed no consistent orientation to the vations (Table 3). direction of tow, were often seen resting on the bottom, Again, when the ROV halogen lamp was switched on, and swam in various directions as the footgear fish were resting on or close to the bottom and in close approached, similar to Beamish’s (1966, 1969) obser proximity to the footgear. They appeared slow to react vations. During night video recordings, with the ROV’s to the approaching footgear and did so in a haphazard halogen lamp on, several skate and flatfish were seen direction with no ordered behaviour pattern. This be colliding with the footgear owing to slow reaction or to haviour was similar to that seen in regular night photo reaction in the wrong direction. graphs. Several night and artificial light photographs showed various roundfish species apparently dashing away, cen Discussion timetres away from the footgear, showing the typical U and S body shapes associated with fast starts (Webb, The results of the experiments led to the rejection of 1976). These startle reactions would generally be fol both null hypotheses and the acceptance of the alternate lowed by cruising behaviour in roundfish or, in the case hypotheses. First, during regular night observations, of flatfish, by settling to the bottom. This may account fish did not show the ordered pattern of reaction behav for the cruising behaviour observed, although less was iour as they did in daylight, so ambient light levels were seen than by day. The increase in startle behaviour from considered to have been below the fish’s vision threshold daylight to night and artificial light fishing hauls may be value of 10-6 lux, in agreement with Glass and Wardle’s the result either of collisions or of “last-second” re (1989) findings. Second, fish did not exhibit the ordered sponse to detection of the footgear. These day and night pattern of behaviour when ambient light levels were photographic observations of various roundfish behav increased by the introduction of artificial light. iours are similar to the behaviour described for haddock

72 Figures 2 and 3. Flash photographs taken in the mouth of the trawl during night observations. The ordered distribution pattern is absent and fish are orientated randomly to the forward direction of the approaching gear.

by Glass and Wardle (1989). Flatfish, on the other hand, visual contact is established (Vyskrebentsev, 1971). The showed little difference in their light and dark behav type of reaction manifested would be a function of iour, and showed no orientation and movement along illuminance and the type of school present, i.e. bottom- the direction of the tow, in agreement with Beamish’s dwelling non-schooling species such as flatfish and skate; (1969) observations. fish scattered in small schools; and dense aggregations or To interpret the absence of the ordered pattern of shoals. behaviour during the night observations, it is necessary As the trawl comes closer, fish probably rely on their to look at the role of the distant-receptor activity of the distant-receptor senses (such as hearing, pressure gradi fish’s senses in the fish capture process. In the study of ent detection, and vision) to assess the situation, make a schooling formation and maintenance in fish, it is gener definitive analysis and react to the stimulus. Vyskrebent ally agreed that vision plays the dominant role, and that sev (1971) concluded that there was a definite link in the absence of vision, schooling and other visually between distance at which danger was detected and controlled behaviour ceases (Glass et al., 1986; Blaxter reaction to it, on the one hand, and schooling on the and Batty, 1987). Noise from the trawl doors and bob other. As light intensity decreases beyond a fish’s visual bins is thought to be clearly audible in the water far threshold, perception of sound and directional pressure ahead of the trawl, possibly leading to a state of gradients from the trawl components are assumed still to increased awareness of the fish which may occur before be perceived. Glass and Wardle (1989) concluded that

73 Figure 4. Flash photograph showing collision of cod with footgear during night observations. Fish are orientated perpendicular to the gear. No reaction is apparent.

these stimuli alone are not sufficient to produce an trawl, at night, was not sufficient stimulation for the fish ordered pattern of behaviour, and our findings support to return to the ordered pattern of behaviour. Also, the this conclusion. use of the single ROV light, which was periodically Nevertheless, artificial light on the footgear of the turned on when adjusting the position of the vehicle, did

Figure 5. Flash photography taken in the mouth of the trawl with artificial lightsticks attached to footgear. The ordered pattern of reaction behaviour is absent. Both haddock arc swimming perpendicular to the forward direction of the trawl, with no evidence of reaction.

74 Figure 6. Flash photograph taken in the mouth of the trawl with artificial lightsticks attached to footgear showing near collision or seconds after collision with the approaching gear. not cause the ordered pattern to resume as Glass and graphed during the artificial light haul than in regular Wardle (1989) reported. night observations. One hypothesis to explain the absence of an ordered Although Glass and Wardle (1989) found a resump pattern of behaviour with use of the artificial light was tion of the ordered pattern in haddock when their that the lights may have blinded the fish, whose eyes are ROV’s 2 x 300 W halogen lamps were activated, acti dark-adapted, and caused immobility. Protasov (1970) vation of our ROV’s single 250 W halogen lamp had no found that the reverse process of dark-to-light adap such effect. The ROV in each study was set up to look tation in a fish’s eye was much faster than light-to-dark across and slightly back at the net to illuminate the trawl adaptation, some aspects of it being completed in 30 s. mouth. Our lamp has a 100 degree x 100 degree beam Such rapid adaptation from a dark to a light cycle could pattern (manufacturer’s specifications). It is expected explain why Glass and Wardle (1989) found an immedi that Glass and Wardle’s (1989) two lamps would have ate resumption of the ordered pattern of behaviour in produced a broader beam pattern across the trawl’s haddock when they switched on their ROV’s powerful path. This might provide better contrast of objects for halogen lamps without startling the fish or blinding the fish, leading to possible earlier detection of the trawl them. The lightsticks provide a string of diffuse, omni and sufficient response time for fish to react in the directional light source because there is no reflector ordered way. This improvement in beam patterns controlling the light projection, as in the ROV’s halogen caused by more powerful light may also explain why lamp. This light string, the illuminance of which de Blaxter et al. (1964) could not stimulate the herding of creases over time, may have been sufficient for visual herring at night when they used underwater lights (up to detection only as fish neared the footgear. Fish further 100 W) mounted on the headline of their bottom trawl. ahead in the trawl path may not have been stimulated A second hypothesis for consideration when explain early enough to react in the ordered way when the net ing the lack of an ordered pattern of behaviour at night, reached them: that would explain the higher variability with and without artificial light, is that there might be a of orientation and swimming behaviour of fish photo diel rhythmicity in awareness levels. Blaxter and Batty

75 (1987) have suggested that dark cycles or dark adap References tations may result in a lower sensory awareness, a form Barham, E. G. 1970. Deep-sea fishes; lethargy and vertical of “sleep”, or a heightened awareness, such as an in orientation. In Proceedings of an International Symposium crease in hearing ability, to compensate for loss of visual on Biological Sound Scatterings in the Ocean, pp. 100-118. abilities, as part of the endogenous diel rhythms of fish. Ed. by G. B. Farquhar. Washington, DC: Department of the They suggested that this reduced level of awareness Navy. might occur because of the absence of predator threats Beamish. F. W. H. 1966. Reactions of fish to otter trawls. Fish. Can., 19: 19-21. and shoaling, both visually dependent behaviours, and Beamish, F. W. H. 1969. Photographic observations on reac the suspected need for energy conservation by fish. tions offish ahead of the otter trawls. FAO Fish. Rep., 62,3: Introduction of diffuse light, such as by our lightsticks, 511-521. or the presence of bioluminescent light may not be Ben-Tuvia, A., and Dickson, W. (Eds.) 1969. Proceedings of the FAO Conference on Fish Behaviour in Relation to sufficient to break a dark cycle adaptation if one exists. Fishing Techniques and Tactics. Bergen, Norway, 19-27 This could explain why, at night (with and without the October 1967. FAO Fish. Rep., 62, 1, 2, 3. 884 pp. artificial lights), several roundfish were seen resting on Blaxter. J. H. S., and Batty. R. S. 1987. Comparisons of herring the bottom both in the still photographs and in video behaviour in the light and dark: changes in activity and observations and, in the latter case, seemed slow to react responses to sound. J. Mar. Biol. Ass. UK. 67: 849-860. Blaxter, J. H. S., Parrish, B. B., and Dickson, W. 1964. The to the approaching footgear. Similarly, Barham (1970) importance of vision in the reaction of fish to drift nets and reportedly observed that some species of mesopelagic trawls. In Modern fishing gear of the world 2, pp. 524-536. myctophids, at night, were immobile, vertically orien Ed. by H. Kristjonsson. Fish News Ltd., London. tated and failed to react to the artificial lights of the Chestnoi, V. N. 1971. Daily rhythmicity in trawl catches offish. In Fish behaviour and fishing techniques, pp. 195-201. Ed. viewing submersible, suggesting a form of sleep. The by A. P. Alekseev. Israel Progr. Sei. Transi.. Jerusalem. 193 hypothesis of diel rhythmicity in awareness may also pp. explain the lack of a herding response in Blaxter et al.'s Glass, C. W., and Wardle, C. S. 1989. Comparison of the (1964) herring experiment with lights mounted on the reactions of fish to a trawl gear, at high and low intensities. trawl headline. Fish. Res., 7: 249-266. Glass. C. W.. Wardle, C. S., and Mojsiewicz, W. R. 1986. A The aim of this study was to look at changes in light intensity threshold for schooling in the Atlantic mack behaviour of demersal fish near bottom trawls as light erel, Scomberscombrus. J. Fish Biol., 29 (Suppl. A): 71-81. conditions changed. Difficulty in trying to artificially Kelly, M. G., and Tett, P. 1978. Bioluminescence in the light a trawl to determine whether fish would react in a ocean. In Bioluminescence in action, pp. 399-417. Ed. by similar manner during both day and night has shown that P. J. Herring. Academic Press, London/New York/San Francisco. the loss of ordered reactions in fish, at low light levels, Parrish, B. B. 1969. A review of some experimental studies of may be more complex than originally thought. Although fish reactions to stationary and moving objects of relevance to at night there is evidently a lack of orientation of fish to fish capture process. FAO Fish. Rep., 62(3): 233-245. the stimulus of the net and a lower activity level, it is not Parrish, B. B., Blaxter, J. H. S., and Dickson, W. 1962. yet clear whether this is a response to low light intensit Photography of fish behaviour in relation to trawls. ICES CM 1962/77: 5 pp. ies, endogenous diel rhythms, or a combination of both. Parrish, B. B., Blaxter, J. H. S., Pope, J. A., and Osborn, More observational and experimental work is re R. H. 1969. Underwater photography of fish behaviour in quired to investigate the effects of appropriate artificial response to trawls. FAO Fish. Rep., 62, 3: 873-884. lights, bioluminescence, and raised background illumi Protasov, V. R. 1970. Vision and near orientation of fish. nation on the reactions of fish to trawls. There is a strong Academy of Sciences of the USSR. Israel Progr. Sei. Transi., Jerusalem. 175 pp. need to study these in the context of diel rhythms and Vyskrebentsev, B. V. 1971. Role of reflex stimuli in the dark adaptative cycles inducing periods of lower aware behaviour of fish near the gear. In Fish behaviour and fishing ness. Of major interest would be the testing of these techniques, pp. 195-201. Ed. by A. P. Alekseev. Israel hypotheses on midwater trawling of pelagic schools Progr. Sei. Transi., Jerusalem. 193 pp. Wardle, C. S. 1983. Fish reactions to towed gears. In Experi during both day and night. mental biology at sea, pp. 167-195. Ed. by A. MacDonald and I. G. Priede. Academic Press, New York. Acknowledgements Wardle, C. S. 1986. Fish behaviour and fishing gear. In The behaviour of teleost fishes, pp. 463-495. Ed. by T. J. Pitcher. This study was supported by funding from the Atlantic Croom Helm, London, Sydney. Fisheries Adjustment Program of the Canadian Depart Wardle, C. S. 1987. Investigation of the behaviour of fish during capture. In Development of fisheries research in ment of Fisheries and Oceans. Special thanks to C. Scotland, pp. 139-155. Ed. by R. S. Bailey and B. B. Parrish. Cooper from Scotia-Fundy Region for support on this Fishing News (Books), London. project, D. Bursey for typing the manuscript, and K. Webb, P. W. 1976. The effect of size on the fast-start perform Youden-Walsh for proofreading comments. This paper ance of rainbow trout, Salmo gairdneri, and a consideration of piscivorous predator-prey interactions. J. Exp. Biol., 65: benefited from reviews by J. Morgan and J. Gibson at 157-171. NAFC. Many thanks to C. Glass for inspirational dis cussions on this topic.