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Effect of seasonal temperature changes on the escape behaviour of haddock, Melanogrammus aeglefinus, from the codend
Fisheries Research 58 (2002) 323–331
Effect of seasonal temperature changes on the escape behaviour of haddock, Melanogrammus aeglefinus, from the codend ¨ zbilgina,*, C.S. Wardleb H. O Fisheries Faculty, Fish Capture Department, Ege University, Bornova, I¨zmir 35100, Turkey b Marine Laboratory, P.O. Box 101, Aberdeen AB11 9DB, UK
a
Received 29 December 2000; received in revised form 14 September 2001; accepted 19 September 2001
Abstract Sea temperature around the coast of Scotland rises from 7 8C in late winter to 12 8C in late summer. Recently, it was shown that the selectivity of trawl codend was poorer in late winter than it was in late summer. A change in water temperature is expected to affect the escape speed of fish, and therefore the selectivity of the gear. Four experiments carried out in this study showed the effect of this seasonal temperature change on the ability of haddock, Melanogrammus aeglefinus, to escape from a codend. A temperature increase from 7 to 12 8C changed the minimum twitch contraction time of the lateral muscle from 38.8 to 27.6 ms. This gives a maximum tail beat frequency of 12.78 Hz at 7 8C rising to 19.12 Hz at 12 8C. Escape reflexes of the fish were significantly slower at 7 8C than at 12 8C ðP < 0:001Þ. The shortest time to complete first body bend (stage 1) was 40 ms at 7 8C and 20 ms at 12 8C. The mean time taken to complete stage 1 at 7 8C (63.9 ms, S.E. 1.2) was significantly (t-test, P < 0:001) longer than at 12 8C (40.1 ms, S.E. 0.92). The mean time taken to complete the propulsive stroke (stage 2) of the fast start was significantly (t-test, P < 0:001) longer at 7 8C (108.5 ms, S.E. 5.3) than at 12 8C (72.6 ms, S.E. 2.6). The maximum speed recorded while competitively swimming for food reward was 7.9 L s1 at 7 8C and 12.5 L s1 at 12 8C. The observed maximum tail beat frequency used by haddock when leaving the mesh of a codend escape panel changed from 12 to 25 Hz. The general underlying physiological effect of a temperature increase of 10 8C was to double the speed of the maximum swimming ability ðQ10 C ¼ 2Þ. The effect of a change of only 5 8C on the ability to manoeuvre out of codend selection devices is discussed. Crown Copyright # 2002 Published by Elsevier Science B.V. All rights reserved. Keywords: Selectivity; Codend; Escape; Haddock; Temperature; Swimming behaviour
1. Introduction Both girth- and length-based selectivity curves of the haddock, Melanogrammus aeglefinus, in the North Sea, demonstrate that some size classes of fish did not escape from the codend at the end of the winter ¨ zbilgin, 1998). whereas they did in late summer (O *
Corresponding author. Tel.: þ90-532-706-19-77; fax: þ90-232-388-36-85. ¨ zbilgin). E-mail address: [email protected] (H. O
Late winter and late summer coincide with the minimum and maximum of the annual cycle of the sea water temperature changing from 7 to 12 8C (Turrell and Slesser, 1992), which is typical for the annual change at the fishing grounds where the selectivity experiments were conducted. The close proximity of fish’s blood with the water at the fish’s gills means body temperatures of most fish closely match the water temperature even when they are actively swimming (Dean, 1976; Robert and Graham, 1979). There are a few larger pelagic cruising species
0165-7836/02/$ – see front matter. Crown Copyright # 2002 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 7 8 3 6 ( 0 1 ) 0 0 3 9 4 - 0
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that have evolved special heat exchangers to keep their red swimming muscle warm (Kishinouye, 1923; Carey and Teal, 1969). As discussed in Wardle (1993), when light levels are above the threshold for vision, there is a sequence of behavioural events mainly limited by swimming ability that leads fish from the mouth of a trawl to the codend. Two major influences on swimming ability are fish size and water temperature (Wardle, 1975, 1980; He, 1993; Videler, 1993; Wardle and He, 1996). Towed trawls move forward at speeds between 1 and 2 m s1 (2–4 knots). Fish of all sizes attempt to swim at the towing speed in the mouth of the net (Main and Sangster, 1981), but there is a difference in ability to do this between small and large fish of the same species (Wardle, 1977). The smallest fish are able to keep up for only brief periods; they tire quickly and drop back into the net. Some of the largest fish may swim easily at the towing speed and never drop back. The time spent in the mouth varies with the size-dependant degree of exhaustion. A particular degree of exhaustion is probably the trigger for this change in reaction behaviour so that fish arriving in the codend are inevitably partially exhausted. There are a few observations at light levels below visual reaction thresholds that show fish not reacting until touched by the gear (Walsh and Hickey, 1993; Glass and Wardle, 1989). The speed of these sudden movements will also be reduced in cold water but further research is needed on the mechanisms involved in escape or otherwise of fish randomly performing fast speeds in darkness. There have recently been many different designs for codends intended to offer increased opportunities for those fish small enough to pass out through the meshes. Conventional codends have only a narrow band of open meshes just ahead of the bulging catch. Ahead of this, there are sometimes open mesh panels built into the wall of the extension designed to provide many more opportunities for the smaller fish to swim out. Direct observations, films and comparative fishing trials have shown that these various arrangements are sometimes but not always successful (Watson et al., 1993; Brewer et al., 1998). Observers have concluded that the degree of stimulus to leave the net, as well as the swimming ability to cope with the complex and sometimes fast water flows, are key variable factors to fish escape attempts (Wardle, 1993; Glass and Wardle, 1995). In order to hold station and then pass out of a fast-moving codend, undersized fish have to swim at or
very close to their maximum speed, which is reduced at colder temperatures (He, 1993; Wardle, 1993). This paper investigates the effect of the temperature change on the ability of haddock to swim and manoeuvre, and addresses the question ‘‘Is a seasonal change from 7 to 12 8C large enough to cause a significant difference in the selectivity of a codend?’’ The effect of temperature is investigated in four different experiments: (a) measurements of the minimum contraction time of the haddock’s swimming muscles and prediction of the maximum tail beat frequency at different temperatures; (b) the time taken for the fast start reaction of haddock at two different water temperatures; (c) the maximum voluntary swimming speed of haddock at two different water temperatures; (d) the tail beat frequency used by fish as they leave the codend mesh in late summer and late winter.
2. Materials and methods 2.1. Measurement of muscle contraction time Experiments were conducted on board Research Vessel, ‘‘Walther Herwig III’’, during a 3-week trawl selectivity trip between 9 and 30 November 1996. Haddock were obtained from 2 h tows in the Start Point and Copinsay areas, 5–20 miles off the East Coast of the Orkney Islands. Live haddock were selected from the codend immediately after hauls and placed in an aerated seawater tank (1 m3) at ambient water temperature of 12 8C. They were rested in the tank for 18–24 h to allow recovery from fatigue. Detailed observations of muscle contraction time at different temperatures were made from 10 selected haddock (total length, TL: 28–32 cm). Each rested haddock was killed by severing the spinal cord behind the head and crushing the brain. The body length was measured and the two muscle blocks representing leftand right-side anterior dorsal white lateral muscles (length 3 cm 1 cm 1 cm) were removed from locations extending tailwards from just behind the operculum. To follow the change in contraction time with temperature, the muscle samples were first placed in a refrigerator at 4 8C to cool. Twenty minutes later, one of the samples was stretched to its original length and impaled on two needle electrodes 1.8 cm apart. The two needles were mounted on a PVC plate
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ð25 mm 10 mm 1 mmÞ of a force transducer similar to that pictured in Wardle (1985, Fig. 4). An electric stimulus pulse (15 V and 2 ms, Digitimer Stimulator DS9A) was applied to the muscle block through the two needles. The muscle between the two needles contracted and the force transducer converted this contraction to a voltage in a resistance bridge connected to an amplifier (Gould universal amplifier, model 13-4615-58). The stimulus pulse and the muscle tension were displayed on an oscilloscope (Gould Data SYS 760) and the time from stimulus to peak force measured as twitch contraction time. To minimise the measurement error, each twitch was repeated three times, with a maximum of 5 s interval, at the same temperature, and the average value was recorded. The temperature of the muscle block was measured using a needle thermometer (P.I. 8013 type K) inserted into its centre. A series of measurements taking a total time of up to 60 min was made in this way as the muscle gradually warmed up to room temperature (14 8C). The second muscle sampled from the same fish was then measured in the same way. Average contraction times were plotted against the muscle temperature, and a logarithmic regression line was fitted to the data. The significance of each regression coefficient was tested through a t-test. 2.2. Measuring fast start reactions at two temperatures Video-taped recordings of fast start reactions of haddock were made in the Fish Behaviour Unit at the Marine Laboratory, Aberdeen, in February 1997 (four fish, TL ¼ 33 2 cm, 7 8C) and September 1997 (four fish, TL ¼ 30 2 cm, 12.3 8C). The fish were acclimatised to the water temperature of the observation tank (1.55 m diameter, 0.6 m depth) for a minimum of 10 days prior to the experiments. They were fed on chopped sandeels three times a week. A video camera (Panasonic, WV CL 350, 8 mm lens) was mounted on a metal frame 0.9 m above the tank looking through the glass bottom of a floating raft at a marked sheet of reflex reflective material coating the tank bottom (3 M, Scotchlite, 3270). Recordings using this camera provided 50 TV frames per second (one frame for each 20 ms). The fish were illuminated with a strobe light (Dawe, Strobetorch, 1222 A) mounted beside the TV camera lens which was flashed (50 ms) every 10 ms
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to give two recorded frozen silhouettes of any fast movements in each TV frame (as used by Wardle, 1975). The Scotchlite was marked with crosses at the corners of 25 cm squares. With this arrangement, during the fast starts, a dark silhouette appearance of 100 fish images per second with sharp outlines on the brightmarked background were recorded on the videotape. A tactile stimulus was applied from a direction posterior to the fish using a plastic rod. Each experimental session elicited 10 recorded escapes. Then the fish were left to rest until the next day. One hundred fast start recordings were made with this method in February and again in September 1997. Recordings were analysed frame-byframe using a standard VHS video recorder (Panasonic AG-7350-B) and a TV monitor (JVC BM-H2000PN) and the times measured for stages 1 and 2 of the C-start (after Weihs, 1973; Webb, 1976). Significance of the mean time taken to complete each stage in February and September was separately determined by t-tests. 2.3. Maximum voluntary swimming speed at two different water temperatures Haddock were trained to race between lights positioned at the two ends of an oval tank (64 m, depth 1 m) and which could be flashed on and off for 0.5 s period to attract the fish (Wardle and Reid, 1977). The conditioned attraction to the lights was established by presentation of a food reward of chopped sandeels at the end where the light is flashing. The races were filmed with the same four haddock in October 1996 (at 11.8 8C, TL ¼ 30 2 cm) and in March 1997 (at 7.2 8C, TL ¼ 33 2 cm). The racing fish were recorded by a TV camera mounted 3 m above the tank. This camera was recorded on videotape with 50 frames per second (20 ms each frame). In both October and March, the racing sessions were carried out for 10 days. Only one session was performed in a day and 10 food presentations were made in each session. Video recordings of the highest swimming speed at each temperature were selected and analysed frame-by-frame. 2.4. Comparison of tail beat frequency used by fish as they leave the codend mesh TV recordings of fish reactions were made using a remote-controlled TV vehicle positioned alongside a
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codend (see Glass and Wardle, 1995). A ‘‘North Sea’’ 600 hp trawl (Galbraith, 1983) was towed from Research Vessel ‘‘Clupea’’ in the Moray Firth in October 1992 and February 1993 at water temperatures of 12.8 and 6.5 8C, respectively. The time taken for each fish to complete one tail beat, after they penetrated through the meshes of the square mesh panel, was measured from the video film analysed frame-by-frame using the video recorder and the TV monitor. The underwater TV camera (Osprey SIT camera) creates 50 frames per second (each frame is 20 ms). The target species was haddock, but occasional whiting, Merlangius merlangus, may have been included in the measurements as the appearance of these two species, when moving fast, was not always clearly separable in these recordings. Total lengths of the fish were estimated by comparing them with the known bar length of the adjacent square mesh netting. From the data obtained, the tail beat frequencies were calculated for each fish. Tail beat frequencies at length were plotted in a graph both for October and February. Then, a logarithmic regression equation line was fitted to each set of data. Significance of each of the regression coefficient was tested by a t-test. 3. Results 3.1. Muscle contraction time and prediction of maximum tail beat frequency The twitch contraction time of muscles dissected from the 10 haddock decreases logarithmically with
Fig. 1. Muscle contraction times of haddock (TL ¼ 2832 cm) measured at temperatures between 6 and 13 8C. A logarithmic regression line is fitted to the data points.
increasing temperature (Fig. 1). At 7 and 12 8C, muscle contraction times of 42 and 29.3 ms are obtained from the regression equation (Fig. 1). However, when considering the maximum swimming speed at a given temperature, the relevant muscle contraction time is the shortest recorded in that temperature, not the mean. The recorded minimum muscle contraction times (MMCTs) for different rounded temperature values between 6 and 13 8C are given in Table 1. The maximum tail beat frequencies (MTBFs) between 6 and 13 8C are calculated (as in
Fig. 2. Predicted maximum tail beat frequencies of haddock between 6 and 13 8C muscle temperature.
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Table 1 MMCTs (in ms) at rounded temperatures (in 8C), and from these values, calculated MTBFs (in Hz) of haddock Temperature
MMCT
MTBF
6 7 8 9 10 11 12 13
45 38.8 37 31 28.8 27.8 27.6 24.4
11.1 12.9 13.5 16.1 17.4 18.0 18.1 20.5
Wardle, 1975) in Table 1 and plotted in Fig. 2. For a 30 cm haddock, the regression equation estimate shown in Fig. 2 provides a maximum tail beat frequency of 12.78 Hz at 7 8C and of 19.12 Hz at 12 8C. 3.2. Fast start reaction at two temperatures The recorded fast start escape responses were usually divisible into three main stages (as described by Weihs, 1973). There was a stereotypical preparatory first body bend (stage 1) in all the responses where the fish bent into a C-shape (Fig. 3b). Stage 1 was usually followed by a propulsive stroke (stage 2) during which the tail was rapidly swept across to the opposite side to create the forward thrust (Fig. 3c). However, this was not a stereotypical movement. In some escapes, the propulsive stroke was cut short and interrupted by a glide when the tail steered the body rather than propelled it. Gliding during stage 2 resulted in a more prolonged tail movement that lasted more than 100 ms. If the fish propelled during stage 2, stage 3 was either a rhythmic tail beat or gliding. As the diameter of the experimental tank (1.55 m) was not large enough to provide space for continuous swimming, stage 3 was very often a glide. Therefore, stage 3 as well as the responses that lasted over 100 ms in stage 2 were not included in the comparison of the two temperatures. Fig. 4 compares the time taken to complete stages 1 and 2 at water temperatures of 7 and 12 8C. Escape responses of the fish were considerably slower at 7 8C than those at 12 8C. The fastest time to complete stage 1 is 40 ms at 7 8C and 20 ms at 12 8C (Fig. 4a). The
Fig. 3. Body bends of a haddock during C-start escape initiated with tactile stimuli: (a) start position; (b) end of stage 1; (c) end of stage 2; (d) end of stage 3.
mean time taken to complete stage 1 at 7 8C (63.9 ms, S.E. 1.2) was significantly slower than that at 12 8C (40.1 ms, S.E. 0.92) (t-test, P < 0:001). The fish were also significantly slower at completing stage 2 at 7 8C than at 12 8C (Fig. 4b). The minimum time to complete stage 2 was 50 ms at 7 8C and 40 ms at
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Fig. 4. (a) Comparison of time taken to complete stage 1 during the C-start escape responses of haddock at water temperatures of 7 and 12 8C. Number of observations are 100 at each temperature. (b) Comparison of time taken to complete stage 2 during the C-start escape responses of haddock at water temperatures of 7 and 12 8C. Total number of observations for the stage 2 duration that were equal to or shorter than 100 ms are 84 at 12 8C (in September) and 63 at 7 8C (in February) out of 100 for each.
12 8C. The mean time taken to complete stage 2 was significantly slower at 7 8C (108.5 ms, S.E. 5.3) than at 12 8C (72.6 ms, S.E. 2.6) (t-test, P < 0:001). 3.3. Maximum voluntary swimming speed at two temperatures While racing each other between the flashing feeding lights, haddock swam a distance of three body lengths in 380 ms at 7 8C (i.e., 7.9 TL s1) and three body lengths in 240 ms at 12 8C (i.e., 12.5 TL s1). An increase from 7 to 12 8C resulted in a 58.2% increase in the maximum voluntary swimming speed of haddock. Mean stride lengths of haddock measured in five sets was 0.734 TL (S.E. 0.0058). This means that a
haddock of 30 cm swims a distance of 22 cm with each completed tail beat cycle. 3.4. Tail beat frequencies used by fish as they leave the codend mesh Haddock showed two different methods of passing through a square mesh window. Some fish slowly work their head out through a mesh opening, give a large amplitude tail beat and then glide away from the netting. Other fish perform a powerful acceleration manoeuvre towards the panel, pass through the mesh and continue with the fast swim as they clear the meshes. Tail beat frequencies were measured only for this second type of escape and compared for the October and February cruises (Fig. 5). The results
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Fig. 5. Comparison of the tail beat frequency used by fish as they swim away from a mesh opening of a square mesh codend escape panel in February at 6.5 8C and in October at 12.8 8C.
showed that haddock in October (water temperature 12.8 8C) are making use of much higher tail beat frequencies than those observed in February (water temperature 6.5 8C). Fig. 5 also indicates that larger fish use slower tail beat frequencies.
4. Discussion Results from all four investigations of haddock performance confirm that a seasonal increase of temperature increases the maximum swimming ability of the fish. The maximum swimming speed was significantly higher for late summer water temperature of 12 8C than for late winter temperature of 7 8C. From the relationship between tail beat frequency and muscle temperature (Fig. 2), it is evident that at 7 8C the frequency is 13 Hz and rises to 19.5 Hz at 12 8C, which indicates that a rise of 5 8C increases frequency by 6.5 Hz or a rise in temperature by 10 8C leads to a rise in frequency by 13 Hz, giving a Q10 8C of 2. This means that general underlying physiological effect of a temperature increase of 10 8C doubles the speed of the maximum swimming ability. Videler and Wardle (1991) calculated Q10 8C values for eight sizes of cod (from 20 to 84.5 cm TL) at seven temperatures between 2 and 15 8C. They found an average value of 2.06 (S.D. 0.1). Q10 8C values of about 2 are common for the rate of biological enzyme-catalysed chemical reactions at the animal’s normal living tem-
perature range (Videler and Wardle, 1991; Videler, 1993). The changes in contraction time with temperature for haddock follow a very similar pattern to those reported for cod, plaice, lemon sole, mackerel and tuna (Wardle, 1980). An experiment comparing the contraction time of muscle from plaice previously held for 8 weeks at 8 and 18 8C showed no compensation of muscle twitch time (Wardle, 1980). In the racing experiment, the fish were kept for 3 months while the natural temperature changed from 12 to 7 8C. The haddock, first trained to swim at 12 8C, grew about 3 cm (approximately 10%) during the 3 months before they were tested at 7 8C. An increase in size slowed the tail beat frequency used by the fish observed in the codend (Fig. 5). On the same lines as the Q10 8C value, Videler and Wardle (1991) calculated an average Q10 cm value of 0.886 (S.D. 0.006) over a size range of 20–84.5 cm TL cod. This value was therefore applied to correct by slowing down the swimming speed due to the length increase in the fish raced at 7 8C. The 3 cm smaller haddock after this correction would be swimming at 8.2 TL s1 instead of 7.9 TL s1. This means that the voluntary maximum swimming speed of haddock at 12 8C (12.5 TL s1) can still be 52.4% higher than it is at 7 8C (8.2 TL s1). Arimoto et al. (1991) also observed an increase in muscle twitch contraction time from 60–80 ms for 20–30 cm fish (walleye pollack, Theregra chalcogramma) to 90–120 ms for 40–50 cm fish, at water temperature of 2 8C.
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Wardle (1975) pointed out that the speed of a steadily swimming fish is the product of stride length and tail beat frequency. A stride length for haddock of 0.734 TL was measured during slow, steady, straight swimming when the fish were resting between the races. The MTBF obtained from Table 1 for 28–32 cm haddock were 12.9 and 18.1 Hz at 7 and 12 8C, respectively. When these numbers are multiplied by 0.734, predicted maximum swimming speeds of haddock are 9.5 TL s1 at 7 8C and 13.3 TL s1 at 12 8C, which are slightly higher than those observed during fish racing experiments. This is expected as fish do not necessarily use their full speed in a voluntary race across a 6 m tank. Webb (1978) investigated the effect of temperature (at 5, 10, 15, 20 and 25 8C) on acceleration performance of small rainbow trout, Oncorhynchus mykiss (of mean weight 23.5 g) during fast starts initiated by an electric shock stimulus. He found that the temperature had little effect on the details of the fast start kinematics, but the time taken for each stage decreased with increasing temperature. Response latencies decreased from 23 ms at 5 8C to 6 ms at 25 8C, and the times to complete the first two stages decreased from 116 ms at 5 8C to 65 ms at 25 8C. Low water temperature does add to the problems of small fish offered opportunities to leave trawl codends. When startled by a tactile stimuli in laboratory tanks, the escape speed of haddock was slower at 7 8C than at 12 8C (Figs. 4 and 5). The results from the analyses of the underwater video camera recordings of the escape speed of fish from the meshes of a trawl gear (Fig. 5) reflect the same effects. The results show that small seasonal changes in the water temperature can have marked effects on the ability of fish to manoeuvre. Trawls are towed at the same speed throughout the different seasons but temperature will alter the swimming performance thresholds of the fish. These thresholds are real limits to the fish’s ability and its behaviour. Changes in temperature could lead to quite different size-related responses throughout the capture process. Temperature reduction will lower the ability of an animal to manoeuvre through the mesh of a rapidly moving net panel. This could lead to failed attempts or absence of attempts if the animal is restricted by its own limitations. Seasonal temperature could explain the higher escape ratio of a given length and/or girth of fish in
¨ zbilgin, 1998). SwimSeptember than in February (O ming faster generates more drag and force required to overcome drag is proportional to velocity-squared. At the end of the summer feeding period, fish have a larger gutted weight and girth for the same length ¨ zbilgin, 1998). This means that (Coull et al., 1989; O fish in September have a larger cross-sectional area of swimming muscle than fish of same length in February. The muscle cross-sectional area is proportional to and the limit of maximum force. More force is needed to make use of the higher tail beat frequencies available due to the higher temperatures in late summer. Doubling the tail beat frequency requires four times the force to overcome the drag. Fish need to be stimulated to escape through windows, especially when these are positioned forwards away from the codend. Both visual stimuli like a change in netting colour or a black tunnel (Glass and Wardle, 1995; Glass et al., 1995) and sound stimuli with hum bars (Watson et al., 1993; Brewer et al., 1998) have been proposed. Even with an appropriate strong stimulus, the problem for the fish is the fast flow parallel to the netting. The faster the flow, the greater is the swimming ability needed to exit a mesh, only just large enough to let it through, without being swept into contact with the downstream side of the mesh. Temperature affects this manoeuvring ability, thus making escapes easier in warm water than in cold water. Effect of water temperature on the swimming ability of fish is clearly one of the factors influencing the escape speed and so the survival probability of fish. Relatively higher water temperature is most likely to be one of the factors leading to a better selectivity of a ¨ zbilgin et al., 1996). This point needs trawl codend (O to be further investigated for some other commercially important species. If similar experimental results are obtained, their significance should be tested. Then, temperature-related selective properties of trawl codends could be taken into account while taking management measures. Similarly, such information could be used in estimating the efficiency of sampling gears used in stock assessment studies.
Acknowledgements Thanks are due to Ben Williamson, Ally Findlay and John Byrne for expert help in maintaining and
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feeding experimental haddock and to the captain and the crew of ‘‘Walther Herwig III’’ and Erdman Dahm of the Institut fur Fangetechnik, Hamburg, for providing the haddock used in muscle contraction ¨ zbilgin acknowledges the Ministry of experiments. O National Education of the Turkish Republic for the support during his Ph.D. studies at the Marine Laboratory, in Aberdeen. References Arimoto, T., Gang, X., Matsushida, Y., 1991. Muscle contraction time of captured walleye pollock, Theregra chalcogramma. Nippon Suisan Gakkaishi 57 (7), 1225–1228. Brewer, D., Rawlinson, N., Eayrs, S., Burridge, C., 1998. An assessment of bycatch reduction devices in a tropical Australian prawn trawl fishery. Fish. Res. 36, 195–215. Carey, F.G., Teal, J.M., 1969. Regulation of body temperature by the bluefin tuna. Comp. Biochem. Physiol. 28, 205–214. Coull, K.A., Jermyn, A.S., Newton, A.W., Henderson, G.I., Hall, W.B., 1989. Length/weight relationships for 88 species of fish encountered in the northeast Atlantic. Scottish Fish. Res. Rep. 43, 81. Dean, J.M., 1976. Temperature of tissues in freshwater fishes. Trans. Am. Fish. Soc. 105, 709–711. Galbraith, R.D., 1983. The Marine Laboratory four panel trawl. Scottish Fish. Inform. Pamphlet 8, 21. Glass, C.W., Wardle, C.S., 1989. Comparison of the reactions of fish to a trawl gear, at high and low light intensities. Fish. Res. 7, 249–266. Glass, C.W., Wardle, C.S., 1995. Studies on the use of visual stimuli to control fish escapes from codends. II. The effect of a black tunnel on the reaction behaviour of fish in otter trawl codends. Fish. Res. 23, 165–174. Glass, C.W., Wardle, C.S., Gosden, S.J., Racey, D., 1995. Studies on the visual stimuli to control fish escape from codends. I. Laboratory studies on the effect of a black tunnel on mesh penetration. Fish. Res. 23, 157–164. He, P., 1993. Swimming speeds of marine fish in relation to fishing gears. ICES Mar. Sci. Symp. 196, 183–189. Kishinouye, K., 1923. Contributions to the comparative study of the so-called scombroid fishes. J. Agric. Im. Univ. Tokyo 8, 293–475. Main, J., Sangster, G.I., 1981. A study of the fish capture process in a bottom trawl by direct observations from a towed underwater vehicle. Scottish Fish. Res. Rep. 23, 23. ¨ zbilgin, H., 1998. The seasonal variation of trawl codend O selectivity and the role of learning in mesh penetration behaviour of fish. Ph.D. Thesis. Aberdeen University, Aberdeen, 206 pp. ¨ zbilgin, H., Ferro, R.S.T., Robertson, J.H.B., Hutcheon, J.R., O Kynoch, R.J., Holtrop, G., 1996. Seasonal variation in codend
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selectivity of haddock. Int. Coun. Explor. Sea CM 1996/B: 18, 7 pp. Robert, J.L., Graham, J.B., 1979. Effect of swimming speed on the excess temperature and activities of heart and red and white muscles in the mackerel, Scomber japonicus. Fish. Bull. US 76, 861–867. Turrell, W.R., Slesser, G., 1992. Annual cycles of physical, chemical and biological parameters in Scottish waters. Scottish Fisheries Working Paper. No. 5/92. Videler, J.J., 1993. Fish Swimming. Chapman & Hall, London, 260 pp. Videler, J.J., Wardle, C.S., 1991. Fish swimming stride by stride: speed limits and endurance. Rev. Fish Biol. Fish. 1, 23–40. Walsh, S.J., Hickey, W.M., 1993. Behavioural reactions of demersal fish to bottom trawls at various light conditions. ICES Mar. Sci. Symp. 96, 68–76. Wardle, C.S., 1975. Limit of fish swimming speed. Nature 255 (5511), 725–727. Wardle, C.S., 1977. Effect of size on the swimming speed of the fish. In: Pedley, T.J. (Ed.), Scale Effects in Animal Locomotion. Academic Press, London, pp. 299–313. Wardle, C.S., 1980. Effects of temperature on the maximum swimming speeds of fishes. In: Ali, M.A. (Ed.), Environmental Physiology of Fishes. NATO Advanced Study Institute Series A 35, pp. 519–532. Wardle, C.S., 1985. Swimming activity in marine fish. In: Laverack, M. (Ed.), Physiological Adaptations of Marine Animals. Proceedings of the Symposium of the Society for Experimental Biology, Vol. 39. The Company of Biologists, pp. 521–540. Wardle, C.S., 1993. Fish behaviour and fishing gears. In: Pitcher, T.J. (Ed.), The Behaviour of Teleost Fishes, 2nd Edition. Fish and Fisheries Series 7. Chapman & Hall, London, pp. 609–643. Wardle, C.S., He, P., 1996. Fish Behaviour near trawls—recent advances. In: Inoue, et al. (Eds.), Proceedings of the Workshop on Co-operation Research in Asian Fishing Technology (CRAFT), Vol. 2. The National Research Institute of Fisheries Engineering, Hasaki, Japan, pp. 35–44. Wardle, C.S., Reid, A., 1977. The application of large amplitude elongated body theory to measure swimming power in fish. In: Steele, J.H. (Ed.), Fisheries Mathematics. Academic Press, London, pp. 171–191. Watson, J., Workman, I., Foster, D., Taylor, C., Shah, A., Barbour, J., Hataway, D., 1993. Status report on the potential of gear modifications to reduce finfish bycatch in shrimp trawls in the southeastern United States 1990–1992. NOAA Technical Memorandum NMFS-SEFSC-327, 131 pp. Webb, P.W., 1976. The effect of size on the fast start performance of rainbow trout, Salmo gairdneri, and a consideration of piscivorous predator–prey interactions. J. Exp. Biol. 65, 157– 177. Webb, P.W., 1978. Temperature effects on acceleration of rainbow trout, Salmo gairdneri. J. Fish. Res. Biol. Can. 35, 1417–1422. Weihs, D., 1973. The mechanism of rapid starting of slender fish. Biorheology 10, 343–350.
Effect of seasonal temperature changes on the escape behaviour of haddock, Melanogrammus aeglefinus, from the codend ¨ zbilgina,*, C.S. Wardleb H. O Fisheries Faculty, Fish Capture Department, Ege University, Bornova, I¨zmir 35100, Turkey b Marine Laboratory, P.O. Box 101, Aberdeen AB11 9DB, UK
a
Received 29 December 2000; received in revised form 14 September 2001; accepted 19 September 2001
Abstract Sea temperature around the coast of Scotland rises from 7 8C in late winter to 12 8C in late summer. Recently, it was shown that the selectivity of trawl codend was poorer in late winter than it was in late summer. A change in water temperature is expected to affect the escape speed of fish, and therefore the selectivity of the gear. Four experiments carried out in this study showed the effect of this seasonal temperature change on the ability of haddock, Melanogrammus aeglefinus, to escape from a codend. A temperature increase from 7 to 12 8C changed the minimum twitch contraction time of the lateral muscle from 38.8 to 27.6 ms. This gives a maximum tail beat frequency of 12.78 Hz at 7 8C rising to 19.12 Hz at 12 8C. Escape reflexes of the fish were significantly slower at 7 8C than at 12 8C ðP < 0:001Þ. The shortest time to complete first body bend (stage 1) was 40 ms at 7 8C and 20 ms at 12 8C. The mean time taken to complete stage 1 at 7 8C (63.9 ms, S.E. 1.2) was significantly (t-test, P < 0:001) longer than at 12 8C (40.1 ms, S.E. 0.92). The mean time taken to complete the propulsive stroke (stage 2) of the fast start was significantly (t-test, P < 0:001) longer at 7 8C (108.5 ms, S.E. 5.3) than at 12 8C (72.6 ms, S.E. 2.6). The maximum speed recorded while competitively swimming for food reward was 7.9 L s1 at 7 8C and 12.5 L s1 at 12 8C. The observed maximum tail beat frequency used by haddock when leaving the mesh of a codend escape panel changed from 12 to 25 Hz. The general underlying physiological effect of a temperature increase of 10 8C was to double the speed of the maximum swimming ability ðQ10 C ¼ 2Þ. The effect of a change of only 5 8C on the ability to manoeuvre out of codend selection devices is discussed. Crown Copyright # 2002 Published by Elsevier Science B.V. All rights reserved. Keywords: Selectivity; Codend; Escape; Haddock; Temperature; Swimming behaviour
1. Introduction Both girth- and length-based selectivity curves of the haddock, Melanogrammus aeglefinus, in the North Sea, demonstrate that some size classes of fish did not escape from the codend at the end of the winter ¨ zbilgin, 1998). whereas they did in late summer (O *
Corresponding author. Tel.: þ90-532-706-19-77; fax: þ90-232-388-36-85. ¨ zbilgin). E-mail address: [email protected] (H. O
Late winter and late summer coincide with the minimum and maximum of the annual cycle of the sea water temperature changing from 7 to 12 8C (Turrell and Slesser, 1992), which is typical for the annual change at the fishing grounds where the selectivity experiments were conducted. The close proximity of fish’s blood with the water at the fish’s gills means body temperatures of most fish closely match the water temperature even when they are actively swimming (Dean, 1976; Robert and Graham, 1979). There are a few larger pelagic cruising species
0165-7836/02/$ – see front matter. Crown Copyright # 2002 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 7 8 3 6 ( 0 1 ) 0 0 3 9 4 - 0
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that have evolved special heat exchangers to keep their red swimming muscle warm (Kishinouye, 1923; Carey and Teal, 1969). As discussed in Wardle (1993), when light levels are above the threshold for vision, there is a sequence of behavioural events mainly limited by swimming ability that leads fish from the mouth of a trawl to the codend. Two major influences on swimming ability are fish size and water temperature (Wardle, 1975, 1980; He, 1993; Videler, 1993; Wardle and He, 1996). Towed trawls move forward at speeds between 1 and 2 m s1 (2–4 knots). Fish of all sizes attempt to swim at the towing speed in the mouth of the net (Main and Sangster, 1981), but there is a difference in ability to do this between small and large fish of the same species (Wardle, 1977). The smallest fish are able to keep up for only brief periods; they tire quickly and drop back into the net. Some of the largest fish may swim easily at the towing speed and never drop back. The time spent in the mouth varies with the size-dependant degree of exhaustion. A particular degree of exhaustion is probably the trigger for this change in reaction behaviour so that fish arriving in the codend are inevitably partially exhausted. There are a few observations at light levels below visual reaction thresholds that show fish not reacting until touched by the gear (Walsh and Hickey, 1993; Glass and Wardle, 1989). The speed of these sudden movements will also be reduced in cold water but further research is needed on the mechanisms involved in escape or otherwise of fish randomly performing fast speeds in darkness. There have recently been many different designs for codends intended to offer increased opportunities for those fish small enough to pass out through the meshes. Conventional codends have only a narrow band of open meshes just ahead of the bulging catch. Ahead of this, there are sometimes open mesh panels built into the wall of the extension designed to provide many more opportunities for the smaller fish to swim out. Direct observations, films and comparative fishing trials have shown that these various arrangements are sometimes but not always successful (Watson et al., 1993; Brewer et al., 1998). Observers have concluded that the degree of stimulus to leave the net, as well as the swimming ability to cope with the complex and sometimes fast water flows, are key variable factors to fish escape attempts (Wardle, 1993; Glass and Wardle, 1995). In order to hold station and then pass out of a fast-moving codend, undersized fish have to swim at or
very close to their maximum speed, which is reduced at colder temperatures (He, 1993; Wardle, 1993). This paper investigates the effect of the temperature change on the ability of haddock to swim and manoeuvre, and addresses the question ‘‘Is a seasonal change from 7 to 12 8C large enough to cause a significant difference in the selectivity of a codend?’’ The effect of temperature is investigated in four different experiments: (a) measurements of the minimum contraction time of the haddock’s swimming muscles and prediction of the maximum tail beat frequency at different temperatures; (b) the time taken for the fast start reaction of haddock at two different water temperatures; (c) the maximum voluntary swimming speed of haddock at two different water temperatures; (d) the tail beat frequency used by fish as they leave the codend mesh in late summer and late winter.
2. Materials and methods 2.1. Measurement of muscle contraction time Experiments were conducted on board Research Vessel, ‘‘Walther Herwig III’’, during a 3-week trawl selectivity trip between 9 and 30 November 1996. Haddock were obtained from 2 h tows in the Start Point and Copinsay areas, 5–20 miles off the East Coast of the Orkney Islands. Live haddock were selected from the codend immediately after hauls and placed in an aerated seawater tank (1 m3) at ambient water temperature of 12 8C. They were rested in the tank for 18–24 h to allow recovery from fatigue. Detailed observations of muscle contraction time at different temperatures were made from 10 selected haddock (total length, TL: 28–32 cm). Each rested haddock was killed by severing the spinal cord behind the head and crushing the brain. The body length was measured and the two muscle blocks representing leftand right-side anterior dorsal white lateral muscles (length 3 cm 1 cm 1 cm) were removed from locations extending tailwards from just behind the operculum. To follow the change in contraction time with temperature, the muscle samples were first placed in a refrigerator at 4 8C to cool. Twenty minutes later, one of the samples was stretched to its original length and impaled on two needle electrodes 1.8 cm apart. The two needles were mounted on a PVC plate
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ð25 mm 10 mm 1 mmÞ of a force transducer similar to that pictured in Wardle (1985, Fig. 4). An electric stimulus pulse (15 V and 2 ms, Digitimer Stimulator DS9A) was applied to the muscle block through the two needles. The muscle between the two needles contracted and the force transducer converted this contraction to a voltage in a resistance bridge connected to an amplifier (Gould universal amplifier, model 13-4615-58). The stimulus pulse and the muscle tension were displayed on an oscilloscope (Gould Data SYS 760) and the time from stimulus to peak force measured as twitch contraction time. To minimise the measurement error, each twitch was repeated three times, with a maximum of 5 s interval, at the same temperature, and the average value was recorded. The temperature of the muscle block was measured using a needle thermometer (P.I. 8013 type K) inserted into its centre. A series of measurements taking a total time of up to 60 min was made in this way as the muscle gradually warmed up to room temperature (14 8C). The second muscle sampled from the same fish was then measured in the same way. Average contraction times were plotted against the muscle temperature, and a logarithmic regression line was fitted to the data. The significance of each regression coefficient was tested through a t-test. 2.2. Measuring fast start reactions at two temperatures Video-taped recordings of fast start reactions of haddock were made in the Fish Behaviour Unit at the Marine Laboratory, Aberdeen, in February 1997 (four fish, TL ¼ 33 2 cm, 7 8C) and September 1997 (four fish, TL ¼ 30 2 cm, 12.3 8C). The fish were acclimatised to the water temperature of the observation tank (1.55 m diameter, 0.6 m depth) for a minimum of 10 days prior to the experiments. They were fed on chopped sandeels three times a week. A video camera (Panasonic, WV CL 350, 8 mm lens) was mounted on a metal frame 0.9 m above the tank looking through the glass bottom of a floating raft at a marked sheet of reflex reflective material coating the tank bottom (3 M, Scotchlite, 3270). Recordings using this camera provided 50 TV frames per second (one frame for each 20 ms). The fish were illuminated with a strobe light (Dawe, Strobetorch, 1222 A) mounted beside the TV camera lens which was flashed (50 ms) every 10 ms
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to give two recorded frozen silhouettes of any fast movements in each TV frame (as used by Wardle, 1975). The Scotchlite was marked with crosses at the corners of 25 cm squares. With this arrangement, during the fast starts, a dark silhouette appearance of 100 fish images per second with sharp outlines on the brightmarked background were recorded on the videotape. A tactile stimulus was applied from a direction posterior to the fish using a plastic rod. Each experimental session elicited 10 recorded escapes. Then the fish were left to rest until the next day. One hundred fast start recordings were made with this method in February and again in September 1997. Recordings were analysed frame-byframe using a standard VHS video recorder (Panasonic AG-7350-B) and a TV monitor (JVC BM-H2000PN) and the times measured for stages 1 and 2 of the C-start (after Weihs, 1973; Webb, 1976). Significance of the mean time taken to complete each stage in February and September was separately determined by t-tests. 2.3. Maximum voluntary swimming speed at two different water temperatures Haddock were trained to race between lights positioned at the two ends of an oval tank (64 m, depth 1 m) and which could be flashed on and off for 0.5 s period to attract the fish (Wardle and Reid, 1977). The conditioned attraction to the lights was established by presentation of a food reward of chopped sandeels at the end where the light is flashing. The races were filmed with the same four haddock in October 1996 (at 11.8 8C, TL ¼ 30 2 cm) and in March 1997 (at 7.2 8C, TL ¼ 33 2 cm). The racing fish were recorded by a TV camera mounted 3 m above the tank. This camera was recorded on videotape with 50 frames per second (20 ms each frame). In both October and March, the racing sessions were carried out for 10 days. Only one session was performed in a day and 10 food presentations were made in each session. Video recordings of the highest swimming speed at each temperature were selected and analysed frame-by-frame. 2.4. Comparison of tail beat frequency used by fish as they leave the codend mesh TV recordings of fish reactions were made using a remote-controlled TV vehicle positioned alongside a
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codend (see Glass and Wardle, 1995). A ‘‘North Sea’’ 600 hp trawl (Galbraith, 1983) was towed from Research Vessel ‘‘Clupea’’ in the Moray Firth in October 1992 and February 1993 at water temperatures of 12.8 and 6.5 8C, respectively. The time taken for each fish to complete one tail beat, after they penetrated through the meshes of the square mesh panel, was measured from the video film analysed frame-by-frame using the video recorder and the TV monitor. The underwater TV camera (Osprey SIT camera) creates 50 frames per second (each frame is 20 ms). The target species was haddock, but occasional whiting, Merlangius merlangus, may have been included in the measurements as the appearance of these two species, when moving fast, was not always clearly separable in these recordings. Total lengths of the fish were estimated by comparing them with the known bar length of the adjacent square mesh netting. From the data obtained, the tail beat frequencies were calculated for each fish. Tail beat frequencies at length were plotted in a graph both for October and February. Then, a logarithmic regression equation line was fitted to each set of data. Significance of each of the regression coefficient was tested by a t-test. 3. Results 3.1. Muscle contraction time and prediction of maximum tail beat frequency The twitch contraction time of muscles dissected from the 10 haddock decreases logarithmically with
Fig. 1. Muscle contraction times of haddock (TL ¼ 2832 cm) measured at temperatures between 6 and 13 8C. A logarithmic regression line is fitted to the data points.
increasing temperature (Fig. 1). At 7 and 12 8C, muscle contraction times of 42 and 29.3 ms are obtained from the regression equation (Fig. 1). However, when considering the maximum swimming speed at a given temperature, the relevant muscle contraction time is the shortest recorded in that temperature, not the mean. The recorded minimum muscle contraction times (MMCTs) for different rounded temperature values between 6 and 13 8C are given in Table 1. The maximum tail beat frequencies (MTBFs) between 6 and 13 8C are calculated (as in
Fig. 2. Predicted maximum tail beat frequencies of haddock between 6 and 13 8C muscle temperature.
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Table 1 MMCTs (in ms) at rounded temperatures (in 8C), and from these values, calculated MTBFs (in Hz) of haddock Temperature
MMCT
MTBF
6 7 8 9 10 11 12 13
45 38.8 37 31 28.8 27.8 27.6 24.4
11.1 12.9 13.5 16.1 17.4 18.0 18.1 20.5
Wardle, 1975) in Table 1 and plotted in Fig. 2. For a 30 cm haddock, the regression equation estimate shown in Fig. 2 provides a maximum tail beat frequency of 12.78 Hz at 7 8C and of 19.12 Hz at 12 8C. 3.2. Fast start reaction at two temperatures The recorded fast start escape responses were usually divisible into three main stages (as described by Weihs, 1973). There was a stereotypical preparatory first body bend (stage 1) in all the responses where the fish bent into a C-shape (Fig. 3b). Stage 1 was usually followed by a propulsive stroke (stage 2) during which the tail was rapidly swept across to the opposite side to create the forward thrust (Fig. 3c). However, this was not a stereotypical movement. In some escapes, the propulsive stroke was cut short and interrupted by a glide when the tail steered the body rather than propelled it. Gliding during stage 2 resulted in a more prolonged tail movement that lasted more than 100 ms. If the fish propelled during stage 2, stage 3 was either a rhythmic tail beat or gliding. As the diameter of the experimental tank (1.55 m) was not large enough to provide space for continuous swimming, stage 3 was very often a glide. Therefore, stage 3 as well as the responses that lasted over 100 ms in stage 2 were not included in the comparison of the two temperatures. Fig. 4 compares the time taken to complete stages 1 and 2 at water temperatures of 7 and 12 8C. Escape responses of the fish were considerably slower at 7 8C than those at 12 8C. The fastest time to complete stage 1 is 40 ms at 7 8C and 20 ms at 12 8C (Fig. 4a). The
Fig. 3. Body bends of a haddock during C-start escape initiated with tactile stimuli: (a) start position; (b) end of stage 1; (c) end of stage 2; (d) end of stage 3.
mean time taken to complete stage 1 at 7 8C (63.9 ms, S.E. 1.2) was significantly slower than that at 12 8C (40.1 ms, S.E. 0.92) (t-test, P < 0:001). The fish were also significantly slower at completing stage 2 at 7 8C than at 12 8C (Fig. 4b). The minimum time to complete stage 2 was 50 ms at 7 8C and 40 ms at
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Fig. 4. (a) Comparison of time taken to complete stage 1 during the C-start escape responses of haddock at water temperatures of 7 and 12 8C. Number of observations are 100 at each temperature. (b) Comparison of time taken to complete stage 2 during the C-start escape responses of haddock at water temperatures of 7 and 12 8C. Total number of observations for the stage 2 duration that were equal to or shorter than 100 ms are 84 at 12 8C (in September) and 63 at 7 8C (in February) out of 100 for each.
12 8C. The mean time taken to complete stage 2 was significantly slower at 7 8C (108.5 ms, S.E. 5.3) than at 12 8C (72.6 ms, S.E. 2.6) (t-test, P < 0:001). 3.3. Maximum voluntary swimming speed at two temperatures While racing each other between the flashing feeding lights, haddock swam a distance of three body lengths in 380 ms at 7 8C (i.e., 7.9 TL s1) and three body lengths in 240 ms at 12 8C (i.e., 12.5 TL s1). An increase from 7 to 12 8C resulted in a 58.2% increase in the maximum voluntary swimming speed of haddock. Mean stride lengths of haddock measured in five sets was 0.734 TL (S.E. 0.0058). This means that a
haddock of 30 cm swims a distance of 22 cm with each completed tail beat cycle. 3.4. Tail beat frequencies used by fish as they leave the codend mesh Haddock showed two different methods of passing through a square mesh window. Some fish slowly work their head out through a mesh opening, give a large amplitude tail beat and then glide away from the netting. Other fish perform a powerful acceleration manoeuvre towards the panel, pass through the mesh and continue with the fast swim as they clear the meshes. Tail beat frequencies were measured only for this second type of escape and compared for the October and February cruises (Fig. 5). The results
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Fig. 5. Comparison of the tail beat frequency used by fish as they swim away from a mesh opening of a square mesh codend escape panel in February at 6.5 8C and in October at 12.8 8C.
showed that haddock in October (water temperature 12.8 8C) are making use of much higher tail beat frequencies than those observed in February (water temperature 6.5 8C). Fig. 5 also indicates that larger fish use slower tail beat frequencies.
4. Discussion Results from all four investigations of haddock performance confirm that a seasonal increase of temperature increases the maximum swimming ability of the fish. The maximum swimming speed was significantly higher for late summer water temperature of 12 8C than for late winter temperature of 7 8C. From the relationship between tail beat frequency and muscle temperature (Fig. 2), it is evident that at 7 8C the frequency is 13 Hz and rises to 19.5 Hz at 12 8C, which indicates that a rise of 5 8C increases frequency by 6.5 Hz or a rise in temperature by 10 8C leads to a rise in frequency by 13 Hz, giving a Q10 8C of 2. This means that general underlying physiological effect of a temperature increase of 10 8C doubles the speed of the maximum swimming ability. Videler and Wardle (1991) calculated Q10 8C values for eight sizes of cod (from 20 to 84.5 cm TL) at seven temperatures between 2 and 15 8C. They found an average value of 2.06 (S.D. 0.1). Q10 8C values of about 2 are common for the rate of biological enzyme-catalysed chemical reactions at the animal’s normal living tem-
perature range (Videler and Wardle, 1991; Videler, 1993). The changes in contraction time with temperature for haddock follow a very similar pattern to those reported for cod, plaice, lemon sole, mackerel and tuna (Wardle, 1980). An experiment comparing the contraction time of muscle from plaice previously held for 8 weeks at 8 and 18 8C showed no compensation of muscle twitch time (Wardle, 1980). In the racing experiment, the fish were kept for 3 months while the natural temperature changed from 12 to 7 8C. The haddock, first trained to swim at 12 8C, grew about 3 cm (approximately 10%) during the 3 months before they were tested at 7 8C. An increase in size slowed the tail beat frequency used by the fish observed in the codend (Fig. 5). On the same lines as the Q10 8C value, Videler and Wardle (1991) calculated an average Q10 cm value of 0.886 (S.D. 0.006) over a size range of 20–84.5 cm TL cod. This value was therefore applied to correct by slowing down the swimming speed due to the length increase in the fish raced at 7 8C. The 3 cm smaller haddock after this correction would be swimming at 8.2 TL s1 instead of 7.9 TL s1. This means that the voluntary maximum swimming speed of haddock at 12 8C (12.5 TL s1) can still be 52.4% higher than it is at 7 8C (8.2 TL s1). Arimoto et al. (1991) also observed an increase in muscle twitch contraction time from 60–80 ms for 20–30 cm fish (walleye pollack, Theregra chalcogramma) to 90–120 ms for 40–50 cm fish, at water temperature of 2 8C.
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Wardle (1975) pointed out that the speed of a steadily swimming fish is the product of stride length and tail beat frequency. A stride length for haddock of 0.734 TL was measured during slow, steady, straight swimming when the fish were resting between the races. The MTBF obtained from Table 1 for 28–32 cm haddock were 12.9 and 18.1 Hz at 7 and 12 8C, respectively. When these numbers are multiplied by 0.734, predicted maximum swimming speeds of haddock are 9.5 TL s1 at 7 8C and 13.3 TL s1 at 12 8C, which are slightly higher than those observed during fish racing experiments. This is expected as fish do not necessarily use their full speed in a voluntary race across a 6 m tank. Webb (1978) investigated the effect of temperature (at 5, 10, 15, 20 and 25 8C) on acceleration performance of small rainbow trout, Oncorhynchus mykiss (of mean weight 23.5 g) during fast starts initiated by an electric shock stimulus. He found that the temperature had little effect on the details of the fast start kinematics, but the time taken for each stage decreased with increasing temperature. Response latencies decreased from 23 ms at 5 8C to 6 ms at 25 8C, and the times to complete the first two stages decreased from 116 ms at 5 8C to 65 ms at 25 8C. Low water temperature does add to the problems of small fish offered opportunities to leave trawl codends. When startled by a tactile stimuli in laboratory tanks, the escape speed of haddock was slower at 7 8C than at 12 8C (Figs. 4 and 5). The results from the analyses of the underwater video camera recordings of the escape speed of fish from the meshes of a trawl gear (Fig. 5) reflect the same effects. The results show that small seasonal changes in the water temperature can have marked effects on the ability of fish to manoeuvre. Trawls are towed at the same speed throughout the different seasons but temperature will alter the swimming performance thresholds of the fish. These thresholds are real limits to the fish’s ability and its behaviour. Changes in temperature could lead to quite different size-related responses throughout the capture process. Temperature reduction will lower the ability of an animal to manoeuvre through the mesh of a rapidly moving net panel. This could lead to failed attempts or absence of attempts if the animal is restricted by its own limitations. Seasonal temperature could explain the higher escape ratio of a given length and/or girth of fish in
¨ zbilgin, 1998). SwimSeptember than in February (O ming faster generates more drag and force required to overcome drag is proportional to velocity-squared. At the end of the summer feeding period, fish have a larger gutted weight and girth for the same length ¨ zbilgin, 1998). This means that (Coull et al., 1989; O fish in September have a larger cross-sectional area of swimming muscle than fish of same length in February. The muscle cross-sectional area is proportional to and the limit of maximum force. More force is needed to make use of the higher tail beat frequencies available due to the higher temperatures in late summer. Doubling the tail beat frequency requires four times the force to overcome the drag. Fish need to be stimulated to escape through windows, especially when these are positioned forwards away from the codend. Both visual stimuli like a change in netting colour or a black tunnel (Glass and Wardle, 1995; Glass et al., 1995) and sound stimuli with hum bars (Watson et al., 1993; Brewer et al., 1998) have been proposed. Even with an appropriate strong stimulus, the problem for the fish is the fast flow parallel to the netting. The faster the flow, the greater is the swimming ability needed to exit a mesh, only just large enough to let it through, without being swept into contact with the downstream side of the mesh. Temperature affects this manoeuvring ability, thus making escapes easier in warm water than in cold water. Effect of water temperature on the swimming ability of fish is clearly one of the factors influencing the escape speed and so the survival probability of fish. Relatively higher water temperature is most likely to be one of the factors leading to a better selectivity of a ¨ zbilgin et al., 1996). This point needs trawl codend (O to be further investigated for some other commercially important species. If similar experimental results are obtained, their significance should be tested. Then, temperature-related selective properties of trawl codends could be taken into account while taking management measures. Similarly, such information could be used in estimating the efficiency of sampling gears used in stock assessment studies.
Acknowledgements Thanks are due to Ben Williamson, Ally Findlay and John Byrne for expert help in maintaining and
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