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SUBSTRATUM PREFERENCES OF JUVENILE FLAT FISH by N.A. NASIR (1) & M.G. POXTON (2) ABSTRACT.!-!Substratum preferences of young plaice (Pleuronectes platessa), turbot (Scophthalmus maximus) and Dover sole (Solea solea) were observed in the laboratory under different levels of illumination. Settlement of these fish on a sandy substratum was significantly more frequent than on other substrata and the particle size of the sand affects the settlement preferences. Furthermore the choices of substrata were also affected by the light intensity. RÉSUMÉ.!-!Préférences vis-à-vis du substrat chez les juvéniles de poissons plats. Les préférences vis-à-vis du substrat chez les jeunes plies (Pleuronectes platessa), turbots (Scophthalmus maximus) et soles communes (Solea solea) ont été observées au laboratoire sous différents niveaux d’éclairement. L’installation de ces espèces est significativement plus fréquente sur les substrats sableux que sur les autres substrats. Les résultats suggèrent également que la taille des particules sableuses affecte les préférences à l’installation sur le fond. Par ailleurs, le choix du substrat est affecté par l’intensité lumineuse. Key words.!- Pleuronectidae - Soleidae - Juveniles - Light intensity - Substratum preferences. Different field studies have attempted to quantify substrate complexity in relation to fish distribution in marine habitats (Bohlin, 1977; Luckhurst and Luckhurst, 1978; Scott, 1982; Rogers, 1991, 1992; Champallert et al., 1992) although some observations have been made under controlled laboratory conditions (Kwain and MacCrimmon, 1969; George and Hadley, 1979; Jacobsen, 1979; Howell and Canario, 1987; Gibson and Batty, 1990; Gibson and Robb, 1992; Ellis et al., 1997). Riley and Holford (1965) reported that young plaice (Pleuronectes platessa) avoided areas where they could not bury themselves. They deduced that suitable substrates, ranged from very fine to coarse sand (> 500 mm) were dependant on size of the plaice: newly-metamorphosed plaice settled on very fine sand and they were able to bury in coarse sand later in their first year. Poxton and Nasir (1985) found that the median diameter of low water sediments was correlated with the mean midsummer catches of O- group plaice in the Firth of Forth. At this time of year the fish were mainly distributed on fine to medium sand (186-480!mm). Our objective was to examine the preferences of common species of juvenile flatfish, such as plaice (Pleuronectes platessa), turbot (Scophthalmus maximus) and Dover sole (Solea solea), when offered different substrate types under different levels of illumination. The pur- pose of this study was also to compare measures of preference based on instantaneous and continuous observation in the field with those found from other studies in the laboratory. (1)!Marine Environmental and Fish Farming Services, 77 Briarcroft Road, Glasgow G33 1RA., SCOTLAND. [[email protected]] (2)!Aquaculture Associates International, The Firs, Glasshouse Hill, Codnor, Derbyshire DE5 9QT, U.K. Cybium 2001, 25(2): 109-117. 110 NASIR & POXTON MATERIALS AND METHODS Plaice of mean total length 28.5!±!6.7!mm were collected from Broad sands Bay in the Firth of Forth and kept in a constant temperature room at 15°C. They were allowed an accli- mation period of 3-4 days before being used in the experiments. Turbot of 22.2!±!5.4!mm were obtained from the Sea Fish Industry Authority at Ardtoe. Dover sole of 41.1!±!5.2!mm were supplied by Sea Fresh Farms Ltd. at Hunterston. They were maintained in 1!m2 fibre- glass tanks in a recirculating seawater system as described by Poxton et al. (1981). Water temperature was maintained at 16.0!±!1°C. Salinity remained steady at 34-36‰ while pH was about 7.0. Electric fluorescent 80!W white tubes provided light to the tanks. These were controlled by a timer with a built in dimmer to simulate dawn and dusk periods. Fish were fed twice a day with moist pellets constituted from fish meal. All tanks were cleaned of dead fish and remaining food on a daily basis. Various sediment types were presented in grey plastic trays measuring 21.8!x!16.3!x!5.1!cm. Eight of these trays were placed on the bottom of a polyvinylchloride tank measuring 79!x!59!x!30!cm, which was then filled with seawater to a depth of 20!cm prior to introducing the fish. At the end of the experimental period, the water was drained to the level of the top of the trays by siphoning. The preferred habitat was then determined by counting the number of fish in each tray, the number of the fish used in each experiment was ranged from 26 to 50, 19 to 48 and 16 to 50 for plaice, turbot and Dover sole, respectively. The positions of the trays were randomised in the tank for each replicate. Each replicate lasted 2-3!h, no single group of fish being used more than once a day. The fish fed some 1-1.5!h before the experiments as unfed fish tended to leave the bottom and swim. Initial experiments for each species were carried out as a basis for determining the duration of the later experiments. If the fish were left for much longer than the above period, they became hungry again and their reaction was to leave the bottom and swim in the water column in search of food. As a consequence 2-3!h was the only practical experimental period to be used during the experiments. Sand was collected from Broad Sands Bay while gravel and pebbles were collected from adjacent place (North Berwick harbour). All of the substrates were ignited in an oven at 600°C for 2!h to remove the organic content (Buchanan and Kain, 1971). Particular sediment types were then obtained using a sieve shaker. This work involved two separate experiments each of which presented the fish with eight choices of substrate types (4!x!2) as follows: Experiment 1.!-!Empty trays, sand (0.062-1.00!mm), gravel (1.00-2.00!mm) and pebbles (2.00-8.00!mm). Experiment 2.!-!Very fine (62-125!mm), fine (125-250!mm), medium (250-500!mm) and coarse sand (500-1000!mm). These above experiments were repeated under a series of light intensities: high light (6.68!mE/m2/s), low light (0.14!mE/m2/s) and dark conditions. The results were compared for significance using the c2 test. They were tested for the null hypothesis, that the fish showed no preference, by using the c2 goodness of fit - test. This parameter was calculated for number of fish on the four substrata types for each experiment under each level of illumination, c2, c2 1 2 and c2 respectively. The distribution of fish on very fine sand was also tested against the null 3 hypothesis of equal distribution using the c2 test. Other c2 form contingency tables were used in order to examine whether they were differences in numbers of fish observed in various substrates in the different experiments. Calculation of exact probability distributions was made for comparison with the c2 test. Substratum preferences of juvenile flat fish 111 Table I.!-!Different trends in settlement of young flatfish species under different levels of illumination (average, range and total of fish settling on four inorganic substrate types, Experiment 1). The probability values given are for c2 with 3 d.f. a: Plaice; b: Turbot; c: Sole. 112 NASIR & POXTON RESULTS Experiment 1 The numbers of fish in each species on the four different substrate types are given in table I. There was a highly significant difference among the numbers of fish on those substrates overall. The results also indicated that, as expected, all three species of flatfish significantly preferred the sand substrates (p!<!0.005) under all light intensities. Indeed the gravel and pebble substrates were so unfavourable that the fish would usually settle in the empty trays in preference to them. The one exception to this was the preference by Dover sole for empty trays in darkness (Table I). Analysis of these results as a contingency table (Table II) showed that the substrate preferences of the fish species were significantly affected by light intensity. Generally it is relevant to note that the attractiveness of the empty trays increased as light intensity decreased. Experiment 2 Comparison of selective preferences between very fine, fine, medium and coarse sand are shown in table III. There was a highly significant difference among the numbers of fish on those sediments overall. Very fine was highly significantly (p!<!0.005) preferred respectively under high light and dark conditions. Most plaice significantly selected very fine sand under all light conditions. Turbot significantly (p!<!0.005) preferred very fine sand under all light intensities (Table III), although under conditions of low light there was a preference (p!<!0.005) for medium sand (92 fishes). In all cases, considerable number of Dover sole was found on the very fine sand substrate under all light condition (p!<!0.005). The random selection of coarse sand by Dover sole was excluded from the conclusion since its ecological preferences actually cover very fine sand and low level of illumination. The analysis of these results from different sand substrates as a contingency table yielded highly significant levels (p!<!0.005) indicating that the choice of any substrate by the fish was strongly affected by the light intensity (Table II). Comparison of the choices Other comparisons were determined with the number of fish settled on substratum trays as opposed to the tank bottom itself. Plaice showed a significantly greater preference for the tank bottom underneath the trays than did the other species (Table IV). Table II.!-!c2 values calculated from analysis of the results of the experiments as a contingency table, showing the influence of light on substrate selection in the flatfish species. All data of the experiments for each species under high, low light and dark conditions were used. Substratum preferences of juvenile flat fish 113 Table III.!-!Different trends in settlement of young flatfish species under different levels of illumination (average, range and total of fish settling on four inorganic substrate types, Experiment 2). The probability values given are for c2 with 3 d.f. a: Plaice; b: Turbot; c: Sole. 114 NASIR & POXTON DISCUSSION These results suggest that the texture of the substratum is important to juvenile flatfish. Some of the Dover soles and plaices settled initially on gravel and pebbles, but later moved onto sand. Shiogaski and Dotsu (1971) and Marliave (1977) also suggested that a change in substrate preferences might be due to differences in textural composition of the substratum. The change in substrate preferences could be due to factors relative to body size. Marliave (1977) stated that Dover sole for example showed preferences for interstitial spaces larger than their cross-sectional body size, so that a gradual movement into larger-sized substrate was observed by the largest fish. Gibson and Robb (1992) indicated that a clear relationship between body length, sediment grain size and the extent to which individuals could bury themselves. This relationship presumably depends upon the force that individuals can exert during the burying process; larger fish can produce greater force and can therefore bury in coarser sediments. The relationship is not a simple one, however, as indicated by the unex- pected failure of many small fish to burry in the finest sediments (Tanda, 1990; Gibson and Robb, 1992). Gibson and Robb (2000) suggested that burial reduces activity and thereby increases the time that fish spend on sediments in which they can bury. Activity level, which is depended on light intensity and endogenous factors, therefore determines the degree of selectivity shown. Ellis et al. (1997) reported that both reared and wild sole selected a sand substratum in preference to a hard substratum. They found that reared sole with no previous experience of sand buried quickly when placed on sand. They also described repeated burial attempts on hard substrata and inhibition of burying on sand may provide a mechanism whereby both the wild and reared sole selected the sand substratum in preference to the hard substratum. Howell (1994) suggested that these features of the hatchery environment may impair the ability of reared flatfishes to bury and colour adept. Ellis et al. (1997) recommended that flatfishes reared for stock enhancement exercisers are conditioned to sand prior to release due to the relatively long time required for crypsis to improve through colour adaptational burying. The results of the present study also indicated that the particle size of the sand affects the settlement preferences. Generally, very fine sand was found to be the most suitable substratum type for plaice, Dover sole and turbot (Table III). Some behaviour differences between species were also noted in relation to substrate types. For example, plaice and Dover sole burrowed into sand when disturbed, whereas turbot in some experiments usually moved to another sand substratum in an adjacent tray. Hatchery-reared turbot are becoming very passive through domestication, whereas wild turbot are not; they live in the surface zone of open sandy shores and sand is not very fine in these areas. Gibson and Robb (1992) stated that the ability of juvenile flatfish to bury depends on many factors other than grain size, particu- Table IV.!-!Different trends in settlement of three species of young flatfish during the experiments. R: Number of replicates; F: Number of fish used; T: Percentage of total fish settled in the substratum trays; B: Percentage of total fish settled on the bottom of the experimental tank. Substratum preferences of juvenile flat fish 115 larly particle density, composition, cohesion, strength of local current and the binding of the sediment by the activities of the benthos. Other experimental studies also showed that juvenile flatfish actively select substrata on basis of grain size (Cook, 1985; Nasir, 1985; Tunda, 1990). Several field studies also found a positive correlation between young flatfish population and grain size (Riley and Holford, 1965; Poxton and Nasir, 1985; Burke et al., 1991). Gibson and Robb (1992) reported that the high burial efficiency decreased with increasing particle size and so a medium/coarse sand (grain size 0.35-0.50!mm) was chosen to facilitate deduction of differences in burying ability. They found that plaice of a similar size were able to bury almost completely in sand of the above-mentioned grain-size. Several previous studies also showed that wild and reared flatfish would attempt to bury in hard substrate (Kruuk, 1963; Arnold and Weihs, 1978; Howell and Canario, 1987; Ellis et al., 1997). The burial efficiency of reared fish increased with time on sand due to changes in muscular efficiency induced by exercise (Pearson et al., 1990), or in technique as a result of learning (Ellis et al., 1997). In fact, sediment characteristics such as grain size, particle density, compaction, cohesion and binding will vary and affect the ease of burial (Gibson and Robb, 1992). Flatfish try to adapt their skin colour to the sediment and also bury in the sediment to reduce vulnerability to predation (Kruuk, 1963; Lanzing, 1977). Ansell and Gibson (1993) confirmed that burial provided only a partial refuge from predation. They stated that the presence of sand reduced predation rates on young plaice by pelagic predators, but did not affect predation rates for benthic predators adapted for locating and capturing buried prey. Rogers (1992) stated that the substrate type appears to be of importance to juvenile sole not only as a suitable environment for its preferred prey species, but also for the protection that it offers. Analysis of the data as a contingency table also indicated that the choice of substrate by young flatfish was significantly affected by the light intensity. Jacobsen (1979) reported that differences in turbulence and light intensity influenced the choice of substrate. Andrews (1946) found, under field and experimental conditions, that the response of certain fish to light varies both with age and with change in light intensity. The changes in the light intensity may alter the type of activity exhibited. Gibson et al. (1978) found from their experiment with plaice that the darkness stimulates swimming off the bottom, both in the laboratory and the field and the rapidly changing light intensities at dawn and dusk act as cues for switching from one type of activity to the other. The activity for Dover sole may be completely suppressed by light. The wild sole in the sea are nocturnal, spending the light hours buried and motionless (Kruuk, 1963). Later, Ellis et al. (1997) confirmed that light motivated burial, and the motiva- tion of reared sole to bury was as strong as that of wild sole. They found both reared and wild sole showed fewer attempts to bury in the light than in the dark. Darkness stimulates activity in juvenile plaice and their time in the dark would be spread more evenly over each substrate type available to them (Burrows et al., 1994). Hence the fish would appear less selective in the dark (Cook, 1985; Neuman and Able, 1998; Gibson and Robb, 2000). Gibson and Robb (2000) summarised from the experimental and field works that the juvenile plaice select sediments preferentially on the basis of grain size. In the absence of other factors such as food or predators, selection is determined principally by the fish ability to bury in the sediment. They also assumed that hunger would increase activity and thereby decrease selectivity. In contrast, the activity might be reduced by the presence of predators and results in an increased selection for substrates in which fish are able to bury. 116 NASIR & POXTON CONCLUSIONS AND RECOMMENDATIONS 1.!- Young plaice, turbot and Dover sole have substrate preferences. Sand substrate seems to be generally suitable settlement area for these species. This study also suggests that the particle size of the sand affects settlement preferences of the fish. The choice of the substrate is affected by the light intensity. Furthermore, settlement preferences have been shown by these species and if they were carried to places with unfavourable substrates, the settlement phase would become a critical time. 2.!- This study has shown a requirement for more fieldwork to study the correlation between sediment type (physical and organic attributes) with the abundance and distribution of fish. 3.!- More laboratory studies are also required on growth studies of young flatfish in different grades of sand to see whether or not sediment type affects the growth pattern of the fish. REFERENCES ANDREWS C.W., 1946.!-!Effect of heat on the light hehaviour of fish. Trans. Roy. Soc. Can., 40: 27-31. ANSELL A.D. & R.N. GIBSON, 1993.!-!The effect of sand and light on predation of juvenile plaice (Pleuronectes platessa) by fishes and crustaceans. J. Fish Biol., 43: 837-845. ARNOLD G.P. & D. WEIHS, 1978.!-!The hydrodynamics of rheotaxis in the plaice (Pleuronectes platessa L.). J. Exp. Biol., 75: 147-169. 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