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After settlement on sandy beaches in spring, juvenile (0-group) plaice Pleuronectes platessa L. spend the summer and autumn months at depths

Juvenile plaice Pleuronectes platessa(n = 1281) were tagged and released at two locations 300 m apart on a 1 km long sandy beach. Most (>90%) of the fish were recaptured within 100 m of the release site (shown by the colour of the tag), with very few caught >200 m distance after 6 weeks. The changing spatial distribution of marked fish was adequately reproduced by a simple dispersal model with a single parameter: a 78% probability of remaining in a 100 m wide zone from one day to the next, with a 22% chance that fish move into an adjacent zone. In a subsequent similar study at the same beach, fish were either released at the point of capture (n = 881) or transplanted to the alternate site (n = 910) 100 m distant. After 6 weeks, transplanted fish moved along the shore towards their sites of original capture. Fish replaced at the point of capture showed no such movement along the shore. Further modification of the dispersal model to allow for a distinction between dispersal from home sites and from sites away from the original point of collection, was sufficient to reproduce the behaviour of the populations of both transplanted and control treatment groups. The likelihood of dispersal from home sites was much less than that seen at sites away from home. Juvenile plaice thus have a degree of long-shore site fidelity not expected of a fish with strong depth-related migration behaviour in a relatively homogenous habitat.

Many shallow-water organisms are subjected to regular, frequent and considerable changes in their environment imposed by the tides. Such changes range from complex alterations in local conditions, as in the intertidal zone, to simpler variations in current speed and direction offshore. To take advantage of the opportunities these changes provide, many animals undertake regular vertical and/or horizontal migratory movements in synchrony with the tidal cycle. Others use tidal currents selectively for transport to new habitats. Such movements vary in extent from a few mm to several km and may be modulated by the diel cycle of light intensity. This paper reviews the existence of tidal migration in marine animals from various taxa and habitats. Crustacean and fish examples illustrate the various functions of tidal migration, including transport, feeding, predator avoidance and reproduction. These functions are not necessarily mutually exclusive and may change throughout ontogeny. The mechanisms involved in migration are discussed in terms of direct reactions to environmental stimuli and the interaction of these responses with endogenous rhythms of activity and responsiveness.

Stratified depth sampling was used to investigate the distributional changes of newly settled plaice Pleuronectes platessa and dabs Limanda limanda, and resident populations of shrimp Crangon crangon on a sandy beach. During the settlement period, the smallest newly settled plaice are found in deeper water than the larger fish that settled earlier. After settlement, plaice concentrate in water < 1 m deep and there is a positive relationship between length and depth. Later in the year the fish gradually migrate into deeper water as they grow. Dabs settle later and in deeper water than plaice. Shrimp are concentrated in deeper water early in the year but migrate onshore in the summer. The movements of plaice are considered to be related to the predation risk caused by the shrimp and other predators in deeper water and to the higher temperatures in shallow water that promote faster growth. Once a size refuge from predators has been reached, the plaice move into deeper water. Intertidal pools probably serve as refuges for the smallest sizes of plaice because predators are less numerous in pools than in the sea. The use of shallow water by plaice represents the occupation of a niche which not only reduces predation and maximises growth, but may also greatly reduce competition for food in the earliest stages when densities are highest and competition with dabs is likely to be greatest.

An individual-based model has been used to predict the effects of size-dependent predation by shrimp Crangon crangon on populations of newly settled 0-group plaice Pleuronectes platessa. The model predicted changes in population size and thus mortality rate of plaice during settlement and the following period until the plaice reached 30 mm in length and were invulnerable to further shrimp predation. Population parameters derived from seven years' catch data for 0-group plaice and shrimp from the west coast of Scotland were supplied to the model for direct comparison of predicted and observed mortality rates for each of the seven years. The contrast of predicted and observed mortality was used to test the hypothesis that predation by shrimp is a major factor regulating populations of juvenile plaice. There was no evidence for control of plaice populations by shrimp predation at the study site, since a negative relationship between plaice mortality and shrimp density was observed. Alternative explanations for the divergence of prediction and observations included (i) a general failure of the model to adequately describe predator and prey populations, (ii) limited effects of consumption of plaice by shrimp, and (iii) responses of both species to a third factor such as temperature producing an apparent association of survival and recruitment in the respective species.

In the laboratory juvenile plaice Pleuronectes platessa (13–114 mm) were consistently found on the finest of four sediments after 24 h both in the light and the dark. Preference when assessed both by the fish's final position and by the amount of time it spent on each sediment agreed on 72% of occasions. Size did not affect preference, which was determined principally by a fish's ability to bury in a sediment. Burial reduces activity and thereby increases the time that fish spend on sediments in which they can bury. Activity level, which is dependent on light intensity and endogenous factors, therefore determines the degree of selectivity shown. Swimming duration and resting by the largest fish (71–114 mm) did not differ among sediments. Field experiments produced results similar to those obtained in the laboratory.

The paper reviews the changes in distribution that take place during the development of flatfishes from the egg to the adult. It describes the behaviour patterns involved in changing habitats, particularly the use of vertical migration to take advantage of tidal currents to aid transport, and the controlling mechanisms underlying these behaviour patterns. The discussion of mechanisms concentrates on the role of possible external (light, temperature, salinity, currents and pressure) and internal (endogenous rhythms, physiological state) cues used to time movements. It also considers the likely clues for directing movement (mainly environmental gradients) and for recognising destinations (food, conspecifics, chemical characteristics, substratum type). Finally, the role of learning in flatfish movement patterns is briefly discussed.

The efficiencies of a 2-m and a 1·5-m beam trawl for catching newly settled flatfish and brown shrimp were compared with that of a 1-m2 drop trap that was assumed to be 100% effective. Low and variable trawl efficiencies were recorded but the trawls were not size selective within the size range of individuals captured.

A study of seasonal and sexual variations of growth, diet, somatic condition (KS), gonadosomatic condition (GSI) and hepatosomatic condition (HSI) of corkwing wrasse Crenilabrus melops (L.), and rock cook Centrolabrus exoletus was made on specimens taken from areas on the west coast of Scotland from May 1992 to February 1994. Capture throughout the year was by baited creel, 3-m beam trawl, fyke net or by anaesthetics applied underwater. Corkwing ranged in size from 31–212 mm and 0.3–131.2 g for males, and 31–203 mm and 0.3–103–5 g for females. Maximum corkwing ages were 6+ years for males, 7+ years for females. Male rock cook ranged in size from 43–165 mm, weighed 0.7–56–5 g, and had a maximum age of 8+ years; females ranged in size from 43–138 mm, weighed 0.7–39.3 g, and had a maximum age of 4+ years. Male growth rates were faster for both species. Male and female corkwing had low KS values during the period of high GSI values; the trends for rock cook KS values were more variable, but levels increased after the reproductive period. Maximum GSI values were 11.37 and 18.94 for male and female corkwing respectively, 9.63 and 16.63 for male and female rock cook. There was no seasonal HSI fluctuation for male corkwing, levels were higher during the reproductive period for female corkwing and male rock cook, lower during the same period for female rock cook. The diets of corkwing and rock cook were both dependent on epibenthos. Corkwing diet was dominated by gastropod molluscs. The management of sustainable fisheries for corkwing and rock cook is discussed in relation to the growth rates of the two species.

Predation by larger fishes is a major cause of mortality for the populations of juvenile fishes on a sandy beach on the west coast of Scotland. Predation was concentrated on the most numerous species (0-group Pleuronectes platessa) in June but with the decline in numbers and growth in size of this species, the fish predators had changed their diet in August to feed principally on small sandeels (Ammodytidae). Six major predatory species had taken 95 per cent of all fish found in predators' stomachs and could be roughly divided into three categories according to their piscivorous tendencies and their abundance. (1) Common species in which fishes formed the main item of their diet (Gadus morhua). (2) Species in which fishes formed only a relatively minor constituent of the diet but which, by virtue of their abundance, potentially had a significant predatory impact (I-group P. platessa). (3) less abundant species whose diet consisted principally of fishes. The intensity of predation was generally greatest at night for G. morhua but during the day for I-group P. platessa. The distributions and movements of the dominant prey species can be interpreted in the light of their predator-prey relationships. The most common benthic species, 0-group P. platessa, move off -and onshore with the ebb and flow of the tides and concentrate in shallow water at night. Such movements would have the effect of maintaining the majority of the population in depths where predation pressure from nocturnally onshore-migrating gadoids and other predators is likely to be minimized.

Fishes were sampled by seine and beam trawl over tidal and diel cycles on a sandy beach on the west coast of Scotland in June and August. Species composition of the catches of the 2 gear types differed. There was a significant positive relationship between depth (0 to 5 m) and species richness but a few species were restricted to depths of

The influences of prey (the bivalve Donax vittatus (de Costa)) and a predator (juvenile cod, Gadus morhua L.) on long-term behaviour patterns of juvenile plaice were investigated by time-lapse video-recording of behaviour in the laboratory. Analysis of the frequencies and durations of behavioural elements revealed two basic types of behaviour. In the absence of food, behaviour was dominated by elements associated with migration to and from the intertidal zone: longer, less frequent swimming movements with occasional off-bottom excursions, especially around the predicted time of high water. Benthic activity was dominated by foraging behaviour when food was present and consisted of frequent short searching movements and occasional attacks on prey which continued throughout the tidal cycle, mixed with the migration component. Migration behaviour was strongly controlled by the endogenous tidally phased activity rhythm, whereas fish foraged only in the presence of food and independently of the phase of the tide. The presence of the predator depressed the expression of migration behaviour and reduced and delayed the onset of foraging, with greatly diminished feeding success. Expression of behaviour in the natural environment reflects the relative contributions of these exogenous and endogenous influences.

The shrimp,Crangon crangon(L.) (Crustacea: Crangonidae), is a significant predator of the smallest sizes of plaice,Pleuronectes platessaL. (Teleostei: Pleuronectidae), during and immediately after the fish settle on sandy beaches when predation rate is strongly dependent on the size of both the predator and the prey. Laboratory experiments showed that this size-dependency is caused principally by the superior escape capabilities of larger fish once captured rather than differences in the ability of different sizes of shrimps to capture their prey. Fish that escape after capture are often wounded and some of these wounds may subsequently be fatal. Many shrimps capture and eat fish that are larger than their stomach volume resulting in long handling times and low prey profitabilities. For all sizes of shrimps used (36–65 mm total length) prey profitability (mg prey ingested min−1) increases with decreasing fish length.

A study of seasonal, locational and sexual variations of growth, diet, somatic condition (Ks), gonadosomatic condition (GSI) and hepatosomatic condition (HSI), of the goldsinny wrasse, Ctenolabrus rupestris, was made on specimens taken from three areas (Millport, Oban and Luing) of the west coast of Scotland from May 1992 to June 1993. Capture throughout the year was by either baited creel or by anaesthetics applied underwater. Male goldsinny of age 1 year and older ranged in total length from 67-155 mm, total weight 3.8-58.8 g, and had a maximum age of 14+ years. Females of 1 year and older ranged in total length from 59-159 mm, total weight 2.4-65.4 g and had a maximum age of 20+ years. Juvenile (0 +) goldsinny were mostly of indeterminate sex, ranged in total length from 16-57 mm, and total weight 0.08-1.97 g. Female growth was slower than that of males for all three locations; growth of male and female goldsinny from Millport was slower than the respective sexes at the other two locations. Length-weight relationships varied with age and location. There were differences in Ks, GSI, HSI and dietary trends associated with variation in season, sex and location. The implications of this variation are discussed in relation to the management of a sustainable goldsinny fishery.

Fish predation on 0-group flatfish (plaice Pleuronectes platessa, dab Limanda limanda and flounder Pleuronectesflesus) populations on Tralee Beach. Scotland (56" 31' N, 5" 29' W) was studied by stomach content analysis during the summers of 1992 and 1993. I-group grey gurnard Eutrigla gurnardus, poor-cod Trisopterus minutus and whiting Merlanyius inerlangus and 0-group cod Gadus rnorhua were the major fish predators of the 0-group flatfishes. Flatfishes >45 mm total length appeared to have a refuge in size from predation by the fishes sampled. Predation by fishes was shown to be negatively size-selective, concentrated on the smaller size-classes of flatfishes. Smaller individuals were eaten by a greater taxonomic variety and a larger size range of predators than larger flatfishes. Smaller flatfishes'were therefore vulnerable to a larger number of predators. The size distributions of predators affected the pattern of predation because only the larger individuals ate flatfishes. It is therefore proposed that the degree of overlap in the size distributions of predators and prey determine the magnitude of predation. Growth rates and the timing of settlement in any one year will therefore have a considerable influence on predation, with the potential to result in a 'match or mismatch' of predator and prey size distributions.

The influences of a light: dark cycle and a persistent endogenous rhythm of activity on foraging (on the bivalve Donax vittatus) and avoiding a predator (juvenile cod, Gadus morhua) were investigated in freshly-caught juvenile plaice, Pleuronectes plalessa. Time lapse video recordings were made of fish in the presence and absence of prey and predators in laboratory tanks over 24-hour periods between the times of successive daytime low waters. Endogenous rhythms of activity were seen in all experimental treatments. Swimming both close to the bottom and in the water column showed a strong circatidal rhythm, with most activity 2 to 3 h after the predicted time of high water. Swimming in the water column was more frequent at night than by day. In the presence of a population of Donax, whose siphon tips could be eaten as food, swimming close to the bottom became more frequent. This increase in benthic swimming was independent of the endogenous cycle of activity and was correlated with the frequency of attacks on siphons. The presence of the cod predator delayed the onset of foraging activity, producing a foraging/predator avoidance trade-off. The independence of foraging from light and endogenous rhythms suggests that this trade-off may be similarly independent. The cod also greatly reduced swimming in the water column in darkness, behaviour apparently unrelated to foraging.

The effects of the major factors contributing to habitat quality (food, predators, temperatures, salinity, oxygen, habitat structure, water depth and hydrodynamics) on the growth and survival of flatfishes during their juveniles stages are described. It is concluded that the first three of these factors are the most important. The impact of habitat quantity on recruitment is also examined and found to play a major role in determining overall population size. Neither habitat quality nor quantity act independently and growth, survival and subsequent recruitment levels of juvenile flatfishes from their nursery grounds must be seen as a result of the interplay between both habitat quality and quantity. The general conclusion is that habitat-related processes on nursery grounds probably serve to dampen rather than generate recruitment variability.

Underwater television cameras were used to observe the movements of bottom-living animals in the intertidal and shallow subtidal zones of a sandy shore over sixteen 24-h periods in the summers of (1991) and (1992). Juvenile (O-group) flatfishes, predominantly Pleuronectes platessa L. with some Limanda limanda (L.), were only seen moving on the bottom by day, with most observed at high water and just before sunset. Their movements were directed offshore in the morning and onshore in the afternoon. They also tended to move onshore with the flood tide and offshore with the ebb. These fishes may use midwater swimming for intertidal migration by night because none was seen moving on the bottom at night. Potential predators of O-group flatfishes, cod Gadus morhua L., crab Carcinus maenas (L.) and the brown shrimp Crangon crangon (L.), were most often seen moving on the bottom at night. Crangon, Carcinus and I-group flatfishes were seen in similar numbers in the subtidal and intertidal zones, while O-group flat-fishes, cod and hermit crabs Pagurus bernhardus (L.) were seen much more frequently under the subtidal camera.

Rates of predation on 0-group plaice, Pleuronectes platessa. in aquaria were compared under four different combinations of conditions to test the hypothesis that the presence of sand in which they may bury affords a refuge from predators. The effect of light and darkness on predation rate was also examined, Two crustaceans, the shrimp, Crangon crangon, and the portunid crab, Liocarcinus holatus, and two fishes, cod, Gadus morhua, and pollack, Pollachius pollachius, were used as predators. Predaton rates were significantly higher in the dark for all predators except pollack. Predation rates in the absence of sand were signifcantly greater only for pollack. The results suggest that predation rates on plaice during their juvenile nursery stage on sandy beaches will be significantly greater during darkness than during the day. Burying in sand appears to provide only a partial refuge from predation, perhaps because natural predators have evolved effective methods of foraging for buried prey.

A field survey of populations of goldsinny wrasse (Ctenolabms rupestris) was undertaken on selected areas of rocky coast of west Scotland. Observations by sub-aqua diving were used to determine distribution, habitat preference, and population density, and their seasonal variations. Availability of the preferred refuge type (crevices on rock faces, or between boulders, where two or more entrances exist) was essential in determining goldsinny presence. Goldsinny were not always present in areas or at depths where the influences of freshwater runoff (low temperatures and salinities) were likely, even if the preferred habitat type was available. In areas remote from these influences, and where there were suitable refuges, depth of water (0–44 m), macroalgal cover and high current speeds (2.1–3.6 ms−1) did not affect goldsinny distribution. Observed densities of goldsinny reached a peak in summer months with a maximum of 4.0 m−2 in areas of shallow boulder scree, but only 1.0 m−2 in shallow areas with little scree, or at deeper study sites. Numbers of goldsinny observed actively swimming decreased after October, with a rapid disappearance in November. A gradual reappearance was recorded in late April, and early May. Changes in activity may be influenced by both water temperature and photoperiod. Young-of-the-year goldsinny were first observed in August in shallow water (0–8 m) areas typified by rock/boulder scree and/or high macroalgal cover. Where adults also inhabited these nursery areas, 0+ fish could make up over 50% of the total population. After their first winter, juvenile goldsinny may migrate away from inshore sites.

Forty-three species of fishes and 16 species of larger crustaceans were caught by trawl and seine during a 4 yr study on a sandy beach on the west coast of Scotland. There were considerable differences in the species composition of the trawl and seine catches such that neither alone provided an adequate description of the sampled cornnlunity. Approximately monthly s a m p h g demonstrated a clear seasonal cycle in abundance and species composition. A few species were resident on the beach all year round; the majority were only present from spring to autumn. Increases in both numbers and species were caused mainly by the recruitment of young of the year individuals whose numbers subsequently declined rapidly probably due to a combination of predation and emigration. There were marked differences in both species composition and abundance between years. Annual cycles in species richness and abundance closely paralleled those of temperature and sahnity. It is suggested that the cycles result mainly from recruitment and mortality rather than from immigration and emigration in response to physical factors.

Abstract. Model prey were used to examine the olfactory and tactile basis of prey selection when crabs feed on resistant items such as bivalves. Shape was an important determinant of handling time which increased with height and hence cross-sectional area of the model. The additional stimulus of odour prolonged persistence time with inedible models and crabs preferred those incorporating the highest concentration of mussel, Mytilus edulis , filtrate. When presented with a sequence of edible models, crabs improved their handling skills during a foraging bout. When models were presented alternately with real mussels, however, the handling skills associated with the mussels were learned more slowly, particularly if a model was encountered first in a sequence. Crabs never rejected edible models that were encountered individually in a sequence, but frequently did so when they were encountered simultaneously in groups. During simultaneous encounters, handling time was shorter than for individual encounters. It appears that chemical cues are important attractants, and may be correlated with the probable yield and vulnerability of the prey. However, whereas olfaction is particularly important early in the attack, shape, which is the main determinant of handling time, becomes important later. Olfactory cues remain potent reinforcers throughout an attack.

Crabs used a limited repertoire of behaviours when attacking resistant prey. Crushing and rotating were the most commonly used behaviours with all prey types. Other behaviours, such as sawing, biting and poking became more important in prolonged predatory acts. The duration of crushing attempts became longer as prey size increased. Crabs were able to improve their handling efficiency, the frequency, but not the duration, of behaviours decreased with experience. Crabs were able to subtly alter their predatory behaviour, within and between predatory acts. Presumably the ability to fine tune predatory behaviour according to prey characteristics, coupled with the ability to learn appropriate handling techniques, leads to more efficient predatory behaviour.

The fifteen-spined stickleback, preying on the mysid, Neomysis integer , and the amphipod, Gammarus locusta , maximized its energy intake by adjusting its predatory behaviour to the prey being attacked. The escape speed of Neomysis placed an upper constraint on the size of the prey available to fish of a given size. The increase in importance of Neomysis in the natural diet of larger sticklebacks reflected size-dependent limitations of the fast-start performance in these fish. The escape speed of Gammarus did not limit its availability to the sticklebacks; the limiting factor for this prey was its cross-sectional area. The fish expended 20 times more energy attacking Neomysis than Gammarus , but Gammarus took up to 159 times longer to ingest. Neomysis always responded to an attack with a tail-flip, which usually resulted in movement at an angle of approximately 90° to the direction of attack. Although Gammarus occasionally used a tail-flip response, it was never used to escape a fish attack. Adoption of a stationary C-shape often deterred the fish from attacking Gammarus .

Annual development of density and mean weight of juvenile plaice Pleuronectes platessa was examined at 2 nursery grounds (Port Erin Bay, Isle of Man, Irish Sea and Admucknish Bay, Firth of Lorne, west coast of Scotland) over a number of years. In both cases there was evidence of 'self-thinning' in the populations when the density was high, although in Port Erin Bay the slope suggested a relationship different from the standard - 4 ⁄3 rule proposed for animal populations. The site specific dynamic thinning lines (dynamic carrying capacities) were consistent with the dynamic thinning line (-4⁄3 rule) for both populations. The 2 nursery grounds were different, with growth rates and densities generally higher on the Ardmucknish Bay nursery ground compared with Port Erin Bay. Similarly, the estimated total biomass of plaice was higher in Ardmucknish Bay than Port Erin Bay, but there was considerable inter-annual variation. Maximum plaice biomass generally occurred from August to September for both sites. In Port Erin Bay this coincided with the maximum consump- tion rates. The size structure was described using the Gini coefficient (G) and the timing of size struc- ture development by the centre of gravity (CG). The size structure of the populations differed and in both cases varied during the season in response to settlement and mortality. Over the study periods, population density was only occasionally high enough to reach the site specific self-thinning line. This suggests that these populations rarely approached the carrying capacity of the nursery grounds.

The ability of Lipophrys pholis to remember the position of a refuge was tested in an artificial habitat under the influence of different visual clues. L. pholis learned the position of the refuge in the presence of a clue consisting only of a small black screen. They responded to this clue by moving towards it and pressing themselves up against it. Lego™ towers and a white screen clue did not provoke such a response. In a further experiment L. pholis continued to respond to the black screen in this way when the screen was moved to another location further from the refuge. After 12 days L. pholis learned to use the black screen in its new position as an indirect clue and navigate to the refuge directly without first approaching the black screen. These results suggested that when placed in a novel habitat the immediate reaction of L. pholis is to move quickly towards the first dark area they see but, with experience, they can use the position of large objects around them to navigate quickly and efficiently to a refuge.