Tidal stream mediation of metapopulation structure in North Sea plaice

Migration is widespread among marine fishes, yet little is known about variation in the migration of individuals within localities. We tested the hypothesis that variation in the migratory behaviour among plaice (Pleuronectes platessa) in the North Sea could be explained by large‐scale differences in the speed and directions of the tidal streams, which the fish use as a transport mechanism. Towards this end, 752 mature female plaice tagged with electronic data storage tags were released at eight locations with contrasting tidal flow properties, between December 1993 and September 1999. The experiment yielded 20 403 days of data from 145 plaice. The position of each fish was determined at intervals throughout the liberty period using the tidal location method. The results show 3 geographically discrete feeding aggregations during the summer, which dispersed over the southern North Sea and Eastern English Channel to spawn during winter. Our results re‐affirmed the major role of the tidal streams in the southern North Sea in structuring plaice dispersion, both by providing transport and guidance, and by delimiting the extent of distribution due to thermal stratification during the summer. These results confirm the prediction that large‐scale variation in migration behaviour can be explained in part by the tidal guidance and transport mechanisms available. They have revealed features of spatial dynamics not previously observed from a century of conventional tagging experiments and illustrate how the study of individual fish can successfully define the migratory characteristics of populations.     

Sex‐biased dispersal in a salmonid fish: implications for growth rate and survival

Migration is widespread among marine fishes, yet little is known about variation in the migration of individuals within localities. We tested the hypothesis that variation in the migratory behaviour among plaice ( Pleuronectes platessa ) in the North Sea could be explained by large‐scale differences in the speed and directions of the tidal streams, which the fish use as a transport mechanism. Towards this end, 752 mature female plaice tagged with electronic data storage tags were released at eight locations with contrasting tidal flow properties, between December 1993 and September 1999. The experiment yielded 20 403 days of data from 145 plaice. The position of each fish was determined at intervals throughout the liberty period using the tidal location method. The results show 3 geographically discrete feeding aggregations during the summer, which dispersed over the southern North Sea and Eastern English Channel to spawn during winter. Our results re‐affirmed the major role of the tidal streams in the southern North Sea in structuring plaice dispersion, both by providing transport and guidance, and by delimiting the extent of distribution due to thermal stratification during the summer. These results confirm the prediction that large‐scale variation in migration behaviour can be explained in part by the tidal guidance and transport mechanisms available. They have revealed features of spatial dynamics not previously observed from a century of conventional tagging experiments and illustrate how the study of individual fish can successfully define the migratory characteristics of populations.     

Being there and getting there: balancing the costs of migration against the benefits of changing habitat for North Sea plaice

To improve survival and reproductive success, many fish species have evolved migratory life‐histories, showing ontogenetic and/or seasonal changes in habitat use. Individuals move between different areas, each of which is 'best' for a particular activity, such as feeding, growing or spawning. The benefits of moving to a different habitat, however, have to be balanced against the costs of migration, so any behaviour that reduces the cost of movement would be expected to expand the migratory range and thereby increase the range of habitats that can be exploited.
Previous tracking experiments in the southern North Sea have shown that plaice selectively exploit the tidal streams to aid their spawning migration. Here we examine whether this behaviour is primarily (1) an energy‐saving strategy, or (2) a transport mechanism by which fish that are unable to navigate over long distances can be carried reliably between feeding and spawning grounds. Because selective tidal stream transport requires that fish remain stationary on the sea‐bed during the 'non‐transporting' tide, energetic calculations predict that this behaviour is beneficial only when the current speed exceeds a critical, size‐dependent, value. We have used detailed information about migratory behaviour from individual fish to calculate the metabolic costs and the likely reproductive benefits of different migratory strategies. Our results show that plaice use selective tidal stream transport only in areas where the tidal streams are suitably fast, indicating that this behaviour is primarily an energy saving strategy.     

The intertidal movements and distribution of young fish on a sandy beach with special reference to the plaice (Pleuronectes platessa L.)

The intertidal movements of young fish were studied on a sandy beach over several tidal cycles during the day and the night. Young plaice (Pleuronectes platessa L.), by virtue of their abundance, received the most attention, and it was found that they migrate up and down the beach with each tide in company with several other less abundant species. The dab (Limanda limanda (L.)), does not make this migration.The vertical distribution of the commoner fish species was charted and each was found to occupy a specific zone in relation to the water, but with large overlaps in many cases. A close relationship between the length of plaice and dabs and the depth in which they were caught was evident during the summer months, but this relationship broke down in the winter due mainly to an offshore movement of larger fish.The food intake of plaice was studied throughout the day and night; feeding occurred mostly during the day. No effect of the tide on feeding could be detected.The possible mechanisms controlling the tide-related movements and the size-depth relationship are discussed and a general hypothesis is put forward in which it is proposed that the fish are able to select and maintain their position at a particular depth throughout a tidal cycle thereby being guided up and down the beach. Attributes of depth which the fish might use as a reference point in their movements are discussed.     

Fish impingement at estuarine power stations and its significance to commercial fishing

The abstraction of cooling water at power stations sited on tidal waters inevitably leads to mortalities of some fish which are drawn in with the cooling water supply and become impinged on the intake screens. These fish are predominantly 0- or I-group juveniles which, owing to their small size, are unable to resist intake currents.
Commercial fishermen often object to the fact that juvenile fish are killed in this way. Their concern stems from the fact that, in order to protect stocks, commercial fishing is restricted to fish which are above a statutory minimum landing size, whereas the majority of fish killed by impingement are below this size.
This paper considers the significance of impingement mortalities at estuarine sites in Britain for six commercially important species: cod, Gadus morhua , whiting, Merlangius merlangus , plaice, Pleuronectes platessa , dab, Limanda limanda , sole, Solea solea and herring, Clupea harengus . Life tables are used to establish expected survival trajectories for each species and to compute reproductive potential. Each fish killed on intake screens is then considered in terms of the fraction of the reproductive potential of a single adult at maturity, and is ascribed an 'adult equivalent' value. Total catches of mixed juveniles and adults are then presented as 'adult equivalent' values. The results are then compared with commercial landings data and it is shown that catches by power stations are trivial in comparison with commercial landings.     

The evolutionary pattern of early life history in water currents

Explanation of the characteristics of the early developmental stage of organisms is an important problem in evolutionary biology. In studies to date, evolutionary biologists have proposed some theories that successfully explain egg size variation. Mesoscale water movements may transport early life stage organisms in the aquatic biosphere. We propose a novel biological view to explain the duration of the retention period at the spawning ground and egg size variations in aquatic organisms with a planktonic stage at least during the early part of their life history. We develop a life history model of the early life stage of such aquatic organisms that takes into account their adaptations to water currents and biotic environmental gradients in the currents. We hypothesize that the distance from the spawning grounds to the nursery grounds and the biological richness of the currents affect the adaptive life history design of these aquatic organisms, including adaptive retention time at the spawning ground and egg size. Various studies of fish biology describe in passing phenomena that suggest the validity of our deductions, but explicit empirical attempts to evaluate our predictions in the field of evolutional biology are needed.     

Vertical migration for horizontal transport while avoiding predators: II. Evidence for the tidal/diel model from two populations of scallop (Placopecten magellanicus) veligers

We examined the vertical migration behavior of scallop (Placopectenmagellanicus) veligers in mesocosms and in previously reportedfield studies. Evidence suggests that these bivalve veligersmigTate in response to both tidal and diurnal stimuli in a mannersimilar to a proposed tidal/diel model. Both populations havea diurnal response to solar cues. The response to tidal cuesdiffers between the Georges Bank and Passamaquoddy Bay populations.Georges Bank veligers appear to utilize the differences in tidalphase that occur with depth to transport them in a northeasterlydirection, thus maintaining the population on the bank. PassamaquoddyBay veligers respond by swimming up at slack water (high andlow tides) and down when currents are strongest. Such behaviorwould minimize dispersal on the strong tidal currents in theBay of Fundy and thus also tend to maintain a population withinan area. Horizontal transport resulting from vertical migrationis the most likely selective pressure to CTeate and maintainthese different behaviors against the homogenizing effects ofmigration between the two populations. The implications of inheriteddifferences in behavior probably require consideration in themanagement of both wild and cultured populations. Common samplingpractices that obscure the tidal part of tidal/diel migration,including averaging the results from several days of sampling,sampling too infrequently to perceive a tidal periodicity, and assuming that only behavior that changes at high and lowtides will affect horizontal transport, need to be avoided instudies of vertical migration.     

Community structure and temporal variability of ichthyoplankton in North Brazilian mangrove creeks

The species composition and dynamics of fish larvae in three mangrove creeks located in the Caeté Estuary (north Brazil) were studied monthly using a trap net during diurnal ebb tides. A total of 109 954 larvae, representing 25 families and 54 species, were collected from October 1996 to October 1997. The community was dominated numerically by a few species, a feature common for other estuarine fish populations. The most abundant taxa were estuarine species, namely the eleotrid Guavina guavina (46·7%) and the engraulid Anchovia clupeoides (14·9%). The sciaenid Cynoscion acoupa was the only marine species that used the mangroves extensively as a nursery site, occurring mainly at the postflexion stage. The size distribution of G. guavina did not produce shifting modes, indicating continuous transport out of the mangroves by tidal currents. Significantly lower species richness was observed in the late rainy season, primarily due to the emigration of marine species. Intermediate seasons were characterized by more complex larval fish assemblages. The temporal trends of the dominant species was influenced to a great degree by their life history strategy.     

The mechanism of retention of pelagic tomcod,Microgadus tomcod,larvae and juveniles in the well-mixed part of the St. Lawrence Estuary

Synopsis We tested the hypothesis that the mechanism of retention of tomcod, Microgadus tomcod, larvae and juveniles in the well-mixed part of the St. Lawrence Estuary is similar to that of sympatric smelt, Osmerus mordax, larvae who actively migrate to the surface during flood tides and to the bottom during ebb tides so as to minimize net downstream displacement. The vertical distribution of tomcod larvae and juveniles was documented during two 98-h sampling series at 2 anchor stations in June and July, 1986. An hourly index of the center of mass of fish in the water column calculated to take into account daytime net avoidance in surface waters suggested that tomcod remained deep in the water column and that their accumulation at the head of the estuary was the result of passive upstream transport by net residual circulation rather than active tidal migrations. For both series, depth of fish was inversely related to density of the water suggesting that the buoyancy of fish influenced their vertical distribution. Tomcod larvae and juveniles were advected by tidal currents. In June, larger larvae were found at low slack water indicating that they were located upstream of smaller larvae. In July, larger juveniles were located downstream of smaller juveniles, the difference in mean length between low and high slack water attaining 20 mm. Ontogenetic buoyancy changes may be responsible for these differences in the vertical distribution of tomcod. Comparisons of the early life-history stages of tomcod and smelt retained in the same area under the same hydrodynamical conditions indicate that more than one mechanism permits retention in a well-mixed estuary and that the observed species-specific patterns of vertical distribution are not simply interpretable as adaptations to retention.     

A quantitative description of the behaviour of wild juvenile plaice (Pleuronectes platessa L.)

The behaviour of young plaice (Pleuronectes platessa L.) as they migrate up and down sandy beaches with the tide is described. Their behaviour during this migration consists mainly of swimming and feeding interspersed with rarer behavioural acts. Over short periods swimming behaviour can be described by a random model in which the probability of a swim occurring remains constant. This probability varies markedly, however, from hour to hour. Two types of swimming movement are recognized: one of very short duration represents searching for food and the other, longer, category serves to transport the fish up and down the shore. Variations in the feeding rate and in the frequency, duration, and direction of swimming movements over the tidal cycle are described and related to the changing physical and biological conditions that the fish experience during their intertidal movements.     

The accumulation of cadmium by the plaice, Pleuronectes platessa L. and the thornback ray, Raja clavata L.

The accumulation of cadmium (^115mCd) from both food and sea water by the plaice, Pleuronectes platessa L., and thornback ray, Raja clavata L., has been studied in relation to their measured cadmium concentrations. Plaice accumulated cadmium from sea water at a faster rate than rays, although the concentrations factors attained by both species as a result of such direct accumulation were very low. Both species retained cadmium from food and accumulated it in the liver, the rays relatively more than the plaice. From analyses of North Sea fish, some evidence was found for a positive linear relation between the concentrations of cadmium in plaice liver and age, but not weight, of the fish.     

Depth selection behaviour during activity cycles of juvenile plaice on a simulated beach slope

Juvenile plaice Pleuronectes platessa on a sand slope in a laboratory tank showed depthselection behaviour consistent with offshore migration by day and onshore by night, as seen in natural conditions. Plaice stayed deep (0·5 m) and avoided the shallowest water (<10 cm depth) during light periods, but ventured up the slope and into the shallows in darkness. The freshly caught fish showed circatidal cycles of activity, but showed no change in depth selection between periods of high (time of expected high water) and low activity (expected low water). This suggests that changing direction of movements in tidal migration is controlled by responses to changing environmental conditions. Further analysis showed movements of fish up the slope to be slower, covering less distance and with shorter pauses in between than moves down the slope. Frequency, distance travelled and speed of moves up and down the slope did not change with tidal or light-dark cycles, and so such modulation could be ruled out as a mechanism for migration.     

An optimal foraging and migration model for juvenile plaice

Summary A state-dependent model has been used to predict daily and tidal patterns of migration, feeding and inactivity in juvenile plaice (Pleuronectes platessa L.) in their intertidal and shallow subtidal nursery areas. Vertical position, quantity of energy reserves and fullness of the gut characterized the state of individual fishes. If feeding is visually mediated, the model predicts that, by night, plaice should move to areas of low predation risk and become inactive, whereas by day, plaice should migrate to feed in areas of high prey encounter rate. Vertical zones adopted by day and night and, hence, patterns of migration, should depend on the distributions of predators and prey. When prey are more abundant in the intertidal zone, plaice should move onshore to feed as the tide rises and when prey are more abundant offshore, plaice should move offshore to feed by day. If predators are equally abundant in all zones, the fish should behave as if no predators were present, having no effective refuge. An increase in the abundance of predators with depth results in the restriction of plaice activities to shallower vertical zones, depending on the magnitude of the predation threat. Zones adopted thus depend on the trade-off between energy intake and predation risk. Concordance between predicted behaviour and observed patterns is evident in contrasting habitats. Migration and feeding in the Wadden Sea, where prey are more abundant on intertidal flats, is dominated by the tidal component, whereas on impoverished exposed beaches of the west coast of Scotland, the diurnal component is dominant. Tidally related behaviour persists in the latter environment, not predicted by the model and may be a consequence of using endogenous rhythms to approach optimal behavioural patterns.     

Recent trends in the abundance of plaice Pleuronectes platessa and cod Gadus morhua in shallow coastal waters of the Northeastern Atlantic continental shelf – a review

Shallow, near-shore water habitats on the continental shelf of the Northeast Atlantic have been productive fishing areas in the past. Here, we review the present knowledge about (i) recent trends in the abundance of plaice and cod in these habitats and (ii) hypotheses regarding the factors responsible for any trends. At present, only a few studies exist on the trends of abundance of plaice or cod, namely from the Bay of Biscay, the North Sea and the Skagerrak/Kattegat. They suggest a declining abundance in coastal, shallow areas and – at least for plaice – a latitudinal gradient with an erosion of the southern distribution boundary in the Bay of Biscay and deepening of stocks in the North Sea. In contrast, no trend in shallow water abundance of plaice similar to a decline in deep-water stocks during the 1970s and their slow recovery during the 2000s is apparent in the Skagerrak/Kattegat. Although shallow habitats fundamentally differ from deeper areas by the prevalence of juvenile stages, the declining trends coincide with decreasing abundance/landings and spatial stock relocations in the deeper areas. Whether this indicates a common trend pointing at connectivity between shallow and deep water remains open. Fundamental differences exist in the suggested causes of the trends in different geographical areas. High fishing pressure together with low local recruitment apparently prevents the recovery of overexploited plaice and cod stocks in the Skagerrak/Kattegat. In contrast, the responses of juveniles and adult fish to increasing seawater temperature are the main hypotheses for changes in distribution and abundance of both fish species in the North Sea/Bay of Biscay. However, temperature alone cannot explain the observed decline of fish in coastal areas, and the causes may be more complex, involving nutrient loading, primary productivity or food availability, although at present, knowledge of these factors is insufficient.     

The host specificity of Lepeophtheirus pectoralis (Müller, 1776) (Copepoda: Caligidae)

The host specificity of Lepeophtheirus pectoralis (Müller) was examined experimentally by exposing different fish species to infection by artificially reared copepodid larvae. Copepodids which were hatched from eggs of adults parasitic on plaice ( platessae copepodids) preferred plaice to all other fishes tested, whereas copepodids which were hatched from eggs of adults parasitic on flounder ( flesi copepodids) preferred flounder to all other fish species. These behavioural differences suggest that two strains of L. pectoralis exist, one ( platessae ) adapted to plaice as its host and the other ( flesi ) to flounder. Comparison of an experimentally derived order of host preference with a table of occurrence obtained from the literature, suggests that a third strain of L. pectoralis , adapted to dab as its host, might also occur.
The process of infection by L. pectoralis copepodids is also described. It comprises a host location phase, during which the copepodid enters the habitat of its flatfish hosts and locates a host individual, and an attachment phase. The host location phase appears to be governed by changes in the activity of the copepodid and by its positively rheotactic response to water currents produced by the host. The attachment phase is probably based on the response of the copepodid to chemical factors produced by the host.     

Progress of migrating Atlantic salmon (Salmo salar) along an estuary, observed by ultrasonic tracking

Adult Atlantic salmon were captured in the Miramichi esturary, tagged with ultrasonic transmitters, and released. Nine fish were tracked for a total of 425 h, spanning 71 flood and ebb tides. There was drifting with the tidal currents, and holding of position relative to land. Fish that achieved overall upstream progress did so by drifting with flood tidal currents and by stemming the ebb currents. Fish which did not achieve upstream progress also drifted with flood tidal currents, but these fish did not stem the ebb currents and dropped back downstream during ebb tides. No differences in movement patterns were apparent for different times of day or wind conditions.     

Macrotidal estuaries: a region of collision between migratory marine animals and tidal power development

Macrotidal estuaries of the inner Bay of Fundy are utilized by large numbers of migratory fishes, particularly dogfish, sturgeon, herring, shad, Atlantic salmon and striped bass as well as by other migratory marine animals, many of which have large body sizes (squid, Lamnid sharks, seals and whales). Tagging experiments indicate the fishes originate from stocks derived over the entire North American Atlantic coast from Florida to Labrador. Population estimates suggest up to 2.0 times 10⁶adult American shad (Alosa sapidissima) migrate through an individual embayment each year. These migrations are an integral part of the life history of the respective species and appear to be controlled in part by the near shore movements of ocean currents. In other regions of the world similar macrotidal estuaries exist (Cook Inlet, Alaska; Severn Estuary, U.K.) and they, like the Bay of Fundy, are linked in continuum to the local ocean currents. We propose that marine animals utilize all these regions in a manner similar to the Bay of Fundy estuaries and properly designed surveys will reveal their presence. Fish passage studies utilizing the Annapolis estuary low-head, tidal turbine on the Bay of Fundy have shown that turbine related mortality of 20–80% per passage occurs depending on fish species, fish size and the efficiency of turbine operation. We suggest that introduction of tidal turbines into open ocean current systems will cause widespread impact on marine populations resulting in significant declines in abundance.     

0-group flatfish growth conditions on a nursery ground (Bay of Canche, Eastern English Channel)

The settlement and growth conditions of three 0-group flatfish species, sole (Solea solea L.), plaice (Pleuronectes platessa L.) and dab (Limanda limanda L.) in the Bay of Canche, French coast of the Eastern English Channel, were investigated during summer 1997. 0-group dab were the more abundant of the three species (75.4%) followed by plaice (21%) and sole (3.6%). Analyses of fish size distribution indicated that settlement of plaice occurred before sole and dab. Growth of sole was faster than that of plaice, which in turn was faster than that of dab (0.68, 0.43 and 0.33 mm d^?1 respectively). Sole differed from plaice and dab by their larger mean length attained at the end of their first summer stay in the nursery ground. Growth performances of 0-group sole and plaice were analysed by comparing estimated growth in the field with predicted maximum growth according to temperature-growth rate models from experimental studies of growth with unlimited food supply. The data of this study suggest that plaice growth may have been limited during late summer, whereas sole growth does appear to have been determined only by water temperature.     

The Interaction between Water Currents and Salmon Swimming Behaviour in Sea Cages

Positioning of sea cages at sites with high water current velocities expose the fish to a largely unknown environmental challenge. In this study we observed the swimming behaviour of Atlantic salmon (Salmo salar L.) at a commercial farm with tidal currents altering between low, moderate and high velocities. At high current velocities the salmon switched from the traditional circular polarized group structure, seen at low and moderate current velocities, to a group structure where all fish kept stations at fixed positions swimming against the current. This type of group behaviour has not been described in sea cages previously. The structural changes could be explained by a preferred swimming speed of salmon spatially restricted in a cage in combination with a behavioural plasticity of the fish.     

Evidence for up-estuary transport of puffer Takifugu larvae (Tetraodontidae) in Ariake Bay, Japan

The present study determined whether puffer Takifugu rubripes and T. xanthopterus larvae use selective tidal stream transport (STST) for migration into the nursery area. The influence of the tidal cycle on the vertical distribution of Thkifugu larvae was investigated during a 24 h sampling period at one location off Shimabara Peninsula in Ariake Bay. Samples were collected in three depth layers, from near the sea floor to near the surface (5, 20 and 30 m depth). The change in vertical distribution in relation to tidal phase was not observed. This data did not support STST hypothesis. Diel vertical migration was observed irrespective of tidal phase, where larvae migrated to the middle layer during the night, and sank to the bottom layer during the day, however, larvae hardly emerged into the surface layer during the study period. In Ariake Bay, the residual current leads to a layered vertically stratified structure, in which surface water flows towards the mouth and the middle-bottom water flows toward inner part of the Bay. It is suggested that Takifugu larvae use not STST but residual currents for transport into the nursery ground, namely, undergoing nocturnal diel vertical migration in the water column between the middle layer and the bottom layer where the net flow is northward.