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.     

Analysis of the enzyme polymorphism in a plaice, Pleuronectes platessa L., population from the north-west coast of Brittany, France

A study of enzyme polymorphism by electrophoretical techniques permitted the genetic structure and the homogeneity degree of the plaice population present in the Abers Wrac'h and Benoit along the north-west coast of Brittany to be determined. Thirteen enzymic systems encoded by 21 loci were analysed and nine of these loci were polymorphic at the 5% level. The comparison of allelic frequencies and of observed heterozygosity h _obs did not show significant differences between these two individual groups. Nei's genetic identity ( I = 0.998) and distance ( D = 0.002) as estimated from allelic frequencies showed that the plaice of the two estuaries are very similar.
Thus, the plaice of A. Wrac'h and of A. Benoit belong to the same population in which the average level of polymorphism per locus P is 47% and the observed average heterozygosity per individual H _obs is 0.163 ± 0.094.     

Temperature and somitogenesis in embryos of the plaice (Pleuronectes platessa)

The rate of somitogenesis was observed in plaice embryos reared at 5 and 12°C. Somite formation occurred relatively later in development at 12 than at 5°C. At any given stage of development 5°C plaice embryos had significantly more somites than embryos reared at 12°C.     

The role of sense organs in the feeding behaviour of juvenile sole and plaice

An infra-red video recording system was used to observe and compare the feeding activity of juvenile sole and plaice. Experiments in light or dark and chemical ablation of the neuromasts with streptomycin sulphate allowed all or a limited set of senses to be used by the fishes. Both species were able to feed on dead prey when only chemical stimuli were available. Plaice showed a greater dependence on vision for feeding but sole relied principally on chemoreception and mechanoreception. When any one sense was removed plaice became less active whereas sole became more active.     

The occurrence of juvenile pigmentation abnormalities in plaice (Pleuronectes platessa) larvae fed on enriched and unenriched Artemia salina nauplii

The effect of diet on the frequency of malpigmentation was investigated in laboratory-reared plaice ( Pleuronectes platessa ) fed on either enriched or unenriched Artemia salina nauplii . The juveniles fed continuously on unenriched Artemia during their larval period showed 29.0% normal pigmentation whereas those fed solely on enriched Artemia , showed 63% normal pigmentation. The possibility of a critical period for pigment development was tested by changing the diet of larvae at different ages. Results suggested that a critical period is less important than the duration of time fed on a nutritionally complete diet.     

Effects of UV-B radiation on embryonic, larval and juvenile stages of North Sea plaice (Pleuronectes platessa) under simulated ozone-hole conditions

Irradiation with artificial quasi-solar light was used to investigate lethal and sublethal effects of enhanced ultraviolet-B (UV-B) radiation on eggs, larval and juvenile stages of North Sea plaice. The irradiation experiments resembled a worst-case scenario with a synchronous occurrence of ozone depletion, sunny weather, and low water turbulence. In eggs, UV-B exposure increased mortality and induced loss of positive buoyancy. UV-B exposures for 1 or 2 days, according to the weather conditions in spring, impaired eggs only if UV-B intensities and doses exceeded those under a further 60% ozone loss. In larvae and juveniles, long-term UV-B exposures during and after metamorphosis affected ventilation rate at normoxia and ventilatory regulation during hypoxic incubations. Oxygen consumption rates of juveniles were not affected by UV-B irradiation. Received in revised form: 20 April 2000 Electronic Publication     

The relationship between body size, sediment grain size and the burying ability of juvenile plaice, Pleuronectes platessa L.

The ability of juvenile plaice (18–190 mm) to bury in sands varying in grain size from 0.062–2 mm was examined. For fish greater than approximately 30 mm in length the relationship between the proportion of the body covered with sand ( C ), total length ( L , mm) and grain size ( S , mm) can be estimated from the equation: logit C = 3.250 + 0.069 L – 6.771 S . Based on their performance in coarser sediments, fish smaller than approximately 30 mm did not bury as well as expected in the finest sediments (0.062 and 0.125 mm).     

Population structure of plaice (Pleuronectes platessa L.) in northern Europe: microsatellites revealed large-scale spatial and temporal homogeneity

Philopatry to spawning grounds combined with well-known migratory patterns in the flatfish Pleuronectes platessa (plaice) has led to the hypothesis that regional populations may reflect relatively discrete, genetic stocks. Using six microsatellite loci we genotyped 240 adult individuals collected from locations in Norway, the Faeroe plateau, the Irish Sea, the Femer Baelt, Denmark, and the southern North Sea, and 240 0-class juveniles collected from five nursery-ground locations in Iceland, northwest Scotland, two sites in the Wadden Sea, and the Bay of Vilaine in Southern Brittany. The mean number of alleles/locus ranged from 5.3 to 20.4, with a mean of 13.9. Expected heterozygosity was uniformly high across all locations (multilocus H(exp)= 0.744 +/- 0.02). Pairwise comparisons of theta; among all 11 locations revealed significant differentiation between Iceland and all other locations (theta = 0.0290*** to 0.0456***), which is consistent with the deep-water barrier to dispersal in plaice. In contrast, no significant differentiation was found among any of the remaining continental-shelf sampling locations. This suggests that regional stocks are themselves composed of several genetic stocks under a model of panmixia which persists even to the spawning grounds. The presence of significant heterozygote deficiencies at all locations (not due to null alleles) suggests a temporal Wahlund effect yet the absence of significant population differentiation among continental shelf localities makes this explanation alone, difficult to reconcile. Sampling of eggs at the spawning grounds will be required to resolve this issue. Causes of the mismatch between genetic and geographical stocks is discussed in the context of high gene flow.     

Alongshore dispersal and site fidelity of juvenile plaice from tagging and transplants

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.     

Scaling of enzyme activity in larval herring and plaice: effects of temperature and individual growth rate on aerobic and anaerobic capacity

Herring Clupea harengus and plaice Pleuronectes platessa were reared at 8 and 12° C from the fertilized egg to a larval age of up to 600 degree-days. Soluble protein as well as the activities of both citrate synthetase (CS) and lactate dehydrogenase (LDH) were measured in homogenate supernatants of individual larvae at 10° C. Scaling factors were calculated using the expression y = ax^b where y is the enzyme activity, x the protein content of the larva a is a constant and b the scaling factor. All scaling factors showed significant differences between the species. Within species, the scaling factors for CS activity were either small or not significantly different between the two rearing temperatures, but the scaling factors for the LDH activities were significantly different at the two temperatures for both species. Herring larvae, which had higher LDH activities when newly hatched, showed smaller scaling factors for LDH ( b =1·42 at 8°C and b =1·07 at 12° C) than plaice ( b =2·11 at 8°C and b =1·45 at 12° C). Activities converged as the larvae grew. The results of the current study together with reanalysis of data from the literature indicate an increasing aerobic and anaerobic capacity during the larval stage of fishes (positive allometry).     

Year-class strength regulation in plaice (Pleuronectes platessa L.) on the Swedish west coast

Year-class strength of plaice (Pleuronectes platessa) has been studied over periods of 10 and 35 years, respectively, in two shallow sandy areas on the Swedish west coast. In one area, Gullmar Bay (N 58 ° 19 – W 11 ° 33), 0-group plaice were sampled quantitatively with a drop trap at 0–0.7 m depth and densities between 0.2 and 3.8 ind./m² were recorded in early summer. In the other area, Laholm Bay (N 56 ° 30 – W 12 ° 55), 0-group plaice were sampled semi-quantitatively in August with young-fish trawl at 1.5 m depths. Densities between 0.001 and 0.28 ind./m² were assessed.The effects of temperature and wind (in the winter and early spring) and predation (in early summer), on the recruitment of 0-group plaice were investigated. In Gullmar Bay high recruitment occurred after severe winters and in years when on-shore winds dominated during spring. No such correlations were found in Laholm Bay. In Gullmar Bay a significant inverse relationship was found between the density of O-group plaice in early summer and the biomass of brown shrimp (Crangon crangon) and the shore crab (Carcinus maenas), the main predators in the nursery area.The importance of physical and biological factors regulating recruitment in plaice are discussed.     

Susceptibility of Atlantic herring and plaice larvae to predation by juvenile cod and herring at two constant temperatures

Predation encounters were staged in the laboratory to compare prey responsiveness, predator error rate, and predator capture success for juvenile cod Gadus morhua (a suction feeder) and herring Clupea harengus (a biting predator) preying on herring and plaice Pleuronectes platessa larvae. Trials were conducted at near natural temperature extremes for the larvae (8 and 13°C) to assess the importance of water temperature to the interaction. Herring larvae were significantly more responsive to attacks by herring than were plaice larvae (5·7 vs 0'0%). The two prey species were equally responsive to attacks by cod (2–6 vs 10%). Cod caught 91% of herring larvae attacked and juvenile herring caught 87%. Cod were successful in 96% of attacks on plaice, but juvenile herring caught significantly fewer (83%) plaice larvae. For each predator species, capture success did not vary significantly with prey species. Overall capture success for herring was significantly lower than that for cod. Responsiveness of herring larvae to attacks by juvenile herring increased with temperature, but predator error rate and capture success were not altered by water temperature.     

Metazoan parasites of Engraulis ringens as tools for stock discrimination along the Chilean coast

The value of the metazoan parasites of the anchoveta Engraulis ringens as biological tags for stock discrimination is assessed. Three hundred and fifty-nine specimens, obtained from six landing ports in northern and central Chile (Arica, Antofagasta, Caldera, Coquimbo, Valparaiso and Talcahuano) covering a latitudinal gradient of c . 18°, were analysed. Six metazoan parasite species (three ectoparasites and three endoparasites) were obtained. A correspondence analysis suggested that anchoveta from northern and central Chile are comprised of two stocks. Identification of stocks was largely based on qualitative differences (absence in southern localities of the monogenean Pseudanthocotyloides heterocotyle , a parasite proper of engraulids, and the copepod Caligus sp.) and quantitative differences in the prevalence of infection for the remaining species. Inertia and mass value for the correspondence analysis supported the hypothesis of two stocks.     

Prolonged-release gonadotrophin-releasing hormone analogue implants enhance oocyte final maturation and ovulation, and increase plasma concentrations of sulphated C21 steroids in North Sea plaice

Reproductively mature female plaice were implanted with or without 50 μg of gonadotrophin-releasing hormone analogue (GnRHa), suspended in either coconut oil or methacrylate resin. The weight of the GnRHa-treated fish increased significantly (due to hydration of the oocytes) and reached a peak between 10 and 14 days. The fish produced several batches of eggs, which were consistently bigger than those produced by control fish. Plasma concentrations of free 17β-oestradiol and glucuronidated testosterone rose briefly (4 days) in response to the GnRHa, but then fell continuously till the end of the experiment (20 days). Plasma concentrations of sulphated 5β-pregnane-3α,17,20β-triol and 5β-pregnane-3β,17,20β-triol (which are putative metabolites of 17,20β-dihydroxy-4-pregnen-3-one, the oocyte maturation-inducing steroid) increased significantly at 4 days and reached a peak between 12 and 16 days. Concentrations were still very elevated on day 20. Plasma concentrations of sulphated 3α,17,21-trihydroxy-5β-pregnan-20-one showed a slight increase on day 4 but did not change thereafter. There was a highly significant difference in the amounts of GnRHa released into the bloodstream by the two methods of administration on day 4. However, this was not matched by significant differences in the concentrations of any of the steroids.     

Distribution and density of populations of goldsinny wrasse (Ctenolabrus rupestris) on the west coast of Scotland

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.     

The use of drop traps to estimate the efficiency of two beam trawls commonly used for sampling juvenile flatfishes

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.     

Abundance,structure, growth and origin of inshore clupeid populations of the west coast of Scotland

The abundance, structure, growth, and origin of clupeid populations in inshore waters of the west coast of Scotland were studied from April 1970 to October 1972. Clupeid populations in the area comprise mainly young fish. 0-group autumn-spawned herring, probably of Minch origin, move into the area about April and spring-spawned ones (Clyde origin) about June. The timing and the body length at which each group arrives in the area during the different years is the same. After the initial immigration, the distribution of both 0-group clupeids becomes localized.Herring populations in the sea-lochs and associated inshore waters are mainly 0-group fish, which are replaced each year by a new incoming brood. The sprat populations of the sea-lochs are dominated by the 0-group; in the more ‘open’ areas the populations comprise older individuals. Year-class distribution in the ‘open’ areas resemble that of the commercial fishery.The rate of increase of 0-group autumn-spawned herring is 3.68 mm/week and that of spring-spawned 0-group herring is 2.83 mm/week. The curves of growth of 0-group sprats of the 1970 and 1971 year-classes are different; the rate of increase in length, however, averages 3.55 mm/week for both year-classes and the differences are not significant.In sprats, after their first year of life a rapid increase in length takes place in the spring. This increase is thought to enable the majority of the population to reach the minimum size (88–90 mm) for initial gonadal maturation and thereby make them capable of reproducing in their second year of life.     

Unravelling the stock structure of the Persian brown trout by otolith and scale shape

The simultaneous use of the scale and otolith morphometry was assessed as a potential tool for the identification of Persian brown trout Salmo trutta stocks of the Lar Lake and five rivers from Lar Basin, Iran. Fourier coefficients (FC) and circularity, rectangularity, roundness, ellipticity and form factor shape indices (SI) were calculated for otolith and scale. Several SIs were significantly different among sites for both structures. Permutational multivariate analysis of variance revealed significant differences between several pairwise comparisons for otolith and scale (FCs and indices separately). Discriminant analysis showed otolith FCs (cross-classification rates: 25–86%) and SI (20–45%) appear to be a relatively acceptable tool to discriminate between several locations. Comparatively, the scale morphometry showed lower discriminatory power (FC = 3–65%; SI = 15–34%), with the exception of SI for Elarm River (60%), Kamardasht River (56%) and Lar Lake (75%). Cross-classification rates improved up to 100% when discriminate analysis incorporating all variables for otolith and scale was performed. The results showed a potential segregation between some water bodies, suggesting that the otolith and scale morphometry could be a useful tool to delimit S. trutta populations in relatively close freshwater environments.