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 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.     

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.     

Adaptive search in juvenile plaice foraging for aggregated and dispersed prey

Juvenile plaice Pleuronectes platessa in a laboratory arena used intensive search behaviour, characterized by short movements and frequent turning, in the five movements before and after attacking a prey in an aggregated distribution. They used extensive search behaviour with, on average, longer movements and less turning at all other times. Intensive search was, apparently, triggered by a high local density of prey but not by isolated prey. This response to local prey density resulted in area-restricted search when prey were aggregated and win-shift behaviour when prey were dispersed. There was no evidence that the use of intensive search increased with experience of aggregated prey. It therefore appears that the fish were able to exploit encountered prey distribution patterns using their immediate perceptions rather than prior experience.     

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.     

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).     

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.     

Otolith microstructure indicating growth and mortality among plaice, Pleuronectes platessa L., post-larval sub-cohorts

Growth and mortality of post-metamorphosed plaice were studied by means of daily increments in the sagittal otoliths. The Gompertz model was the best fit to length-at-age data and there were no significant differences between length-at-age and back-calculated lengths. The microstructure pattern of the otoliths at metamorphosis was also used to estimate hatching and settlement distributions. Differential growth and mortality occurred among sub-cohorts; growth rates and mortality were higher in fish that settled earlier. In 1986, the best survival was for a sub-cohort settling in late May to early June. In contrast, in the warmer season of 1987, survival was highest for the second and third sub-cohorts settling in late April and mid May.     

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.     

The stock structure and migrations of plaice populations on the west coast of England and Wales

Plaice Pleuronectes platessa populations on the west coast of England and Wales are currently managed as two stocks: in ICES Division VIIa (Irish Sea, Cardigan Bay and St George's Channel), and ICES Divisions VIIf&g (Bristol Channel and Celtic Sea). A total of 13,784 plaice were fitted with Petersen tags and released in these areas during 1979–1980 and 1993–1996. Analysis of the 2788 recaptures received by June 2000 confirmed known spawning and feeding grounds in the region. It showed also that plaice >25 cm L _T tended to undertake extensive spatial movements. At this size, female plaice were likely to be mature or maturing for the first time, whilst males were probably mature. Tag recaptures indicated resident sub-stocks of plaice in the north-east Irish Sea, the south-east Irish Sea, Cardigan Bay and Bristol Channel, a contingent of plaice in all areas that undertook permanent dispersal to other areas, and a contingent which originated in the south-east Irish Sea and migrated to spawn in the Bristol Channel. Plaice originating in the Bristol Channel rarely moved north into ICES Vila. A general hypothesis of plaice population structure in the region is presented and discussed in relation to stock assessment.     

Nest site fidelity and dispersal of Rio Grande wild turkey hens in Texas

Rio Grande wild turkey (Meleagris gallopavo intermedia) nests suffer high predation rates exceeding 65%, which may limit recruitment. We evaluated post-nesting movements of reproductively active female Rio Grande wild turkeys. We monitored 194 nesting attempts between 2005 and 2010 and documented 17% and 32% overall apparent nest success for the Edwards Plateau and Central Rio Grande Plains study regions, respectively. Rio Grande wild turkey hens move approximately 1.2 km (SD = 0.7) between nesting attempts within a nesting season and approximately 1.4 km (SD = 1.6) between initial nesting attempts among years. Rio Grande wild turkey hens selected open areas with moderate woody cover for nesting ( = 37.7%; range = 3.0–88.2%). Patchiness of vegetation in the nesting landscape also was borne out by typically low edge-to-area ratios ( = 0.20; range = 0.040–0.732). We found no clear pattern in movement distance and either landscape composition or edge-to-area ratio for within or between breeding season nest site selection for either the Edwards Plateau or Central Rio Grande Plains study region. Based on our results, movement distances post-nest failure do not seem to influence habitat selection. © 2012 The Wildlife Society.     

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).     

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.     

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.     

Habitat use, migration pattern and population dynamics of chevron snakehead Channa striata in a rainfed rice farming landscape

Habitat use, migration, mortality and growth of the chevron snakehead Channa striata in a rainfed rice farming landscape of north‐east Thailand were studied through a tagging experiment. A total of 751 fish were captured, tagged and released during three distinct events in the late dry season, and in the beginning and at the end of the wet season. Rice fields provided the major wet season habitat for C. striata . Small trap ponds built to provide dry season habitat on farms provided 20% of catches and, if not harvested, could increase recruitment to the spawning stock by >30% despite accounting for <1% of dry season habitat by area. Migrations were localized (mostly <500 m). Up‐migration from perennial to seasonal water bodies at the beginning of the wet season involved longer distances and took place over a longer time than down‐migration at the end of the wet season. Natural mortality rates were extremely high, particularly during the period of down‐migration. Fishing mortality rates were high in absolute terms, but contributed only 6–36% to the total mortality. Growth was seasonal with a maximum towards the end of the wet season. Snakeheads have successfully colonized the rainfed rice farming landscape, where populations can withstand intensive exploitation and respond well to aquatic habitat management on farmland.     

Assessment and control of non‐indigenous brown bullhead Ameiurus nebulosus populations using fyke nets in shallow ponds

In order to evaluate fyke nets as a tool for population assessment and mass removal of brown bullhead Ameiurus nebulosus populations, small, shallow ponds were sampled during late summer in 1999 and 2000. Catch efficiency, species and size selectivity of fyke nets were investigated with the mark‐recapture method. Two different strategies were applied: (1) on six occasions, the mark‐recapture method was applied to entire populations, and (2) in six other cases, sampling was done in two replicated enclosed areas (625 m²). When applied to entire populations, fyke nets were found to be size selective, and capture efficiency significantly increased with standard length. Conversely, no size selectivity of fyke nets was found in enclosed areas. Sampling in replicated enclosures yielded reproducible estimates of population density and biomass. The results suggest that double fyke nets, when combined with the mark‐recapture technique, are a very useful tool for the efficient and reliable assessment of brown bullhead populations. The results of the study also suggest that double fyke nets may potentially be a cost‐effective tool for the mass removal of non‐indigenous brown bullhead populations.     

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.     

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.     

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.