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.     

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.     

The distribution of 0-group flatfish in relation to abiotic factors on the tidal flats in the brackish Dollard (Ems Estuary, Wadden Sea)

The distribution of 0-group flatfish was investigated in 1992 in the Dollard (Ems–Dollard estuary, Wadden Sea). 0-Group plaice, flounder and sole were not evenly distributed over the sampled locations. The spatial distribution pattern of 0-group flatfish in the Dollard changed during the investigation period. In the first week of sole presence, when the mean length of sole was 24–30 mm, salinity correlated significantly with sole density. The distribution of juvenile sole larger than 40 mm total length was affected by the elevation of the location: 0-group sole was restricted to the sampled site with the lowest elevation. The distribution of 0-group plaice was related to sediment: no juvenile plaice were caught at locations with more than 10% mud fraction in the sediment. The distribution of 0-group flounder was also correlated with sediment. Later in the year, salinity correlated negatively with the distribution of 0-group flounder. The influence of sediment composition is probably indirect and linked to the abundance of preferred food items, such as Corophium volutator . Abiotic conditions were suitable to 0-group plaice, flounder and sole.     

Plaice nurseries: effects on recruitment

The function of coastal nursery areas is discussed in relation to the variability in North Sea plaice recruitment. Since 60% of the recruitment of juveniles originates from the Wadden Sea, special attention is paid to this area. The concentration of juveniles in a restricted area seems to evoke only adverse effects: an increased risk of food limitation and hence reduced growth, and an increased vulnerability to predation. Despite these expectations, growth of most of the plaice in the Wadden Sea has always been optimal within the wide range of year-class strength observed and depends only on ambient water temperature. The same situation is indicated for some British bays, where the growth of 0-group plaice is far lower than in the Wadden Sea, because of lower temperatures. Mortality through predation seems to be relatively low in the Wadden Sea and restricted to only a short period, because of the absence of almost all potential predators. In the more open British bays higher mortalities are found, probably due to the presence of a number of predatory fish species. The low variability in the recruitment of plaice might be the combined result of optimal growth with the absence of between-year fluctuations in predator abundance in the Wadden Sea. As a result, observed mortality only depends on prey abundance and is therefore density-dependent both within one year and especially between years, reducing variations in recruitment. This suggestion is supported by the situation in more open British bays: here too, growth is maximal, but the abundance of predators shows larger fluctuations between years and, as a result, greater fluctuations in mortality are observed.     

Monitoring juvenile stocks of flatfish in the Wadden Sea and the coastal areas of the southeastern North Sea

Results are presented of an on-going monitoring program, started in 1970, of the demersal fish population in the Wadden Sea, Scheldt estuary, and along the continental coast between the Belgian-French border and Esbjerg. Particular attention is given to long-term trends in overall abundance and annual variations in spatial distribution of 0- and 1-group plaice and sole in relation to year-class strength and to variations in growth rate. Recruitment level in both plaice and sole appeared to be higher in the 1980s than in the 1970s. The most important nursery areas for plaice are the German Bight and the Wadden Sea, and the relative contributions of these areas to the total plaice stock in different years are relatively stable. In contrast, the most important nurseries for sole are along the continental coast, and the contribution of different parts of these nurseries vary considerably from year to year. Abundant year-classes of sole can originate from northern as well as from southern parts of the nurseries. Relations between survey results and year-class estimates from Virtual Population Analysis (VPA) for plaice are better compared to sole. No trends have been observed in mean length at age 0 and 1 during the years the surveys have been carried out. It is concluded that the quality of the continental coastal area and the Wadden Sea as nursery area for plaice and sole has not declined during the period of investigation. Presented at the VI International Wadden Sea Symposium (Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-2282 List, FRG, 1–4 November 1988)     

A modified drop-net method for sampling mobile epifauna on marine shallow sandy bottoms

The seasonal variations in numbers and biomass of the mobile epifauna of a shallow, sandy bay in Gullmar Fjord (Sweden) was investigated during 1976–1977 using the drop-net technique. Juvenile plaice, Pleuronectes platessa L., sand goby, Pomatoschistus minutus (Pallas), and brown shrimp Crangon crangon L. are the dominant epifaunal species. A net enclosing an area of 100 m² was used, and the organisms captured were collected with a small trawl. Three stations with a water depth≤ 1 m were regularly sampled with a total number of 73 drops. Maximum abundance was for plaice in June (6 ind m^-2), for the shrimp in July-August (61 ind m^-2), and for sand goby in September (2 ind m^-2). Standing stocks (dry wt) were 0.2, 1.5 and 0.2 g m^-2 respectively. Production estimates were based on seasonal changes in size frequency and information on specific growth rates. The production rate was for plaice (0-group) 0.3 g m^-2 yr^-1, for the shrimp 1.5, and for sand goby 0.2 g m^-2 yr^-1. Reliable estimates of the absolute abundance can be derived from the mean catch per unit area only when the efficiency of the fishing gear is known. After 6 hauls with the trawl in the drop-net enclosure more than 85% of the shrimps were caught and after the fifteenth haul less than 1% were still left. The fishing efficiency of a push-net in relation to the drop-net was for the shrimp 50%, for plaice 35% and for sand goby 17%.     

The food web in Red Wharf Bay (North Wales) with particular reference to young plaice(Pleuronectes platessa)

Summary 1. About twenty-five species of fish occur in Red Wharf Bay; plaice, dabs and gobies predominate in the beam-trawl catches. 0-group plaice comprise about 70% of the catch in a push-net at low-water mark.2. The 0-group plaice first arrive in late April or May and recruitment continues into July or August. In the succeeding months, the mortality rate is about 40% per month. 0-group dabs arrive in June and their monthly mortality rate is about 44%.3. Growth rate of plaice and dabs is high in summer but low from November to April.4. The food of the earliest 0-group plaice differed from year to year and included both plankton and benthos.5. Pairs of fish species having similar feeding habits could be discerned but one of each pair had a wider depth distribution than the other. Plaice and dabs had similar diets but, when examined in detail, differences were apparent.6. Marking experiments indicated that, in summer and autumn, O-group plaice tended to keep to their own part of the bay and that, if transported offshore, they returned to the shallow-water areas.

---

Die Nahrungskette in der Red Wharf Bucht (Nord-Wales) mit besonderer Berücksichtigung junger Schollen(Pleuronectes platessa)

Kurzfassung Ökologische Aspekte und qualitative Gesichtspunkte der Ernährung vonPleuronectes platessa undLimanda limanda aus der Red Wharf Bucht wurden untersucht. Die jungen Schollen treten in der Bucht in der Zeit von April bis Juli (oder August) auf; von August bis zum Winter beträgt die monatliche Verlustquote ungefähr 40%. Zunächst ist die Verbreitung der Jungfische auf die Ebbe-Linie beschränkt. Jungfische der Kliesche erscheinen von Juni an; ihre Verlustrate beträgt ungefähr 44% pro Monat. Im Sommer wachsen die beiden Arten schnell; in der Zeit von November bis April ist die Wachstumsgeschwindigkeit jedoch verringert. Bei Arten mit ähnlichem Nahrungsverhalten wurden Unterschiede hinsichtlich der Tiefenverteilung festgestellt, so daß sich die Areale der Arten nur teilweise überdecken. Die Klieschen haben ein Nahrungsspektrum, das dem der Schollen aus der 0-Gruppe ähnelt, doch bestehen, wie eingehende Untersuchungen gezeigt haben, bestimmte Unterschiede. Sowohl Schollen als auch Klieschen der 0-Gruppe wurden von anderen Tieren, vor allem Artgenossen (Tiere der I-Gruppe), gefressen. Markierungsexperimente zeigten, daß Jungschollen der 0-Gruppe — zumindest im Sommer und Herbst — in das flachere Wasser zurückkehren, nachdem sie markiert und in tieferes Wasser ausgesetzt worwaren.

     

Seasonal Ichthyodiversity and Growth Patterns of Juvenile Flatfish on a Nursery Ground in the Southern Bight of the North Sea (France)

The demersal fish community on a flatfish nursery ground, of the Southern Bight of the North Sea, was sampled monthly between May 1998 and 1999. The studied coastal area is a multispecific nursery area. Although 32 fish species were caught, only nine species had a major influence on the variation in total densities through the year and can be considered as key species. Juvenile sole and plaice exhibited similar seasonal growth patterns with rapid growth during late spring and summer, a growth arrest during autumn and winter and a restart of growth in March. Comparisons of observed growth in length with predicted maximal growth under optimal food conditions suggested that during the summer, growth of 0-group sole and plaice was only determined by prevailing mean water temperature. During autumn and winter, while growth in length of sole followed model predictions, observed length of plaice was less than model predictions, suggesting growth limitation. Analyses of the factors that may be responsible for differences between observed and expected length growth indicated that autumn and winter growth arrest of plaice was not only related to low winter water temperature. It is suggested that for visual feeders such as plaice, the interaction of decreasing food availability and day length during autumn and winter can reduce the access to food resource and therefore feeding success and growth.     

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.     

The feeding ecology of Pleuronectes platessa L., Limanda limanda (L.) and Scophthalmus rhombus (L.) in Carmarthen Bay, South Wales, U.K.

To assess the feeding habits of plaice, Pleuronectes platessa L., and dab, Limanda limanda (L.), within Carmarthen Bay, three sampling techniques were used: benthic trawling and surf-zone push-netting were undertaken during the feeding season for the gut contents of plaice and dab (age groups 0-IV) and of 0-group brill, Scophthalmus rhombus , to be analysed in detail, and a benthic macro-fauna grab survey covered the trawled ground to assess the distribution of food species in relation to gut contents. Plaice consumed commonly-occurring food species whereas dab took any available food. The main items were the same for both species, but secondary food preferences differed. Variations in gut contents within species, for the same sized fish, chiefly resulted from the spatial distribution of fish and not seasonal variations in the benthic macro-fauna. The length and not the age of fish primarily determined the selection of food organisms.     

Developmental defects in pelagic fish embryos from the western Baltic

In February/March 1983 and 1984 a survey of pelagic fish eggs was conducted in the western Baltic (Kiel Bight), employing a horizontally towed plankton net (1 m Ø and 300 μm mesh). Maximum egg numbers in the upper meter of the S=21×10^?3 salinity layer were 200·100 m^?3. The most abundant eggs were cod (up to 142 eggs·100 m^?3), followed by plaice (up to 74 eggs·100 m^?3) and flounder (20 eggs·100 m^?3). A considerable percentage of embryos of all species displayed aberrant development. In 1983 18% of cod, 22% of flounder and 24% of plaice eggs caught contained defective embryos; in 1984 this number was larger, ranging from 28% in plaice over 32% in cod to 44% in flounder. Early developmental stages showed the highest malformation rates (up to 51% in the case of early flounder embryos). With progressive development, malformations decreased in numbers, being lowest prior to hatching. Highest rates of malformations were recorded in the Mecklenburg Bight in 1983. A second area with high incidence of malformation rates was located south and east of the island of Langeland. Several reasons, including environmental and anthropogenic factors, for the occurrence of malformed embryos in pelagic fish eggs are discussed. The potential of malformation rates in embryos of pelagic fish eggs as a tool for monitoring is considered.     

The effect of sand and light on predation of juvenile plaice (Pleuronectes platessa) by fishes and crustaceans

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.     

Movements of 0-group plaice Pleuronectes platessa L. as shown by latex tagging

0-group plaice were caught in Red Wharf Bay, Anglesey, North Wales, tagged with coloured liquid latex, and released in the bay or, after transplantation, at a position on a sandbank 3–5 km offshore. The pattern of movement shown by recaptures of plaice caught and released at the same position in the bay was one of extremely restricted movement of less than 0–5 km over several months, with only a slight interchange of fish between the sides of the bay and the centre. These restricted movements contrasted sharply with relatively long distances travelled by fish displaced to an unusual depth and position offshore and to a position at the other side of the bay 3 km away. In the first case a high proportion returned rapidly to the usual depth preference inshore and in the second a recapture pattern significantly different from that of control fish was demonstrated. Both results indicated a return to a preferred or familiar situation.     

Feeding ecology and growth of O-group flatfish (sole, dab and plaice) on a nursery ground (Southern Bight of the North Sea)

The food composition of O-group sole Solea solea , dab Limanda limanda and plaice Pleuronectes platessa on a nursery ground at Gravelines, France, included 17–25 taxa. Sole (new settlers) fed mainly on harpacticoid copepods and when 50 mm in size, on polychaetes (Terebellidae). Dab (<40 mm) consumed mainly polychaetes (Magelonidae and Spionidae), and later amphipods, polychaetes (Spionidae) and Hydrozoa. O-group plaice diet was dominated by polychaetes (Terebellidae), crustaceans and molluscs at all sizes. O-group sole, dab and plaice did not compete for food resources, each species being specialized in different prey items. Growth rates during May-July 1998 varied between 0·5 and 0·67 mm day⁻¹ for sole, 0·12 and 0·24 mm day⁻¹ for dab and 0·55 and 0·81 mm day⁻¹ for plaice. For sole and plaice, these estimates were similar to those recorded in other nurseries and also close to the maximal growth predicted by experimental models. This suggests that their growth was not limited by food during the first summer of life.     

Ecological significance of 0-group fish in the Barents Sea ecosystem

Investigations into the 0-group fish in the Barents Sea have been carried out since 1965, with the goal of estimating the abundance of 0-group fish. 0-group abundance indices have been used in the assessment of the recruitment level and in recruitment variability studies. However, the ecological importance of the 0-group fish in the Barents Sea has been less studied. Although 0-group capelin, herring, cod and haddock are widely distributed in the Barents Sea, the central area seems to be the most important, accounting for approximately 50–80% of the annual biomass. The total biomass of the four most abundant 0-group fish species can be up to 3.3 million tonnes, with an average of 1.3 million tonnes (1993–2009). Wide distribution and high biomass of pelagically distributed 0-group fish make these fishes an important element in the energy transport between different trophic levels and different geographical areas, having a critical impact on the entire Barents Sea ecosystem. In recent years, capelin have shown a pronounced northward shift in biomass distribution, and several successive strong year classes occurred during warm temperature conditions. Cod biomasses were unexpectedly low during warm years and were positively correlated with spawning stock biomass, while the correlation with temperature was not significant. Haddock and herring show, as expected, increasing biomass with increased temperature when the spawning stock is at a sufficiently high level.     

Karyotypes of the Echinogammarus berilloni-group (Crustacea,Amphipoda) from Spain

The chromosome number and karyological features of Iberian populations of seven species of the Echinogammarus berilloni-group (Crustacea, Amphipoda) have been studied. For comparison, Gammarus gauthieri, belonging to the G. pulex-group was also studied. Four species of the berilloni-group share the same number n=27. Other numbers found were n=25, n=26 and n=28. The chromosomes of these species have a centromeric region surrounded by large heterochromatic blocks at late prophase. There-after, the centromere splits precociously. C. gauthieri has n=26, the same number found in the pulex-group. This species has larger chromosomes than the former but it does not show these karyological peculiarities.Cytological observations confirm both the close relationship among species of the berilloni-group and the differentiation with respect to the pulex-group. On the other hand, the existence of different numbers confirms other results that reject the older idea that speciation within Gammaridea occurred with minor changes in karyotypic characters.     

Biomass of scyphozoan jellyfish, and its spatial association with 0-group fish in the Barents Sea

An 0-group fish survey is conducted annually in the Barents Sea in order to estimate fish population abundance. Data on jellyfish by-catch have been recorded since 1980, although this dataset has never been analysed. In recent years, however, the ecological importance of jellyfish medusae has become widely recognized. In this paper the biomass of jellyfish (medusae) in 0–60 m depths is calculated for the period 1980–2010. During this period the climate changed from cold to warm, and changes in zooplankton and fish distribution and abundance were observed. This paper discusses the less well known ecosystem component; jellyfish medusae within the Phylum Cnidaria, and their spatial and temporal variation. The long term average was ca. 9×10⁸ kg, with some years showing biomasses in excess of 5×10⁹ kg. The biomasses were low during 1980s, increased during 1990s, and were highest in early 2000s with a subsequent decline. The bulk of the jellyfish were observed in the central parts of the Barents Sea, which is a core area for most 0-group fishes. Jellyfish were associated with haddock in the western area, with haddock and herring in the central and coastal area, and with capelin in the northern area of the Barents Sea. The jellyfish were present in the temperature interval 1°C<T<10°C, with peak densities at ca. 5.5°C, and the greatest proportion of the jellyfish occurring between 4.0–7.0°C. It seems that the ongoing warming trend may be favourable for Barents Sea jellyfish medusae; however their biomass has showed a recent moderate decline during years with record high temperatures in the Barents Sea. Jellyfish are undoubtedly an important component of the Barents Sea ecosystem, and the data presented here represent the best summary of jellyfish biomass and distribution yet published for the region.     

Small-scale distribution of juvenile plaice and flounder in relation to predatory shrimp in a shallow Swedish bay

Metamorphosed plaice and flounder were sampled in a small non-tidal nursery bay with a non-selective drop trap from April to August in 1991 and 1992. Post-settlement patterns were mapped in time and space and related to wind stress and the distribution of a potential predator Crangon crangon L. Both plaice and flounder had a patchy distribution at a sample scale of 1 m². Plaice settlement coincided with timing of onshore winds in 1991 and 1992, while flounder settlement seemed to be unrelated. Plaice settled earlier than flounder and during the early phase of 1-group C. crangon immigration. The first appearance of flounder occurred simultaneously with the peak biomass of predatory C. crangon . Plaice occurred in the deeper part of the bay together with C. crangon , while flounder preferred shallow shore waters. We propose that different temporal (plaice) and spatial (flounder) strategies have evolved in order to minimize the risk of shrimp predation after settlement.     

Influence of sowing windows and genotypes on growth,radiation interception,conversion efficiency and yield of guar

Crop growth largely depends on radiation. Radiation is the main impetus for photosynthesis and movement of photosynthates from source to sink. Therefore, identification of the optimum sowing windows and suitable cultivars for efficient utilization of radiation is of prime importance. A field study was conducted in red clay soil during 2014 and 2015 Kharif season and the treatments consisted of three genotypes and three sowing windows by using randomized complete block design with three replications. The effect of genotypes and sowing windows was found significant with respect to number of trifoliate leaves, leaf area ratio, dry matter production, grain numbers, pod length, test weight, grain yield, and stover yield of guar during 2014 as compared to 2015 sown crop. Statistically significant plant height, number of trifoliate leaves, number of branches, leaf area ratio, absolute growth rate, leaf area index, dry matter, grain number, pod length, grain yield, stover yield and a higher cumulative radiation interception were recorded with 15th August sown crop as compared to other sowing windows. The plant height, number of trifoliate leaves, number of branches, leaf area ratio, absolute growth rate, leaf area index, dry matter, grain number, pod length, grain yield, stover yield and maximum cumulative interception of radiation were significant with RGC-1003 as compared to RGC-936 and HG-365. It is observed that the incident PAR to dry matter accumulation conversion efficiency was varied with cultivars and different sowing windows which ranges from 0.74 g MJ⁻¹ to 0.79 g MJ⁻¹.     

Robertsonian polymorphism in plaice, Pleuronectes platessa L., and cod, Gadus morhua L., (Pisces Pleuronectiformes and Gadiformes)

Karyotypic analysis of plaice, Pleuronectes platessa L., and cod, Gadus morhua L., revealed that the chromosome numbers in both species vary but that chromosome arm numbers (NF) stay constant. The C-, Q- and R-banding patterns also confirmed that the population of plaice studied shows Robertsonian polymorphism. The tendency of reduction in chromosome number in fishes suggests that Robertsonian fusions play a role in karyotype evolution in fishes.