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Is summer growth reduction related to feeding guild? A test for a benthic juvenile flatfish sole (Solea solea) in a temperate coastal area, the western Wadden Sea
... The first dataset consists of microscopic images of plaice (Pleuronectes platessa) otoliths (i.e., hearing stones) in which concentric rings are visible. These rings represent daily growth layers and are used to estimate the age of the fish to reconstruct egg and larval drift and calculate the contribution of various spawning grounds to different settling areas 6,7 . Plaice eggs and larvae are transported from their North Sea spawning grounds towards the coast of the North Sea and into the Wadden Sea (pelagic phase), where they settle (benthic phase). ...
... Each individual fish was measured, the sagittal otoliths were removed and microscopic images of two zoom levels ( 10 × 20 and 10 × 10 , depending on fish length) were made. Post-settlement daily growth rings outside the accessory growth centre were then counted by eye 6,7 . In this dataset, images of otoliths with less than 16 and more than 45 rings were scarce (Fig. 6). ...
Many ecological studies rely on count data and involve manual counting of objects of interest, which is time-consuming and especially disadvantageous when time in the field or lab is limited. However, an increasing number of works uses digital imagery, which opens opportunities to automatise counting tasks. In this study, we use machine learning to automate counting objects of interest without the need to label individual objects. By leveraging already existing image-level annotations, this approach can also give value to historical data that were collected and annotated over longer time series (typical for many ecological studies), without the aim of deep learning applications. We demonstrate deep learning regression on two fundamentally different counting tasks: (i) daily growth rings from microscopic images of fish otolith (i.e., hearing stone) and (ii) hauled out seals from highly variable aerial imagery. In the otolith images, our deep learning-based regressor yields an RMSE of 3.40 day-rings and an R2 of 0.92. Initial performance in the seal images is lower (RMSE of 23.46 seals and R2 of 0.72), which can be attributed to a lack of images with a high number of seals in the initial training set, compared to the test set. We then show how to improve performance substantially (RMSE of 19.03 seals and R2 of 0.77) by carefully selecting and relabelling just 100 additional training images based on initial model prediction discrepancy. The regression-based approach used here returns accurate counts (R2 of 0.92 and 0.77 for the rings and seals, respectively), directly usable in ecological research.
... More examples for this relation are provided by Dupont et al. (2020) and Trenkel et al. (2015). For juvenile fish, the relationship between food abundance and fish length has been shown in many studies (Fonds et al., 1992;Freitas et al., 2016;Poiesz et al., 2020a). Reduced growth rates, compared to ad libitum laboratory growth rates or maximum growth predicted by bioenergetics modelling, have been observed in juvenile fish, suggesting food limitation (Ciotti et al., 2014;Poiesz et al., 2019;van der Veer et al., 2016), which may be more severe at higher temperatures (Freitas et al., 2012;Teal et al., 2008Teal et al., , 2012. ...
Coastal areas in north-western Europe have been influenced by elevated nutrient levels starting in the 1960s. Due to efficient measures, both nitrate and phosphate levels decreased since the mid-1980s. The co-occurring declines in nutrient loadings and fish productivity are often presumed to be causally linked. We investigated whether four resident fish species (twaite shad, bull-rout, thick-lipped grey mullet and eelpout), that spend the majority of their life in the vicinity of the coast, differed in growth between the historic eutrophication period compared to the recent lower nutrient-level period. Based on Von Bertalanffy growth models of length at age, and the analysis of annual otolith increments, we investigated the difference in sex-specific growth patterns and related these to temperature, eutrophication level (Chlorophyll a), growth window and fish density. In all four species, annual otolith growth rates during the early life stages differed between the two periods, mostly resulting in larger lengths at age in the recent period. All species showed significant correlations between increment size and temperature, explaining the observed period differences. The lack of an effect of total fish biomass provided no evidence for density dependent growth. A correlation with chlorophyll was found in bull-rout, but the relationship was negative, thus not supporting the idea of growth enhanced by high nutrient levels. In conclusion, we found no evidence for reduced growth related to de-eutrophication. Our results indicate that temperature rise due to climate change had a greater impact on growth than reduced food availability due to de-eutrophication. We discuss potential consequences of growth changes for length-based indicators used in management.
... Instead, the scalar was used to scale the attained length of the juveniles at the end of the growth season, set at Julian day 273, for the reference temperature scenario to match observed values from early September surveys of $11 cm (Teal et al., 2008). By setting the value of f such that the modelled length matches observed length, we implicitly mimic food limitation as has been demonstrated for sole in the field (Le Pape and Bonhommeau, 2015; Poiesz et al., 2020). For simplicity reasons, we use a fixed value, for adult sole little seasonality in gut fullness was demonstrated (Teal et al., 2012). ...
This study shows the effect of climate change on the growth and survival of early life history stages of common sole (Solea solea) in different nursery areas of the North Sea, by combining a larval transport model with an individual-based growth model (Dynamic Energy Budget) to assess the fate from egg to young of the year at the end of the first growth season. Three scenarios of climate change, inspired by the 2040 Intergovernmental Panel on Climate Change projections, are tested and results are compared to a reference situation representative of current climate conditions. Under climate change scenarios where wind changes, water temperature increases and earlier spawning are considered, the early arrival of fish larvae in their nurseries results in larger young of the year at the end of summer. However, early arrival leads to higher mortality due to initially slow growth in spring. Future climate scenarios result in higher biomass and reduced first-year survival. How this result translates into changes at population level and stock management needs further investigation. Nonetheless, this study illustrates that processes linking life stages are paramount to understand and predict possible consequences of future climate conditions on population dynamics.
Arctica islandica is the longest-living non-colonial animal known at present. It inhabits coastal waters in the North Atlantic and its annual shell increments are widely used for paleoclimatic reconstructions. There is no consensus, however, about the intra-annual timing of its feeding activity and growth. This research aims to identify the main environmental drivers of A. islandica valve gape to clarify the ambiguity surrounding its seasonal activity. A lander was deployed from February 2014 to September 2015 on the sea bottom at Ingøya, Norway (71°03′N, 24°05′E) containing living A. islandica specimens (70.17 ± 0.95 mm SE) in individual containers. Each individual was attached to an electrode unit that measured the distance between their valves (valve gape) every minute. Individuals were followed for various lengths of time, and in some cases replaced by smaller individuals (54.34 ± 0.63 mm SE). The lander was also equipped with instruments to simultaneously monitor temperature, salinity, [Chl-a], turbidity, and light. There was a significant difference in the average monthly valve gape (P value < 0.01), with monthly means of 19–84% of the total valve gape magnitude. The experimental population was largely inactive October–January, with an average daily gape <23%. During this period the clams opened at high amplitude once or twice a month for 1–3 days. Seasonal cycles of sea water temperature and [Chl-a] were temporally offset from each other, with temperature lagging [Chl-a] by about 2 months. Multiple regression analyses showed that bivalve gaping activity was most closely correlated with variable [Chl-a], and to a much smaller degree with photoperiod and temperature.
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The habitat quality of European shallow-water nurseries was studied for 3 common flatfish species based on juvenile growth conditions. Field growth of 0-group plaice Pleuronectes platessa, flounder Platichthys flesus and sole Solea solea, from both published and unpublished studies, was compared with maximum growth predicted by a bioenergetics model based on the dynamic energy budget theory. In plaice and flounder, realized growth ratio decreased consistently during the growing season in most of the nurseries analyzed, indicating a widespread pattern of declining conditions. In sole, growth performance was not maximal, but as opposed to the other species, no clear temporal trend in realized growth ratio was observed. A latitudinal comparison of realized growth ratio over the various nurseries indicated clear positive trends for plaice and flounder, with better growth conditions at northern latitudes. In sole, despite some variability, the same trend was found during part of the summer. In the absence of clear gradients in benthic prey biomass, we hypothesize that increased food limitation in southern locations is most likely caused by interspecific competition reducing maximum individual intake rates. These results suggest that, in the context of global warming, habitat quality of southern European nurseries for juvenile growth may be particularly affected by the combined interaction of food and thermal constraints.
This study concludes that declines in growth rates of young-of-the-year European plaice (Pleuronectes platessa) (YOY plaice) during summer vary spatially and between years and that these dynamics are not driven by temperature, body size, or competition. RNA-predicted growth rates of YOY plaice on the west coast of Scotland declined linearly between mid-July and mid-September, with faster declines at beaches and in years where growth rates were initially high. Absolute growth estimates rarely approached ad libitum laboratory rates, and relative declines in growth rates were unrelated to temperature or body size allometry, indicating that maximum growth was rare. However, the absence of inverse relationships between spatial or temporal growth variation and YOY plaice or brown shrimp (Crangon crangon) densities suggests that competition did not limit growth. There were no consistent trends in morphometric condition factor, biochemical composition, or energy content during summer, indicating that apparent growth declines did not result from energy storage and did not result in depletion of energy stores. Influences of extrinsic factors (such as prey conditions, physical disturbance, and predator densities) on the diet, mediated by behavioral decisions to optimize growth with other ecological constraints, require further investigation as causes of growth variation in YOY plaice.
Mortality factors operating in a temporarily isolated 0-group plaice population in the western Wadden Sea have been examined from information on larval immigration, subsequent settlement and changes in demersal densities. Larvae entered the western Wadden Sea every year from the end of February to the beginning of May. In years of strong larval abundance the main perlod of arrival seemed somewhat delayed from March to April. As a result, between-years mean length and density of the settled population showed an inverse relation on June 1. Density-dependent mortality was observed among plaice up to about 35 mm, with instantaneous mortality rates ranging up to 0.06 d-l during and shortly after settling of the larvae. Arguments are given that, although in the coastal zone pelagic predators are present (several coelenterate species), the main dens~ty-dependent mortality processes operate in the demersal phase. The larval data further showed that level of year-class strength was established early in life, during the planktonic stage in the open sea. Observed density-dependent mortality in the Wadden Sea reduced the between-year variation in year-class strength generated in the open sea egg and larval stages from a coefficient of variation of 62 % during larval immigration to 30 % after settling in June.
Larval immigration and subsequent settlement, abundance fluctuations, growth, and mortality of a 0-group flounder population were studied in a tidial flat area, the Balgzand, in the western Wadden Sea. Larvae entered the area from early April to early May. The first settled flounder were found about 1 mo after larval immigration started. Settled flounder were restricted to the siltier areas of the tidal flats. Growth did not seem to be food-limited within the density range observed over the years and depended only on prevailing water temperatures. The life history pattern of flounder was similar to that of plaice in the same area: year-class strength was generated in the open sea and after settlement a period of density-dependent mortality followed, most probably due to predation by crustaceans. The instantaneous mortality rate Z during summer seemed to be higher (0.08 d-1) than in plaice (0.03 d-1), probably due to a combination of higher water temperature and higher abundance of predators in summer than during the settlement of plaice in spring. No relationship between the year-class strengths of plaice and flounder was found.
A test of the 'maximum growth/optimal food condition' (MG/OFC) hypothesis is described for 0-group plaice Pleuronectes platessa L. in the western Dutch Wadden Sea. The MG/OFC hypo-thesis assumes optimal food conditions (i.e. no competition for superfluous food and hence no negative density-dependent growth) and consequently a maximum growth rate only determined by prevailing water temperature conditions. Growth of 0-group plaice subpopulations was determined for a number of areas differing in food abundance and composition (benthic biomass of potential food items) and density of 0-group plaice in both the intertidal and the sublittoral. Growth showed a positive relation-ship with food abundance, in both the intertidal and the sublittoral, falsifying the MG/OFC hypothesis. At similar food abundance, growth in the intertidal was always higher than in the sublittoral due to the presence of the lugworm Arenicola marina in the intertidal only. In the intertidal, growth was positively related to density of A. marina. The assumption of the MG/OFC hypothesis of no negative density-dependent growth was confirmed both in the intertidal and in the sublittoral, although it did not result in maximum growth rate for 0-group plaice everywhere in the field Differences in growth rate there-fore do not necessarily reflect competition for food and density-dependent growth, but they might reflect differences in food composition and availability (searching and handling time).
Temperature tolerance and sensitivity were examined for some North Atlantic marine species and linked to their energetics in terms of species-specific parameters described by dynamic energy budget (DEB) theory. There was a general lack of basic information on temperature tolerance and sensitivity for many species. Available data indicated that the ranges in tolerable temperatures were positively related to optimal growth temperatures. However, no clear relationships with temperature sensitivity were established and no clear differences between pelagic and demersal species were observed. The analysis was complicated by the fact that for pelagic species, experimental data were completely absent and even for well-studied species, information was incomplete and sometimes contradictory. Nevertheless, differences in life-history strategies were clearly reflected in parameter differences between related species. Two approaches were used in the estimation of DEB parameters: one based on the assumption that reserve hardly contributes to physical volume; the other does not make this assumption, but relies on body-size scaling relationships, using parameter values of a generalized animal as pseudo-data. Temperature tolerance and sensitivity seemed to be linked with the energetics of a species. In terms of growth, relatively high temperature optima, sensitivity and/or tolerance were related to lower relative assimilation rates as well as lower maintenance costs. Making the step from limited observations to underlying mechanisms is complicated and extrapolations should be carefully interpreted. Special attention should be devoted to the estimation of parameters using body-size scaling relationships predicted by the DEB theory.
The Balgzand intertidal is an important nursery area for early life stages of various epibenthic crustacean and fish species. Especially in summer, extremely high numbers of individuals occur. This study analyses whether these high densities in summer lead to food limitation using 0-group plaice Pleuronectes platessa L. as a model species. Between 1975 and 2007, this species was quantitatively monitored during 20 yr. The aim of this study is twofold: (1) a statistical analysis of field growth in relation to density, whereby negative density-dependent growth is considered as an indication of intraspecific competition, and (2) a comparison of observed field growth with predicted maximum growth according to the dynamic energy budget (DEB) model, to detect whether growth reduction occurs during the growing season as an indication of interspecies competition. The statistical analysis indicated no negative density-dependent growth during the whole growing season, suggesting the absence of intraspecies competition for food. The comparison of observed growth with DEB-predicted maximum growth showed that field growth was lower than the possible maximum, and that the difference increased over time until about the end of July, suggesting interspecies competition for food in summer. The stabilization in growth rate from July onwards might be related to a change in food quality: a shift from small bivalve siphons as main food items to larger tail tips of the lugworm Arenicola marina. These findings illustrate that not only food quantity but also food quality affects growth rates, at least in 0-group plaice.
Habitat quality of a subarctic nursery ground in northern Norway for 0-group plaice Pleuronectes platessa was investigated by following settlement, mortality and growth during 2005 and 2006. Newly settled individuals were first observed in the end of May to early June and settlement lasted until mid-July. Densities peaked in early July and were comparable to those reported in temperate nursery grounds. Mortality estimates after settlement differed between 0.062 d−1 in 2005 and 0.025 d−1 in 2006. Potential predators appeared to be rather similar as those reported in other areas: the brown shrimp Crangoncrangon, the shore crab Carcinus maenas and demersal fish species (gadoids). Population mean growth indicated linear growth until August leveling-off afterwards. 0-group plaice reached a lower mean size (5–6 cm) at the end of the growing season than in temperate areas probably due to later settlement timing in combination with lower summer–autumn water temperatures. The comparison of observed growth rates with predictions of maximum growth models indicated a similar pattern as observed in temperate nursery grounds: Growth appeared to be maximal except for the period after summer. Whether or not this was related to changes in food quality throughout the season, to interspecies competition or to emigration remains to be elucidated.
Michael W. Beck, Kenneth L. Heck, Jr., Kenneth W. Able, Daniel L. Childers, David B. Eggleston, Bronwyn M. Gillanders, Benjamin Halpern, Cynthia G. Hays, Kaho Hoshino, Thomas J. Minello, Robert J. Orth, Peter F. Sheridan and Michael P. Weinstein
Summer growth reduction in juvenile flatfish has been observed in various temperate coastal areas, suggesting a general mechanism. One possible mechanism that might explain this phenomenon is related to trophic limitation. After the spring phytoplankton bloom macrozoobenthic infauna becomes less active above the sediment, thereby affecting the time spent by predatory flatfish on searching for prey and hence, causing a reduction in food intake and in growth. Here, our aim is to gather evidence to substantiate this so-called “summer growth reduction” hypothesis by analyzing summer growth for 0-group flounder Platichthys flesus at the Balgzand intertidal area in the western Dutch Wadden Sea, under the prediction that flounder, as a more epibenthic predator, would suffer less or not at all from summer growth reduction in contrast to 0-group plaice Pleuronectes platessa, a more benthic feeder. Summer growth was studied for three contrasting years with respect to preceding winter water temperature conditions (cold, average and warm year) to exclude possible irreversible non-genetic adaptation of growth to water temperature conditions. Growth performance was analyzed by combining information on individual growth based on otolith daily ring analysis with predictions of maximum growth (= under optimal food conditions) based on a Dynamic Energy Budget model. In line with expectations and in contrast to 0-group plaice, no trend in growth performance over time was found suggesting that 0-group flounder showed no growth reduction after summer, providing further basis for a future testing of the trophic limitation hypothesis.
Fluctuations in species composition can have major effects on the functionality of an ecosystem; however, studying such impacts is often complicated because fluctuations coincide with changes in other parts of the ecosystem. In this study, we explored long-term (1975 to 2014) changes in predator-prey interactions following concurrent changes in predatory epibenthic fish and crustaceans as well as their prey, macrozoobenthic bivalves and polychaetes, in the western Wadden Sea. Historical and recent invasions have resulted in an increase in relatively large and long-lived bivalves (Mya arenaria, Ensis directus, Crassostrea gigas) which have found a size refuge from epibenthic predators in the Wadden Sea. While bivalves dominated the macrozoobenthic biomass, polychaetes were the main food source of epibenthic predators, with the invasive polychaete Marenzelleria viridis probably becoming an important food source during the early 2000s. Food requirements of epibenthic crustaceans, mainly Crangon crangon, almost doubled from 5 to 10 g ash-free dry mass (AFDM) m⁻² yr⁻¹, and requirements of epibenthic fish decreased by more than 80%, mainly due to the local disappearance of Pleuronectes platessa. While the overall food requirements of the epibenthic predators stayed more or less constant, the edible fraction of the macrozoobenthic biomass increased from 5 to 20 g AFDM m⁻² until the 2000s and decreased to 10 g AFDM m⁻² thereafter. This was the result of changes in native (Nereis diversicolor and Heteromastus filiformis) and invasive (M. viridis) polychaetes. These findings illustrate that coinciding species-specific changes and interactions of both predators and prey should be taken into account to determine the impact of invasions on the food web structure and functioning of coastal systems.
Growth studies based on population-based growth estimates are limited by the fact that they do not take into account differences in age/size structure within the population. To overcome these problems, otolith microstruc-ture analysis is often used to estimate individual growth. Here, we analyse growth of 0-group plaice in the western Wadden Sea in two years: a year preceded by a mild winter (1995) and a year preceded by a severe winter (1996). Growth was analysed by combining information on individual growth based on otolith analysis with predictions of maximum growth (= under optimal food conditions) based on a Dynamic Energy Budget model. Otolith analysis revealed that settlement occurred earlier in 1995 than in 1996. In both years, one main cohort was found, followed by a group of late settlers. No differences in mean length-at-age were found between these groups. DEB modelling suggested that growth was not maximal during the whole growing season: realized growth (the fraction of maximum growth realized by 0-group plaice) declined in the summer, although this decline was relatively small. In addition, late settling individuals exhibited lower realized growth than individuals from the main cohort. This study confirms that growth conditions for 0-group plaice are not optimal and that a growth reduction occurs in summer, as suggested in previous studies.
Growth variability within individuals and among groups and locations and the phenomenon of summer growth reduction has been described for juvenile flatfish in a variety of European coastal areas whereby the underlying causes still remain elusive. Potential mechanisms were tested for juvenile plaice Pleuronectes platessa L. in the western Dutch Wadden Sea, by analysing published and unpublished information from long-term investigations (1986–present). Growth variability did occur and could be explained by differences induced by environmental variability (water temperature), and by non-genetic irreversible adaptation and sex. Dynamic Energy Budget analysis indicated that especially sexually-dimorphic growth in combination with variability in sex ratio could explain most of the variability in growth and the increase in the range of the size of individuals within the population over time. Summer growth reduction was not only observed among 0-group plaice in the intertidal, but also in the subtidal and tidal gullies as well as among I-and II-group plaice. Intraspecific competition for food was not detected but some support for interspecific competition with other predators was found. Also resource competition (due to crowding) with the other abundant epibenthic species (0-, I-and II-group flounder Platichthys flesus; the brown shrimp Crangon crangon; the shore crab Carcinus maenas; the goby species Pomatoschistus minutus and Pomatoschistus microps) could not explain the summer growth reduction. The observed growth reduction coincided with a decrease in stomach content, especially of regenerating body parts of benthic prey items. It is hypothesised that macrozoobenthos becomes less active after the spring phytoplankton bloom, reducing prey availability for juvenile plaice in summer, causing a reduction in food intake and hence in growth.
Since the mid-1980s major changes in spatiotemporal patterns of distribution of juvenile plaice have occurred in the Wadden Sea. Large juvenile (I- and II-group) plaice have almost completely disappeared from the intertidal flats in spring and summer and are no longer found in subtidal and tidal channels in autumn, consistent with an offshore movement reported in the nearshore coastal zone. In this paper we evaluate the present functional importance of the western Wadden Sea as a nursery for young-of-the-year (0-group) plaice and the changes over time compared to the mid-1980s period by analyzing abundance, growth and distribution patterns in various intertidal, subtidal and tidal channel areas. Spatiotemporal changes in habitat use were observed compared to 1986 based on a depth-stratified sampling repeated two decades later, in 2009. Present results showed that the utilization of the western Wadden Sea has greatly changed, with changing patterns of depth distribution of the 0-group. Settlement of 0-group plaice still occurred in the intertidal, however, shortly thereafter, they moved to deeper waters. Such shift in habitat use did not seem to have affected growth rates. Overall, it seems that the western Wadden Sea can still support young-of-the-year plaice population and in spite of changes in habitat use, the functional importance of the area for this group has not been affected.
Growth studies based on population-based growth estimates are limited by the fact that they do not take into account differences in age/size structure within the population. To overcome these problems, otolith microstructure analysis is often used to estimate individual growth. Here, we analyse growth of 0-group plaice in the western Wadden Sea in two years: a year preceded by a mild winter (1995) and a year preceded by a severe winter (1996). Growth was analysed by combining information on individual growth based on otolith analysis with predictions of maximum growth (= under optimal food conditions) based on a Dynamic Energy Budget model. Otolith analysis revealed that settlement occurred earlier in 1995 than in 1996. In both years, one main cohort was found, followed by a group of late settlers. No differences in mean length-at-age were found between these groups. DEB modelling suggested that growth was not maximal during the whole growing season: realized growth (the fraction of maximum growth realized by 0-group plaice) declined in the summer, although this decline was relatively small. In addition, late settling individuals exhibited lower realized growth than individuals from the main cohort. This study confirms that growth conditions for 0-group plaice are not optimal and that a growth reduction occurs in summer, as suggested in previous studies.
Since the mid-1980s major changes in spatiotemporal patterns of distribution of juvenile plaice have occurred in the Wadden Sea. Large juvenile (I- and II-group) plaice have almost completely disappeared from the intertidal flats in spring and summer and are no longer found in subtidal and tidal channels in autumn, consistent with an offshore movement reported in the nearshore coastal zone. In this paper we evaluate the present functional importance of the western Wadden Sea as a nursery for young-of-the-year (0-group) plaice and the changes over time compared to the mid-1980s period by analyzing abundance, growth and distribution patterns in various intertidal, subtidal and tidal channel areas. Spatiotemporal changes in habitat use were observed compared to 1986 based on a depth-stratified sampling repeated two decades later, in 2009. Present results showed that the utilization of the western Wadden Sea has greatly changed, with changing patterns of depth distribution of the 0-group. Settlement of 0-group plaice still occurred in the intertidal, however, shortly thereafter, they moved to deeper waters. Such shift in habitat use did not seem to have affected growth rates. Overall, it seems that the western Wadden Sea can still support young-of-the-year plaice population and in spite of changes in habitat use, the functional importance of the area for this group has not been affected.
The Dynamic Energy Budget theory unifies the commonalities between organisms, as prescribed by the implications of energetics, and links different levels of biological organisation (cells, organisms and populations). The theory presents simple mechanistic rules that describe the uptake and use of energy and nutrients and the consequences for physiological organisation throughout an organism's life cycle, including the energetics of ageing and contact with toxic compounds. This new edition includes a new chapter on evolutionary aspects, and discusses methods to quantify entropy for living individuals, isotope dynamics, a mechanism behind reserve dynamics, and toxicity of complex mixtures of compounds. An updated ageing module now also applies to demand systems, new methods for parameter estimation, adaptation of substrate uptake, the use of otiliths for reconstruction of food level trajectories, the differentiated growth of body parts (such as tumours and organs) linked to their function, and many more topics. © Cambridge University Press 1993, 2000 and
Growth variability within individuals and among groups and locations and the phenomenon of summer growth reduction has been described for juvenile flatfish in a variety of European coastal areas whereby the underlying causes still remain elusive. Potential mechanisms were tested for juvenile plaice Pleuronectes platessa L. in the western Dutch Wadden Sea, by analysing published and unpublished information from long-term investigations (1986-present). Growth variability did occur and could be explained by differences induced by environmental variability (water temperature), and by non-genetic irreversible adaptation and sex. Dynamic Energy Budget analysis indicated that especially sexually-dimorphic growth in combination with variability in sex ratio could explain most of the variability in growth and the increase in the range of the size of individuals within the population over time. Summer growth reduction was not only observed among 0-group plaice in the intertidal, but also in the subtidal and tidal gullies as well as among I- and II-group plaice. Intraspecific competition for food was not detected but some support for interspecific competition with other predators was found. Also resource competition (due to crowding) with the other abundant epibenthic species (0-, I- and II-group flounder Platichthys flesus; the brown shrimp Crangon crangon; the shore crab Carcinus maenas; the goby species Pomatoschistus minutus and Pomatoschistus microps) could not explain the summer growth reduction. The observed growth reduction coincided with a decrease in stomach content, especially of regenerating body parts of benthic prey items. It is hypothesised that macrozoobenthos becomes less active after the spring phytoplankton bloom, reducing prey availability for juvenile plaice in summer, causing a reduction in food intake and hence in growth.
Patterns and causes of spatial variation in RNA-predicted growth rates in mid-August were examined in young-of-the-year European plaice Pleuronectes platessa ('YOY plaice') at 22 beaches along a 300 km stretch of coastline in west Scotland in 3 consecutive years. According to restricted maximum likelihood models, growth rates varied among beaches 925 km scale), but these spatial patterns were not consistent across years. We found no evidence for spatial variation in growth at the scale of subregions 950 km) or regions 9100 km). Growth rate was positively correlated with total length, both within and among beaches and years. In general, YOY plaice in mid-August grew more slowly than estimated ad libitum laboratory rates. Average growth rates by beach and year were inversely related to intraspecific competitor densities, but not interspecific competitor densities (brown shrimp Crangon crangon) or 2 environmental productivity metrics (nearshore chlorophyll a concentration and lugworm Arenicola marina cast density). Physical beach characteristics also explained a significant source of spatial growth variation, with fish growing faster at beaches with larger tidal range and wave fetch. Therefore, the hypothesis of sub-maximum growth due to intraspecific competition (density-dependent growth) was supported, but additional, previously unexplored processes related to physical beach characteristics appear to have important influences on the spatial growth dynamics of YOY plaice.
The interannual variability in settlement and mortality of juvenile plaice Pleuronectes platessa L. was investigated between 1992 and 1998 on Port Erin Bay, west side of the Isle of Man, Irish Sea. The dampening influence of factors operating on the nursery grounds was especially obvious in 1992 and 1996. In these years extremely high numbers of individuals settled, yet the population sizes in July were similar to other years. Thus the nursery ground processes were likely to be density dependent. Shrimp and crab densities were low in Port Erin Bay and probably had little predatory impact on young plaice. Crustacean densities were not significantly related to winter temperatures. In the Irish Sea, year-class strength is determined during the nursery ground phase, in contrast to the North Sea where determination of year-class strength occurs prior to the nursery ground phase.
The population dynamics of three 0-group species, plaice (Pleuronectes platessa L.), flounder (Platichthys flesus L.) and sole (Solea solea L.) in the Dollard (Ems Estuary, Wadden Sea) were investigated in 1992. The instantaneous rate of decrease in catch density of plaice was 0.011·d−1, which corresponded with other calculated mortality rates of plaice in the western Wadden Sea. Catch densities of 0-group flounder decreased at a rate of 0.018·d−1. The rate of decrease in catch density of 0-group sole was estimated at 0.011·d−1, but was less accurate and probably reflected migration. The rate of increase in mean length of 0-group sole was in agreement with experimental growth studies under excess of food. The observed rate of increase in mean length of plaice and flounder appeared to decline from the beginning of June onwards in comparison with simulated growth in length. A number of factors that may be responsible for the observed differences are discussed.
Some measurements on the fishing efficiency of a 2 metre beam trawl used in plaice studies in the Wadden Sea are described.Experimental hauls were made in order to measure the reducing effect of specific characteristics of the fishing procedure on the catch. In a second approach catches by the standard sampling method were compared to those obtained by an enclosure net. Both methods led to about the same efficiency estimate for II-group plaice (approximately 20%), whereas, as a result of the comparative hauls, a negative correlation between fish length and efficiency was found.
The previously (1974-1982) observed inverse relationship between the February seawater temperature at the spawning grounds in the Southern Bight of the North Sea and subsequent year-class strength of 0-group plaice on the Balgzand tidal flats (western Dutch Wadden Sea) was revalidated for the period 1991-1998. However, seawater temperatures determined from NOAA satellite images showed that water temperatures during egg and larval drift in the open sea were more constant than the February temperatures at the spawning grounds suggested. In all years, and under all conditions, there was a persistent tongue of English Channel water entering through the Dover Strait into the Southern Bight of the North Sea, in which plaice eggs and larvae developed and were transported. The intensity of this plume varied between years, but even during the coldest period it penetrated as far as offshore Texel and the Marsdiep inlet The absence of interruptions in the pattern of larval immigrations at the nursery implies a link between variations in water temperatures and in the period of drift. Between and within years, the number of settling larvae at Balgzand was inversely related to the median temperature during drift. Overall, also year-class strength estimates were still inversely related to water temperature during drift. However, the relationship broke down for the very strong 1996 year-class, suggesting that other factors were acting too. Cold winters (reflected in low temperatures in the coastal zone are associated with persistent winds from the east which induce an atypical water circulation pattern that affects the strength and direction of the residual current. Easterly winds might also generate upwelling near the coast stimulating larval immigration.
Burial in sediment-dwelling clams is affected by morphological features, such as shell shape and size, but also by biotic and abiotic factors, such as predator presence, oxygen deficiency and sediment characteristics. In the Baltic Sea, oxygen deficiency is a severe problem not only in the deep basins, but also in the shallow coastal areas, due to eutrophication. In the species-poor Baltic Sea, the bivalve h Macoma balthica (L.) is a key species in both shallow and deep bottoms. This paper analyzes the impact of biotic and abiotic factors on the burrowing behaviour of M, balthica. Experiments were conducted to study the importance of sediment type, hypoxia, predator presence and algal mats on the burrowing behaviour (start of burial and burial velocity) of M. balthica. Results show that sediment type and the presence of the predatory isopod Saduria entomon did not affect the burrowing behaviour of M, balthica. In contrast, the burrowing behaviour was negatively influenced by hypoxia and drifting algae. Physical disturbance and oxygen decifiency are important forces that may displace M. balthica in the sediment and make it susceptible to predators at the sediment surface. Burial in the sediment is the only way in which infauna bivalves may escape predators, and this study shows that burrowing capability can be affected by poor environmental conditions.
A method for estimating the daily food consumption of 0-group plaice in the sea is described. Eight series of samples were
taken, four in the summer and four in the autumn. Each series was comprised of eight pairs of samples taken at 3-h intervals.
One of each pair was preserved immediately, the other was held in deck tanks until the next pair of samples was taken. The
difference between the mean stomach contents of the sample held in the deck tank and that of the sample caught three hours
later, and preserved at the time of capture, is a measure of the food consumed in the 3-h interval. The fish show a tendency
to commence feeding on the ebb tide after day break. They cease to feed shortly after sunset, except when there is a full
moon. Then they may continue to feed through the night. In July the fish consumed 8 to 11 % their body weight per day and
in September–October 1 to 3 %. This decrease in feeding level was not related to sea temperature.
Flatfishes (Pleuronectiformes), belonging to the families Bothidae, Pleuronectidae and Soleidae were studied. A division could be made into fish-feeders, Crustacea feeders and polychaeta-mollusca feeders. This division is based on experiments in which the behaviour of the fish was studied in relation to different sensory factors (olfaction and vision) and was confirmed by morphological study of the digestive tract and gill rakers. As a rule the Bothidae are fish feeders, the Pleuronectidae Crustacea feeders, and the Soleidae polychaeta-mollusca feeders. However exceptions occur, especially in the Pleuronectidae.
The possibility of prey limitations on the growth performance of age‐0 year northern rock sole Lepidopsetta polyxystra was evaluated at three sites along the north‐east coast of Kodiak Island, Alaska, U.S.A., by comparison of observed to potential growth rates. Growth potential was measured in the laboratory across the range of temperatures encountered by this species during the first summer of life. Growth potential (gL, mm day−1) increased with water temperature (T) between 2 and 13° C, according to: gL = 0·0151 + 0·3673·log10(T). There were significant differences in growth rate between the three field sites such that Holiday Beach fish were 7·1 mm longer than Shakmanof Beach fish by mid‐September, with Pillar Creek Cove fish of intermediate size. Temperature differences between sites accounted for less than half of this variation. The remainder may have been related to differences in prey availability among the sites in association with observed differences in sediment characteristics. In addition to the spatial variability, there was significant monthly variation in growth performance. Realized growth rates between July and August were in excess of 85% of potential. Between August and September, however, realized growth fell to 43–71% of potential indicating a decline in conditions for growth. The spatial variation in growth rates was not density‐dependent as the site with the highest fish densities (Holiday Beach) also supported the highest growth rates. The available data indicates that for this subtidal species, interannual variation in growth may be more important than site variation.
The populations of 0-group plaice and common dabs have been studied in a bay in Loch Ewe on the west of Scotland during the years 1964 to 1967. After metamorphosis the plaice settle on the bottom from April till June and the dabs only during June. Both species show a rapid mortality rate during the succeeding six months. The depth distribution of the plaice changed from month to month but in general the population inhabited water shallower than 4 m and the fish migrated into the littoral zone with the rising tide. The dabs almost always remained sublittoral with the maximum abundance at 4 m and with the population extending to a depth of 6 to 8 m.The food of the two species overlap to some extent but are quantitatively different. Immediately after settlement the plaice feed mainly on Tellina siphons and the dabs on polychaete tentacles, except in 1967 when the plaice also fed on the polychaete tentacles. After this initial feeding period both species feed on whole polychaetes, amphipods, and cumaceans.
1.1. The feeding stimulant requirement of the Dover sole was very specific, only betaine and dimethylthetin being effective.2.2. Substances tasting bitter to the human tongue acted as feeding deterrents for the sole.3.3. Of the other feeding deterrents identified, anthraquinone pigments, azo dyes and saponins were the most potent.4.4. The results are discussed in the context of both aquaculture and chemical ecology.
article i nfo Daily observations of the sea surface temperature in the Marsdiep tidal inlet, which connects the shallow Dutch western Wadden Sea with the deeper North Sea, already started in the summer of 1860, over 140 years ago. Since the year 2000 the sampling frequency has strongly increased because of the use of electronic sensors and data logging by computer. Analysis of these temperature data has revealed variations with time scales from tidal, daily, seasonal, inter-annual, to centennial. The tidal temperature variations are generated by advection of the seasonally varying temperature gradient between Wadden Sea and North Sea, while the daily variations are mainly caused by the daily variation of solar radiation. The seasonal variation in sea surface temperature only lags a few days behind the coastal surface air temperature, contrary to the sea surface temperature in the deeper nearby North Sea, which is delayed with about 1 month. The North Atlantic Oscillation index has been used as large-scale proxy for the atmospheric forcing of the Wadden Sea temperature. Only for the winter and spring a significant correlation is found between temperature and the winter index. However, this correlation is so strong that also the annual mean temperature is correlated significantly with the North Atlantic Oscillation. At longer time scales, from decadal to centennial, also large temperature variations are observed, of the order of 1.5 °C. However, these are not related to long-term changes of the North Atlantic oscillation. These long-term temperature changes involve a cooling of about 1.5 °C in the first 30 years of the record and a similar warming in the last 25 years. In between, these long- term changes were smaller and more irregular. Similar conclusions can also be applied to individual seasons as well as to the date of the onset of spring.
The function of coastal nursery areas is discussed in relation to the variability in North Sea plaice recuitment. 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.
A resumé is given of the sampling techniques applied in juvenile flatfish research in the countries around the North Sea in the course of time with the emphasis on small trawl nets. A preliminary comparison of the relative efficiencies of otter-trawl and beam-trawl under different environmental conditions is presented. Beam-trawls show a higher and less variable efficiency than otter-trawls. The fixed width of a beam-trawl may be an important factor. Based on these results, a standardization of juvenile flatfish sampling techniques is suggested, based on the 1-m and 2-m beam-trawl.
Density of 0-group plaice (Pleuronectes platessa L.) during peak abundance in spring (June) was recorded in 22 coastal nursery areas on the Swedish west coast. Relationships between plaice density and some local environmental factors as exposure, habitat structure, food availability and predation pressure in the nursery areas were studied, and the importance of these factors was evaluated. Exposure affects habitat structure (sediment and vegetation) which might influence settling behaviour and survival of juvenile plaice. Highest plaice densities were observed in semi-exposed bays. Shortly after settling of plaice larvae, predation by crustaceans will be a key factor controlling population density of juvenile plaice, as high predation densities went together with low plaice densities. Food availability in the nursery areas was not found to be related to plaice density.
In this review, the impact of juvenile growth on subsequent recruitment in flatfish is discussed. Recruitment is defined as the number of specimens of a specific year class that survives to attain sexual maturity and joins the reproductive population. Theoretically, variability in growth rate can have an impact on recruitment either by mean of size-selective mortality during juvenile life and/or by means of size-dependent onset of maturation. In flatfish up to about 10 cm, growth depends on size in such a way that variability in size within a population increases during the first year of life, and decreases again in the subsequent part of juvenile life. Temporal variability in size within local populations appears to be lower than spatial variability. Due to the prolonged spawning period, and hence period of settlement, variability in size of juvenile flatfish increases with decreasing latitude. As a consequence of these patterns, size-selective mortality appears to be mainly restricted to the 0-group and to gain importance with decreasing latitude. A literature search for field data yielded only a few references suggesting size-selective mortality. In none of the studies was any relationship with ultimate recruitment studied or even suggested. Size-dependent onset of maturation has been found in some flatfish species, with slow-growing individuals or cohorts showing delayed maturation. Size-dependent onset of maturation has a clear effect on the level of recruitment. However, in the species studied, the main traits in year-class strength still existed at the moment of recruitment to the reproducing stock. Size-dependent onset of maturation also appeared to affect the year-to-year variability in recruitment, but different effects were observed among species..It is argued that both size-selective mortality and size-dependent onset of maturation are more likely to dampen than to generate variability in recruitment. The study of the impact of juvenile growth on recruitment in flatfish is hampered by the absence of long-term data sets on recruitment. Especially comparable series of (sub)tropical species and of populations covering the total range of distribution of a species are lacking.
The settlement, growth and mortality of plaice (Pleuronectes platessa L) were examined on a small nursery ground (Port Erin Bay, Isle of Man) in the Irish Sea in 1993 and between 1996 and 2000. The timing, duration and strength of juvenile settlement varied between years and were positively correlated with the duration and quantity of egg production. Otolith increment counts were used to determine the age and metamorphosis and/or settlement dates of fish and to compare settlement patterns inferred from catch data with those inferred from age data. The catch data suggested two 'pulses' of settlement in Port Erin Bay whereas the otolith age data indicated three main settlement events. Over the years the first sub-cohort was generally the largest and overall this sub-cohort suffered the highest mortality. This first sub-cohort may have a "high risk" strategy and swamp potential predators early in the settlement period, with the result that the second sub-cohort generally has faster growth rate and lower mortality. A release of predatory pressure on the second sub-cohort could be a cause of large year classes in plaice populations. Growth rates were lowest for fish in the first sub-cohort, likely reflecting density-dependent effects and less optimal environmental conditions early in the year. The variations in instantaneous mortality rate between sub-cohorts, as well as inter-annually within sub-cohorts, illustrate the complex dynamics in the structure of these juvenile plaice populations on the nursery grounds. Global environment change effects are visible in the wider Irish Sea plaice population dynamics, with temperature dependent nursery ground processes as one of the contributing mechanisms. (C) 2011 Elsevier B.V. All rights reserved.
It is difficult to estimate natural mortality for many marine fish populations, especially during the transition period from larvae to juveniles, because the appropriate data are scarce. Plaice (Pleuronectes platessa) is an exception since it has been studied extensively. The study of mortality rates in juveniles is made easier because the nursery grounds are inshore and generally less than 5 m deep. This contribution considers the factors affecting mortality rates for eggs and larvae, and for settlement and nursery ground phases. There are problems associated with estimating mortality rates, especially for juveniles because immigration into nursery areas at the beginning of the season and emigration of larger individuals off nursery grounds in the latter part of the season confound losses due to mortality. The shifts in mortality schedules and the causes through the early life history are investigated in relation to concepts such as 'nursery ground carrying capacity' and 'self-thinning'. Predation in the pelagic phase is probably density independent and a source of inter-annual variability in survival of early life-history stages. Predation mortality during the nursery ground phase is most likely density dependent. However, there is a need for further in-depth study, especially during the period of settlement. (C) 2011 Elsevier B.V. All rights reserved.
Climate change is currently one of the main driving forces behind changes in species distributions, and understand-ing the mechanisms that underpin macroecological patterns is necessary for a more predictive science. Warming sea water temperatures are expected to drive changes in ectothermic marine species ranges due to their thermal tolerance levels. Here, we develop a mechanistic tool to predict size-and season-specific distributions based on the physiology of the species and the temperature and food conditions in the sea. The effects of climate conditions on physiological-based habitat utilization was then examined for different size-classes of two commercially important fish species in the North Sea, plaice, Pleuronectes platessa, and sole, Solea solea. The two species provide an attractive comparison as they differ in their physiology (e.g. preferred temperature range). Combining dynamic energy budget (DEB) models with the temperature and food conditions estimated by an ecosystem model (ERSEM), allowed spatial differences in potential growth (as a proxy for habitat quality) to be estimated for 2 years with contrasting temperature and food conditions. The resulting habitat quality maps were in broad agreement with observed ontogenetic and seasonal changes in distribution as well as with the recent changes in distribution which could be attributed to an increase in coastal temperatures. Our physiological-based model provides a powerful tool to explore the effect of climate change on the spatio-temporal fish dynamics, predict effects of local or broad-scale environmental changes and provide a physiological basis for observed changes in species distributions.
Errors in estimates of year class strength and mortality among 0-group plaice can result from migrations. In order to reliably determine the number present, tidal flats and channels in a large and clearly delimited Wadden Sea area (715 km2) were sampled simultaneously using different fishing methods (by-catch, beam trawls, pushnets, and fyke nets). The numbers of 0-group plaice calculated for each depth stratum reflected changes in the distribution patterns during the complete diurnal cycle and during the course of the season and proved to be most reliable for estimating the total number in the whole area. In contrast to this, the traditional way of calculating population size by multiplying the arithmetic mean of simultaneous catches resulted in very different numbers which would entail an overestimation of mortality.
The changes during the life cycle in the relative length of certain parts of the alimentary tract, buccal-pharyngeal cavity, oesophagus-stomach, intestine, rectum—are investigated in the five North Sea flatfish species, viz. turbot, brill, plaice, dab, sole. According to the average ratio between the length of the alimentary tract and the food preference three main groups are distinguished.
Growth variability and condition of juvenile soles Solea solea and Solea senegalensis, were assessed through RNA : DNA estimates and compared to absolute growth rates. Higher mean cohort RNA : DNA ratios were observed for cohort I at the beginning of estuarine occurrence for both species (4·42 and 4·87, for S. solea and S. senegalensis respectively). Despite different estuarine colonization habits, no significant differences were observed between RNA : DNA monthly variation for both sole species within the same year (P > 0·05 for 2003 and 2004). Juvenile S. senegalensis showed significant differences between RNA : DNA ratios obtained for the two nursery areas (P < 0·001). The decrease of seasonal growth rates with fish age was similar to seasonal variation of mean RNA : DNA values. Thus the RNA : DNA pattern of juvenile S. solea and S. senegalensis reflected growth and estuarine colonization patterns.