Juvenile Flatfish Research Articles

Assessment of resource selection models to predict occurrence of five juvenile flatfish species (Pleuronectidae) over the continental shelf in the western Gulf of Alaska

According to the nursery size hypothesis, flatfish recruitment is constrained by nursery area. Thus, if resource selection models can be shown to accurately predict the location and geographic extent of flatfish nursery areas, they will become important tools in the management and study of flatfish population dynamics. We demonstrate that some resource selection models derived previously to predict the presence and absence of juvenile flatfishes near shore were applicable to the broader continental shelf. For other age-species groups, derivation of new models for the continental shelf was necessary. Our study was conducted in the western Gulf of Alaska (GoA) during October 2011 on four groups of age-0 juvenile flatfishes: Pacific halibut (Hippoglossus stenolepis), arrowtooth flounder (Atheresthes stomias), northern rock sole (Lepidopsetta polyxystra), and flathead sole (Hippoglossoides elassodon); and three groups of age-1 juvenile flatfishes: northern rock sole, flathead sole, and yellowfin sole (Limanda aspera). Sampling occurred at 33 sites across the continental shelf. Fish were collected using a 3-m beam trawl, and a midwater trawl. Environmental data were collected on sediment composition and water temperature and depth. Many of the age-species groups co-occurred in the Shumagin and Barnabus sea valleys; however, age-0 arrowtooth flounder occurred at more locations than other juveniles, perhaps due to a relatively broad tolerance of environmental conditions and to the utilization of midwater habitat. Thus, the large nursery area of arrowtooth flounder may be one reason why they are currently the most abundant GoA flatfish. In fact, among all species, mean recruitment at age 3 increased with the percent occurrence of age-0 juveniles at the 33 sites, a proxy for relative nursery area, in accordance with the nursery size hypothesis, suggesting that mean recruitment among GoA flatfishes is structured by nursery size.

Possible causes for growth variability and summer growth reduction in juvenile plaice Pleuronectes platessa L. in the western Dutch Wadden Sea

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.

Shallow-water habitat use by Bering Sea flatfishes along the central Alaska Peninsula

Flatfishes support a number of important fisheries in Alaskan waters and represent major pathways of energy flow through the ecosystem. Despite their economic and ecological importance, little is known about the use of habitat by juvenile flatfishes in the eastern Bering Sea. This study describes the habitat characteristics of juvenile flatfishes in coastal waters along the Alaska Peninsula and within the Port Moller–Herendeen Bay system, the largest marine embayment in the southern Bering Sea. The two most abundant species, northern rock sole and yellowfin sole, differed slightly in habitat use with the latter occupying slightly muddier substrates. Both were more common along the open coastline than they were within the bay, whereas juvenile Alaska plaice were more abundant within the bay than along the coast and used shallow waters with muddy, high organic content sediments. Juvenile Pacific halibut showed the greatest shift in distribution between age classes: age-0 fish were found in deeper waters (~ 30m) along the coast, whereas older juveniles were found in the warmer, shallow waters within the bay, possibly due to increased thermal opportunities for growth in this temperature-sensitive species. Three other species, starry flounder, flathead sole, and arrowtooth flounder, were also present, but at much lower densities. In addition, the habitat use patterns of spring-spawning flatfishes (northern rock sole, Pacific halibut, and Alaska plaice) in this region appear to be strongly influenced by oceanographic processes that influence delivery of larvae to coastal habitats. Overall, use of the coastal embayment habitats appears to be less important to juvenile flatfishes in the Bering Sea than in the Gulf of Alaska.

Alterations in juvenile flatfish gill epithelia induced by sediment-bound toxicants: A comparative in situ and ex situ study

Juvenile Solea senegalensis were exposed in the laboratory (ex situ) and field (in situ) to different sediments of a moderately impacted estuary (the Sado, Portugal) for 28 days. A qualitative histopathological screening yielded scant lesions to gills, albeit alterations such as epithelial hyperplasia being evident and more frequent in fish exposed ex situ. Fully quantitative traits, namely chloride and goblet cell count and size revealed differences between the two bioassay approaches, with ex situ experiments likely enhancing bioavailability of toxicants. Chloride cells endured autolytic processes that could, at least in part, relate to contamination by mixed metals and polycyclic aromatic hydrocarbons (PAHs). Goblet cells did not reveal changes in the chemistry of mucous. Still, their number and size was reduced in fish exposed ex situ to the sediments most contaminated by PAHs, with evidence for adaptation. Also, copper histochemistry revealed the potential role of mucocytes in the regulation of metals.

Diet overlap between juvenile flatfish and the invasive round goby in the central Baltic Sea

The present study offers a comprehensive analysis of changes in the abundance and diet composition of juvenile flounder (Platichthys flesus) and turbot (Psetta maxima), along with other dominant coastal fish species, before and after the establishment of the alien round goby off an exposed stretch of coast in the eastern Baltic Sea.In the study area, the round goby (Neogobius melanostomus) was recorded for the first time in 2009. After a few years of low abundance, a sharp increase in the population occurred. After the round goby invasion, flatfish juveniles exhibited an increased diet overlap with other species and had a lower feeding success, reflecting an increase in resource competition. For juvenile turbot, the increase was mainly caused by the round goby, while for flounder it was due to both the round goby and the lesser sandeel (Ammodytes tobianus).Juvenile turbot, whose dominant food item before the round goby establishment had been mysids, shifted their diet towards Crangon crangon, reflecting a decrease in mysid abundance by three orders of magnitude and a concurrent doubling in C. crangon abundance in the habitat. At the same time a significant decrease in turbot recruitment was observed.Juvenile flounder had the widest food spectrum of the studied species. When the availability of the primary food item, Bathyporeia pilosa, decreased, flounder juveniles adapted by increasing the share of zooplankton in their diets. No changes in flounder feeding success and recruitment were observed. However, the recruitment estimates of flounder and turbot show an increasing co-variation after the round goby invasion, suggesting that recruitment of the species may currently be regulated by processes in the common nursery habitat.

Juvenile flatfish in the northern Baltic Sea — long-term decline and potential links to habitat characteristics

Flatfish in the northern Baltic Sea are facing multiple environmental pressures due to on-going large-scale ecosystem changes linked to eutrophication and climate change. Shallow juvenile habitats of flatfishes are expected to be especially susceptible to these environmental pressures. Using previously unpublished historical and present-state data on juvenile flatfish in nursery areas along the Finnish coast we demonstrate a drastic (up to 40×) decline in 1-Y-O flounder densities since the 1980s and a particularly low current occurrence of both flounders and turbots in several known juvenile habitats. As a consequence of ongoing coastal eutrophication vegetation coverage and filamentous algae have generally increased in shallow areas. We examined the predicted negative effect of vegetation/algae by exploring quantitative relationships between juvenile flatfish (flounder and turbot) occurrence and vegetation/algae among other environmental factors in shallow juvenile habitats. Despite sparse occurrence of juveniles we found a significant negative relationship between flatfish abundance and vegetation cover, implicating eutrophication as a potential major driver affecting the value of juvenile habitat. Shallow littoral habitats play a particularly central role for flatfish due to the spatial concentration of fish in these areas during the critical juvenile stage. Despite their importance, these areas have been relatively poorly studied in the northern Baltic Sea, which makes it difficult to quantify overall changes in environmental conditions and to relate these changes to flatfish recruitment. The low present-state flatfish densities recorded preclude strong inferences of the role of habitat quality to be drawn. Our study does, however, provide a baseline for future assessment. Based on existing evidence, we cannot thus establish any bottlenecks but hypothesize that the current low occurrence of juvenile flatfish, and the population decline of flounder on the Finnish coast, might have resulted from a combination of altered larval supply and reduced nursery value.

Role of predation on sandy beaches: Predation pressure and prey selectivity estimated by laboratory experiments

Predation is known to play an important role in structuring communities. In rocky intertidal communities, both environmental variables and the structuring role of predation determine species zonation and distribution patterns. However, on intertidal sandy beaches, little is known on the presence and the role of predation. In this study, laboratory experiments were used to examine prey consumption, prey selectivity and predation pressure of the two main epibenthic predators, being shrimp and juvenile flatfish, present on the intertidal beach at high tide. Results show that macrobenthos is important in the diet of these epibenthic predators and that prey selectivity is present. As predation pressure on the intertidal beach is high, predation is probably an important structuring factor for the sandy beach macrobenthos community. Hence, the macrobenthos zonation pattern is likely to be steered by the combination of abiotic and biotic factors: while the upper limit of a species zone is defined by the species physiological response to abiotic environmental variables, the lower limit is defined by biotic factors such as predation pressure. Furthermore, the intertidal zone functions as an important nursery area for commercially important species like shrimp and flatfish.

A quantitative estimate of the function of soft-bottom sheltered coastal areas as essential flatfish nursery habitat

Essential fish habitat suitability (EFHS) models and geographic information system (GIS) were combined to describe nursery habitats for three flatfish species (Solea solea, Pleuronectes platessa, Dicologlossa cuneata) in the Bay of Biscay (Western Europe), using physical parameters known or suspected to influence juvenile flatfish spatial distribution and density (i.e. bathymetry, sediment, estuarine influence and wave exposure). The effects of habitat features on juvenile distribution were first calculated from EFHS models, used to identify the habitats in which juvenile are concentrated. The EFHS model for S. solea confirmed previous findings regarding its preference for shallow soft bottom areas and provided new insights relating to the significant effect of wave exposure on nursery habitat suitability. The two other models extended these conclusions with some discrepancies among species related to their respective niches. Using a GIS, quantitative density maps were produced from EFHS models predictions. The respective areas of the different habitats were determined and their relative contributions (density × area) to the total amount of juveniles were calculated at the scale of stock management, in the Bay of Biscay. Shallow and muddy areas contributed to 70% of total juvenile relative abundance whereas only representing 16% of the coastal area, suggesting that they should be considered as essential habitats for these three flatfish species. For S. solea and P. platessa, wave exposure explained the propensity for sheltered areas, where concentration of juveniles was higher. Distribution maps of P. platessa and D. cuneata juveniles also revealed opposite spatial and temporal trends which were explained by the respective biogeographical distributions of these two species, close to their southern and northern limit respectively, and by their responses to hydroclimatic trends.

Polychaete worm tubes modify juvenile northern rock sole Lepidopsetta polyxystra depth distribution in Kodiak nurseries

We have observed inter-annual variability in the depth distribution of juvenile northern rock sole Lepidopsetta polyxystra on their nursery grounds around Kodiak Island, Alaska. This study evaluates whether this variability is a response to inter-annual changes in the availability of habitat created by polychaete tubes; principally Sabellides sibirica. We suspect that worm tubes constitute an alternative refuge and/or feeding habitat for juvenile flatfish. Accordingly, we hypothesized that during years of low worm abundance, fish would concentrate in the shallows (<10m depth) where they find refuge from predation, but would move to greater depths (>15m, where the worms occur) during years when the worms were abundant. Using data on worm abundance and fish density over 5yr, we tested this hypothesis at 2 Kodiak nursery embayments. Whether worms were abundant in a given year or embayment had no influence on overall fish abundance, however, worm abundance did influence juvenile flatfish depth distributions. At Holiday Beach, where worms tended to be scarce, fish were typically concentrated in shallow water. However, during the 1year when worms were abundant, fish were concentrated in deeper water. At Pillar Creek Cove, where worms are more regularly found, fish tended to concentrate in deeper water, the exception being the one year when worms were nearly absent. Regression analysis for both sites and all years indicated that the percent of fish occupying shallow water (<10m) decreased with increasing worm abundance. When worms were prevalent, fish were most commonly found on bottom with sparse to moderate worm cover, but avoided bottom where the worms were so dense as to form a ‘turf’. These results demonstrate that the geographic and inter-annual variation in worm tube abundance has significant influence over the distribution of juvenile northern rock sole.

Influence of Hydrocarbons Exposure on Survival, Growth and Condition of Juvenile Flatfish: A Mesocosm Experiment

Juveniles of numerous commercial marine flatfish species use coastal and estuarine habitats as nurseries. Hence, they are likely to be exposed to a number of anthropogenic stressors such as accidental and chronic exposure to chemical contaminants. Little is known about their response to such pollutants at the individual level and about the consequences on their population dynamics. Mesocosm experiments were conducted to determine whether short (24 h) but high exposure to petroleum hydrocarbons (1/1000 v: v water: fuel), similar to what happened after an oil spill on coastal areas, affects survival and biological (growth, body condition and lipid reserve) performances of juvenile common sole, which live on near shore and estuarine nursery grounds. Results demonstrated that this type of exposure significantly reduce survival, growth (size, recent otolith increment and body condition), and especially energy storage (triacylglycerol to free sterol ratio) of the juvenile fish on the medium-term (three months after the exposure). These medium-term consequences affect future recruitment of this long- lived species.

Effects of natural and anthropogenic disturbance on polychaete worm tubes and age-0 flatfish distribution

ABSTRACT: Tubes of the ampharetid polychaete Sabellides sibirica are a prominent yet spatiallyvariable habitat feature in shallow-water flatfish nurseries around Kodiak, Alaska, USA. Juvenileflatfish associate with the edges of worm tube regions but seldom use the dense ‘turf-like’ wormbeds that sometimes form on the bottom in the late summer. The present study used a fine-scaleanalysis (2 to 3 m) to examine how juvenile flatfish distribution changed with worm tube hetero-geneity, i.e. density and patchiness. Using a video sled, 8 transect lines (~250 m each) were repeat-edly surveyed from late summer to mid-winter in a worm tube region of Pillar Creek Cove, half ofwhich were experimentally disturbed using simulated trawl gear. Results indicated that juvenileflatfish (mainly northern rock sole Lepidopsetta polyxystra) increasingly use patches of bare sub-strate as worm tube densities increase. However, the simulated trawl disturbance resulted in aunique kind of patchiness, typified by long, thin exposed regions of bare substrate referred to as‘combing’. Unlike natural patches, evidence of combing disappeared 2.5 mo after the initial distur-bance, whereas natural patches persisted throughout the entire study. Flatfish abundance in-creased in trawl-disturbed worm tube beds for only a short period (2 d), possibly due to episodicforaging opportunities rather than physical changes in habitat. These results indicate that wormtube habitat is provisionally resilient to disturbance, and its heterogeneity (density, patchiness, andpatch type) is an important component of habitat quality for juvenile flatfish in Alaska.KEY WORDS: Coastal habitats · Benthic recovery · Patchiness · Habitat heterogeneity · Northernrock sole

Colonization and nursery habitat use patterns of larval and juvenile flatfish species in a small temperate estuary

Migrations between coastal and estuarine nursery areas are essential for successful completion of the life cycle of several marine fish. The present study evaluates the use of a small temperate estuary, the Mondego, Portugal, as a nursery habitat for several flatfishes during their early life stages. Data from seasonal and diel larval sampling at the mouth of the estuary and both larvae and juvenile monthly spatial distribution in the estuary (2005–2009) were gathered in order to investigate the life cycle of Platichthys flesus, Solea solea and Solea senegalensis. Larvae entrance in the estuary occurred mainly during summer and autumn with no evidence for diel or tidal vertical stratification. S. senegalensis larvae were present in all seasons at downstream areas presenting low successful settlement and juveniles' densities inside the estuary. Conversely, P. flesus and S. solea were mainly present as juveniles with upstream areas being preferred by flounder. Both species larvae seemed to settle in nearby coastal areas. The importance of the Mondego estuary for flatfishes differed according to the species, playing an important role mainly during the first year for all species. The present study highlights the importance of integrating larval and juvenile stages of fish to assess the very important role of estuaries as nursery areas.

The role of structuring benthos for juvenile flatfish

Within coastal nurseries, the distribution of juvenile flatfish may depend on small-scale habitat variability. The presence of ecosystem engineers is known to have important impacts in coastal sediments. Lanice conchilega is a well-known marine ecosystem engineer of shallow soft bottom ecosystems, shaping the macrobenthic community and attracting flatfish. The present study examines the relation between juvenile flatfish and L. conchilega reefs through two experiments. In a field experiment in the Dutch part of the North Sea, the benthic habitat is evaluated by comparing relative differences in numbers of juvenile flatfish between ecosystem engineered habitats and adjacent bare sand (i.e. non-ecosystem engineered) habitats. The hypothetical shelter seeking behaviour was further examined using stomach content analyses. Results show that juvenile plaice Pleuronectes platessa was the dominant species within the tube worm habitat and the species selects specifically for this biogenic habitat. This selection was explained as feeding behaviour. In a complementary laboratory study, food was excluded and the shelter function of the ecosystem engineered habitat was investigated. This experiment quantifies the selection for this habitat by juveniles of the common sole Solea solea. Results from the flume experiment, manipulating the number of tube worms, show that distribution of sole was not random when current velocities are high. The selected habitat is the one with low density tube worm aggregations. Overall, we conclude that structuring benthos plays an important role for juvenile flatfish, both as refuge and as feeding ground.

Growth and distributional correlates of behavior in three co-occurring juvenile flatfishes

We explored whether anti-predator behavior and intrinsic growth are co-evolved traits in 3 co-occurring juvenile flatfish species: English sole Pleuronectes vetulus, Pacific halibut Hippoglossus stenolepis and northern rock sole Lepidopsetta polyxystra. English sole are risk prone, adopting behavior that renders them more vulnerable to predation, while northern rock sole are more risk averse. Pacific halibut are risk sensitive and modulate behavior to match per- ceived threats. We hypothesized that risk-taking behavior and intrinsic growth rate would be pos- itively correlated in these species. We examined the willingness of each species to feed, both in the presence and absence of risk, and in separate experiments we measured their intrinsic growth rates under a range of temperatures. As predicted, risky behavior in English sole was accompa- nied by high intrinsic growth, while risk aversion in rock sole was accompanied by low growth. Pacific halibut confounded predictions, combining moderate risk-taking behavior with high growth. Lastly, we examined the depth distribution of each species. We expected English sole would be restricted to the shallows (<5 m), where predators are less common, while rock sole would have a deeper distribution, being better able to co-exist with predators. Halibut were expected to present an intermediate depth distribution. Again, English sole and rock sole con- formed to predictions, while Pacific halibut confounded them by having a deeper depth distribu- tion, comparable to that of rock sole. We suspect that the behavioral plasticity of Pacific halibut, combined with refuging behavior and well-developed escape capabilities, may allow them to have both high intrinsic growth as well as the ability to coexist with the predators. Thus, our results provide only partial support for our hypothesis that behavior and growth are co-evolved traits that together control the distribution of juvenile flatfishes across predation gradients.

Habitat characteristics promoting high density and condition of juvenile flatfish at nursery grounds on the west coast of Ireland

Coastal zones are essential nursery habitats for most juvenile flatfish species. Understanding the habitat requirements promoting high abundance and growth of juvenile flatfish is important for evaluating nursery habitat quality. The present study aimed to assess nursery ground quality for the most common flatfish species: dab (Limanda limanda) and plaice (Pleuronectes platessa), in Galway Bay, on the west coast of Ireland. Monthly surveys were carried out in the period after peak settlement over two years. Variability in flatfish density and Fulton's K condition was explained in relation to biotic and abiotic habitat characteristics, differing within and between distinct nursery grounds. Dab were concentrated in deeper waters, were negatively associated with shrimp densities and salinity and their abundance showed a decrease from June to September combined with interannual variation. Plaice densities were highly associated with shallower depths and high polychaete and shrimp densities and were negatively related with increased macroalgal cover and organic content. Most of the variability in Fulton's condition was explained by the same set of variables for both species; year and densities of crab and malacostracans. This analysis revealed important ecological mechanisms allowing the co-existence of two flatfish species in nursery grounds. However, high quality nursery grounds for dab and plaice differed and anthropogenic and climatic impacts on flatfish nurseries are likely to have a different impact on plaice and dab populations. Knowledge gained about the quality of nursery habitat for commercially important fish species provides a basis for mapping essential flatfish habitats to inform management plans for coastal areas.

Trophic ecology of juvenile flatfish in a coastal nursery ground: contributions of intertidal primary production and freshwater particulate organic matter

Coastal and estuarine nurseries are essential habitats for juvenile flatfish. These small but productive areas provide food supply and sustain adult fish populations. The Mont-Saint-Michel Bay (MSMB) supports an important flatfish nursery ground but differs from many other nursery habitats due to limited freshwater inputs. The objectives of the present study in the MSMB were to (1) use gut content analysis to identify prey of the 2 most abundant flatfish species (common sole Solea solea and plaice Pleuronectes platessa) for different juvenile age-classes (0-group and 1-group for sole, 0-group for plaice), (2) use C and N stable isotope analysis to model the production sources sustaining juvenile flatfish production, and (3) compare these results with previous knowledge of estuarine nursery grounds located in western Europe. Items found in gut contents differed between species and size classes, with juvenile plaice having a larger prey spectrum. Despite accounting for sensitivity to large uncertainties in source signatures and trophic enrichment factors, stable isotope mixing models led to robust outputs. In contrast to previous studies in estuarine nurseries, we found that microphytobenthos was the major carbon source contribution to juvenile flatfish. The contribution of freshwater particulate organic matter was nonetheless significant, in spite of the very limited river inputs to MSMB.

Move Over! Studying Flatfish Travel Patterns to Profile Fish Behavior

The way an animal moves from place to place can inform us about its life and environment. In this lesson, students examine the travel patterns of juvenile flatfishes in an estuary. The process of sampling bottom-dwelling fishes is explained, and data from a university-based marine science laboratory are evaluated. Students compare the distance traveled by juvenile fish to human movement by determining their own average step length. Comparing step length to the distance-to-body-length traveled by flatfish enables students to put in perspective the journey taken by the fish.

Dependence of RNA:DNA ratios and Fulton’s K condition indices on environmental characteristics of plaice and dab nursery grounds

Abstract This field study showed a lack of a correlation between a morphometric (Fulton’s K ) and biochemical (RNA:DNA ratio) condition index in juvenile plaice ( Pleuronectes platessa ) and dab ( Limanda limanda ) studied to assess habitat quality in four sandy beach nursery grounds in Galway Bay, Ireland. Based on monthly surveys from June to September in 2008 and 2009, fish growth, indicated by RNA:DNA ratios and Fulton’s K , displayed considerable spatio-temporal variability. Site-related patterns in Fulton’s K for plaice and dab were consistent between years whereas RNA:DNA ratios displayed annual and interspecific variability among nursery habitats. This indicates a higher sensitivity of RNA:DNA ratios to short-term environmental fluctuations which is not apparent in Fulton’s K measurements of juvenile flatfish. Generalized Additive Modelling (GAM) revealed non-linear relationships between the condition indices and (biotic and abiotic) habitat characteristics as well as diet features, derived from gut content analyses. Density of predators, sediment grain size and salinity were the most important predictors of both condition indices. Temperature also affected condition indices in dab whereas plaice condition indices varied with depth. Diet features did not contribute to the explained variability in the models predicting RNA:DNA ratios whereas certain prey groups significantly improved the explained variability in the models predicting Fulton’s K of plaice and dab. The value of both indices for assessing fish condition and habitat quality in field studies is discussed. These findings aid understanding of the biological and physical mechanisms promoting fast growth and high survival which will help to identify high quality nursery areas for juvenile plaice and dab.

Variability in the early life stages of juvenile plaice (Pleuronectes platessa) on west of Ireland nursery grounds: 2000–2007

Plaice (Pleuronectes platessa) are the most abundant juvenile flatfish present on west of Ireland nursery grounds. 0-group plaice were collected from eleven sandy beach nursery areas along the west coast of Ireland by beach seining (2000–2007). In the majority of years examined, certain beaches were found to support a higher abundance of juvenile plaice, or individuals that were larger in size and exhibited good condition, suggesting that differences in habitat quality exist between beaches. Monthly mean densities of plaice ranged from 2.4 (± 2.4) to 100 (± 1.3) individuals 1000 m−2. Mean lengths of plaice ranged between 6.0 (± 1.0) and 10.1 (± 0.5) cm. No evidence of density dependent growth was detected over the eight years of the study. The length and condition of juvenile plaice also differed between hauls, indicating the existence of microhabitat effects and emphasizing the importance of selecting an appropriate spatial resolution to adequately assess an area. Similar to other locations in Europe, juvenile plaice abundances on nursery grounds were inversely related to mean spring seawater temperatures during the pelagic stage. Abundances of 0-group plaice were not related to the number of either 1- or 2-group individuals collected during bottom-trawl surveys off the Irish west coast, demonstrating the difficulties associated with developing a recruitment index for plaice.

Long-term trends in juvenile flatfish indicate a dramatic reduction in nursery function of the Balgzand intertidal, Dutch Wadden Sea

Since 1975, juvenile flatfish (plaice and flounder) populations have been monitored at the Balgzand intertidal and, based on this data series (1975-2007), the interannual patterns in preda- tion pressure were quantified. Temporal patterns in abundance have changed greatly, especially for plaice. Up to the early 1980s, 3 year classes (0-, I- and II-group) were present and growing up in the area, but from the late 1980s onwards, densities of the I- and II-group plaice dropped from a few hun- dred individuals per 1000 m 2 to levels close to zero. It appears that the Balgzand intertidal has lost its nursery function for I- and II-group plaice, although feeding conditions have remained the same or even slightly increased since the late 1970s. The absence of I- and II-group flatfish in the intertidal cannot be explained at present; however, processes operating offshore are most likely involved. As a consequence, the annual predation pressure by juvenile flatfish upon the intertidal macrozoobenthos dropped by 94%, declining from an average of approximately 5 to 0.25 g ash free dry mass m -2 . Such a decline in top-down predation may not only have directly influenced the macrozoobenthic commu- nity, but may also have indirectly affected food availability and accessibility for other top predators (e.g. shorebirds).