Predation by juvenile Platichthys flesus (L.) on shelled prey species in a bare sand and a drift algae habitat

Due to increasing eutrophication of the coastal Baltic waters, drifting algae are a common phenomenon. Drifting algal mats accumulate on shallow sandy bottoms in late summer and autumn, and affect the ambient fauna. Juvenile flounder, Platichthys flesus, utilize these habitats during their first few years. They feed on benthic meio- and macrofauna; part of their diet consists of shelled species, such as Ostracods, and juvenile Hydrobia spp. and Macoma balthica. Earlier studies have shown that up to 75% of ostracods and 92% of hydrobiids survive the gut passage of juvenile flounder, while all M. balthica are digested by the fish. We conducted laboratory experiments to study how the shelled prey responded to a drift algal mat, and the predation efficiency of juvenile P. flesus on these prey species on bare sand and with drifting algae (50% coverage). Hydrobia spp. utilized the drift algae as a habitat and, after 1 h, 50% had moved into the algae; ostracods and M. balthica were more stationary and, after 96 h, only 23 and 12%, respectively, were found in the algae. For the predation efficiency of P. flesus, a two-way ANOVA with habitat (algae, bare sand) and predation (fish, no fish) as factors revealed that both algae and predation affected negatively the survival of all three prey species. The algae, thus, affected the predation efficiency of juvenile P. flesus and the consumption of prey was much reduced in the algal treatments compared to the bare sand. This was due probably to increased habitat complexity and the ability of prey, especially hydrobiids, to use the algal mat as a refuge. Altered habitat structure due to drift algae, together with the resultant changes in habitat (refuge) value for different prey species, may profoundly change the structure of benthic communities.     

Feeding habits of young predatory fishes in marsh creeks situated along the salinity gradient of the Schelde estuary, Belgium and The Netherlands

Fish and macrobenthos were sampled in four different marshes along the salinity gradient of the Schelde estuary, Belgium/Netherlands, to investigate the importance of marsh creeks as foraging grounds for the dominant, larger fish species. The total density and biomass of all the main macrobenthic taxa (Corophium volutator, Nereis diversicolor, Oligochaeta, Macoma baltica and Heteromastus fliliformis) were measured. The feeding habits of the larger predatory fishes (Platichthys flesus, Dicentrarchus labrax) were investigated. Qualitative and quantitative stomach analyses included the calculation of different indices, showing the niche breadth (as diet diversity) and the niche overlap (as similarity between the predators diet) for this habitat. These analyses showed that the two most important benthic prey species for P. flesus were C. volutator and N. diversicolor. D. labrax preyed upon a wider range of species, including C. volutator, N. diversicolor, Crangon crangon, Carcinus maenas and Orchestia spp. The stomach diversity of D. labrax and P. flesus showed differences between the marshes although there was no consistent pattern in diet composition, reflecting the opportunistic nature of feeding by these large predators. The fullness indices of both flounder and sea bass did not differ significantly along the salinity gradient and the estimated minimum consumption by these predators did not indicate a top-down control of the macrobenthic community. The salt marsh creeks seem to provide excess food for the visiting fish species. The benthic prey was present in very high abundances, which may suggest that the typical nursery species such as C. crangon and C. maenas, and early juveniles of P. flesus, D. labrax and Pomatoschistus microps were not preyed upon significantly. This supports the hypothesis that salt marsh creeks provide good refuge areas for nursery species against predation by larger fish.     

Estuarine colonization, population structure and nursery functioning for 0-group sea bass (Dicentrarchus labrax), flounder (Platichthys flesus) and sole (Solea solea) in a mesotidal temperate estuary

The function of the Mondego estuary as a fish nursery habitat was investigated from June 2003 to June 2004 by comparing the timing of estuarine colonization with juveniles of sea bass Dicentrarchus labrax, flounder Platichthys flesus, and sole Solea solea, while also analysing their population structure, growth and diet composition. Differences in the onset of estuarine colonization were observed, since sole juveniles were the first to enter the estuary (in January), followed by flounder in April and sea bass in June. The estuarine population of these species consisted of several age‐groups, although the majority of individuals belonged to age‐groups 0 and 1. The growth rates determined for 0‐group fish were within the range of those reported for other European estuarine systems. Some differences were also recognized regarding the timing of estuarine colonization and the length of the growing season. Diet of 0‐group sea bass consisted mainly of Crustacea, Polychaeta and Mollusca. Flounder juveniles fed chiefly on Amphipoda (especially Corophium spp.), with Polychaeta, Isopoda and Decapoda also being common prey. The diet of 0‐group sole was dominated by Polychaeta, with Amphipoda, Mollusca and Decapoda ranking highest, with other important benthic organisms also being present. Dietary overlap among these species was relatively low.     

Relationship between body shape and trophic niche segregation in two closely related sympatric fishes

Diet, shape and their covariation were analysed in juvenile flounder Platichthys flesus and plaice Pleuronectes platessa specimens from a tidal inshore habitat (Dublin Bay) in the Irish Sea. Stomach content analysis revealed differences in the diet, suggesting a substantial degree of resource partitioning between the species. Geometric morphometrics detected significant and discriminating differences in body shape, particularly in the head region. Most interestingly, a high degree of correlation was detected between diet and shape. The ontogenetic and ecological implications of the diet and shape covariation pattern were discussed, suggesting that morphological differences, in particular in the head region, may represent the functional platform which allows for the existence of ecologically significant differences in prey search and capture between P. platessa and P. flesus.     

The forgotten feeding ground: patterns in seasonal and depth-specific food intake of adult cod Gadus morhua in the western Baltic Sea

This study presents the diet composition of western Baltic cod Gadus morhua based on 3150 stomachs sampled year-round between 2016 and 2017 using angling, gillnetting and bottom trawling, which enhanced the spatio-temporal coverage of cod habitats. Cod diet composition in shallow areas (<20 m depth) was dominated by benthic invertebrate species, mainly the common shore crab Carcinus maneas. Compared to historic diet data from the 1960s and 1980s (limited to depth >20 m), the contribution of herring Clupea harengus decreased and round goby Neogobius melanostomus occurred as a new prey species. Statistical modelling revealed significant relationships between diet composition, catch depth, fish length and season. Generalized additive modelling identified a negative relationship between catch depth and stomach content weight, suggesting reduced food intake in winter when cod use deeper areas for spawning and during peak summer when cod tend to avoid high water temperatures. The results of this study highlight the importance of shallow coastal areas as major feeding habitats of adult cod in the western Baltic Sea, which were previously unknown because samples were restricted to deeper trawlable areas. The results strongly suggest that historic stomach analyses overestimated the role of forage fish and underestimated the role of invertebrate prey. Eventually, this study shows the importance of a comprehensive habitat coverage for unbiased stomach sampling programmes to provide a more reliable estimation of top predator diet, a key information for food web analyses and multispecies models.     

The use of trophic resources by Argyrosomus regius (Asso, 1801) escaped from Mediterranean offshore fish farms

The population of meagre (Argyrosomus regius) has increased dramatically around the Western Mediterranean, accompanied by a higher seasonal incidence of the species in fishery landings. Presumably because of escapes, this new predator is gaining ground in marine habitats where it was normally not found. Given the lack of knowledge about the potential impact of this locally absent species on ecosystems and fishery resources, the objective of this study was to analyse the diet and predation behaviour of A. regius in order to evaluate its capability to adapt to a new environment as well as to identify its main prey. Stomach contents of 159 individuals were analysed and the identifiable prey items classified into major groups. Results showed that the diet of meagre was remarkably size‐dependent. The commercially important decapod Melicertus kerathurus (Index of Relative Importance IRI: 798.53), and to a lesser extent other crustaceans, were preferred to smaller sizes, while larger individuals gradually shifted to a piscivorous diet, particularly to Platichthys flesus (77.99 IRI). The potential ecological effects on the prey species are discussed, and management measures preventing the establishment of new local populations are suggested in order to harmonize coastal zone management policies with a sustainable production of meagre.     

The diet of whiting Merlangius merlangus in the western Baltic Sea

The diet of whiting Merlangius merlangus in the western Baltic Sea was investigated and compared to the diet in the southern North Sea. Clupeids were important prey in both areas, but especially in the western Baltic Sea where they constituted up to 90% of the diet of larger individuals. Gobies, brown shrimps and polychaetes were the main prey of juveniles in the western Baltic Sea, while a wider range of species were consumed in the North Sea. The shift to piscivory occurred at smaller sizes in the western Baltic Sea and the fish prey consumed was proportionately larger than in the southern North Sea. Estimates of prey abundance and food intake of M. merlangus are required to evaluate its predatory significance in the western Baltic Sea, but its diet suggests that it could be just as significant a fish predator here as in the southern North Sea.     

Food partitioning and spatial subsidy in shelter‐limited fishes inhabiting patchy reefs of Patagonia

The diets of the most conspicuous reef‐fish species from northern Patagonia, the carnivorous species Pseudopercis semifasciata, Acanthistius patachonicus, Pinguipes brasilianus and Sebastes oculatus were studied. Pinguipes brasilianus had the narrowest diet and most specialized feeding strategy, preying mostly on reef‐dwelling organisms such as sea urchins, limpets, bivalves, crabs and polychaetes. The diet of A. patachonicus was characterized by the presence of reef and soft‐bottom benthic organisms, mainly polychaetes, crabs and fishes. Pseudopercis semifasciata showed the broadest spectrum of prey items, preying upon reef, soft‐bottom and transient organism (mainly fishes, cephalopods and crabs). All S. oculatus guts were empty, but stable‐isotope analyses suggested that this species consumed small fishes and crabs. In general, P. brasilianus depended on local prey populations and ate different reef‐dwelling prey than the other species. Pseudopercis semifasciata, A. patachonicus and probably S. oculatus, however, had overlapping trophic niches and consumed resources from adjacent environments. The latter probably reduces the importance of food as a limiting resource for these reef‐fish populations, facilitating their coexistence in spite of their high trophic overlap.     

Niche overlap between juvenile flatfishes, Platichthys flesus and Solea solea, in a southern European estuary and adjacent coastal waters

The distribution and abundance patterns as well as the feeding ecology of two flatfishes, Platichthys flesus (Linnaeus, 1758) and Solea solea (Linnaeus, 1758), were studied in the Douro estuary and adjacent coastal waters (Portugal) in order to evaluate the niche overlap between these species. Fish were collected during sampling surveys conducted every 2 months between November 2000 and March 2002 using a 12‐m otter‐trawl with 18 mm stretched mesh size, at 12 sites within the estuary and six sites in the adjacent coastal area. The highest densities of P. flesus were registered in the middle estuary; a peak density of 9.12 ind 1000 m^?2 was obtained in October 2001. Solea solea densities were low compared with P. flesus; the highest values were also recorded in the middle estuary in October 2001. For both species, the proportion of juveniles was above 90% within the estuary. Highest densities of P. flesus and S. solea were recorded at sites with low water depth and high densities of crustaceans and polychaetes, the main food items in the diet of these species. High values of spatial and diet niche overlap were found between these two flatfishes (0.97 and 0.92, respectively).     

Fish feeding guilds in shallow rocky and soft bottom areas on the Swedish west coast

Multivariate analyses of the diet relationships within fish assemblages resulted in the recognition of five major feeding guilds in shallow (0-9 m) rocky- and soft-bottom habitats on the Swedish west coast. The two habitats had similar guilds that could be broadly described as piscivores, decapod feeders, copepod feeders and amphipod and mollusc feeders. The last guild on the rocky bottom was more strictly one of amphipod feeders, whereas the last guild on soft bottoms consisted of infauna feeders (mainly polychaetes and bivalves). A comparison of the diet of the entire fish assemblages showed that amphipods and gastropods were more prevalent as prey taxa on rocky bottoms, whereas bivalves and teleosts were more common on soft bottoms. A large proportion (12 out of 27 species) of the fish species investigated was found in both habitats. These fish species generally seemed to rely on the same major prey groups in the two habitats, but also included prey taxa characteristic for the habitat in which they resided. Sea trout Salmo trutta , mackerel Scombrus scombrus , saithe Pollachius virens , whiting Merlangius merlangus and cod Gadus morhua were found to be the major piscivores in both the investigated habitats. Herring Clupea harengus and members of the family gobiidae were the most prevalent prey species for piscivores within the rocky- as well as the soft-bottom fish assemblage. A comparison of feeding modes suggested that the fish assemblage on rocky bottoms predominantly (60% of the food items) relied on food found in vegetation. On soft bottoms, the average distribution of food categories among fish species was 44% benthic, 35% pelagic and 21% vegetation-associated food items.     

Warming alters the body shape of European perch Perca fluviatilis

The consequences of elevated temperature on body shape were investigated by comparing European perch Perca fluviatilis from the Forsmark area of the Baltic Sea to P. fluviatilis from a nearby Biotest enclosure. The Biotest is a man‐made enclosure within the Baltic Sea that has received warm water from a nuclear power plant since 1980, resulting in temperatures that are elevated 5–10° C relative to the surrounding Baltic Sea. Sampled fish ranged from young‐of‐the‐year to 14 years. Geometric morphometrics and multivariate statistical analysis revealed significant morphological differences between individuals of P. fluviatilis from these two habitats. Most importantly, relative shape changed with size, with small individuals of P. fluviatilis from Biotest being characterized by a deeper body shape and a larger caudal peduncle than the smaller Baltic individuals. In large specimens, smaller differences were found with Biotest individuals being more slender than Baltic individuals. These results show that, in order to have a full understanding of the biological effects of elevated temperatures, studies that cover the entire size range of organisms will be important. Apart from the direct influence of temperature on growth rate and body shape, other ecological factors affected by temperature are discussed as possible contributors to the observed differences between the two populations.     

Feeding ecology of the shortnose greeneye Chlorophthalmus agassizi Bonaparte, 1840 (Pisces: Chlorophthalmidae) in the eastern Ionian Sea (eastern Mediterranean)

The feeding ecology of Chlorophthalmus agassizi from the North Ionian Sea was examined in specimens caught during daylight hours on a seasonal basis from December 1996 to November 1997, at depths between 250 and 800 m (92 stations, total). Chlorophthalmus agassizi showed a mixed feeding strategy, exploiting a wide range of prey including mesopelagic, benthic and endobenthic organisms. Crustaceans and fishes were dominant food items in the diet year round. Diet composition showed slight seasonal fluctuations; these variations were correlated with food availability and reproductive activity. However, ontogenetic changes in the diet were relatively clear, despite the high overlap observed between small, medium and large individuals. Larger individuals are more efficient hunters that exhibit selection of prey with good swimming capabilities; smaller individuals consume prey with low mobility. Chlorophthalmus agassizi seem to be adapted to a food‐scarce environment, as typified in the deep‐water habitats of the Ionian Sea, and exploit all available niches.     

Salinity gradient shapes distance decay of similarity among parasite communities in three marine fishes

Published data were used to compare the distance decay of similarity in parasite communities of three marine fish hosts: Atlantic cod Gadus morhua, the dab Limanda limanda and the flounder Platichthys flesus in two adjacent areas that differ with respect to the strength of a salinity gradient. In the Baltic Sea, which exhibits a strong salinity gradient from its connection with the North Sea in the west to its head in the north‐east, parasite communities in all three fish hosts showed a significant decline of similarity with increasing distance. In contrast, among host populations in the North Sea, which is a fully marine environment, there was no such decline or only a weak relationship. The results suggest that environmental gradients like salinity can be strong driving forces behind patterns of distance decay in parasite communities of fishes.     

Zoogeographical classification of the macroparasite faunas of four flatfish species from the northeastern Atlantic

The macroparasite faunas of four righteye flounder species (flounder Platichthys flesus, witch flounder Glyptocephalus cynoglossus, American plaice Hippoglossoides platessoides and Atlantic halibut Hippoglossus hippoglossus) caught off the northern Norwegian coast at 70°N, were classified zoogeographically. The parasite faunas of all four host species were dominated by boreal parasites. This appears to be the situation with the entire area of distribution of these flatfish species. The relationship between the extension of the range of the host species towards the Arctic and the proportion of parasites belonging to the arctic group is indicated.     

Einnischung von fünf Grundeln (Teleostei, Gobiidae) der Ostsee und ihre Deutung mit Hilfe der Präadaptationstheorie

Niche occupation by 5 goby species (Teleostei, Gobiidae) in the Baltic Sea and its interpretation by means of the theory of pre-adaptation The habitats and prey of five goby species were investigated near the shore of the Lübeck Bight (SW Baltic) during the period 1989–1991. Observations showed that Gobius niger is a macrobenthos feeder, Pomatoschistus microps a meiobenthos feeder; Pomatoschistus pictus prefers meiobenthos as well as plankton while Pomatoschistus minutus feeds on benthos of every size, as well as planktonic cope-pods when offered in surplus; Gobiusculus flavescens is generally a plankton feeder but, when this component is in short supply in the summer, changes to macrobenthos. P. pictus and P. microps existed at different depths; G. flavescens and P. minutus presented countercurrent densities in shallow habitats during the course of a year. This adaptability of behaviour in habitat and prey choice can be explained by the modification ability of pre-adaptional genes in the goby genomes. After having invaded species-poor habitats such as the brackish Baltic Sea, wide niche dimensions are initially occupied. The niche is here considered to be an attribute of the species as well as of the habitat according to GÜNTHER (1950). Hence, innidation is defined as a process which implies specialization by loss of most preadaptations and leads to the optimal use of the ecosystem by the species. This process of a single species corresponds to the maturation of successions of whole biocoenoses.     

Spatial variation in abundance of prey and diet of trumpeter (Pelates sexlineatus: Teraponidae) associated with Zostera capricorni seagrass meadows

Abstract The diet of juveniles of Pelates sexlineatus was examined at six estuaries, separated by tens to hundreds of kilometres, and two sites within each estuary, separated by hundreds of metres to kilometres. Fish were collected in Zostera capricorni seagrass meadows along the coast of New South Wales (Australia). Spatial variability in diet was compared with the abundance of prey. Pelates sexlineatus had a broad diet (27 different prey) but generally preferred crustaceans (harpacticoid copepods, gammarid amphipods, ostracods and tanaids), although at some sites other prey items were important (e.g. polychaetes, nematodes and foraminiferans). Composition of the diet varied among estuaries and between sites. Proportional representation of the different size of prey eaten by the three sizes of juveniles (40‐ 54 , 55–74 and 75–94 mm standard length) was similar. Fish generally preferred prey smaller than 1 mm. Abundance of prey also varied at both spatial scales. At five of the 12 sites, there was a significant correlation between abundance of prey in the seagrass meadows and abundance of prey in the diet. Variation in the composition of the diet was partly explained by spatial variation in abundance of prey. When crustaceans were not abundant in the seagrass, P. sexlineatus had a broad diet, taking both benthic and planktonic prey items. It is concluded therefore that trophic linkages between P. sexlineatus and benthic invertebrates may vary greatly with spatial scales from hundreds of metres to hundreds of kilometres, and are strongly related to availability of prey in seagrass habitats.     

Diet reveals links between morphology and foraging in a cryptic temperate reef fish

Predators select prey so as to maximize energy and minimize manipulation time. In order to reduce prey detection and handling time, individuals must actively select their foraging space (microhabitat) and populations exhibit morphologies that are best suited for capturing locally available prey. We explored how variation in diet correlates with habitat type, and how these factors influence key morphological structures (mouth gape, eye diameter, fin length, fin area, and pectoral fin ratio) in a common microcarnivorous cryptic reef fish species, the triplefin Helcogrammoides cunninghami. In a mensurative experiment carried out at six kelp‐dominated sites, we observed considerable differences in diet along 400 km of the Chilean coast coincident with variation in habitat availability and prey distributions. Triplefins preferred a single prey type (bivalves or barnacles) at northern sites, coincident with a low diversity of foraging habitats. In contrast, southern sites presented varied and heterogeneous habitats, where triplefin diets were more diverse and included amphipods, decapods, and cumaceans. Allometry‐corrected results indicated that some morphological structures were consistently correlated with different prey items. Specifically, large mouth gape was associated with the capture of highly mobile prey such as decapods, while small mouth gape was more associated with cumaceans and copepods. In contrast, triplefins that capture sessile prey such as hydroids tend to have larger eyes. Therefore, morphological structures co‐vary with habitat selection and prey usage in this species. Our study shows how an abundant generalist reef fish exhibits variable feeding morphologies in response to the distribution of potential habitats and prey throughout its range.     

Population specific sperm production in European flounder Platichthys flesus: Adaptation to salinity at spawning

Marine teleosts inhabiting the brackish Baltic Sea have adapted to the less saline water with activation of spermatozoa at low salinity hypo‐osmotic conditions but with shorter longevity and lower swimming speed that affect the fertilization capacity. Aiming to elucidate if the fertilization capacity may be maintained by increasing the number of spermatozoa produced, testis size for the euryhaline flounder Platichthys flesus with external fertilization was assessed along a salinity gradient; with spawning at a salinity of c. 7, 10–18 and 30–35. Fulton's condition factor K = 0.881 ± 0.085 (mean ± S.D .), 0.833 ± 0.096 and 0.851 ± 0.086, for fish spawning at salinities of c. 7, 10–18 and 30–35, respectively, with no difference between areas, i.e. analysed fish were in similar nutritional condition. A general linear model, with testes dry mass as the dependent variable and somatic mass as covariate resulted in a significant difference between areas–populations with larger testes for P. flesus spawning at a salinity of c. 7 but no difference between fish spawning at a salinity of 10–18 and 30–35. The result suggests that adaptation by increasing the number of spermatozoa produced may be a key mechanism for marine teleosts spawning in areas with low salinities to sustain the fertilization capacity as shown here for the euryhaline P. flesus.     

Substratum stability associated with the riverine macrophyte Justicia americana

1. Patches of stable substratum in streams may be important refugia for benthic organisms during scouring floods. Streambed stone stability, packing and embeddedness were assessed within and adjacent to beds of the macrophyte Justicia americana in five Alabama streams. 2. The force needed to dislodge stones and embeddedness was about two times lower outside Justicia beds than within them. Significant positive correlations between stone stability and (i) degree of embeddedness, and (ii) the abundance of binding rhizomes and the presence of attached roots indicate that Justicia may physically modify the local streambed, indirectly enhancing substratum stability and reducing flow, thereby increasing sand deposition. 3. Despite higher stability (i.e. physical refugia during bed‐moving spates) within Justicia beds, the abundance of epilithic plants (moss and Podostemum ceratophyllum) and pleurocerid snails (Elimia spp.) was similar both inside and outside the macrophyte beds. Several physical characteristics within macrophyte beds, such as low light, reduced current and increased sand intrusion, may create suboptimal conditions for benthic organisms in these habitats. 4. Additional work is needed to determine if Justicia biogenically enhances substratum stability or if its presence merely reflects patches of stable substratum within the streambed. Regardless of the mechanism, there is an association between Justicia beds and streambed characteristics.     

Using short-term growth of enclosed 0-group European flounder, Platichthys flesus, to assess habitat quality in a Danish bay

Instantaneous growth rates of 0‐group European flounder, Platichthys flesus, obtained from two 9‐day enclosure experiments were used to assess the relative quality of three eelgrass and three bare sand habitats in the Bay of Aarhus, Denmark. Measurements of temperature, dissolved oxygen, salinity, sediment structure and biomass of benthic macrofauna were made during the experiments. Gut content analysis was performed after retrieval of the fish. Growth rates of flounder at two bare sand sites were higher or equal to growth rates at the three eelgrass sites, and at a muddy bare sand site in both experiments. The variation in gut fullness of flounder between vegetated and bare sand sites followed the pattern observed in growth rates, but diverged from the spatial variation in prey abundance, which indicates that there were differences in the feeding efficiency between sites. Overall, habitat complexity, i.e. the presence or absence of vegetation seems to be the most important factor determining the growth rates and, by corollary, habitat quality for juvenile European flounder in the Bay of Aarhus.