Environmental factors and interactions affecting the temporal abundance of juvenile flatfish in the Thames Estuary

Samples taken regularly from the intake screens of West Thurrock power station between January 1977 and November 1992 were used to investigate the factors controlling flatfish abundance (flounder: Platichthys flesus; dab: Limanda limanda; plaice: Pleuronectes platessa; sole: Solea solea) in the middle Thames estuary. Most sampled fish were age-0. All species followed regular patterns of seasonal occurrence, had distinctive seasonal abundance peaks and demonstrated only minimal temporal overlap. Multiple-regression models were used to study the relationship between physico-chemical variables, physico-chemical variable interactions, trends and seasonal factors on fluctuations in flatfish sample abundance. Flounder abundance was significantly influenced by temperature, suspended solids, oxygen-flow and oxygen-temperature interactions, trend and seasonal variables. Dab abundance was significantly related to suspended solids, oxygen-flow, oxygen-temperature and flow-chlorinity interactions, Crangon crangon abundance and seasonal variables. Sole abundance was controlled only by seasonal variables, whilst plaice abundance could not be adequately explained by the available physico-chemical data. The combination of variables affecting flatfish abundance indicates species using the estuary were affected by a complex set of events within the estuary. The interactions between routinely measured environmental variables further suggest that estuarine monitoring studies must attempt to understand the nature of possible interactions between variables if an improved understanding of the changes in estuarine fish communities resulting from human activity is to be gained.     

Estimated intensity needed for sampling flatfish assemblages in reference areas of tidal mud flats systems may be disproportionately costly and deleterious

Beam trawl surveys were carried out in a tidal stream system of the German Wadden Sea on two to three successive days of comparable weather conditions in three summers. The variability in time and space detected for plaice (Pleuronectes platessa), flounder (Platichthys flesus), dab (Limanda limanda), sole (Solea solea) and lemon sole (Microstomus kitt) was considerable even for species with a very high abundance. Other flatfish species such as turbot (Scophthalmus maximus) and brill (Scophthalmus rhombus) occurred only occasionally in the catches. Sampling effort calculated to detect a 50 or 20% difference between the surveys caused by immigration, emigration or mortality appears to be disproportionate. It is argued that such a major sampling effort of reference areas in tidal mud flat systems is likely to influence the results, to harm the environment, and to be very expensive.     

A comparative analysis of recruitment variability in North Atlantic flatfishes — testing the species range hypothesis

The hypothesis that recruitment variation in flatfishes should be most variable at the northern edge of the species range, least near the centre of the range, and intermediate near the southern limit was tested using stock and recruitment data generated from sequential population analysis for several different flatfish stocks involving four species (plaice Pleuronectes platessa, sole Solea vulgaris from the eastern Atlantic, American plaice Hippoglossoides platessoides, and yellowtail flounder Limanda ferruginae from the western Atlantic). Several groundfish species have been found to conform to this so-called species range hypothesis with the suggestion that density-independent processes predominate at the edges of the distributional range and density-dependent processes dominate in the centre of the range. Our results were generally inconsistent with the hypothesis: the coefficient of variation (CV) of recruitment for plaice in the eastern Atlantic was independent of latitude, the CV of recruitment for sole exhibited a dome-shaped relationship with latitude with the highest CVs occurring at the mid-point of the range, and the CV of recruitment for the western Atlantic stocks exhibited a monotonic decrease with latitude. We extended our latitudinal analyses by assessing both the degree of dependency of recruitment on spawning stock biomass and the spatial and temporal scales of variability in recruitment and pre-recruit survival for the eastern Atlantic stocks. In general our analysis revealed no evidence of a strong stock and recruitment relationship for any of the stocks examined, and previously published analyses revealed no such patterns with latitude. Analysis of both de-trended recruitment and pre-recruit survival time series over the species ranges of sole and plaice revealed strong positive correlations among adjacent stocks and inverse correlations among stocks at the extremes of the range. Recruitment variation in the flatfish stocks examined appears to be dominated by density-independent factors, operating at a local scale, on the egg and larval stages.     

The impact of intrapopulation variability in reproductive traits on population reproductive potential of Grand Bank American plaice (Hippoglossoides platessoides) and yellowtail flounder (Limanda ferruginea)

In geographically extensive fish populations the potential exists for reproductive traits to vary over the population’s range but, the impact that such intrapopulation variability has on overall population reproductive potential has not been formally assessed. Here intrapopulation spatial variability in size at maturity and fecundity are demonstrated for Grand Bank American plaice (Hippoglossoides platessoides) and yellowtail flounder (Limanda ferruginea). Recognition of intrapopulation variability in these reproductive traits coupled with spatial variability in abundance resulted in an increase in estimated population total egg production (TEP) by as much as 10¹⁴ eggs for American plaice and 10¹⁵ eggs for yellowtail flounder as compared to assessment of TEP for the population as a whole. Results highlight the need to explore variability in life history traits not only between but, also within populations and emphasizes the need for sufficient spatial coverage during sampling in order to assess the reproductive potential of fish populations.     

Reduction of flatfish habitat as a consequence of the proliferation of an invasive mollusc

Coastal bays provide habitats for juveniles and adults of many marine species. Mont Saint-Michel Bay (MSMB, France) hosts a highly diversified fish community and constitutes one of the most important nursery grounds for many commercially exploited marine species, such as sea bass, flatfish, clupeids and rays in the English Channel. Besides, MSMB also suffers from the massive invasion of an exotic mollusc, the American slipper-limpet (Crepidula fornicata, L.). This species arrived four decades ago and now represents the main filter-feeder biomass in the bay (150 Mt), an order of magnitude larger than local farmed and natural shellfishes. Recent analyses underlined the impact of this small gastropod on the trophic structure of this bay and its negative influence on juvenile sole densities in the nursery grounds. The present study uses a geostatistical approach to explore the effect of the extension of the slipper-limpet on flatfish (common sole Solea solea, L.; plaice Pleuronectes platessa, L.; brill Scophthalmus rhombus, L. and flounder Platichthys flesus, L.) spatial distribution. Data collected during survey of the MSMB at the end of the 1970s and three decades later have been used to build interpolated maps of (1) slipper-limpet and (2) flatfish spatial distributions. Slipper-limpets were concentrated in a small area, in the western part of the MSMB, in the 1970s while today they occupy half of the bay. This rapid proliferation led to the decrease of available surface for flatfishes, which previously occupied the whole bay and are now restricted to its eastern part. The present study highlighted that the negative influence on fish habitat in MSMB is apparently more related to changes in the substratum than to trophic interactions. This invasion has possible consequences on flatfish population renewal at a large scale and may also damage other benthic or demersal species, such as rays.     

Histopathological observations on liver,kidney and gonad of plaice (Pleuronectes platessa) taken from the Mersey estuary

The Mersey estuary is the most contaminated estuary in British waters. Detailed studies are underway on the pathology of flounder (Platichthys flesus) from the Mersey and on flounder from the nearby, but less contaminated, Dee estuary. Flounder breed offshore but spend a lot of time in the estuaries, penetrating into freshwater. These flatfish are in close contact with sediments and will be exposed to exenobiotic stored in the sediments. Plaice (Pleuronectes platessa) are flatfish which enter the Mersey with the tidal flow but spend less time in the estuary than flounder and are less likely to show evidence of pollutant exposure. Over 20 plaice were collected from the Mersey. Samples of the liver, kidney and gonad were fixed in 10% formal saline, processed into paraffin wax and 5 microm sections cut and stained with haematoxylin and eosin. Liver changes consisted of variable amounts of glycogen/lipid storage product, minor perivenular and perivascular fibrosis, and helminth parasitisation. No tumours or foci of cellular alteration, necrosis and inflammation were seen. In the gonad only changes consistent with normal gonadal cycling was seen. These studies provide evidence that plaice show little or no evidence of toxic pathological damage resulting from their stay in the estuary.     

Fish populations of the Avon‐Heathcote Estuary

Breeding seasons and sizes at first maturity for nine species of fish in the Avon‐Heathcote Estuary, Christchurch, New Zealand, were: sand flounder, Rhombosolea plebeia, mid‐winter to spring at 200 mm+; yellow‐bellied flounder, Rhombosolea leporina, winter and spring at 260 mm+; common sole, Peltorhamphus novaeieelandiae, did not breed in the estuary but reached first maturity at 220 mm +; yellow‐eyed mullet, Aldrichetta forsteri, spawned twice a year, winter and summer, at 220 mm +; kahawai, Arripis trutta, bred outside the estuary at 520–540 mm +; spotty, Pseudolabrus celidotus, spring to autumn at 200 mm for males and 160 mm for females; cockabully, Tripterygion nigripenne, spring and early summer at 52–57 mm +; common bully, Gobiomorphus basalis, spring and early summer at 45–50 mm+; and globefish, Speroides richei, spring to autumn at 147 mm + for males and 120 mm + for females. The successive stages in gonad development are described for these species.

In pelagic species, e.g., flatfish and kahawai, x² analyses showed that for most months females are more numerous than males, and that migration is of major importance. In (littoral species, e.g., common bully and cockabully, there is a 1 : 1 sex ratio in the breeding season, but this ratio subsequently breaks down.

The Avon‐Heathcote estuary was used mainly as a nursery area by juvenile fish, although some species, such as sand flounder, yellow‐bellied flounder, yellow‐eyed mullet, and cockabully spawned in the estuary.     

Regulation of hepatic glutathione S-transferase expression in flounder

The major glutathione S-transferase isoform of flounder liver, an antigenically related structural homologue of plaice GST-A, also displays mRNA homology. A 901bp cRNA probe for plaice GST-A cross-hybridised to a 1100bp flounder mRNA on northern blot analysis. The plaice antibody and cRNA probes were used to study effects of inducer treatment on GST-A expression in flounder liver. Six days after PAH treatment (3-methylcholanthrene) total hepatic GST activity was halved, levels of GST-A were 80% and GST-A mRNA levels were 25% of controls. A commercial PCB mixture (Aroclor 1254^TM) had little effect on total GST or GST-A levels despite halving GST-A mRNA levels. An epoxide, trans-stilbene oxide induced total GST activity 1·4 fold and GST-A protein levels 1·8-fold and its mRNA levels 3-fold. This reduced expression of the major flounder hepatic GST by agents which induce cytochrome P4501A1 may modulate cytoxicity of environmental pollutants in this species.     

Mesh selection studies on flatfish in relation to the Otago trawl fishery

Cod‐end mesh selection experiments were conducted on three species of flatfish (common or English sole Peltorhamphus novaezeelandiae, lemon sole Pelotretis flavilatus, and sand flounder Rhombosolea plebeia) trawled off the Otago coast. It is suggested that better yields would be obtained from stocks of these fish if the minimum cod‐end mesh size was increased from 10.2 cm (4 in.) to 11.4 cm (4.5 in.).     

Quantitation of CYP1A expression in two flatfish species showing different prevalences of contaminant-induced hepatic disease

English sole (Pleuronectes vetulus) and starry flounder (Platichthys stellatus) are two sympatric flatfish species which show markedly different responses to chemical contaminant exposure. Whereas English sole develop hepatic neoplasms, the prevalences of which are strongly linked to exposure to polycyclic aromatic hydrocarbons (PAHs), evidence of neoplasia is virtually nonexistent in starry flounder, even those residing in areas with very high levels of PAH in the sediments. Because PAHs are activated to genotoxic forms by the action of cytochrome P450 1A (CYP1A) in teleosts, we are examining the hypothesis that variation in the hepatic expression of this major inducible cytochrome P450 isozyme may contribute to the differential contaminant response. These two species were captured in the Duwamish Waterway, a contaminated area of Puget Sound, Washington. Catalytic activity of CYP1A [determined as aryl hydrocarbon hydroxylase (AHH) activity] was measured in the liver to quantitatively assess the relative induction of CYP1A in these two species, and AHH activity was significantly higher in English sole than in starry flounder (p = 0.015). Cellular expression of CYP1A was determined by immunohistochemical localization of tissues, using an avidin-biotin peroxidase complex method, with polyclonal rabbit anti-cod CYP1A. The results showed a markedly reduced expression of CYP1A in hepatocytes of starry flounder, which is consistent with the apparent resistance of this species to the development of hepatocellular neoplasia. This reduced expression of CYP1A in hepatocytes of contaminant-exposed fish was previously seen in oyster toadfish from the Elizabeth River, Virginia, and this species is also apparently resistant to hepatocellular neoplasia.     

Age compositions of flatfish stocks as determined by a new otolithometric method,its application in the estimation of growth,spawning potential and fisheries management

We developed an ageing methodology and examined age composition of three flatfish stocks inhabiting the Seto Inland Sea, Japan. Ages were difficult to determine for three-lined tongue sole (Cynoglossus abbreviates) and ridged-eye flounder (Pleuronichthys cornutus) because the first year annulus ring was often indistinct; therefore, we used directional change in otolith growth to distinguish it. Sectioning and etching methods were powerful tools for identifying annual checks for red tongue sole (Cynoglossus joyneri). Using these ageing methods, we determined age–length relationships and growth curves. The age composition of the populations studied and of the landings showed that a large proportion of the latter consisted of individuals under the mean age of sexual maturity, thereby reducing the percent spawning potential ratio (%SPR) to ≈ 20% for all species. These findings suggest that fishing pressure on immature fish is leading to overfishing of these flatfish stocks.     

Age compositions of flatfish stocks as determined by a new otolithometric method, its application in the estimation of growth, spawning potential and fisheries management

We developed an ageing methodology and examined age composition of three flatfish stocks inhabiting the Seto Inland Sea, Japan. Ages were difficult to determine for three-lined tongue sole (Cynoglossus abbreviates) and ridged-eye flounder (Pleuronichthys cornutus) because the first year annulus ring was often indistinct; therefore, we used directional change in otolith growth to distinguish it. Sectioning and etching methods were powerful tools for identifying annual checks for red tongue sole (Cynoglossus joyneri). Using these ageing methods, we determined age–length relationships and growth curves. The age composition of the populations studied and of the landings showed that a large proportion of the latter consisted of individuals under the mean age of sexual maturity, thereby reducing the percent spawning potential ratio (%SPR) to ≈ 20% for all species. These findings suggest that fishing pressure on immature fish is leading to overfishing of these flatfish stocks.     

Polychaete assemblage as surrogate for prey availability in assessing southeastern Bering Sea flatfish habitat

The flatfish yellowfin sole (Limanda aspera), northern rock sole (Lepidopsetta polyxystra), and Alaska plaice (Pleuronectes quadrituberculatus) in the southeastern Bering Sea prey mainly on infauna. Spatial correspondence between their stomach contents and infauna assemblages across habitat types was examined to identify indices of prey availability for flatfish habitat characterization and quality assessment. Benthic samples and flatfish stomachs were collected in 2009 near the Alaska Peninsula in the southeastern Bering Sea. Polychaetes and bivalves were the most dominant infauna groups, each comprising 35–60% by weight in each infauna sample. These two were also the only prey groups that frequently averaged > 50% of stomach content by weight. Bivalves dominated the infauna biomass on the relatively sandy inner shelf (0–50 m depth). The muddier middle shelf (50–100 m) had the highest infauna biomass, which was dominated by polychaetes. Diet compositions of the flatfish varied spatially in correspondence with the infauna assemblage. Polychaetes were prevalent in all flatfish diets on the middle shelf, even yellowfin sole whose typical primary prey are amphipods and bivalves. Polychaete-rich habitats are potentially prime for flatfish as polychaetes are readily utilized where available and generally have high nutritional value. Flatfish did not select for specific polychaete taxa, so an index of habitat quality could be based on the biomass of aggregate polychaetes or on dominant polychaete families of the region. Under normal environmental conditions, the three flatfish have slightly-offset spatial distributions, enabling each to utilize different infauna assemblages across the shelf. However, during cold phases in the Bering Sea ecosystem, as when this study was conducted, a cold pool of < 2 °C bottom water from the spring ice melt extends over the middle shelf in summer. This physiological barrier displaces all three flatfish to the inner shelf, intensifying competition for prey resources.     

The food and growth of 0-group flatfish on nursery grounds in the Clyde Sea Area

In early summer plaice (Pleuronectes platessa L.) in the Clyde Sea Area eat Tellina siphons and spionid palps. As they grow, they take progressively larger siphons, fragments of spionids and some crustaceans. In late summer they eat entire spionids and other sedentary polychaetes. There is no significant predation on fish. The common dab [Limanda limanda (L.)] took a wider range of prey species than the plaice, but also depended mainly on polychaetes, crustaceans and bivalves. Depth distribution may limit both intra- and interspecific competition between these fish and also influence the choice of prey. The results from the main nursery areas of Irvine and Ayr Bays were similar to those from the other bays throughout the Clyde Sea Area and support earlier work at Firemore Bay in north-west Scotland and in the Dutch Wadden Sea.The growth rate of the fish increases during the early summer and may be related to increasing food availability, feeding experience of the fish and longer day length. Growth rate between areas appeared to be correlated with benthic productivity. 0-group plaice in Irvine, Ayr and Firemore Bays grew to different mean lengths by the end of the summer growth season, being 82 mm, 73 mm and 61 mm, respectively, for the period 1972–1974. This agrees with the known differences in standing stocks of intertidal macrobenthos in these bays (17·0, 11·1 and 1·3 g m^?2 dry weight, respectively). The significance of this in relation to survival is discussed. Offshore migration of the larger fish in the autumn may result in apparent slowing of growth rates prematurely in inshore areas. The year-class strength depends more on the food availability in the nursery areas than on the number of plaice that settle on the bottom.     

Estimation of dynamic energy budget parameters for the Mediterranean toothcarp (Aphanius fasciatus)

Organisms adopt different sets of physiological, behavioural and morphological trade-offs in order to cope with natural environmental fluctuations. This has consequential rebounds on ecological processes and population dynamics. Such aspects become crucial for sex-dimorphic species, where sex-specific growth variation could mirror different tactics both in energy acquisition and investment between maximum female and male body size with cascading effects on population demography. To date, different approaches have been used in order to understand the causes of individual growth rate changes in ectotherm indeterminate growers, most of which failed. Here, we propose the use of a mechanistic model based on the Dynamic Energy Budget theory (DEB; Koojiman, 2010) to investigate potential differences in energy allocation strategies adopted by individuals of different genders with the Mediterranean toothcarp Aphanius fasciatus (Valenciennes, 1821) as the model species. We collected literature and field data in order to study differences in energy allocation strategies between females and males of the same species by generating projections of possible growth performances: (1) throughout their entire life span and (2) under a context of varying functional responses. Generally, the present exercise of simulations returned different patterns of growth performance among females and males of A. fasciatus, with the former being able to better optimize energetic trade-offs under optimal environmental conditions. The present DEB parameterization exercise represents an essential step towards developing a mechanistic approach to depict metabolic strategies, which are at the base of observed sexual differences, and how such differences may impair ultimate fitness at individual and, therefore, population levels.     

Continental shelf nurseries and recruitment variability in American plaice and yellowtail flounder on the Grand Bank: insights into stock resiliency

In 1994, a directed fishing moratorium was declared for Grand Bank American plaice (Hippoglossoides platessoides) and yellowtail flounder (Limanda ferruginea) stocks because both stocks showed severe declines in abundance from heavy exploitation during the mid 1980s and early 1990s. Four years later, the fishery for yellowtail re-opened while the plaice stock has shown little recovery and the moratorium is still in effect. To assess the possible causes of the differences in recovery between species, we examined the spatial structure and environmental characteristics of the continental shelf nursery habitats of plaice and yellowtail, and their relationship to recruitment variability and overall population size. Depth plays a major influential role determining the spatial pattern and the abundance of juveniles of both species and in the case of plaice the spatial structure of the adult population also determines the amount of nursery area utilised by juveniles. Recruitment variability was higher in plaice than in yellowtail. We found year class synchrony in both species indicating that common environmental conditions and/or biological processes are affecting recruitment in a similar manner. Density-dependent regulation appears to be more severe in yellowtail and this should contribute to a more stable population when compared to plaice. These results are discussed in terms of resiliency of both stocks to over-exploitation.     

Recruitment variability in dab (Limanda limanda) in the southeastern North Sea

Data from three annual surveys, covering inshore and offshore waters of the southeastern North Sea, were analysed to study recruitment variability in dab (Limanda limanda) over the period 1978–1997. Geometric mean abundance of 0- to 5-group dab was estimated using general linear models. Juvenile dab (0- and 1-group) were found over the entire area, from inside the estuaries to 50 m depth offshore. Environmental conditions (water temperature, wind stress, turbidity) affected the catch rates. The potential errors in the estimates of year-class strength, caused by differences in catchability, are discussed. The inter-annual pattern of year-class strength appeared to be established between ages 1 and 2, suggesting that factors determining recruitment are not restricted to the pelagic early life phase only, but also operate during the demersal juvenile phase. Recruitment variability at age 2 was in the order of 50–60% and appears to be equal to, or lower than, recruitment variability in plaice and sole. These results contradict expectations based on the concentration hypothesis, which states that the degree of variation in recruitment is inversely related to the degree of concentration during early life phases.     

DNA-based identification methods of prey fish from stomach contents of 12 species of eastern North Pacific groundfish

Stomach content analysis of eastern North Pacific groundfish has been conducted routinely by researchers interested in understanding trophic interactions between key predator species and their prey. Identification of prey by traditional morphological methods has limitations however, due to the loss of identifiable characters from digestion and morphological similarities between taxa. Furthermore, some forage fish (e.g., osmerids, ammodytids, and juvenile gadids), common prey of Bering Sea and Gulf of Alaska groundfish, are difficult to distinguish because of their slender or fusiform shape, disarticulating easily during digestion. DNA-based identification methods were developed to differentiate among 18 fish species, some that are found at depths greater than 200 m, from four taxonomic families: Ammodytidae and Osmeridae (forage fish), Pleuronectidae (flatfish), and Gadidae (gadid fish). Polymerase chain reaction (PCR) amplification of a 739 basepair section of mitochondrial DNA cytochrome c oxidase I and an 862 basepair section of mitochondrial DNA cytochrome b was followed by restriction digest assays and resulted in species level resolution for 16 of 18 species of interest. PCR restriction digest assays applied to fish prey from stomach contents of groundfish indicated the presence of several target species, eulachon (Thaleichthys pacificus), walleye pollock (Gadus chalcogrammus), searcher (Bathymaster signatus), rock sole (Lepidopsetta bilineata), yellowfin sole (Limanda aspera) and either Bering flounder (Hippoglossoides robustus) or flathead sole (H. elassodon). The PCR restriction digest protocols improved the identification rate of predated fish from stomach contents compared to identification by conventional taxonomic methods alone, and DNA sequence analysis further resolved identification of unknown prey fish samples.