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

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

Factors affecting the post‐release survival of cultured juvenile Pseudopleuronectes americanus

Laboratory experiments were performed with cultured and wild juvenile winter flounder Pseudopleuronectes americanus to evaluate differences in behaviour and adaptation affecting post‐release vulnerability to predation. Studies revealed that the cryptic abilities of cultured winter flounder increased over time. Sediment‐naïve, cultured fish required a minimum of 2 days to improve their burying skills and at least 90 days for colour adaptation to match the sediment. Cultured winter flounder selected sediments consisting of small grains and colours matching their own pigment. Cultured winter flounder, regardless of their colour, were significantly more vulnerable to predation by birds. Additionally, cultured fish reacted differently than wild winter flounder when exposed to cues from a potential predator.     

Patterns in abundance and distribution of juvenile flounder, Platichthys flesus, in Minho estuary (NW Iberian Peninsula)

For 2 years, the flatfish community of Minho estuary (Northwest of Iberian Peninsula) was surveyed monthly with a beam trawl. Flounder, Platichthys flesus, was the most representative species contributing the most (74%) to the total flatfish abundance and was the only species occurring throughout the sampling period and along the entire salinity gradient. The majority of flounder sampled consisted of 0- and 1-year group individuals. In both years, a peak in abundance was recorded in late spring–summer months (from May to August) which coincided with seasonal settlement events. Higher densities of newly-settled flounder were obtained in the most upstream site, located in the tidal freshwater wetland, indicating that this area is an important nursery area. Of the environmental variables tested, temperature and salinity were the best predictors for 0-year group abundance explaining around 30% of the variability, whereas sediment was also important when considering all age groups together. This study supports the generally prevalent idea that estuaries act as nurseries for the early life stages of flounder, with particular relevance to the tidal freshwater estuarine wetlands.     

Immunological detection of winter flounder (Pseudopleuronectes americanus) eggs and juveniles in the stomach contents of crustacean predators

Predation on the early life history of fish is an important factor regulating year-class strength. Verifying predation events, however, is difficult when analyses rely on visually identifying the remnants of partially digested fish in the stomachs of suspected predators. The objective of this study was to assess the utility of using immunological assays to detect the presence of winter flounder eggs and juveniles (Pseudopleuronectes americanus) in the gut contents of sand shrimp (Crangon septemspinosa) and green crab (Carcinus maenas). After defining assay capabilities, the stomach contents of field-collected shrimp and crabs were examined to determine if these predator-prey relationships occur under natural conditions. Winter flounder-specific antisera developed and used in this study successfully identified homologous antigens (egg or juvenile flounder extracts) without appreciably cross-reacting with antigenic material from predators or nontarget prey. Moreover, antisera detected flounder eggs 10.8-16.4 h after initial feeding by various sized shrimp, and identified juvenile flounder 9.4 and 7.8 h after initial ingestion by shrimp and crabs, respectively. Immuonological dietary analysis of decapod crustaceans collected from Niantic River, Connecticut, revealed that C. septemspinosa and C. maenas are potentially important predators on the early life stages of winter flounder. The temporal trends and magnitude of flounder predator-induced mortality was affected primarily by the spatial and temporal overlap between predator and prey (egg mortality), and the size-dependent relationships underlying crustacean and flatfish predator-prey interactions (juvenile mortality).     

Adaptive coloration, behavior and predation vulnerability in three juvenile north Pacific flatfishes

Adaptive color change in flatfish has long been of interest to scientists, yet rarely studied from an ecological perspective. Because color change can take a day or so in some species, movement between sediments with differing color or texture may render fish more conspicuous to predators. We conducted laboratory experiments to test the following hypotheses related to adaptive color change in flatfish: 1) fish which do not cryptically match sediment will be more vulnerable to predation, 2) fish will reduce activity and bury to minimize conspicuousness when on a sediment they mismatch, and 3) fish will choose a sediment they match when given a choice. Experiments were conducted using three co-occurring north Pacific juvenile flatfishes: English sole Parophrys vetulus, northern rock sole Lepidopsetta polyxystra and Pacific halibut Hippoglossus stenolepis. As per expectations, juvenile flatfish were more vulnerable to visual predators when they mismatched sediment. Mismatched fish tended to behave differently than fish which matched the sediment. Rather than burying and becoming inactive, they became more active and less likely to bury, perhaps contributing to their predation vulnerability. This increased activity may have represented search for better matching sediment, a stress response, or conspicuousness-related density dependent behavior. Fish which had acclimated to light colored sediment preferred light over dark sediment in choice trials. In contrast, fish acclimated to dark sediment demonstrated no preference. These experiments demonstrate that adaptive coloration is an integral part of the flatfish detection minimization strategy and that movement between habitats can increase risk of predation.     

Size-related shifts in the habitat associations of young-of-the-year winter flounder (Pseudopleuronectes americanus): field observations and laboratory experiments with sediments and prey

Field surveys and laboratory studies were used to determine the role of substrata in habitat selection by young-of-the year winter flounder. A synoptic field survey of winter flounder and sediments in the Navesink River-Sandy Hook Bay estuarine system in New Jersey demonstrated that winter flounder distribution was related to sediment grain size. Analysis using a generalized additive model indicated that the probability of capturing 10-49 mm SL winter flounder was high on sediments with a mean grain diameter of /=40 mm SL) preferred coarse-grained sediments. Burying ability increased with size and all flounders avoided sediments that prevented burial. Subsequent laboratory experiments revealed that the presence of live prey (Mya arenaria) can over-ride sediment choice by winter flounder (50-68 mm SL) indicating the complexity of interrelated factors in habitat choice.     

Predatory impact of the green crab (Carcinus maenas Linnaeus) on post-settlement winter flounder (Pseudopleuronectes americanus Walbaum) as revealed by immunological dietary analysis

Predation on flatfish during the early juvenile stage is an important factor regulating year-class strength and recruitment. In this study, immunological dietary analysis was performed on green crabs (Carcinus maenas) collected from the Niantic River, Connecticut, in an effort to evaluate the predatory impact of this species on post-settlement winter flounder (Pseudopleuronectes americanus). Through the use of species-specific antiserum, winter flounder proteins were identified in 4.8% of the green crab stomachs analyzed (n = 313, size range = 14-74 mm carapace width, CW), revealing that crabs ≥ 29 mm CW are predators of post-settlement winter flounder in natural populations. The most significant factor underlying the predator-prey interaction was the relative size relationship between species, such that the incidence of winter flounder remains in the stomach contents of green crabs was positively correlated with predator-to-prey size ratio. Results from dietary analysis were incorporated into a deterministic model to estimate the average daily instantaneous mortality and cumulative mortality of winter flounder owing to green crab predation. Accordingly, green crabs may account for 0.4% to 7.7% (mean = 2.2%) of the daily mortality of winter flounder and consume 1.1% to 32.3% (mean = 10.2%) of the flounder year-class. Model simulations further indicate that variations in green crab abundance and size-structure account for the greatest variability in winter flounder mortality. Relative to other macro-crustacean predators, however, predation by green crabs has a minimal effect on winter flounder survival, due in large part to the low densities of these crabs in temperate estuaries.     

Early development and Juvenile Culture Technique of Speckled Flounder,Paralichthys woolmani (Jordan & Williams, 1897) under ambient seawater

The speckled flounder Paralichthys woolmani is a flatfish species found in Ecuadorian coastal waters, consumed by local coastal communities and likely an excellent candidate for aquaculture production in indoor facilities. This study describes the early development and culture technique of P. woolmani under experimental conditions. Broodstocks were obtained from the wild and kept under laboratory conditions for this study. A total of four effective spontaneous spawns were registered at water temperature of 24.1°C, resulting in 1.3 million eggs and 129.2 thousand newly hatched larvae. Larval pelagic life lasted 28 days post hatch. Survival at benthic juvenile stage ranged 0.3%–46.9%. Pseudoalbinism was present in all juvenile batches. However, different pigmentation patterns in juveniles were recovered with addition of semi‐moist diet. There was no presence of ectoparasites or abnormal eye migration. At 370 days post hatch, flatfish reached 294.6 ± 86.9 g of body weight. These results can be of practical use for optimizing culture conditions to increase the yield of P. woolmani in captivity.     

Shallow water predation risk for a juvenile flatfish (winter flounder; Pseudopleuronectes americanus, Walbaum) in a northwest Atlantic estuary

Many small fish, including several juvenile Atlantic flatfish, are most abundant in shallow areas presumable because these habitats enhance survivorship and/or growth. In this study, we investigated size-dependent depth distributions and the role of shallow habitats as predator refuges for age-0 winter flounder (Pseudopleuronectes americanus) in a northwest Atlantic estuarine nursery. Analysis of trawl surveys performed during the larval settlement period throughout the Navesink River and Sandy Hook Bay, New Jersey, showed that as fish increased in size, depth of occurrence gradually decreased, so that individuals >35 mm standard length (SL) were concentrated in habitats ∼1 m deep. Tethering in structurally simple and adjacent shallow and deep habitats showed that predation risk for flounder (30-50 mm SL) was low in shallow water (<1 m) and increased rapidly with depth. Summer flounder (Paralychthys dentatus), which were more abundant in trammel nets in deep habitats and included winter flounder in their diets, appeared to be important consumers of tethered fish. Our results indicate that following larval settlement, winter flounder emigrate from or suffer high mortality in deeper water to become concentrated in shallow habitats that can serve as predator refuges even when they lack complex physical structures. These results highlight the potential for functional habitat loss when natural and/or anthropogenic factors make shallow habitats unavailable to young fish.     

A New PCR-Based Method Shows That Blue Crabs (Callinectes sapidus (Rathbun)) Consume Winter Flounder (Pseudopleuronectes americanus (Walbaum))

Winter flounder (Pseudopleuronectes americanus) once supported robust commercial and recreational fisheries in the New York (USA) region, but since the 1990s populations have been in decline. Available data show that settlement of young-of-the-year winter flounder has not declined as sharply as adult abundance, suggesting that juveniles are experiencing higher mortality following settlement. The recent increase of blue crab (Callinectes sapidus) abundance in the New York region raises the possibility that new sources of predation may be contributing to juvenile winter flounder mortality. To investigate this possibility we developed and validated a method to specifically detect winter flounder mitochondrial control region DNA sequences in the gut contents of blue crabs. A survey of 55 crabs collected from Shinnecock Bay (along the south shore of Long Island, New York) in July, August, and September of 2011 showed that 12 of 42 blue crabs (28.6%) from which PCR-amplifiable DNA was recovered had consumed winter flounder in the wild, empirically supporting the trophic link between these species that has been widely speculated to exist. This technique overcomes difficulties with visual identification of the often unrecognizable gut contents of decapod crustaceans, and modifications of this approach offer valuable tools to more broadly address their feeding habits on a wide variety of species.     

Which way to turn? Effect of direction of body asymmetry on turning and prey strike orientation in starry flounder Platichthys stellatus (Pallas) (Pleuronectidae)

Starry flounder Platichthys stellatus , a rare polymorphic flatfish exhibiting a large-scale geographic cline in the frequency of right-eyed (dextral) and left-eyed (sinistral) morphs, was studied to investigate whether foraging behaviour (turning angle and prey strike orientation) differed between dextral and sinistral laboratory-raised juveniles. Platichthys stellatus foraging on brine shrimp Artemia sp. nauplii tended to strike dorsally at prey ('left' to an observer for dextral flounder and 'right' to an observer for sinistral flounder), although this effect was stronger for sinistral fish. This dorsal tendency also increased with body size. Non-strike behaviours (movements between strikes) were ventrally biased for both morphs. Maximum turn angles were larger for both morphs towards the dorsal side than the ventral side during prey strikes but were the same during non-strike behaviours. The positioning of the eyes of the juvenile starry flounder was skewed towards the dorsal midline rather than being symmetrically placed between dorsal and ventral margins on the eyed side of each fish. The migrating eyes of dextral fish, however, were significantly closer to the dorsal midline than in sinistral fish. This, in addition to the more dorsally oriented prey strikes in sinistral fish, suggests that the morphs are not simple behavioural mirror images of one another and therefore may differ ecologically.     

Influence of mass release of hatchery-reared Japanese flounder on the feeding and growth of wild juveniles in a nursery ground in the Japan Sea

The feeding and growth of wild juvenile Japanese flounder Paralichthys olivaceus (Temminck et Shlegel), collected after experimental mass releases of marked hatchery-reared juveniles of the same species, was examined at Wada Beach, Wakasa Bay, the Japan Sea, in 1997 and 1998. The feeding and growth of wild juveniles in the eastern area where the hatchery-reared juveniles were released and recaptured were compared with those in the western area (control area) where no hatchery-reared juveniles were recaptured. The stomach contents of both hatchery-reared and wild juveniles collected in the eastern and western areas consisted mainly of mysids in the early season (late-May to mid-June), and fish larvae in the late season (late-June to early July) in both 1997 and 1998. The stomach content index (SI) of juvenile flounder collected in the eastern and western areas showed that the feeding conditions followed similar patterns. However, the percent of empty stomachs in the wild flounder juveniles collected within 4 days after the mass release in the eastern area was as high as 10-30% in 1997 and 40% in 1998, whereas it was 0% in the western area in both years. Otolith microstructure analysis showed that the growth rate in the eastern area was not significantly different before and after release in both years. These results demonstrate that the mass release of hatchery-reared juvenile flounder temporarily affects the feeding efficiency of the cohabiting wild juveniles, but not to an extent that significantly reduces the growth rate.     

Offshore Habitat Preference of Overwintering Juvenile and Adult Black Sea Bass,Centropristis striata,and the Relationship to Year-Class Success

Black sea bass (Centropristis striata) migrations are believed to play a role in overwinter survival and connectivity between juvenile and adult populations. This study investigated oceanographic drivers of winter habitat choice and regional differences between populations of juvenile and adult black sea bass. Trends in cohort strength, as a result of juvenile survival, were also identified. Oceanographic and fisheries survey data were analyzed using generalized additive models. Among the oceanographic variables investigated, salinity was the main driver in habitat selection with an optimal range of 33–35 practical salinity units (PSU) for both juveniles and adults. Preferred temperature ranges varied between juveniles and adults, but held a similar minimum preference of >8°C. Salinity and temperature ranges also differed by regions north and south of Hudson Canyon. Shelf water volume had less of an effect than temperature or salinity, but showed an overall negative relationship with survey catch. The effect of winter conditions on juvenile abundance was also observed across state and federal survey index trends. A lack of correlation observed among surveys in the fall paired with a strong correlation in the spring identifies the winter period as a factor determining year-class strength of new recruits to the population. A rank order analysis of spring indices identified three of the largest year classes occurring during years with reduced shelf water volumes, warmer winter shelf waters, and a 34 PSU isohaline aligned farther inshore. While greater catches of black sea bass in the northwest Atlantic Ocean remain south of Hudson Canyon, the species’ range has expanded north in recent years.     

Predator-prey interaction between hatchery-reared Japanese flounder juvenile, Paralichthysolivaceus, and sandy shore crab, Matutalunaris: daily rhythms, anti-predator conditioning and starvation

Predator-prey interaction between sandy shore crab, Matuta lunaris (Forskål, 1775), and juvenile Japanese flounder, Paralichthys olivaceus (Temminck et Schlegel), was investigated under controlled laboratory conditions. Possibility of training and conditioning hatchery-reared flounder to avoid predators was also examined. Crabs took over 75% of their daily ration at night when they were given access to prey 24 h a day. Large (64.8±5.4 g)- and medium (30.68±3.33 g)-sized crabs ate ca. 5.5±1.45 and 3.9±1.99 individuals of flounder (TL=4.96±0.23 cm) a day, respectively. When flounder juveniles that have experienced predation pressure by crabs encountered predators again, they exhibited better survival compared to the naive fish. Flounder juveniles were also conditioned either using small and, thus, benign predators, or large crabs over fence. The conditioned fish with either method were better able to avoid capture by crabs than naive fish, revealing that learning process should play an important role in their predator avoidance. Anti-predator performance was also compared between starved and fed flounder juveniles. Fed fish were rarely eaten by predators after 3 h of exposure, whereas starved fish continued to be eaten. Our results suggest that stock-enhancement program of Japanese flounder can be improved by applying proper feeding protocol and conditioning to avoid predators prior to release. Present research supports the idea that behavioural and ecological consideration for the target species is indispensable for the success of stock enhancement.     

Distribution and microhabitat associations of the juveniles of a high-value sea cucumber, <Emphasis Type="Italic">Stichopus</Emphasis> cf. <Emphasis Type="Italic">horrens,</Emphasis> in northern Philippines

There is considerable global interest in rebuilding depleted populations of sea cucumbers (Echinodermata, Holothuroidea) to address conservation and economic goals. For the vast majority of holothurian species, the habitat and food requirements of the juvenile stage are poorly understood. We investigated the distribution and microhabitat associations of juveniles of a commercially important sea cucumber, Stichopus cf. horrens, in the shallow, shoreward side of a coral reef (or backreef) in northern Philippines (16°21′38.7″N, 119°59′47.9″E). Relationships between juvenile density and the physical, chemical and biological characteristics of their habitat were examined. Potential food sources of the juveniles were also investigated using elemental and stable isotope analysis. Results showed that juveniles are more abundant in seagrass areas and the transition zone between seagrass and the rubble-dominated reef flat. A non-metric multidimensional scaling (MDS) plot indicated that juvenile density was most positively associated with coarser sand and rubble (>0.5 mm) and seagrass (Thalassia hemprichii) abundance (2D stress = 0.11). Juvenile density was also positively associated with sediment organic matter from plant detritus to a lesser extent. Elemental and isotope analysis of one site indicated that epiphytes were the primary food source of juveniles, while sediment detritus from microalgae and seagrass was a secondary food source. This study corroborates anecdotal evidence regarding the importance of seagrass to S. cf. horrens as potential refugia and source of high-quality food for its juveniles. These findings underscore the need to protect the nursery habitats of wild juveniles and provide critical information for the selection of suitable natural habitats for releasing cultured juveniles of this important species.     

Post‐release predation on hatchery‐reared Japanese flounder Paralichthys olivaceus in the coast of Fukushima,Japan

To determine the predators of 100 mm total length hatchery‐reared juvenile Japanese flounder Paralichthys olivaceus, fishes and crabs were collected using gillnets and a small trawl net off the coast of Fukushima Prefecture, Japan. Predation on juvenile P. olivaceus by older conspecifics, the snailfish Liparis tanakai, ocellate spot skate Okamejei kenojei and the swimming crab Ovalipes punctatus, was detected based on analogical observation and molecular techniques. These predators are nocturnal feeders except for P. olivaceus. Liparis tanakai with body sizes large enough to consume juveniles only appeared in winter, whereas the large O. punctatus was abundant in early summer and in late autumn. Such seasonal variation in predator abundance indicates that the release season can be optimized for reducing predation mortality.     

The fish assemblage of the intertidal salt marsh creeks in North Bull Island, Dublin Bay: seasonal and tidal changes in composition, distribution and abundance

An intertidal salt marsh fish assemblage inhabiting two creeks on North Bull Island, Dublin Bay was sampled monthly from June 2000 until May 2002. Water temperature and salinity were recorded in situ and samples were also taken for Suspended Particulate Matter (SPM) and Chlorophyll a. All fish caught were weighed and measured and classified into functional guilds. A total of 6,549 individuals comprising 10 fish species from 10 families were recorded within the two creeks. The community was dominated by a few species, a feature common to other estuarine fish populations. Of the 10 species found, the common goby, Pomatoschistus microps, the 3-spined stickleback, Gasterosteus aculeatus, the thick-lipped grey mullet, Chelon labrosus and the flounder, Platichthys flesus contributed 98.4% of all fish sampled. The fish population of the channels at Bull Island, Dublin, was dominated by the resident gobies (true estuarine resident species), but also hosted juveniles of species such as the bass, Dicentrarchus labrax (marine juvenile migrant species). In turn, the nekton populations were dominated by the brown shrimp, Crangon crangon and the fairy shrimp, Palaemonetes varians especially in winter when fewer fish (numbers of species and abundance) were found. Multivariate analysis of fish diversity and abundance revealed a strong seasonal pattern but there was little evidence of difference between creeks, nor of tidal (spring/neap) effects. The estuarine fish using the intertidal marsh creeks have been little studied in Europe yet they play a major role with the decapods in these habitats. This role needs to be quantified for a proper understanding of the system’s function.     

Identifying spawning behavior in Pacific halibut, <Emphasis Type="BoldItalic">Hippoglossus stenolepis</Emphasis>, using electronic tags

Synopsis Identifying spawning behavior in Pacific halibut, Hippoglossus stenolepis, is particularly challenging because they occupy a deep, remote environment during the spawning season. To identify spawning events, a method is needed in which direct observation by humans is not employed. Spawning behavior of seven other flatfish, species has been directly observed in their natural environment by investigators using SCUBA. All of these flatfish species display almost identical spawning behavior that follows a routine. Therefore, it is reasonable to believe that this spawning behavior occurs in other flatfish species, including Pacific halibut. As part of a larger study, we recaptured two Pacific halibut on which Pop-up Archival Transmitting (PAT) tags had been attached during the winter spawning season. Because the tags were physically retrieved, we were able to collect minute-by-minute depth records for 135 and 155 days. We used these depth data to tentatively identify spawning events. On seven separate occasions between 20 January 2001 and 9 February 2001, one fish displayed a conspicuous routine only seen during the spawning season of Pacific halibut and the routine parallels the actions of other spawning flatfish directly observed by humans using SCUBA. Therefore, we propose this routine represents spawning behavior in Pacific halibut. The second tagged fish did not display the conspicuous routine, thus challenging the assumption that Pacific halibut are annual spawners. PAT tags may prove to be a useful tool for identifying spawning events of Pacific halibut, and that knowledge may be used for improved management in the future.     

Distribution and microhabitat use by flatfishes in a Louisiana estuary

We used a 1 m beam trawl to characterize microhabitat use of flatfishes in monthly samples collected in Barataria Bay, Louisiana. Six strata were established along a salinity gradient from the nearshore zone along the Gulf of Mexico to approximately 30 km inland. Randomized sampling within strata characterized flatfish distributions and environmental conditions throughout the bay. Microhabitats were characterized by salinity, temperature, dissolved oxygen, depth, distance from shore, and substrate type. In 594 short-duration, beam-trawl samples collected from October 1992 through September 1994, we identified seven flatfish species from 7046 specimens, most of which were juveniles. Four species accounted for 98.3% of all flatfishes. The two most abundant, offshore tonguefish and bay whiff, were euryhaline and widely distributed over the salinity gradient; however, offshore tonguefish were concentrated in the lowermost stratum where bay whiff were uncommon. The third species, fringed flounder, was more abundant in middle and lower bay strata at higher salinities. The fourth species, blackcheek tonguefish, was most common in middle and upper bay strata and declined in abundance in coastal strata. A rotated factor analysis resolved six environmental variables into three major axes that explained 69 percent of the variance and were characterized as seasonal, depth-distance, and substrate-salinity axes. Separation of species and life history intervals in three-dimensional factor space reflected temporal and spatial segregation. Within the four common flatfishes, 14 of 24 variable comparisons showed clear ontogenetic trends in which at least two size-class means differed significantly. Among the common species, temporal differences were reflected by mean temperatures and dissolved oxygen concentrations and spatial differences were reflected by other environmental variables, including depth, distance, substrate, and salinity. This revised version was published online in July 2006 with corrections to the Cover Date.