Observations on the distribution of meroplankton during a downwelling event and associated intrusion of the Chesapeake Bay estuarine plume

We investigated the dispersal of larvae of benthic invertebratesand tested the hypothesis that larvae behaved as if they werepassive particles. Observations were made off Duck, North Carolina,USA during a period of wind driven downwelling at the coastand an intrusion of estuarine water from the Chesapeake Bay.The plume of estuarine water (salinity < 30 psu) was strongestat the shoreward stations in the more northern transects. Winddriven shoreward surface flow converged at the seaward edgeof the plume and downwelled. Offshore flow was present belowthe thermocline and caused the thermocline to bend downwardand contact the bottom at between 5 and 10 km offshore. In the zooplankton samples, we enumerated 33 taxa of larvae(17 taxa of bivalve veligers, 10 taxa of gastropod veligers,and 6 taxa of polychaete larvae). Using cluster analysis, larvaewere separated into groups with similar patterns of distribution.If larvae were acting as passive particles then we hypothesizedthat: 1) Their distribution should remain tied to a water massand 2) around a convergence or divergence, there should be nochange in larval concentration. The distributions of larvaein Clusters 1, 4, 5, and 6 were consistent with the hypothesisthat they were acting as passive particles. Larvae in Clusters2 and 3, however, did not appear to be acting as passive particles.Larvae in Cluster 2 did not remain tied to a water mass. Theyentered the study area in the estuarine plume waters, but within20 km they were nearly absent from the plume water and werefound seaward of the plume and at greater depth. Larvae in Cluster3 were most abundant in areas of converging currents where theshoreward flowing surface waters downwelled at the plume frontor against the shore. We hypothesized that larvae of organisms which as adults livein the intertidal or shallow subtidal zones would have morenearshore distributions than the larvae of adults that are broadlydistributed across the shelf. We compared the depth of the habitatof the adult bivalves from which the bivalve larvae in the differentclusters were derived. The results were consistent with thehypothesis; larvae with distributions closer to shore tendedto come from adults found at shallower depths or in the intertidalzone.     

Mesoscale advection of Upogebia pusilla larvae through an upwelling filament in the Canaries Coastal Transition Zone (CTZ)

Larval transport has a strong influence on marine populations and ecosystem function. Traditional hypotheses establish that larvae of coastal species are swept offshore during strong upwelling periods producing low recruitment rates; however, recent studies have demonstrated that this hypothesis is not supported by larval distribution data. The present study examines the influence of upwelling filaments on larval advection and wastage. During August 1993, the transport of Upogebia pusilla was analysed in relation to an offshore filament off Northwest Africa. This mesoscale structure was generated by a trapped, quasi-permanent cyclonic eddy located between the Canary Islands and the African shelf and extended 150?km westward into the oligotrophic open ocean waters. The horizontal distribution depicted by U. pusilla larvae was strongly influenced by the oceanographic features of the Canaries Coastal Transition Zone (CTZ). Specifically, the larvae were closely associated with upwelling front and filament position, showing that these neritic larvae can be advected 10–100s of kilometres offshore through the filament. However, the low larval densities observed in the samples suggest that many larvae might remain close to the coast, thus avoiding seaward transport. This implies that filaments probably do not significantly affect the recruitment success of intertidal invertebrates such us U. pusilla in upwelling systems.     

Temporal and spatial patterns in ichthyoplankton assemblages in bay and open coastal environments

The larval fish fauna occurring in temperate bay and shelf waters off Victoria, southern Australia, was found to be diverse, comprising taxa from 52 families. The most abundant groups collected were gobiids, tripterygiids, gobiesocids and clupeids. Fish egg concentrations were highest during spring and summer (September to February). Eggs of the Australian anchovy Engraulis australis occurred mainly during spring (September to November). Total larval fish concentrations were highest during summer (December to February), and were significantly higher at 1 km than 2 and 5 km from shore in offshore samples. Larval concentrations of a number of families, mainly reef‐associated taxa that attach their eggs to hard substrata, were also higher nearer to shore. These larvae are more developed upon hatching than those of pelagic spawners and more capable of avoiding passive drift. Multivariate analyses found that larval taxonomic composition did not vary significantly with distance from shore, but that seasonal and monthly groupings were evident, with different taxa dominating at different times of the year. Larvae of the families Gobiidae and Tripterygiidae occurred in all months, but were less abundant during winter. Spatial differences in the larval fish assemblage between offshore samples and samples taken in the bay were only apparent during summer. This was primarily due to a higher abundance of seagrass‐associated species, such as syngnathids and hemiramphids, utilizing specific habitats in the bay.     

IMPROVING MANAGEMENT OF OVERLAPPING BOTTLENOSE DOLPHIN ECOTYPES THROUGH SPATIAL ANALYSIS AND GENETICS

In the Northwest Atlantic the distribution of coastal bottlenose dolphins ( Tursiops truncatus ) overlaps with that of the offshore ecotype. We hypothesized that the distribution of the two ecotypes could be delineated by depth and/or distance from shore, facilitating their identification during surveys. We obtained 304 skin biopsy samples and identified each as either coastal or offshore using analysis of mitochondrial DNA. We then interpreted the spatial distribution of coastal and offshore forms using spatial analysis. Using a Classification and Regression Tree (CART) analysis, we found a statistically significant break in ecotype distribution at 34 km from shore. In waters beyond 34 km from shore and deeper than 34 m, all bottlenose dolphins were of the offshore ecotype. Within 7.5 km of shore, all 65 samples were of the coastal ecotype. Between these two areas only nine samples were collected, so the genetic composition of bottlenose dolphins in this area remains poorly known. To enhance our understanding of the spatial distribution of the two ecotypes, future research should obtain more biopsy samples in this zone. Nevertheless, our results indicate that a conservative abundance estimate for the coastal ecotype could be generated from surveys of bottlenose dolphins within 7.5 km of shore.     

Reef Odor: A Wake Up Call for Navigation in Reef Fish Larvae

The behavior of reef fish larvae, equipped with a complex toolbox of sensory apparatus, has become a central issue in understanding their transport in the ocean. In this study pelagic reef fish larvae were monitored using an unmanned open-ocean tracking device, the drifting in-situ chamber (DISC), deployed sequentially in oceanic waters and in reef-born odor plumes propagating offshore with the ebb flow. A total of 83 larvae of two taxonomic groups of the families Pomacentridae and Apogonidae were observed in the two water masses around One Tree Island, southern Great Barrier Reef. The study provides the first in-situ evidence that pelagic reef fish larvae discriminate reef odor and respond by changing their swimming speed and direction. It concludes that reef fish larvae smell the presence of coral reefs from several kilometers offshore and this odor is a primary component of their navigational system and activates other directional sensory cues. The two families expressed differences in their response that could be adapted to maintain a position close to the reef. In particular, damselfish larvae embedded in the odor plume detected the location of the reef crest and swam westward and parallel to shore on both sides of the island. This study underlines the critical importance of in situ Lagrangian observations to provide unique information on larval fish behavioral decisions. From an ecological perspective the central role of olfactory signals in marine population connectivity raises concerns about the effects of pollution and acidification of oceans, which can alter chemical cues and olfactory responses.     

Effects of temperature on larval fish swimming performance: the importance of physics to physiology

Temperature influences both the physiology offish larvae and the physics of the flow conditions under which they swim. For small larvae in low Reynolds number (Re) hydrodynamic environments dominated by frictional drag, temperature‐induced changes in the physics of water flow have the greatest effect on swimming performance. For larger larvae, in higher Re environments, temperature‐induced changes in physiology become more important as larvae swim faster and changes in swimming patterns and mechanics occur. Physiological rates at different temperatures have been quantified using Q₁₀s with the assumption that temperature only affected physiological variables. Consequently, Q₁₀s that did not consider temperature‐induced changes in viscosity overestimated the effect of temperature on physiology by 58% and 56% in cold‐water herring and cod larvae respectively. In contrast, in warm‐water Danube bleak larvae, Q₁₀s overestimated temperature‐induced effects on physiology by only 5–7%. This may be because in warm water, temperature‐induced changes affect viscosity to a smaller degree than in cold water. Temperature also affects muscle contractility and efficiency and at high swimming velocities, efficiency decreases more rapidly in cold‐exposed than in warm‐exposed muscle fibres. Further experiments are needed to determine whether temperature acts differently on swimming metabolism in different thermal environments. While hydrodynamic factors appear to be very important to larval fish swimming performance in cold water, they appear to lose importance in warm water where temperature effects on physiology dominate. This may suggest that major differences exist among locomotory capacities of larval fish that inhabit cold, temperate waters compared to those that live in warm tropical waters. It is possible that fish larvae may have developed strategies that affect dispersal and recruitment in different aquatic habitats in order to cope not only with temperature‐induced physiological challenges, but physical challenges as well.     

Ontogeny of behaviour in larvae of marine demersal fishes

The development of behaviours that are relevant to larval dispersal of marine, demersal fishes is poorly understood. This review focuses on recent work that attempts to quantify the development of swimming, orientation, vertical distribution and sensory abilities. These behaviours are developed enough to influence dispersal outcomes during most of the pelagic larval stage. Larvae swim in the ocean at speeds similar to the currents found in many locations and at 3–15 body lengths per second (BL s⁻¹), although, based on laboratory measurements, species from cold environments swim slower than those from warm environments. At least in warm-water species, larvae swim in an inertial hydrodynamic environment for most of their pelagic period. Unfed swimming endurance is >10 km from about 8–10 mm, and reaches more than 50 km before settlement in several species. Larval fishes are efficient swimmers. In most species, a large majority of larvae have orientated swimming in the ocean, but the precision of orientation does not improve with growth. Swimming direction of the larvae frequently changes ontogenetically. Vertical distribution changes ontogenetically in most species, and both ontogenetic ascents and descents are found. Development of schooling is poorly understood, but it may influence speed, orientation and vertical distribution. Sensory abilities (hearing, olfaction, vision) form early, are well developed and are able to detect cues relevant to orientation for most of the pelagic larval stage. All this indicates that the passive portion of the pelagic larval duration will be short, at least in most warm-water species, and that behaviour must be taken into account when considering dispersal, and in particular in dispersal models. Although quantitative information on the ontogeny of some behaviours is available for a relatively small number of species, more research in this field is required, especially on species from colder waters.     

Assessment of an ichthyoplankton net operated at different velocities and durations in the Paraná River

The aim of this work was to evaluate the performance of a conical ichthyoplankton net under the most common range of sampling conditions found in the Paraná River. Three sampling velocities in the range of 0.3–1.4 m seg^?1 and four sample durations (2, 4, 8 and 16 min) were simulated from two different experiments carried out in surface and bottom waters of the main channel during daylight hours. Neither significant differences nor trends were found in larval fish densities in various stages of development under the conditions studied. Results are discussed in the context of the natural ichthyoplankton abundance variability. The results support the hypothesis that in turbid waters larvae are not visually able to detect the net and that in the size range sampled, they either cannot mechanically detect the gear or have not developed enough swimming ability to efficiently evade it. The results of this work may contribute to the interpretation of the abundance data in ichthyoplankton studies in South American rivers with similar ranges of transparency, flow and larvae development.     

Spatiotemporal occurrence and feeding habits of tonguefish,Cynoglossus lighti Norman, 1925, larvae in Ariake Bay,Japan

Spatiotemporal occurrence and feeding habits of tonguefish (Cynoglossus lighti Norman, 1925) larvae were investigated in an offshore area (>5 m in depth) of the inner part of Ariake Bay, Japan. All specimens were symmetric, free‐swimming larvae. Although their seasonal abundance and distribution in the study site varied from year to year, spawning started in June and the larval abundance was high in August and September with a wide distribution in the inner part of the bay. Both present and previous study results strongly suggest that larvae may settle primarily in the estuary and near‐shore areas of Ariake Bay after their wide distribution in the offshore area of the bay during the free‐swimming stage. Larvae showed a clear feeding rhythm in which they fed on prey mainly during the daytime. Larvae fed exclusively on copepods, and identified prey were mostly Paracalanidae (mainly Parvocalanus crassirostris), Microsetella norvegica, and Oithona davisae. Pre‐metamorphosis larvae fed primarily on Paracalanidae and O. davisae, whereas O. davisae formed a smaller proportion of the early‐metamorphosis diet. In early metamorphoses, larvae fed preferentially on Paracalanidae and M. norvegica.     

Seasonal distribution and duration of the planktonic stage of Dover sole, Solea solea, larvae in the Bay of Biscay: an hypothesis

The possibility of deriving a prediction about the effect of seasonal variations in the duration of the planktonic larval phase on the dispersal of larval Dover sole was investigated. During six cruises, from February to May 1992, the distribution of sole larvae was studied along a 100-km transect, from the offshore spawning grounds to the coastal nurseries of the Bay of Biscay (France). Samples ( n = 189) were collected with a suprabenthic sampler, and vertical profiles of water temperature and salinity were recorded simultaneously. Counts of otolith increments of larval stage 4b (onset of metamorphosis) were used to estimate the duration of the planktonic life. Duration of the larval phase decreases by about 15 days (37%) with water temperature increase (between 8° in February and 11.2° C in May). Sole larvae occur from the coastal area to 100 km offshore. Within the same cruise, no difference in the duration of the planktonic life was observed between the larvae caught in the onshore and the offshore area. In spite of seasonal differences in abundance, the extent and the shape of the larval distributions during the period of study suggest that the seasonal variations in the duration of the planktonic life did not affect the larval distribution.     

Across-Shelf Transport of Bivalve Larvae: Can the Interface between a Coastal Current and Inshore Waters Act as an Ecological Barrier to Larval Dispersal?

Using an integrated physical and biological approach, we examined across-shelf advection and exchange and the associated transport of bivalve larvae in the presence of a strong coastal current separated from the coast by a stratified inshore environment. We tested the hypothesis that the interface of the coastal current and inshore waters can act as an ecological barrier to across-shelf transport of larvae but can be overcome by wind- or tidally-induced transport. Our study region in the Gulf of Maine encompasses a coastal current that diverges from the coast as it moves downshelf. The region inshore of this current is home to several species that exhibit limited recruitment in spite of extensive upshelf larval sources. Analysis of surface water temperatures and wind velocities revealed episodic decreases in temperature along the coast correlated with alongshelf (but not upwelling) winds, indicating wind-forced onshore movement of the cold coastal current. Such wind-driven onshore migrations are more common along the northern portion of the study region where the coastal current is near the coast, tidal currents are strong, and wind directions are more conducive to onshore migration, but rarer further south where the interface between inshore waters and the coastal current is further offshore and suitable wind events are less common. The distribution of bivalve larvae was consistent with the physical measurements. There was little across-shelf variation in larval abundance where the current abuts the coast, indicating strong across-shelf exchange of larvae, but strong across-shelf variation in larval density where the stratified inshore waters separate the current from the coast, indicating weak across-shelf transport of larvae. Our results suggest that the interface between the coastal current and inshore waters may constitute a major ecological barrier to larval dispersal in the southern part of the region that may only be overcome by rare, strong wind-forced events.     

Orientation Cues Used by Hatchling Loggerhead Sea Turtles (Caretta caretta L.) During their Offshore Migration

Sea turtle hatchlings ernerge from underground nests, crawl to the ocean and swim out to sea. The orientation cues used to maintain offshore headings are unknown. Our field experiments suggest loggerhead hatchlings on the Atlantic coast of Florida continue on offshore headings by swimming into oceanic swells and wind-generated waves. Dependence on these cues appears complete when hatchlings are 3.0 km from shore. Because waves and swells usually approach from seaward directions, they are generally reliable guideposts.     

Good eaters, poor swimmers: compromises in larval form

Compromises between swimming and feeding affect larval formand behavior. Two hypotheses, with supporting examples, illustratethese feeding-swimming trade-offs. (1) Extension of ciliatedbands into long loops increases maximum clearance rates in feedingbut can decrease stability of swimming in shear flows. A hydromechanicalmodel of swimming by ciliated bands on arms indicates that morphologieswith high performance in swimming speed and weight-carryingability in still water differ from morphologies conferring highstability to external disturbances such as shear flows. Instabilityincludes movement across flow lines from upwelling to downwellingwater in vertical shear. Thus a hypothesis for the high armelevation angles of sea urchin larvae, which reduce speed instill water, is that they reduce a downward bias imposed bythe vertical shear in turbulence. Observations of sea urchinlarvae in vertical shear and comparisons among brittle starlarvae are consistent with the performance trade-offs predictedby the model. (2) Structures and behaviors that reduce swimmingspeed can enhance filtering for feeding. In the opposed-bandfeeding mechanisms of veligers and many trochophores, ciliapush water to swim but movement of cilia relative to the wateroccurs when cilia overtake and capture particles. Features thatmay increase clearance rates at the expense of speed and weightcapacity include structures that increase drag or body weightand a ciliary band that beats in opposition to the feeding-swimmingcurrent. Larval feeding mechanisms inherited from distant ancestorsresult in different swimming-feeding trade-offs. The differenttrade-offs further diversify larval form and behavior.     

Topographically generated fronts, very nearshore oceanography and the distribution and settlement of mussel larvae and barnacle cyprids

Flow patterns adjacent to shore may prevent or aid shorewardmigration of benthic invertebrate larvae. We hypothesized thata front at the mouth of Sunset Bay, Oregon, prevents shorewarddispersal of larvae, significantly altering settlement of mussellarvae and barnacle cyprids. Settlement was measured at threesets of moorings (three moorings per site) distributed acrossthe front at Sunset Bay. From 6 July to 4 September 2000, sampleswere collected roughly every other day. Concurrently, we madevertical zooplankton tows adjacent to each mooring site andcollected physical oceanographic data. During upwelling-favorablewinds, the front was always present at the bay mouth, separatingsignificantly cooler, saltier and denser offshore water fromthat within the bay. During downwelling winds, the front brokedown and we found no significant difference in the surface physicaloceanographic parameters across the bay mouth. During upwelling,the concentration of mussel larvae was higher seaward of thefront than landward, but there was no significant differencein concentration during downwelling, suggesting that the frontmay act as a barrier to the shoreward dispersal of mussels.Mussel settlement was too low and sporadic to allow statisticalanalysis. There was no difference in cyprid concentrations acrossthe bay mouth whether the front was present or not. Cyprid settlementwas, however, nearly an order of magnitude lower at mooringsseaward of the front than at those landward. A significant cross-correlationwas found between settlement at the offshore mooring and tidalrange (r = 0.464, lag = 0 days) and between settlement at themid and inner moorings and downwelling winds (r = 0.532 midbay, r = 0.532 inner bay, lag = 0 days). Seaward of the front,settlement varied with tidal range, while landward of the front,most settlement occurred as brief pulses during downwellingwinds, periods when the front was not present. We found largedifferences in the distribution of cyprids, and mussel larvaeand cyprid settlement relative to the front; larval distributionsand settlement varied with upwelling versus downwelling windsand was due to differences in the very nearshore (i.e. within100–1000 m of shore) coastal oceanography.     

Distribution and drift of the crab Carcinus maenas (L.) (Decapoda, Portunidae) larvae over the continental shelf off northern Portugal in April 1991

The coastal distribution and abundance of Carcinus maenas larvalstages were studied from plankton samples collected in a gridof 79 sampling station organized into six cross-shore transectsextending from the coast to –170 km offshore, betweenthe Mondego and Ave rivers, on the northwest coast of Portugal.The samples were collected in April 1991 with a modified Bémultinet plankton sampler, which was towed from a maximum depthof 200 m and provided a vertical resolution of up to five standardstrata at each station. Current and wind data were availablefor a period that extended beyond the period covered by theobservations All the species' larval stages were found in thesamples, but their distribution was confined to the inner andmiddle shelf stations Vertically, 88% of the larvae occurredin the top 30 m and another 11% in the 30–60 m stratum.The zonal stages I and II were concentrated (–90%) inthe surface layer, but a gradual ontogenic displacement to deeperwaters was observed from then on, the megalopa being equallydistributed between the 0–30 and the 30–60 m strata.Horizontally, there was a dear association of the first zoeawith the estuarine inlet while the older meal stages were dispersedprogressively offshore. Evidence was found that the megalopaexperienced an onshore transport that did not affect the previousstages.This transport is wnsistent with the observation of anonshore flow component at 40 m. It was not possible, however,to examine the hypothesis that this flow conveys the last larvalstage to the coast, but not the previous ones. The dependenceof the along-shore flow component on wind stress lends supportto the hypothesis that the larvae are advected from the northas the upwelling season progresses.     

Why no red tide was observed on the West Florida Continental Shelf in 2010

Over the past two decades, the two most anomalous years for water properties on the west Florida continental shelf were 1998 and 2010. In both instances, the shelf was ventilated by relatively cold, nutrient-rich waters of deep ocean origin, which reset the background state underlying shelf ecology. The ventilation in both of these years derived from prolonged interactions of the Gulf of Mexico Loop Current with the shelf slope near the Dry Tortugas located on the southwest corner of the shelf. By contacting relatively shallow isobaths, the boundary current interactions there set the entire shelf into motion, facilitating upwelling across the shelf break, even to DeSoto Canyon some 500 km away, and then across the shelf to the near shore. Such prolonged and intense upwelling of nutrient-rich water in 2010 contrasted with the more typically occurring locally wind driven upwelling conditions, whereby waters upwelled at the near shore are from the inner shelf, versus the deep ocean. Thus not all upwelling scenarios have similar consequences. Whereas the typical wind driven upwelling scenario is necessary for Karenia brevis red tide blooms to manifest along the coastline, the rarer, deep ocean induced upwelling scenario (as occurred in 1998 and particularly in 2010) acts to suppress K. brevis red tides because of the elevated inorganic nutrient conditions that they facilitate. Hence, minimal cell counts above background were observed in 1998, and no cell counts above background were observed in 2010. We conclude that the lack of red tide along the west coast of Florida in 2010 was due to anomalously large and protracted upwelling of nutrient-rich waters of deep ocean origin caused by Loop Current and eddy interactions with the shelf slope.     

Close encounters with eddies: oceanographic features increase growth of larval reef fishes during their journey to the reef

Like most benthic marine organisms, coral reef fishes produce larvae that traverse open ocean waters before settling and metamorphosing into juveniles. Where larvae are transported and how they survive is a central question in marine and fisheries ecology. While there is increasing success in modelling potential larval trajectories, our knowledge of the physical and biological processes contributing to larval survivorship during dispersal remains relatively poor. Mesoscale eddies (MEs) are ubiquitous throughout the world''s oceans and their propagation is often accompanied by upwelling and increased productivity. Enhanced production suggests that eddies may serve as important habitat for the larval stages of marine organisms, yet there is a lack of empirical data on the growth rates of larvae associated with these eddies. During three cruises in the Straits of Florida, we sampled larval fishes inside and outside five cyclonic MEs. Otolith microstructure analysis revealed that four of five species of reef fish examined had consistently faster growth inside these eddies. Because increased larval growth often leads to higher survivorship, larvae that encounter MEs during transit are more likely to contribute to reef populations. Successful dispersal in oligotrophic waters may rely on larval encounter with such oceanographic features.     

Occurrence of fecal indicator bacteria in surface waters and the subsurface aquifer in Key Largo, Florida.

Sewage waste disposal facilities in the Florida Keys include septic tanks and individual package plants in place of municipal collection facilities in most locations. In Key Largo, both facilities discharge into the extremely porous Key Largo limestone. To determine whether there was potential contamination of the subsurface aquifer and nearby coastal surface waters by such waste disposal practices, we examined the presence of microbial indicators commonly found in sewage (fecal coliforms, Clostridium perfringens, and enterococci) and aquatic microbial parameters (viral direct counts, bacterial direct counts, chlorophyll a, and marine vibriophage) in injection well effluent, monitoring wells that followed a transect from onshore to offshore, and surface waters above these wells in two separate locations in Key Largo in August 1993 and March 1994. Effluent and waters from onshore shallow monitoring wells (1.8- to 3.7-m depth) contained two or all three of the fecal indicators in all three samples taken, whereas deeper wells (10.7- to 12.2-m depth) at these same sites contained few or none. The presence of fecal indicators was found in two of five nearshore wells (i.e., those that were < or = 1.8 miles [< or = 2.9 km] from shore), whereas offshore wells (> or = 2.1 to 5.7 miles [< or = 3.4 to 9.2 km] from shore) showed little sign of contamination. Indicators were also found in surface waters in a canal in Key Largo and in offshore surface waters in March but not in August. Collectively, these results suggest that fecal contamination of the shallow onshore aquifer, parts of the nearshore aquifer, and certain surface waters has occurred.(ABSTRACT TRUNCATED AT 250 WORDS)     

Seasonal variation in horizontal and vertical structure of larval fish assemblages off south-western Australia, with implications for larval transport

The coastal ocean off south-western Australia is characterisedby the southward-flowing Leeuwin Current, which suppresses theupwelling typically associated with other eastern boundary currentsin the southern hemisphere. This results in a unique environmentfor the transport and survival of planktonic fish larvae. Thehorizontal and vertical structure of larval fish assemblagesoff south-western Australia was investigated during winter (August2003) and summer (January 2004), and related to these unusualregional oceanographic and biological processes. Larval fishwere sampled along a four station transect running from theinner continental shelf to offshore waters, using depth-integratedbongo net tows and depth-stratified EZ net tows. The distributionof taxa across the shelf and offshore was strongly influencedby the current regime at the time of sampling. Larval fish assemblagestructure reflected the distinctive oceanographic conditionsfound during each season, and vertical depth distributions oflarvae affected their horizontal location. Continental shelfsamples were dominated by larvae of pelagic fishes, such asclupeiform species (e.g. Sardinops sagax), whereas offshoreassemblages were characterised by larvae of oceanic families,such as Myctophidae and Phosichthyidae. The winter cruise (August2003) was completed during a time of strong, southerly LeeuwinCurrent flow, whereas the northward-flowing Capes Current, incombination with surface offshore Ekman transport, predominatedduring summer. The vertical depth preferences of larvae wereparticularly influential in affecting their horizontal position;especially so for surface-dwelling larval fishes found duringsummer. This study represents the first documentation of thevertical structure of ichthyoplankton assemblages in the oligotrophicwaters off south-western Australia.     

Ontogenic migration of Pleuronectes flesus larvae in the eastern English Channel

Along the French coast of the eastern English Channel there is a strong separation in the hydrology, zooplankton community and ichthyoplankton assemblages, between a coastal ecosystem of continental influence and an offshore ecosystem of Atlantic influence. During April and May 1995, two surveys (60 stations) were conducted to describe the early life history of fishes in this area, and especially the influence of hydrodynamics on Pleuronectes flesus larval transfer from the spawning grounds to the nurseries. Ontogenic changes in larval distributions were described by variograms and path analysis. P. flesus eggs and young larvae were found in the southern offshore area. Their distributions were mainly influenced by sediment and cold temperature found in the central part of the Channel. Older larvae were found at more northern stations, showing a clumped distribution near estuaries, in areas of low salinity. Apparently, young stages of this species were transported from the south central spawning area to coastal waters by residual currents. Behavioural changes in older larvae facilitate the move to nurseries along the French coast.