Ichthyoplankton Assemblages Research Articles

Larval fish distribution in two different hydrographic situations in the Gulf of Cádiz

Ichthyoplankton assemblages and individual larval distributions of several mesopelagic and shelf-associated species were studied in two consecutive spring surveys in the Gulf of Cádiz (2001), covering both coastal and oceanic domains. The two surveys were characterised by markedly different hydrographical conditions, partly attributable to a shift from westerly to easterly winds. At the scale studied, larvae of coastal and shelf-dwelling species were always clearly separated from mesopelagic species, probably as a result of well-defined coastal-oceanic habitats related to a wide shelf and the prevailing currents. However, underlying these associations, short-term hydrographic changes induced by wind forcing seemed able to severely affect the distribution patterns of the early stages of some shelf-associated species. In particular, species like the European anchovy Engraulis encrasicolus were transported off the usual areas of maximal concentrations following a strengthened northwestern coastal countercurrent.

Zooplankton and ichthyoplankton communities in a temperate estuary: spatial and temporal patterns

Zooplankton and ichthyoplankton assemblages were studied from January 2003 to June 2004 in a temperate shallow estuary (Mondego estuary, Portugal). Monthly sampling was performed at five stations at high and low tides, with subsurface tows with 335 and 500 µm mesh Bongo nets. Analysis of variance (ANOVA) showed a significant effect (P < 0.05) of the mesh size of the net on the abundance of main zooplankton groups. On average, the abundance of the 500 µm taxocenosis was 67 and 102 times lower than the 335 µm taxocenosis at high and low tides respectively, especially in the south arm. More than 80 species were identified in the zooplankton samples. The upper reaches of northern arm are dominated by freshwater crustacean mesozooplankton like Acanthocyclops robustus and Diaptomus spp. and the cladocerans Daphnia, Ceriodaphnia and Bosmina, often being codominant. In the southern arm, the resident estuarine copepod Acartia tonsa was dominant, eventually being the most abundant taxon. Marine reaches of estuary are usually dominated by the marine calanoid copepods Acartia clausi and Temora longicornis and the siphonophores Muggiaea atlantica. Concerning the ichthyoplankton, this was dominated by the larvae of estuarine resident species, mainly Pomatoschistus sp., and eggs of Engraulis encrasicolus. The abundance of Pomatoschistus sp. larvae was positively correlated with water temperature. Statistical analysis (canonical correspondence analysis) used to determine the spatiotemporal structure of the zooplankton assemblages and its correlation with environmental variables showed that salinity and temperature were the main factors influencing the distribution of zooplankton. The results obtained also showed that abundance was strongly influenced by the hydrological circulation pattern and direct or indirect human impacts that occur in each arm of the estuary.

Spatial and temporal patterns in summer ichthyoplankton assemblages on the eastern Bering Sea shelf 1996–2000

AbstractLarval and early juvenile fishes were sampled from the eastern Bering Sea (EBS) shelf during summer from 1996 to 2000. Data from these collections were used to examine spatial and temporal patterns in species assemblage structure and abundance. Cluster analyses based on Bray–Curtis dissimilarity coefficients were used to group species and stations according to similar abundance and species composition. Ordination techniques were used to verify groupings, and a non‐parametric stepwise procedure using a Spearman correlation coefficient (BIO‐ENV) was used to relate groupings to predominant environmental variables. These approaches revealed a pattern of station groupings that were generally related to bathymetry in 1996, 1997, 1999, and 2000, although no obvious relationship to geographic boundaries was observed in 1998. Significant differences in species associations were observed in 1997 and 1998, and depressions in abundance were also noted among many species between 1997 and 1999. A regional, full primitive equation model was used to simulate float trajectories on the EBS shelf in each year to better relate fish distributional observations to prevailing current patterns. Model results indicated general variations in flow in several years, although 1998 stood out with stronger northeast flow than in any of the other years examined. Observed disruptions of larval and early juvenile fish assemblages could be related to the strong El‐Niño event of 1997–98 in the EBS. If this idea is confirmed, our study suggests that larval and juvenile fish are sensitive and respond relatively quickly (1–2 yr) to environmental perturbations, and as such, may be timely indicators of environmental change.

Ichthyoplankton abundance, distribution, and assemblage structure in the Gulf of Alaska during September 2000 and 2001

Autumn ichthyoplankton assemblage structure in the Gulf of Alaska (GOA) region has not previously been characterized. Ichthyoplankton data from September 2000 and 2001 survey collections were analyzed to describe assemblages in the western GOA, to examine interannual variation in assemblages, and to relate observations to oceanographic conditions. Taxa with the highest frequency of occurrence (2000/2001) included Osmeridae (94/87%), Hexagrammos lagocephalus (57/45%), Sebastes spp. (35/47%) and Bathymaster spp. (27/57%). Through the use of clustering techniques and ordination, three identifiable larval assemblages common to both years were identified. Slope assemblages appeared well defined in both years, while nearshore and mid-shelf assemblages tended to be more loosely associated and variable between years. Analyses of environmental variables (temperature, salinity, water depth, distance from shore, and water transport) suggest ichthyoplankton assemblage structure is affected primarily by bathymetry and circulation. We demonstrate that early autumn ichthyoplankton assemblage structure in the GOA is fundamentally linked to variations in macro- (slope vs. shelf) and meso-scale topography (on-shelf proximity to land masses), and suggest that these intrinsic associations may be modified by spatial and temporal variations in local hydrographic conditions.

Spatial patterns of ichthyoplankton assemblages in the Rı́o de la Plata Estuary (Argentina–Uruguay)

Two oceanographic surveys were conducted across the salinity gradient of the Rı́o de la Plata Estuary during austral late spring and summer. Ichthyoplankton was dominated by families Sciaenidae, Clupeidae, Engraulidae, Cynoglossidae and Carangidae. The distribution of larval fish assemblages was strongly associated to the salinity structure. Oceanographic conditions were characterized by a large salt wedge (180 km long and a vertical salinity gradient up to 10 units per meter). During both sampling periods each ichthyoplankton assemblage was related to the same region of the salt wedge: the fresh water environment, the bottom salinity front, the mixohaline zone and the outer portion (surface salinity front) of the estuary. Larval assemblages were not related to the thermal structure. Reproductive activity of fishes inside the Rı́o de la Plata Estuary is relatively common (60% of the teleosts sampled use this estuary as a nursery ground). Moreover, the high incidence of pelagic eggs spawners (75% of the species present in the ichthyoplankton) in the Rı́o de la Plata contrasts to the vast majority of estuaries, where this reproductive strategy is poorly represented. Estuarine dynamics plays a dominant role in allowing this reproductive style in this estuary. Spring–summer wind pattern should generate a retentive environment which could minimize advective losses of eggs and larvae to the adjacent ocean.

Spatial patterns of ichthyoplankton assemblages in the R�o de la Plata Estuary (Argentina?Uruguay)

Two oceanographic surveys were conducted across the salinity gradient of the Rı́o de la Plata Estuary during austral late spring and summer. Ichthyoplankton was dominated by families Sciaenidae, Clupeidae, Engraulidae, Cynoglossidae and Carangidae. The distribution of larval fish assemblages was strongly associated to the salinity structure. Oceanographic conditions were characterized by a large salt wedge (180 km long and a vertical salinity gradient up to 10 units per meter). During both sampling periods each ichthyoplankton assemblage was related to the same region of the salt wedge: the fresh water environment, the bottom salinity front, the mixohaline zone and the outer portion (surface salinity front) of the estuary. Larval assemblages were not related to the thermal structure. Reproductive activity of fishes inside the Rı́o de la Plata Estuary is relatively common (60% of the teleosts sampled use this estuary as a nursery ground). Moreover, the high incidence of pelagic eggs spawners (75% of the species present in the ichthyoplankton) in the Rı́o de la Plata contrasts to the vast majority of estuaries, where this reproductive strategy is poorly represented. Estuarine dynamics plays a dominant role in allowing this reproductive style in this estuary. Spring–summer wind pattern should generate a retentive environment which could minimize advective losses of eggs and larvae to the adjacent ocean.

Cross-shelf and vertical structure of ichthyoplankton assemblages in continental shelf waters of the Southern Gulf of Mexico

The horizontal and vertical variation of ichthyoplankton assemblages in continental shelf waters of the Southern Gulf of Mexico was studied during an annual cycle (1994–1995) at 22 stations distributed across four transects located off the main fluvio-lagoon systems of the area. Samples were collected from five levels of the water column, from the surface to a depth of 105 m. The Bray–Curtis Dissimilarity Index identified five assemblages: Coastal, Inner Neritic, Outer Neritic, Oceanic and a Transition Group. The main species of each assemblage reflected the habitat, season, and spawning area of adult organisms. The Coastal Assemblage occupied the uppermost water column levels of the most nearshore stations. The Oceanic Assemblage was restricted to the deepest samplings of the stations of the outer shelf and the Neritic Assemblages were located on the continental shelf between the Coastal and Oceanic Assemblages, embracing all the sampling levels in some periods. Variation in composition, location and abundance of larvae in each assemblage was determined by a series of physical processes that act at different temporal and spatial scales, especially the mixing process generated by the wind stress and continental water discharges acting at a mesoscale that influences the distribution of the assemblages along the whole continental shelf as well as a fine scale affecting the composition and vertical distribution of organisms in a more reduced area of just tens of meters in the water column. Canonical Correspondence Analysis revealed that the depth of the sample, the potential energy, and the temperature generated the greatest variability in the system. Vertical and horizontal salinity gradients play a determinant role in delimiting the Coastal and Neritic Assemblages.

Diversity of ichthyoplankton in the central Mexican Pacific: a seasonal survey

The seasonal variation in the diversity of ichthyoplankton was analyzed from samples collected monthly during a year cycle off the central Pacific coast of Mexico. Samples were collected using a Bongo net at 12 stations during 11 months, from December 1995 through December 1996. The most dominant species was Bregmaceros bathymaster, its relative density varied between 77 and 98%. Two main seasonal patterns were revealed after analyzing the dynamics of the coastal water mass and the ichthyoplankton assemblage ordination in relation to diversity. Diversity values (jack-knifed H=0.24) were relatively low from January to May, and indicated a first seasonal pattern, related to a period dominated by the influence of the California Current, with surface temperatures ranging between 21 and 24 °C. The second pattern was featured by diversity values five-fold higher than in the winter-spring ( H=0.92). These were related to the influence of warm, tropical waters from the Equatorial Countercurrent, with associated temperatures ranging between 26 and 29.2 °C. A transitional period was identified in June and December; it represented a mixed assemblage and yielded the highest richness. Diversity null models were useful as tools to confirm the major seasonal patterns of the surveyed ichthyoplankton community.

Spawning patterns of four species of predominantly temperate pelagic fishes in the sub-tropical waters of southern Queensland

The diverse pelagic fish assemblage of sub-tropical southern Queensland includes fishes with predominantly temperate distributions, such as tailor Pomatomus saltatrix, sardine Sardinops sagax, round herring Etrumeus teres, and Australian anchovy Engraulis australis. The peak spawning seasons of P. saltatrix, S. sagax and E. teres occur during late winter and early spring (June–October). Eggs and larvae of these three species are widely distributed in shelf waters and comprise >50% of the ichthyoplankton assemblage during this period. Mean monthly sea surface temperatures (SSTs) during late winter and early spring range from 21 to 23 °C, and are thus similar to those recorded in southern Australia during summer and autumn, which is the spawning season of these three species in those temperate waters. E. australis eggs occur mainly in inshore waters, and comprise >50% of fish eggs collected during summer and autumn when mean monthly SSTs in southern Queensland exceed 27 °C. E. australis also spawns mainly during summer and autumn in temperate Australia. Hence, water temperature may be less important as a determinant of the spawning season of E. australis than it is for the other three species. The suitability of southern Queensland for spawning by predominantly temperate species during late winter and early spring may contribute to the high diversity of the region's pelagic fish assemblage. Adult P. saltatrix, S. sagax and E. teres appear to migrate northwards into southern Queensland during early winter to spawn, and larvae may be transported southwards into temperate waters by the East Australian Current. This dispersal-migration pattern is similar to those observed for several species, including P. saltatrix, in the western boundary current systems off the east coasts of North America and Africa. Hence, pelagic fishes in ecosystems off the east coast of three continents migrate into sub-tropical waters to spawn, and larvae are transported back into temperate nursery areas by the prevailing current.

독도 주변해역에서 봄과 가을철 어란과 자치어의 종조성에 관한 연구

A study on the ichthyoplankton assemblage was conducted around Dokdo in the East Sea in September 1999 and May 2000. Sixteen species of fish larvae including eggs were collected during the surveys. Anchovy (Engraulis japonicus) eggs and larvae were comprised of 2.0% and 94.5% of total samples in number, respectively. Muller's pearlside (Maurolicus muelleri) eggs and larvae were comprised of 95.0% and 3.2%, respectively. The other species mostly composed of warm water species were less than 2% of total samples in number. The number of species and abundance of the fish lawae and eggs were higher in September than in May. Although there were limited data on the ichthyoplankton around Dokdo in summer and winter, this study area seems to be used for the spawning or nursery ground of fishes.

Larval fish assemblages in waters off the central Pacific coast of Mexico

Spatial and seasonal variation of the ichthyoplankton assemblages was analysed using monthly samples collected during a 1 year cycle off the central Pacific coast of Mexico. Zooplankton samples were collected at 12 stations during 11 months, from December 1995 to December 1996. The analysis of coastal oceanographic conditions revealed two main seasonal patterns, one related to the California Current influence and the other a tropical one. A temporal recomposition of the ichthyoplankton assemblages was found to be related to each oceanographic pattern. During the California Current period (January–May) the larval fish assemblage was respresented mainly by Auxis sp., Bentosema panamense, Cetengraulis mysticetus, Opistonema sp., Symphurus elongatus, Gobionellus sp., Dormitator latifrons and Bregmaceros bathymaster. The tropical season assemblage (July, October and November) was represented by Harengula thrissina, Euthynnus lineatus, Vinciguerria lucetia, Syacium ovale, and representatives of the Eleotridae and Pomacentridae. An inshore–offshore gradient was observed during the tropical oceanographic periods, with a thermic stratification of the water column over the shelf. Conversely, during the California Current period, with coastal advective processes, the assemblages of fish larvae were more homogeneous. Although a mixed, richer faunistic composition was expected in this transitional area, the overall biogeographic affinity of the surveyed community remained stable even in contrasting conditions.

Use of ichthyoplankton ecology to evaluate ecosystem changes: a case study in a large, semi-enclosed Australian bay

Intensive night sampling was conducted fortnightly in 1995/96 to investigate the ichthyoplankton assemblage of Port Phillip Bay. Results are compared with those from a similar survey in 1983/84, and are used to ascertain whether major changes have occurred in the composition and abundance of fish eggs and larvae, and whether these are related to ecosystem changes in the bay between 1969 and 1995. The 17 157 larvae caught during this study belonged to 60 taxa from 32 teleost fish families, with Gobiidae (54.2%), Engraulidae (16.7%), Clinidae (10.0%) and Odacidae (5.9%) dominating the catches. Larval concentrations peaked only in summer, in contrast to summer and winter in 1983/84. Larvae from 13 families recorded in 1995/96 did not occur in 1983/84, including Gobiesocidae (9 spp.) and Odacidae (3 spp.). Larvae absent in 1995/96 but present in 1983/84 included Acanthopegasus lancifer (Pegasidae) and taxa from another seven families. Neoodax balteatus larvae ranked fourth in 1995/96 but were absent in 1983/84, while Engraulis australis eggs and Gymnapistes marmoratuslarvae were comparatively fewer in 1995/96. We suggest that some of the main differences between the two surveys may be attributable to major ecosystem changes in the bay, particularly the introduction and establishment of exotic marine species.

Spatial Patterns of Ichthyoplankton Assemblages Along the Eastern English Channel French Coast during Spring 1995

Two surveys were conducted during April and May 1995 along the French coast of the eastern English Channel. They aimed to describe the influence of environmental features—water advection through the Straits of Dover and the hydrological coastal front—on ichthyoplankton assemblages.Multivariate analysis, using the non-metric multi-dimensional scaling and based on the Bray-Curtis dissimilarity index, was associated with multiple regression to characterize the spatial larval distribution and to relate it to the environmental factors. During the April survey only young larvae were found. They were separated into a coastal larvae assemblage, characterized by high water temperature and fluorescence of chlorophyll a and two offshore assemblages limited to high salinity water. During the May survey, young larvae were mainly located in offshore waters, in the south of the Picarde Bay and in the Straits, where high offshore chlorophyll a fluorescence was found. In contrast old larvae were found along the coast, near estuaries. This pattern shows a spatial segregation of developmental stages of larvae in this area due to adult spawning migration, larval drift and ontogentic migration. Larval distributions can be explained by salinity, temperature and concentration of chlorophyll a, but the spatial evolution of those parameters during spring have also to be taken into account for the comparison of the distributions between both surveys.

Cross-frontal transport and distribution of ichthyoplankton associated with Chesapeake Bay plume dynamics

Cross-frontal transport associated with upwelling conditions was responsible for modifying ichthyoplankton distributions across the inner continental shelf waters off the Chesapeake Bay during late August 1988. Two ichthyoplankton assemblages characteristic of the Chesapeake Bay plume and inner-continental shelf waters were defined using multivariate analysis. Members of the plume assemblage ( Anchoa spp., Menticirrhus spp., and Micropogonias undulatus) were not retained within the Chesapeake Bay plume, but were instead advected 60 km onto the shelf within a low-salinity water mass. A second assemblage, dominated by several shelf-spawned taxa including Etropus microstomus, Prionotus spp. and Centropristis striata was distributed across the shelf. Atlantic croaker, M. undulatus, previously thought to be shelf-spawned exhibited a length-frequency distribution that increased from inshore to offshore, and paralleled that of Anchoa spp. This cross-shelf distribution, and the abundance of small (<3.5 mm), pre-flexion larvae inshore suggests that M. undulatus spawned near the plume front and were subsequently transported offshore with the plume assemblage. A mechanism for rapid cross-frontal transport is described. These findings suggest that the transport and recruitment patterns previously described for this taxon in Chesapeake Bay need to be re-examined. Larval survival and recruitment success of shelf-spawned estuarine species, like M. undulatus, are likely tied to oceanographic conditions on the inner shelf related to upwelling and downwelling conditions and plume dynamics, rather than to simple, wind-driven recruitment mechanisms.

Displacement of diverse ichthyoplankton assemblages by a coastal upwelling event on the Sydney shelf

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 176:49-62 (1999) - doi:10.3354/meps176049 Displacement of diverse ichthyoplankton assemblages by a coastal upwelling event on the Sydney shelf Kimberley A. Smith*, Iain M. Suthers School of Biological Science, University of New South Wales, Sydney, New South Wales 2052, Australia *E-mail: marsci@unsw.edu.au ABSTRACT: The influences of upwelling-favourable winds and thermocline displacement on the distribution of temperate/tropical ichthyoplankton assemblages were determined from vertically stratified plankton hauls across the Sydney shelf, southeastern Australia. Five stations were sampled along a shore-normal transect off Sydney, Australia, on 3 or 4 consecutive nights during January and April 1994. High taxonomic diversity (111 ichthyoplankton families) was attributed to the convergence of temperate and tropical waters in this region. Total larval abundance was higher in January than in April, reflecting the spawning times of many coastal taxa. In both months, highest larval density and taxonomic diversity were associated with the interface between the mixed layer and the thermocline. In April, a period of upwelling-favourable winds resulted in the offshore displacement of the nearshore mixed layer, as well as numerous shelf-spawned larvae (e.g. Centroberyx affinis, Chromis hypsilepsis, Arripis trutta) from the nearshore region, and coincided with the injection of deeply distributed mesopelagic larvae into the nearshore zone (e.g. Scopelosaurus sp., Melamphaes sp.). Shelf-spawned larvae were displaced to the outer shelf, where they were still associated with the mixed layer/thermocline interface. Dynamic ichthyoplankton distributions are interpretable, at least at a scale of weeks, by considering larvae as tracers of the local hydrography. KEY WORDS: Ichthyoplankton assemblages · Hydrography · Upwelling · Thermocline Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 176. Publication date: January 18, 1999 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1999 Inter-Research.

Short term variability in larval fish assemblages of the Sydney shelf:tracers of hydrographic variability

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 178:1-15 (1999) - doi:10.3354/meps178001 Short term variability in larval fish assemblages of the Sydney shelf: tracers of hydrographic variability Kimberley A. Smith1,*, Mark T. Gibbs2,3, Jason H. Middleton2, Iain M. Suthers1 1School of Biological Science, University of New South Wales, Sydney, New South Wales 2052, Australia 2School of Mathematics, University of New South Wales, Sydney, New South Wales 2052, Australia 3Department of Marine Science, University of Otago, PO Box 56, Dunedin, New Zealand *E-mail: marsci@unsw.edu.au ABSTRACT: The among- and within-night changes in the vertical and horizontal distribution of ichthyoplankton assemblages across the Sydney continental shelf, SE Australia, were examined in relation to concurrent changes in hydrography. Of the environmental factors examined, spatial position (sampling depth, distance from shore), water temperature and sampling depth relative to the base of the mixed layer gave the best explanation for variability in assemblage distributions. There was good agreement between observed changes in ichthyoplankton distributions and among-night variability in the cross-shelf temperature and current fields. Ichthyoplankton assemblages functioned as short term, biological tracers of hydrographic variability. Our observations suggest several cross-shelf advective processes, including a recirculation cell over the shelf, could retain passively drifting ichthyoplankton within the shelf region and also minimise longshore drift. The multivariate analysis used was an effective method of examining patterns within and between large, highly variable biological and environmental data sets. KEY WORDS: Ichthyoplankton · Hydrography · Biological tracer · Larval retention · Multivariate Full text in pdf format NextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 178. Publication date: March 17, 1999 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1999 Inter-Research.

Evaluation of the performance of light traps for sampling fish larvae in inshore temperate waters

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 186:293-302 (1999) - doi:10.3354/meps186293 Evaluation of the performance of light traps for sampling fish larvae in inshore temperate waters Michael J. H. Hickford*, David R. Schiel Marine Ecology Research Group, Department of Zoology, University of Canterbury, Private Bag 4800, Christchurch, New Zealand *E-mail: mike@zool.canterbury.ac.nz ABSTRACT: We compared 2 methods of collecting fish larvae in inshore temperate waters near the Kaikoura Peninsula, New Zealand: night collection with light traps and with a plankton net. The sampling design incorporated seasons (summer and autumn), moon phases (full and new) and habitats (reef and beach). The 2 methods were simultaneously deployed over 2 nights in replicates of 3 within each factor. The resulting 96 samples captured 8086 larvae from 14 families. The plankton net captured twice as many taxa from twice as many families as the light traps. No taxa were caught exclusively by the light traps. For all taxa, the fish larvae collected by the light traps were larger than those in the plankton net samples. Most taxa were more abundant in the summer and new moon samples taken by both methods. The 2 methods indicated different abundance patterns between habitats for most taxa. The light traps collected more of most taxa in the reef habitat, while the plankton net collected more in the beach habitat. The light trap samples complemented those taken by the plankton net. Both sampling methods could be combined in a sampling procedure to provide a more comprehensive picture of inshore ichthyoplankton assemblages. KEY WORDS: Fish larvae · Light traps · Rocky reef · Sampling methods · New Zealand Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 186. Publication date: September 17, 1999 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1999 Inter-Research.

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.

The Ichthyoplankton of Selected Estuaries in Sarawak and Sabah: Composition, Distribution and Habitat Affinities

The ichthyoplankton assemblages of 23 estuaries in Sarawak and Sabah were surveyed with particular reference to the culturally and commercially important clupeidTenualosa toli. The species composition, distribution and density of all fish larvae were recorded, together with the physical characteristics of each estuary. A more detailed study was made of the Lupar and Lassa estuaries, which are the ‘core area’ forT. toli.The estuaries fall into two groups. The first consists of the estuaries of the ‘core area’ from the Sebako in the west to the Lassa in the east, and the Labuk and Kinabatangan estuaries of eastern Sabah. They are large and deep, have middle-range salinities with no halocline, are highly turbid and have tidal ranges of >3·5m and strong currents. There is little seasonal change in freshwater input and their waters are not peat-stained. Their total zooplankton biomasses (approximately 0·05gm−3dry weight, excluding fish larvae) are an order of magnitude greater than biomasses in the second group of estuaries. The second group consists of all estuaries east of the Lassa as far as the Papar in Sabah. They are mostly smaller, shallower and have more variable salinities than the ‘core area’ estuaries, with marked haloclines and seasonal changes in freshwater inflow, lower turbidities, weaker currents, tidal ranges of <2m and low overall zooplankton biomasses.The composition of the ichthyoplankton is different in these two groups of estuaries. The assemblage in the first group (‘core area’) of estuaries consists primarily of taxa associated with estuarine and/or turbid water conditions, whereas those in the smaller estuaries of the second group have mainly marine and clearer water affinities. Only the Gobiidae are ubiquitous. Very few larvae of freshwater species were recorded in any of the estuaries. The number of fish larvae was highly variable, but the mean densities (0·01–9·23m−3) were similar, and similarly variable, to the densities reported for other tropical estuaries.The diversity of the ichthyoplankton assemblage in Sarawak and Sabah estuaries (56 taxa, 26 families) is lower than in most other tropical estuaries of the Indo-west Pacific. This is possibly because of their rigorous physical nature, particularly the very high turbidities and current speeds, or in smaller, less physically rigorous estuaries, the low biomass of zooplankton available as food for the larvae.

A preliminary study of the influence of a coastal frontal structure on ichthyoplankton assemblages in the English Channel

Larval fish surveys were made during spring 1993 along the Opale coast (northern France) to describe the hydrological influences on inshore–offshore ichthyoplankton assemblages. The French coast of the north-eastern English Channel is characterised by a coastal water mass, 3 to 5 miles wide, that is separated from offshore water by the presence of an unstable tidal front. Multivariate analysis was used to identify larval fish assemblages. Three kinds of larval distributions were found: (1) offshore larvae, whose distribution was defined by the position of the spawning area, (2) coastal larvae, which were mainly older larvae moving to coastal nursery grounds, and (3) inshore–offshore larvae, depending on the change of spawning areas during the spring. The influence of a tidally-dependent hydrological front was related to inshore–offshore larval exchange and larval development. The changing structure of the frontal zone influenced larval distribution and may be regarded as a regulating factor for the annual recruitment of different species.