Springtime ichthyoplankton of the slope region off the north-eastern United States of America: larval assemblages, relation to hydrography and implications for larval transport

Larval transport in the slope region off north‐eastern North America influences recruitment to juvenile habitats for a variety of fishes that inhabit the continental shelf. In this study, collections of larval fishes were made during springtime over the continental slope to provide insights into larval distributions and transport. Ichthyoplankton composition and distribution mirrored the physical complexity of the region. Three larval fish assemblages were defined, each with different water mass distributions. A Gulf Stream assemblage was found predominantly in the Gulf Stream and associated with filaments of discharged Gulf Stream water in the Slope Sea. Larvae of this assemblage originated from oceanic and shelf regions south of Cape Hatteras. Several members of this assemblage utilize habitats in the Middle Atlantic Bight (MAB) as juveniles (Pomatomus saltatrix, Peprilus triacanthus) and other members of the assemblage may share this life cycle (Mugil curema, Sphyraena borealis, Urophycis regia). A Slope Sea assemblage was found in all water masses, and was composed of epi‐ and mesopelagic fish larvae, as well as larvae of benthic shelf/slope residents. Larvae of one member of this assemblage (U. tenuis) are spawned in the Slope Sea but cross the shelf‐slope front and use nearshore habitats for juvenile nurseries. A MAB shelf assemblage was found in MAB shelf water and was composed of larvae that were spawned on the shelf. Some of these species may cross into the Slope Sea before returning to MAB shelf habitats (e.g. Enchelyopus cimbrius, Glyptocephalus cynoglossus). Previous studies have examined the effect of warm‐core rings on larval distributions, but this study identifies the importance of smaller‐scale features of the MAB shelf/slope front and of filaments associated with Gulf Stream meanders. In combination with these advective processes, the dynamic nature of larval distributions in the Slope Sea appears to be influenced, to varying degrees, by both vertical and horizontal behaviour of larvae and pelagic juveniles themselves.     

Variations in larval walleye pollock feeding and condition: a synthesis

There was a strong association among concentrations of microzooplankton prey sampled from the walleye pollock, Theragra chalcogramma , larval habitat, gut contents of larvae, and their nutritional condition. Subsequently, hypothesized survival potentials linked to food availability were validated by independently determined mortality rates. We present evidence that a significant number of walleye pollock larvae were starving in 1991 but that fewer were starving in 1992. At some stations where prey levels were anomalously low in 1991, up to 40% of the larvae were in poor condition. There appears to be a 2-week period after first feeding when walleye pollock are vulnerable to starvation.     

Anomalous transport of walleye pollock larvae linked to ocean and atmospheric patterns in May 1996

Larvae from a large aggregation of walleye pollock spawning in early spring in Shelikof Strait, Gulf of Alaska, are normally transported to the south-west in the vigorous Alaska Coastal Current. In the spring of 1996, anomalous winds resulted in unusually weak transport in the Shelikof Strait sea valley. The main aggregation of larval pollock in the Shelikof region was surveyed four times in 1996 over a period of about 40 days, including finer-scale sampling of the leading south-western edge of the larval distribution. The south-western edge of the larval distribution showed weak transport up the sea valley for a period of about 10 days, corresponding to the observations of currents, after which many larvae were transported over the shelf region to the west. These observations are unique in over 15 years of monitoring larval transport patterns and demonstrate how anomalous weather, and hence current patterns, influence variability in larval transport.     

Ubiquitous eddies of the eastern Bering Sea and their coincidence with concentrations of larval pollock

Between 1988 and 1993, 12 satellite-tracked buoys were deployed in four eddies in the south-eastern Bering Sea. Our success in finding eddies resulted from placing buoys in high concentrations of walleye pollock (Them-gra chalcogramma) larvae. We utilize data from hydro-graphic surveys, satellite-tracked buoys and moored current meters to describe the eddies. Small (< 25 km diameter) eddies likely transit along the slope of the eastern Bering Sea every 45–60 days. In previous studies such small features were not observed because their size fell within typical separation of hydrographic stations and the weak sea surface temperature gradients are not resolved by satellite-borne infrared imagery.     

Larval fish distribution and retention in the Canary Current system during the weak upwelling season

The spatial distribution of fish larvae was studied in the Canaries‐African Coastal Transition Zone, outside the strong upwelling season. An onshore–offshore transition in the larval fish community structure was observed, from a coastal assemblage dominated by small pelagics (sardine, anchovy, mackerel), bounded by the upwelling front, to an offshore assemblage dominated by mesopelagic species (mainly Myctophidae, Phosichthydae, Gonostomatidae). Distribution of the neritic larvae was deeply influenced by the intense mesoscale activity found in the area, both horizontally (larvae were advected offshore but were always retained within the upwelling area) and vertically (larvae were deepened in the vicinity of two anticyclonic eddies). A combined effect of the upwelling front and a cyclonic–anticyclonic eddy dipole is likely the successful retention mechanism for these larvae. These results support the current belief that retention may be higher than previously thought in upwelling areas. Oceanic larvae were also collected in higher abundances near the front and an anticyclonic eddy. Neritic and oceanic larvae frequently showed a differentiated position in the water column, although they sometimes coexisted. Finally, larval connectivity between Islands within the Canary archipelago is suggested. The present study thus contributes to the understanding of the complex dispersal and retention processes in the Canaries‐African Coastal Transition Zone. However, results also highlight the poor knowledge of this region compared with the other three main Eastern Boundary Upwelling Systems in terms of ichthyoplankton dynamics. The importance of routine monitoring programs of commercial and non‐commercial species in the area is emphasized.     

Acoustic detection of mesoscale biophysical features in the Shelikof sea valley, and their relevance to pollock larvae in the Gulf of Alaska

Mesoscale features such as fronts and eddies can act to retain larval walleye pollock ( Theragra chalcogramma ) within the continental shelf zone in the western Gulf of Alaska. During two May cruises, we observed unusual patterns of backscattering with a 38 kHz acoustic system. Characteristics of this signal were a strong scattering layer at the surface and in midwater, with the water column between nearly void of sound scattering organisms. This signal appeared in several transects where satellite remote sensing indicated the presence of an eddy. Analysis of concomitant water properties and ADCP (153 kHz) data confirmed the existence of an anomalous physical feature at this location. Biological properties (chlorophyll and zooplankton) showed marked changes across the edge of the feature. Larval pollock densities were estimated to be an order of magnitude higher within this feature compared to those outside. Acoustic backscatter signals can be used to identify and characterize mesoscale biophysical features in the ocean, thereby enabling real-time studies of these features.     

Walleye pollock recruitment in Shelikof Strait: applied fisheries oceanography

Fisheries-Oceanography Coordinated Investigations (FOCI) is a National Oceanic and Atmospheric Administration (NOAA) research programme seeking to understand recruitment processes of commercially exploited Alaskan fishes. The FOCI is mainly comprised of scientists at the Pacific Marine Environmental Laboratory and the Alaska Fisheries Science Center who study both the biotic and abiotic environment, including processes within larval patches through integrated field, laboratory, and modelling studies. The initial focus of studies was walleye pollock ( Theragra chakogramma ) spawning in Shelikof Strait, Gulf of Alaska. The choice of this population for our research was based on development of a large fishery and the substantial variation in recruitment that was observed in the late 1970s and early 1980s. Also, the early life history of this population is quite predictable and restricted both temporally and spatially. Walleye pollock spawn consistently in a small part of Shelikof Strait in early spring from which a large patch of eggs and later larvae is produced. In most years this concentration of larvae drifts to the south-west through the strait during April and May. Large numbers of larvae are often found in eddies which frequent the area and we have observed improved feeding conditions for larvae, in as opposed to out of eddies. We have found that first-feeding larvae have higher survival rates during calm periods, rather than in storms, and that in many years recruitment is largely set by the end of the larval period, although in some years age-0 juvenile mortality is also important. FOCI now generates information that is being used for management of this resource.     

Distribution of larval fishes among water masses in Onslow Bay, North Carolina: implications for cross-shelf exchange

The Gulf Stream (GS) is a major oceanographic feature with potential to influence the recruitment of larval fishes to continental shelf habitats in the southeastern United States. To test the hypothesis that the GS is a source of certain larval fishes to Onslow Bay, North Carolina, we (i) classified water masses as shelf, GS, GS front (GSF), or GS/shelf mixture (GS/S); (ii) compared larval fish assemblages and concentrations among these water masses; and (iii) compared length–frequency distributions and length–concentration relationships of indicator and commercially important taxa among water masses. A total of 21,222 larvae were collected with bongo and neuston nets from April 2000 to December 2001. Non-metric multidimensional scaling analyses revealed distinct larval assemblages associated with different water masses. For bongo catches, bothids were abundant in all water masses, gobiids, callionymids, and labrids were abundant in shelf waters, and myctophids and scombrids were abundant in the GS. For neuston catches, carangids dominated in GS/S, GSF, and GS waters, whereas triglids were abundant in shelf water. Larval concentrations in neuston catches were lower in shelf waters and higher in GS and GSF waters. Concentrations of most taxa in bongo catches were low in the GS and higher in shelf waters. We used trends in myctophid (offshore/GS) and gobiid (shelf) length–concentration data as indicators of the sources of commercially important serranids. Length distributions and concentrations of larval indicator taxa suggested local, shelf spawning, and transport of larvae from offshore.     

Simulating larval supply to estuarine nursery areas: how important are physical processes to the supply of larvae to the Aransas Pass Inlet?

Factors controlling the movement of fish larvae from coastal spawning environments to estuarine nursery areas are important to fish recruitment. In this paper, the role of physical processes in larval transport to estuarine nursery areas in the Aransas Pass region, Texas, is examined using a circulation model coupled with a fixed‐depth particle transport model. Two phases of transport are examined: transport on the shelf to the tidal inlet and transport through the inlet to estuarine nursery areas. Observed pulsing in the supply of red drum (Sciaenops ocellatus) larvae to the tidal inlet is significantly correlated with modeled particle supply. This pulsing is not correlated with a specific physical process, but results from the interaction of several factors affecting water movement, including low‐frequency variations in water level and wind forcing. Simulations suggest that the primary spawning region for red drum larvae that utilize nursery habitat in the Aransas Pass region is located north of the inlet. Patterns in the trajectories of particles that successfully enter the inlet reveal that they move alongshelf in the nearshore region and then move into the inlet, rather than moving directly across the shelf to the inlet. The approach path of particles outside the inlet determines the spatial transport patterns for inlets with branched channels and multiple bays. This study demonstrates that physical processes play an important role in determining larval supply to a tidal inlet.     

Survival and growth of sardine larvae in the offshore side of the Kuroshio

After 1984 the major spawning of the Japanese sardine, Sardinops melanostictus, has been observed to occur in the offshore waters, where the survival of early-stage larvae (4–10 mm in length) is questionable. The main objective of this study is to estimate the growth and survival rates of the early-stage larval cohort in the offshore side of the Kuroshio Current. A radar-reflecting buoy with a surface drogue was launched to tag a patch of larvae, and the patch was traced for 3 days in March 1991. The survival rate of the early-stage larval cohort was calculated from the change in density during the survey. The range of the instantaneous mortality rate was from 0.83 to 1.11 day, a survival rate of32-44%day-¹.The mean density of the smaller larvae (4–6 mm in length) decreased more rapidly than that of the larger larvae (6–10 mm in length) during the sampling period. The survival rate of the first-feeding larvae in the offshore region seemed to be lower than that of the post-flrst-feeding larvae. However, the growth rates of the first-feeding larvae's survivors in the region were higher than those of the post-first-feeding larvae's survivors. The first-feeding larvae in the offshore region seemed to survive when the growth rate of the larvae was high.     

Simulation of physically mediated variability in prey resources of a larval fish: a three-dimensional NPZ model

A three-dimensional biophysical nutrient–phytoplankton–zooplankton model was used to investigate the spatial and temporal dynamics of food resources for young walleye pollock in the western Gulf of Alaska, to further understanding of recruitment processes for pollock. We modeled nitrogen, phytoplankton, a large herbivorous grazer parameterized as Neocalanus spp. (the biomass dominant copepod in the Gulf), and the 13 stages (egg, naupliar and copepodite) of Pseudocalanus spp. (a major constituent of the diet of pollock) so that the appropriate size class of food for each size of larval pollock was represented. Model results identified an area between the Semidi and Shumagin Islands that may not be suitable as a nursery area early in the year due to low prey abundance. Modeled mesoscale eddies, previously hypothesized to be important for larval pollock retention in Shelikof Strait, contained higher prey concentrations than the surrounding waters when they were cyclonic. This work also help to understand the consistency of pollock spawning in time and space in Shelikof Strait, by examining the timing and location of prey availability which, along with transport, narrows the window for optimal spawning.     

Observed patches of walleye pollock eggs and larvae in Shelikof Strait, Alaska: their characteristics, formation and persistence

Using observations from 38 ichthyoplankton surveys conducted near Shelikof Strait, Alaska between 1979 and 1992, we characterized the horizontal distribution and spatial patchiness of the early life stages of walleye pollock ( Theragra chalcogtamma ). Lloyd's index of patchiness ranged from 3.9-6.1 for eggs and 3.9–16.2 for larvae. This index was size (age) dependent: low for eggs, high for newly hatched larvae, then decreasing through late larval stage. By the early juvenile stage, patchiness increased as pollock began to school. The percentage of larvae in a patch (defined as the percentage of larvae present at stations where larval counts exceeded the mean by one standard deviation during the given survey) varied greatly (26–92%). Larval distributions were used to deduce physical mechanisms responsible for patches. Three categories of patches were identified: those created by interaction of larvae with time-dependent currents, those in the vicinity of Sutwik Island, and those associated with eddies. Simulation experiments were utilized to examine processes influencing patch formation and the role of larval swimming. Between 5 and 6 weeks after hatching, larvae have swimming abilities that enable them to maintain a patch already created by physical mechanisms.     

Effects of tank colour on larval survival and development of mud crab Scylla serrata (Forskål)

Hatchery culture of mud crabs has not yet achieved commercial viability despite decades of research efforts. Further research is therefore needed to better understand larval culture requirements of the crab. Based on anecdotal observations, an experiment was carried out to test whether the background colour of the culture vessel affected larval culture success. Newly hatched larvae of Scylla serrata were reared in culture vessels of five colours, i.e., black, dark green, maroon, sky blue and white. Larval survival and development were monitored daily until all of them either moulted to the first crab stage or died. The results showed clear effects of background colour on larval survival. A general tendency of higher larval survival in darker‐coloured backgrounds was evident. In particular, overall zoeal survival for larvae reared in black vessels was significantly higher than those reared in white ones. Background colour also appeared to affect larval development. Larvae reared in darker backgrounds generally had shorter development times and more synchronized moulting. A significant delay in zoeal development was observed in larvae reared in white vessels. Dark backgrounds possibly facilitated more efficient feeding, reduced settlement of larvae at the bottom of the vessels as well as minimized stress. This result appears to be the first to demonstrate that background colour can significantly affect larval survival and development of a crustacean species.     

Effect of stocking density and feeding regime on larval growth,survival, and larval development of Japanese flounder,Paralichthys olivaceus,using live feeds

This research examined the effect of initial stocking density and feeding regime on larval growth and survival of Japanese flounder, Paralichthys olivaceus. Larval rearing trials were conducted in nine 50‐L tanks with different initial stocking densities combined with different feed rations (20 larvae/L with standard feed ration [LD], 80 larvae/L with standard feed ration [HD], and 80 larvae/L with four times the standard feed ration [HD+]). Larvae were stocked on 0 days posthatch (DPH) following hatching of the fertilized embryos. Larval total length (TL), survival rates, and final densities were observed on larval settlement (32 DPH) to evaluate larval rearing performance. At 32 DPH, there were no significant differences (p > .05) in TL or survival rates between the LD (46.5 ± 17.0%) and HD+ (40.3 ± 9.4%). The TL and survival rate of HD (23.1 ± 3.5%) were significantly lower than that of LD and HD+ (p < .05). However, the larval density of HD was significantly higher than that of LD (p < .05). HD+ achieved the best larvae production (32.27 ± 7.51 larvae/L), supported by sufficient food source, high water exchange, and proper water quality management (routine siphoning, surface skimming). The larval‐rearing protocols and larval development from hatching to metamorphosis is described in detail, with corresponding photographs taken during the experiment.     

Eastern Bering Sea pollock recruitment,abundance, distribution and approach to fishery management

Eastern Bering Sea pollock have two distinctly different stable spawning grounds—along the shelf and in the eastern and central Aleutian Islands between 400 and 500 m water columns. Pollock spawning behavior supports the hypothesis that the shelf and deepwater “basin” spawning pollock are completely independent reproductive stocks. Deepwater pollock inhabit the shelf and, once mature at age 5–6 years, migrate from the shelf onto the continental slope into the Zhemchug, Pribilof, and Bering canyons by the end of winter. Bering Sea pollock recruitment and year class abundance have high annual variability, but there are no clear relationships between pollock year class strength and water temperature, ice distribution or survival on early ontogenesis stages (eggs and larvae). Young-of-the-year fish survival varies dramatically during winter supporting the hypothesis that the Bering Sea pollock recruitment and strength of year class have high annual variability depending on young-of-the-year fish survival during winter. The annual change of physical oceanography condition, productivity and species composition of zooplankton community are associated with great differences in pollock seasonal migrations and distribution, reproduction, survival of recruits at early stages of development and finally with abundance of year classes and total biomass. Implementation of ecosystem-based fishery management most important for application of pollock research both of Russian national program and on base of International Agreements.     

Advection of anchovy (Engraulis encrasicolus) larvae along the Catalan continental slope (NW Mediterranean)

The Gulf of Lions is one of the main anchovy (Engraulis encrasicolus) spawning areas in the NW Mediterranean. During the spring, low‐salinity surface water from the outflow of the Rhône is advected by the shelf‐slope current along the continental slope off the Catalan coast. In June 2000, a Lagrangian experiment tracking these low‐salinity surface waters was conducted to assess the importance of this transport mechanism for anchovy larvae and to determine the suitability of the tracked surface waters for survival of anchovy larvae. The experiment consisted of sampling the tracked water parcel for 10 days with three drifters launched at the core of the shelf‐slope current where low‐salinity surface waters were detected. The survey was completed by sampling the surrounding waters. Anchovy larvae from the spawning area in the Gulf of Lions were advected towards the south in the low‐salinity waters. The size increase of anchovy larvae throughout the Lagrangian tracking closely followed the general growth rate calculated by otolith analysis (0.65 mm day⁻¹). However, advection by the current was not the only mechanism of anchovy larval transport. A series of anticyclonic eddies, originated in the Gulf of Lions and advected southwards, seemed to play a complementary role in the transport of larvae from the spawning ground towards the nursery areas. These eddies not only contributed to larval transport but also prevented their dispersion. These transport and aggregation mechanisms may be important for anchovy populations along the Catalan coast and require further study.     

Mortality of larval walleye pollock Theragra chalcogramma in the western Gulf of Alaska, 1988–91

Mortality rates of larval walleye pollock Theragra chakogramma were estimated from larval survey data from 1988 to 1991. Mortality estimates were based on cohort-specific losses between occupations of survey grids. Interannually, estimates of early feeding stage larval mortality rates ranged over an order of magnitude, from 0.045–0.43 day^-1, and declined sharply with age. There is some evidence that mortality rates of early feeding larvae tend to be negatively correlated with temperature and postively correlated with wind mixing.     

Entrainment and advection of larval sardine,Sardinops sagax,by the East Australian Current and retention in the western Tasman Front

The poleward flowing East Australian Current (EAC) drives sporadic upwelling, entrains coastal water and forms the western Tasman Front (wTF), creating a mosaic of water types and larval transport routes along south eastern Australia. The spatial distribution, otolith chemistry and growth rates of larval sardine (Sardinops sagax) were examined to infer spawning location and larval transport. A gradient of increasing larval size from north to south along the shelf was not detected but was evident between the shelf and offshore in the wTF. Here larvae were larger and older. Based on the occurrence of newly hatched larvae, spawning by S. sagax between southern Queensland and mid New South Wales (NSW) was more extensive than previously reported. The otolith chemistry from two wTF larval size classes differed, implying different origins. The otolith chemistry of wTF post‐flexion larvae was similar to larvae from northern NSW, whereas wTF flexion larvae were similar to larvae observed nearby from mid‐NSW. Two possible larval transport routes, direct and indirect, are inferred from otolith chemistry, current velocities and a previously published particle tracking study. Either larvae from northern NSW were advected south and entrained with younger larvae directly into the wTF, or larvae from a range of shelf regions were advected around the southern edge of an anticyclonic eddy, to join younger larvae directly entrained into the wTF. Based on the co‐occurrence of larval ages and sizes in the wTF and their advection routes, the wTF appears to be an important larval retention zone.     

Effects of ontogeny, temperature, and light on vertical movements of larval Pacific cod (Gadus macrocephalus)

The role of behavior, especially vertical migration, is recognized as a critical component of realistic models of larval fish dispersion. Unfortunately, our understanding of these behaviors lags well behind our ability to construct three-dimensional flow-field models. Previous field studies of vertical behavior of larval Pacific cod ( Gadus macrocephalus ) were limited to small, preflexion stages (≤11 mm SL) in a narrow range of thermal conditions. To develop a more complete picture of larval behavior, we examined the effects of ontogeny, temperature, and light on vertical responses of larval Pacific cod in experimental columns. While eggs were strictly demersal, yolk-sac larvae displayed a strong surface orientation as early as 1 day post hatch (∼ 5 mm SL). Consistent with field observations, small preflexion larvae (<10 mm SL) showed no response to varying light levels. However, there was a direct effect of temperature on larval behavior: Pacific cod larvae exhibited a stronger surface orientation at 4°C than at 8°C. The behavior of larger, postflexion larvae (>15 mm SL) in experimental columns was consistent with a diel vertical migration and independent of water temperature: fish were more widely distributed in the column, and median positions were consistently deeper at higher light levels. These laboratory observations are combined with observations from discrete-depth (MOCNESS) sampling in the Gulf of Alaska to characterize the vertical distribution of larval Pacific cod and contrast ontogenetic patterns with walleye pollock ( Theragra chalcogramma ). The vertical movements of larval Pacific cod described here will be applied in the development of dispersal projections from Gulf of Alaska spawning grounds.