Physical oceanography of the North Carolina continental shelf during the fall and winter seasons: implications for the transport of larval menhaden

The circulation over the continental shelf off the southern Middle Atlantic Bight (MAB) and northern South Atlantic Bight (SAB) is examined for the fall and winter periods. Observational data are compared with results from a three-dimensional numerical model to identify the dominant processes on the shelf. By considering wind-forcing, tides, and a specified upstream inflow (into the MAB), the observed and modelled flow fields are in close agreement in the mid- and inner shelf regions. The resulting larval drift indicates a seasonal dependence of transport pathways from spawning grounds to estuarine nursery areas for menhaden larvae and other offshore-spawning estuarine-dependent fish. Specifically, the physical oceanography of the MAB and SAB during the fall and winter months suggests a north-to-south shift in spawning areas providing recruits to the Carolina estuaries, in agreement with the observed migration of the spawning populations.     

Larval transport during winter in the SABRE study area: results of a coupled vertical larval behaviour–three-dimensional circulation model

A three-dimensional circulation model was used in conjunction with larval fish vertical behaviour models to study the interaction between larval vertical distribution, advection and the outcome of larval transport along the central portion of the east coast of the United States. The circulation model was forced by tides, a northern boundary inflow, and winds. Vertical behaviour models were developed for Atlantic menhaden ( Brevoortia tyrannus ) and spot ( Leiostomus xanthurus ). The purpose of this modelling effort was to investigate the transport pathways of Atlantic menhaden and spot larvae from offshore spawning grounds to estuarine nursery habitats. The coupled circulation and behavioural model demonstrated the importance of along-shelf transport in what is generally thought to be a 'cross-shelf' problem. Cross-shelf transport was associated with bathymetric features, such as shoals. Both physical (e.g. wind) and biological (e.g. changes in larval behaviour) events were responsible for many of the observed patterns in larval transport. Overall, larval transport was determined by circulation but was modified by larval vertical distributions.     

Spawning habitat of the Atlantic menhaden in Onslow Bay, North Carolina

The Continuous, Underway Fish Egg Sampler (CUFES) was used to sample pelagic eggs of the Atlantic menhaden ( Brevoortia tyrannus ) from 3-m depth off North Carolina in winter 1993–94 and 1994–95. Simultaneous measurements were made of temperature, salinity, and the concentration of chlorophyll a . The maximal concentration of eggs was 346 eggs m⁻³. Eggs were highly aggregated in patches which occurred between the Gulf Stream and mid-shelf fronts (17–23°C, 36.0–36.4‰). Unexpectedly, eggs were found almost exclusively in water of 20–60 m (mode 20 m) bottom depth. Thus, spawning appears related to bathymetry as well as hydrography. Variograms for egg concentration indicated a mean (± SE) patch scale of 3.6 ± 1.7 km and a high degree of spatial variance explained by CUFES sampling. Lagrangian modelling of particles moving in response to tides, winds, and a prescribed flow from the north indicated that the region of observed, maximal occurrence of eggs is favourable for the retention of eggs and larvae on the shelf adjacent to inlets used to enter nursery areas.     

Vertical distribution of larval Atlantic menhaden (Brevoortia tyrannus) and Atlantic croaker (Micropogonias undulatus): Implications for vertical migratory behaviour and transport

Understanding the interactions among biological and physical processes is essential to determining how the environment affects transport and survival of fishes. We examined vertical distribution in larval Atlantic menhaden (Brevoortia tyrannus) and Atlantic croaker (Micropogonias undulatus) using 126 depth stratified tows in Delaware Bay, USA, during two cruises, in December 2007 and February 2008. Menhaden larvae were 16.8–24.6 and 20.5–26.2 mm standard length in December and February. Corresponding lengths for croaker were 9.3–17.9 and 8.6–19.6 mm. Using empirical observations, and statistically derived models, we explored larval concentration for both species as a function of location, depth, diel period, tidal period, size, and pairwise interactions. Menhaden concentration was best modeled as a function of station, cruise, and interactions between depth and size as well as between station and cruise. No significant differences in larval menhaden concentration were present among tidal and diel periods. Croaker concentration was best modeled as a function of size and interactions between station and diel period, depth and size, cruise and size. Despite tidal period not emerging as a significant model parameter, we observed larger croaker larvae during nighttime flood tides. Our statistical models are consistent with processes of up‐estuary transport for both species, suggesting larvae are increasingly affected by behavioral responses as larvae grow, exhibiting stronger patterns in vertical distribution. The results refine our understanding of the potential importance of size‐related differences in vertical distribution for larval transport in these species. Future research should examine the interactions among size‐specific vertical migratory capabilities, vertical distribution, transport, and retention.     

Eulerian views of layered water currents, vertical distribution of some larval fishes, and inferred advective transport over the continental shelf off North Carolina, USA, in winter

Currents that effect the shoreward transport of the larvae of estuarine-dependent fishes spawned in winter in Onslow Bay, North Carolina, USA, were driven by winds and pressure gradients, and influenced by the Gulf Stream. Aside from storms, winds over the continental shelf in Onslow Bay blew predominantly alongshore with velocities approaching 14 m s^-1 during February and March 1986, and January and February 1989. Water masses and currents observed at two current-meter moorings, one at mid-shelf and the other on the outer shelf, reflected the onshore (or offshore) advection of interior water in compensation for the offshore (or onshore) advection of wind-driven surface water. Winds and currents reversed direction approximately every 4 to 6 days. The larvae of Atlantic menhaden, Brevoortia tyrannus , spot, Leiostomus xan- thurus, and Atlantic croaker, Micropogonias undulatus , were most abundant in 17–19^oC and 20–21^oC water of the outer shelf and Gulf Stream fronts. There was little indication of diel vertical migration; larval Atlantic menhaden were most abundant in mid- and surface water, while spot and Atlantic croaker were most abundant in mid- and deep water. Given this distribution, the inferred advective transport of larvae was at times onshore, but at other times it was offshore. Within a spawning season, the prevalence of either reciprocation could determine the number of larvae that reach coastal inlets.     

Spawning and transport dynamics of Atlantic menhaden: inferences from characteristics of immigrating larvae and predictions of a hydrodynamic model

We examined patterns of abundance, age and spawning date distributions of Atlantic menhaden Brevoortia tyrannus larvae immigrating during two seasons through three North Carolina inlets—Oregon, Ocracoke and Beaufort—to elucidate their spawning and transport dynamics. These patterns were examined in conjunction with corresponding predictions from a three-dimensional, wind-and tide-driven hydrodynamic model. Larvae immigrating through different inlets showed consistent similarities as well as marked differences in temporal patterns of abundance, spawning dates and transport times. Intraseasonal patterns in abundance and spawning date distributions among inlets suggest that, in both study years, the spatio-temporal dynamics of menhaden immigration were driven by large-scale patterns along the Atlantic coast, rather than by localized variation in spawning activity. Interannual differences in the temporal patterns of spawning dates and larval immigration indicate interannual differences in transport dynamics and/or the spatial-temporal distribution of spawning. When the spawning locations predicted by the hydrodynamic model are interpreted in conjunction with advanced very high resolution radiometer sea-surface temperature information, the results are consistent with the limited historical information available on spatio-temporal distribution of Atlantic menhaden eggs and larvae. The transport model also predicted distributions of arrival times for immigrating larvae that were comparable in range and variability with observed patterns. Our use of data from immigrating larvae, coupled with a hydrodynamic transport model and sea-surface temperatures, allowed us to uncover relationships between spatio-temporal patterns of Atlantic menhaden spawning and transport dynamics that could not have been identified by either approach alone.     

Modelling the transport of larval sandeels on the north-west European shelf

This study considers the interannual variability in the advection of sandeel ( Ammodytes marinus ) larvae on the north-west European shelf. We use recently collated data on spawning location, age of larvae and the time of hatching (between 1955 and 1993) as input to a particle-tracking model. Utilizing hourly currents from a 39 year run of a two-dimensional sea circulation model, cohorts of sandeel larvae were represented as a distribution of particles in space and time, and particle drift was calculated for the larval life stage. The final distributions of particles were compared with actual distributions of 0-group sandeels in the North Sea, as determined from surveys conducted at around the end of the larval phase. Preliminary examination of the data between 1973 and 1993 indicates that differences in predicted drift could explain some of the large interannual variability in 0-group distributions. This approach may help to improve our understanding of the frequent colonization events that are typical of this species.     

Late winter larval mesopelagic fish assemblage in the Kuroshio waters of the western North Pacific

The larval mesopelagic fish assemblage and its distribution patterns were investigated in the Kuroshio region off southern Japan in late winter. A total of 8690 fish larvae was collected, 85.8% of which were mesopelagic fish larvae. Mesopelagic fish larvae were significantly more abundant in the area east of the Kuroshio axis than west of the Kuroshio axis (660.6 versus 194.5 ind 10 m^?2). Sigmops gracile, Bathylagus ochotensis, Notoscopelus japonicus, Diaphus slender type and Myctophum asperum were the five most abundant larvae and accounted for 16.9, 16.4, 15.2, 13.9 and 9.3% of the total catch in numbers, respectively. We conclude that these larvae were transported by the Kuroshio Current to the more productive transition waters, where they spend their juvenile stage from spring to early summer. The possibility of expatriation and southward long‐distance spawning migrations of N. japonicus and B. ochotensis are discussed, based on the geographic distribution patterns of their larvae, juveniles and adults.     

Predictive mapping for management and conservation of seagrass beds in North Carolina

An Erratum has been published for this article in Aquatic Conservation: Marine and Freshwater Ecosystems 12(2), 2002 577

• 1. This research extends techniques of predictive mapping from their application in terrestrial environments to marine landscapes by investigating the relationship between seagrass and hydrodynamics in Core Sound, North Carolina, USA.
• 2. An empirically derived logistic multiple regression model and a Boolean logic suitability model were used to produce several predictive map products, including: susceptibility of seagrasses to storms, probability of seagrass cover, and suitability of areas for restoration of seagrasses. A visual comparison between these maps and conventional seagrass polygon maps allows for a discussion of ‘field’ versus ‘object’ mapping, and the ramifications for management based on different cartographic techniques.
• 3. The predictive method used here showed that only a small portion (19%) of the seagrass bed in the study area would be expected to have a high probability of seagrass coverage. The majority of the seagrass habitat in the study area was predicted to have less than 50% probability of seagrass cover. In addition, 16% of the nearly 2000 ha of seagrass within the study area were predicted to be highly susceptible to acute storm events. Moreover, using a conservative set of site selection criteria, only 7% of the study area encompassed by seagrass habitat was predicted to have a high probability of successful restoration if injured.
• 4. This method provides for an inexpensive way to scale‐up from high‐resolution data to a coarser scale that is often required for conservation and management.

Copyright © 2001 John Wiley & Sons, Ltd.     

Modelling the transport of particles in the North Sea with reference to sandeel larvae

The transport of particles representing sandeel larvae in the North Sea is simulated with a three-dimensional circulation model for the years 1976 to 1990. A great year-to-year variability in drift patterns is demonstrated. The results may explain some of the observed differences in recruitment between the main sandeel areas in the North Sea. In the northern sandeel area it seems that strong year classes are unlikely if the retention due to unfavourable currents is poor, and that a high retention in the summer may favour a good year class. In the southern sandeel area no clear coupling between year class strength and larval drift is found, possibly because the retention always seems large enough for a potentially good year class. For more quantitative use of such models in relation to sandeel recruitment, more biological knowledge is obviously needed on larval vertical distribution and timing of sandeel hatching and settling.     

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.     

Searching for sensitivity in the life history of Atlantic menhaden: inferences from a matrix model

We used modified Leslie matrix models to explore the life history of Atlantic menhaden ( Brevoortia tyrannus ). By examining the sensitivity of long-term population growth rates to changes in vital rates, we identified those life history components which can cause large population level responses. Our models subdivide the first year of life into five stages (eggs, early larvae, late larvae, juveniles, and 'peanuts' or subadults), and population growth rate responds most strongly to changes in juvenile and late larval stages. The relative ranking between these stages is dependent on the magnitude of mortality during the prejuvenile stages relative to juvenile mortality. An examination of low-level model parameters indicates that the population growth rate is influenced by the growth and mortality rates during the time when young-of-the-year menhaden are gaining access to and residing in the estuaries. Sensitivity to changes in many adult metrics, such as fishing mortality, were relatively low. We conclude that a better understanding of biotic and abiotic factors that influence the late larval and juvenile stages will further our understanding of population dynamics in this species.     

Ocean transport paths for the early life history stages of offshore-spawning flatfishes: a case study in the Gulf of Alaska

Offshore‐ and deepwater‐spawning flatfish species face the problem of transport of their planktonic stages to shallow juvenile nursery grounds that are often far shoreward in bays or estuaries. We compare life history attributes of four offshore‐spawning flatfish species in the Gulf of Alaska: Pacific halibut (Hippoglossus stenolepis), arrowtooth flounder (Atheresthes stomias), rex sole (Glyptocephalus zachirus) and Dover sole (Microstomus pacificus) to examine how their larvae get from a spawning location at the edge or beyond the continental shelf to specific inshore nursery zones. We utilize historical records of survey catches of different life stages to characterize the stage‐specific changes in distribution of spawning, planktonic stages and juvenile nursery areas. We infer transport mechanisms based on the shifts in distribution of the life stages and in comparison with local physical oceanography. This comparison provides insight into the different mechanisms marine species may use to solve the common ‘problem’ of planktonic drift and juvenile settlement.     

Vertical distribution and migration of fish larvae in the NW Iberian upwelling system during the winter mixing period: implications for cross‐shelf distribution

The vertical distribution and vertical migrations of fish larvae and implications for their cross‐shelf distribution were investigated in the northern limit of the NE Atlantic upwelling region during the late winter mixing period of 2012. The average positive values of the upwelling index for February and March of this year were far from normal, although the average hydrographic conditions during the period of study were of downwelling and the water column was completely mixed. Fish larvae, most in the preflexion stage, were concentrated in the upper layers of the water column and their distribution was depth stratified, both day and night. However, the larval fish community was not structured in the vertical plane and fish larvae did not show significant diel vertical migration (DVM), although five species showed ontogenetic vertical migration. In regions of coastal upwelling and in the absence of DVM, the location of fish larvae in the water column is crucial for their cross‐shelf distribution. Thus, the cross‐shelf distribution of the six most abundant species collected in this study can be explained by the surface onshore flow associated with coastal downwelling, retaining larvae of the coastal spawning species with a relatively shallow distribution in the shelf region and transporting larvae of slope spawning species onto the shelf. The wide vertical distribution shown by larvae of the offshore spawning species could be an adaptation of these species to ensure that some larvae reach the inshore nursery areas.     

Fluctuation in the distribution of low-salinity water in the North Equatorial Current and its effect on the larval transport of the Japanese eel

Surface water in the North Equatorial Current (NEC) is composed of southern low‐salinity water diluted by precipitation to less than 34.2 psu and northern, high‐salinity tropical water greater than 34.8 psu. Analyses of 27‐year historical data, observed in winter and summer along the longitude 137°E by the Japan Meteorological Agency, shows that an obvious salinity front (34.5 psu) generated by the two water masses was usually located around 15°N. However, the salinity front has been moving northward during the past three decades. El Niño/Southern Oscillation (ENSO) affected salinity in the surface layer, while temperature changed in the middle layer. The salinity front sometimes moved southward, mainly south of 5°N, and the movement was well correlated with the southern oscillation index (SOI). Because precipitation at Yap (9.5°N, 138.1°E) fluctuated with SOI, this spike‐like southward movement of the salinity front was probably affected by reduction of low‐salinity water during El Niño in the north‐western Pacific Ocean. However, ENSO only induced such large southward movements of the salinity front when the time lag between the low precipitation and low SOI was short (within four months). This salinity front is quite important for long‐distance migrating fish such as the Japanese eel because the eels spawn just south of the salinity front in the NEC. This behaviour suggests that the movement of the salinity front associated with ENSO may control the success of larval transport from the spawning ground in the NEC to the nursery ground in East Asia. In fact, catch of the Japanese eel larvae in Japan was well correlated with fluctuation of SOI and the location of the salinity front, and lower catch occurred during El Niño. The salinity front has moved from 13°N to 17°N during the past three decades. Considering that conditions of larval transport are worse north of 15°N, we suggest that decadal‐scale linear decrease of glass eel catch during the past three decades also can be explained by the displacement of the salinity front.     

Influence of larval transport and temperature on recruitment dynamics of North Sea cod (Gadus morhua) across spatial scales of observation

The survival of fish eggs and larvae, and therefore recruitment success, can be critically affected by transport in ocean currents. Combining a model of early‐life stage dispersal with statistical stock–recruitment models, we investigated the role of larval transport for recruitment variability across spatial scales for the population complex of North Sea cod (Gadus morhua). By using a coupled physical–biological model, we estimated the egg and larval transport over a 44‐year period. The oceanographic component of the model, capable of capturing the interannual variability of temperature and ocean current patterns, was coupled to the biological component, an individual‐based model (IBM) that simulated the cod eggs and larvae development and mortality. This study proposes a novel method to account for larval transport and success in stock–recruitment models: weighting the spawning stock biomass by retention rate and, in the case of multiple populations, their connectivity. Our method provides an estimate of the stock biomass contributing to recruitment and the effect of larval transport on recruitment variability. Our results indicate an effect, albeit small, in some populations at the local level. Including transport anomaly as an environmental covariate in traditional stock–recruitment models in turn captures recruitment variability at larger scales. Our study aims to quantify the role of larval transport for recruitment across spatial scales, and disentangle the roles of temperature and larval transport on effective connectivity between populations, thus informing about the potential impacts of climate change on the cod population structure in the North Sea.     

Circulation of the eastern North Atlantic and north-west European continental shelf – a hydrodynamic modelling study

As part of the interdisciplinary investigation of Calanus finmarchicus migrations between oceanic and shelf areas off north-west Europe (the ICOS project), a three-dimensional, prognostic baroclinic circulation model system, consisting of a coarse and a nested fine scale model, was constructed. The aim was to simulate the flow fields around Iceland, the Faroe Islands and the north-west European continental shelf and slope, including the northern North Sea. The coarse scale model covered the northern North Atlantic and provided the far field flow to the finer scale model. The latter was initialized and forced with climatological seasonal (winter and spring) means of temperature, salinity, river run-off, and other parameters necessary for the calculation of heat fluxes. Seasonal average simulations of flow and hydrographic conditions were performed, together with simulations of specific scenarios of uniform wind forcing from different directions. A feature common to all the results was a topographically guided northwards flow along the continental shelf edge, with a southwards counter-flow in the vicinity of the Faroe–Shetland Channel. The extent and strength of the shelf edge current, of its counter flow, and of the re-circulation cell in the northern North Sea, were dependent on the prevailing wind direction. The modelling results were consistent with the accepted circulation patterns in the area, and a comparison with field measurements in the Faroe–Shetland Channel showed that the main characteristics of the flow were well replicated, although the estimated strength of the slope current was somewhat less than in the field observations.     

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.     

From spawning grounds to the estuary: using linked individual-based and hydrodynamic models to interpret patterns and processes in the oceanic phase of Atlantic menhaden Brevoortia tyrannus life history

Present theory suggests that population regulation in marine fishes cannot be resolved until an understanding of the processes involved in shaping the overall distribution (the number of populations, geographical extent, mean abundance and temporal changes in abundance) is developed. Here, we present a step toward understanding Atlantic menhaden population patterns, by studying processes in the Middle and South Atlantic Bights, which shape those patterns. We use individual-based and hydrodynamic models to reinterpret the 'mechanics' of the menhaden life history, and put forward several potentially testable hypotheses. The success of the menhaden reproductive strategy seems to depend on the seasonal changes in the mean flow field of the Middle and South Atlantic Bights, suggesting that their life history may have been strongly structured by the regional physics of the system. Because the annual menhaden migration is size-based and spawning occurs throughout the population's range, the size distribution of the adult population may influence the supply of larvae to particular estuaries along the coast. Recruitment of larvae into Delaware and Chesapeake Bays may be dependent on spawning to the north of the bays' mouths, owing to coastline shape and orientation in the vicinity of the bays. Our results suggest that management of this resource might be improved by consideration of the spatial and temporal variability in both the biological and the physical system.