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.     

Atlantic menhaden recruitment to a southern estuary: defining potential spawning regions

Atlantic menhaden ( Brevoortia tyrannus ) is an estuarine-dependent fish that spawns in coastal waters of the Middle and South Atlantic Bights. Circulation modelling studies of larval transport suggest that recruitment of larvae into the Albemarle-Pamlico Estuarine System, North Carolina, is linked to dynamics on the shelf from New York to South Carolina. Field-collected menhaden egg data (from MARMAP and SABRE) define a range of temperatures within which menhaden eggs have been found. In this study we refine the transport model-predicted spawning grounds for the 1994–95 season by using satellite-derived sea surface temperature data to highlight regions that are outside the observed spawning temperature range. We also use transport pathways leading from source locations to the estuarine system to characterize the temperature field experienced by particles/larvae during their spawning-ground to inlet transit. The modelled nearshore location of source regions agrees well with MARMAP and SABRE egg data, and points to the importance of understanding biological and physical linkages between the Middle and South Atlantic Bights. The combination of modelled transport and synoptic temperature maps can provide useful guidance to future sampling efforts as well as help refine our understanding of menhaden ecology.     

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.     

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.     

Predator–prey analysis of striped bass and Atlantic menhaden in upper Chesapeake Bay

Between 1997 and 2000, an outbreak of skin lesions and observations of emaciated striped bass, Morone saxatilis (Walbaum), in upper Chesapeake Bay were attributed to a perceived shortage of its main prey, Atlantic menhaden, Brevoortia tyrannus Latrobe. Abundance estimates, Atlantic menhaden consumption per recruit analysis (modified yield‐per‐recruit), bioenergetics analysis and predator–prey theory were combined to explore whether an imbalance between striped bass and Atlantic menhaden in upper Chesapeake Bay was plausible. Reduced fishing mortality and higher size limits that underpinned the effort to restore striped bass during the 1980s and 1990s lead to more abundant and larger striped bass in upper Chesapeake Bay, increasing predatory demand for forage‐sized Atlantic menhaden, the abundance of which declined to a historic low. Nominal losses of age 0–2 Atlantic menhaden to harvest and potential striped bass predation exceeded supply after 1998. The outbreak of lesions in upper Chesapeake Bay striped bass coincided with increased variation in weight‐at‐length, decreased length‐at‐age, and decreased presence of Atlantic menhaden in diets. Decreased attack success, inferred from a 97% decline in ratios of forage‐sized Atlantic menhaden to striped bass between 1983 and 1998, would have expected to have been followed by a deterioration of striped bass nutritional state. Transmission of disease would have been aided by high density of striped bass in poor nutritional condition residing in degraded habitat (Chesapeake Bay was the most hypoxic estuary in the mid‐Atlantic region in the late 1990s). These problems suggest that striped bass exceeded their carrying capacity in Chesapeake Bay during the late 1990s.     

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.     

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.     

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.     

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.     

Physical transport of young pollock larvae (Theragra chalcogramma) near Shelikof Strait as inferred from a hydrodynamic model

An advective model was used to simulate the drift of larval walleye pollock ( Theragra chalcogramma ) over a 40-day period (late April through early June) near Shelikof Strait, Alaska. This model was used: (i) to assess how much of the observed change in larval positions during that period can be explained by transport at fixed depth; (ii) to demonstrate that observed change can be related to mean large-scale meteorological forcing; and (iii) to investigate accumulation of larvae in specific areas near the coast. Based on availability of larval and circulation data, three years were studied: 1988, 1989 and 1991. Velocity fields generated from a hydrodynamic model driven by winds and runoff were used to advect particles seeded in accordance with observed larval distributions in late April of each year. The modelled larvae were tracked at 40 m depth, corresponding to the mean depth of sampled larvae and the depth of neutrally buoyant drifters employed in field studies. Specific features observed in late May larval surveys were reproduced by the model, such as the accumulation of larvae in a shoal area downstream of the strait. Differences among the modelled years include extensive flushing of larvae to the south-west in 1988 and 1991, vs. limited flushing in 1989. These differences appear related to the mean large-scale atmospheric pressure patterns for April-May of those years.     

Using a biophysical model to investigate connectivity between spawning grounds and nursery areas of King George whiting (Sillaginodes punctatus: Perciformes) in South Australia’s gulfs

Many demersal marine fish species depend on a dispersive larval stage that connects geographically discrete sub‐populations. Understanding connectivity between these sub‐populations is necessary to determine stock structure, which identifies the appropriate spatial scale for fishery management. Such connectivity is poorly understood for King George whiting (Sillaginodes punctatus; Perciformes) in South Australia's gulf system, even though spawning grounds and nursery areas are adequately defined. In response to declines in commercial catches and estimated biomass, this study aimed to determine the most important spawning grounds and nursery areas to recruitment, and the connectivity between them. A biophysical model was seeded with particles according to the distribution and density of eggs throughout the spawning area in 2017 and 2018. Despite inter‐annual differences in the origins of particles, dispersal pathways and predicted settlement areas remained consistent between years. Predicted settlement was generally highest to nursery areas only short distances from regional spawning grounds, consistent with previous hydrodynamic models. However, the model also predicted that spawning in one region could contribute to recruitment in an adjacent region later in the spawning season, which aligned with the breakdown of thermohaline fronts at the entrance of each gulf. The connectivity between spawning grounds and nursery areas predicted by the model is supported by spatio‐temporal patterns in the otolith chemistry of pre‐flexion larvae and settled juveniles. Consequently, the most parsimonious explanation is that the populations of King George whiting in South Australia's gulf system constitute a single, panmictic stock, which has implications for fishery management.     

Linking growth to environmental histories in central Baltic young-of-the-year sprat, Sprattus sprattus: an approach based on otolith microstructure analysis and hydrodynamic modelling

Otolith microstructure analysis and hydrodynamic modelling were combined to study growth patterns in young‐of‐the‐year (YoY) sprat, Sprattus sprattus, which were sampled in October 2002 in the central Baltic Sea. The observed ‘window of survival’, approximated by the distribution of back‐calculated days of first feeding (DFF), was narrow compared to the extended spawning season of sprat in the Baltic Sea (mean± SD = 22 June ± 14.1 days) and indicated that only individuals born in summer survived until October 2002. Within the group of survivors, individuals born later in the season exhibited faster larval, but more rapidly decreasing juvenile growth rates than earlier born conspecifics. Back‐calculated larval growth rates of survivors (0.48–0.69 mm day^?1) were notably higher than those previously reported for average larval sprat populations, suggesting that the YoY population was predominantly comprised of individuals which grew faster during the larval stage. Daily mean temperatures, experienced across the entire YoY population, were derived from Lagrangian particle simulations and correlated with (1) detrended otolith growth and (2) back‐calculated, daily somatic growth rates of survivors. The results showed that abrupt changes in ambient temperature can be detected in the seasonal pattern of otolith growth, and that higher temperatures led to significantly faster growth throughout the entire age range of YoY sprat.     

Modeling the transport and survival of Japanese sardine larvae in and around the Kuroshio Current

We have numerically modeled the advection and diffusion of sardine eggs and larvae to investigate the larval transport processes of Japanese sardine from the spawning grounds by the Kuroshio.
The results indicated that the offshore drift current induced by the winter monsoon and the location of the spawning ground have significant effects on the survival of the Japanese sardine. The contribution of the drift current, the distance of the spawning ground from the Kuroshio axis, and the eddy diffusivity to the larval retention in the coastal area is approximately expressed by the following equation: where R is the retention rate in the coastal area, a the variance of initial distribution of eggs, T the time after the eggs were spawned, – V₀ the velocity of the wind-induced offshore current, y₀ the distance of the center of the spawning area from the Kuroshio axis, and K the coefficient of horizontal eddy diffusivity.
The year-to-year variation in larval survival rates stimulated by the two-dimensional model are consistent with those estimated previously by using field data of egg and larval abundance during 1978–1988.     

Sensitivity analysis of sea scallop (Placopecten magellanicus) larvae trajectories to hydrodynamic model configuration on Georges Bank and adjacent coastal regions

The previous larval-trajectory modeling studies on Georges Bank were assessed through process-oriented Lagrangian-tracking comparison experiments using the high-resolution Gulf of Maine/Georges Bank Finite-Volume Coastal Ocean Model (GOM-FVCOM). The results indicate that in a strong nonlinear system such as Georges Bank, the passive tracer movement is driven by a fully three-dimensional Lagrangian flow field that varies in space and time due to large tidal excursion and steep bottom topography. The particle-tracking methods developed based on the assumption of weak nonlinearity of the flow field are not applicable to Georges Bank. The results of previous larval transport studies driven by circulation fields constructed under the weak-nonlinearity assumption need to be interpreted with caution. In the present work, the influence of model physical setups on sea scallop larval dispersal and settlement on Georges Bank and adjacent shelf regions is examined. Distinct differences in the spatial distribution of the passive larvae predicted by the model under various physical conditions suggest that a fully nonlinear model driven by realistic spatially and temporally varying forcing should be employed for Lagrangian-based studies of fishery population dynamics on Georges Bank.     

Modeled transport of winter flounder larvae spawned in coastal waters of the Gulf of Maine

Winter flounder, Pseudopleuronectes americanus, from southern New England and the Gulf of Maine were historically considered to be obligate estuarine spawners. However, recent experiments and observations document that winter flounder in the Gulf of Maine also utilize coastal waters for spawning. An individual‐based modeling approach was used to investigate the transport of winter flounder larvae from three hypothesized coastal spawning grounds in the Gulf of Maine. Transport success rates were greatest for larvae released from Ipswich Bay, intermediate for Stellwagen Bank and least successful for those released from Jeffreys Ledge. There was substantial interannual variability in larval transport and geographic patterns of potential connectivity. Furthermore, the date of spawning had an important influence on transport success. Model results suggest that certain coastal spawning grounds used by winter flounder may serve as an important source of larvae to estuaries and nearshore nursery areas. The potential influx of coastal spawned larvae could have implications for the resilience, productivity and gene flow in local populations. Model results provide further support for the conclusion that winter flounder in the Gulf of Maine may not be solely dependent upon estuaries for spawning. Results also suggest that coastal spawning groups should be considered explicitly in the management of winter flounder, and protected under Essential Fish Habitat regulations.     

Processes controlling retention of spring‐spawned Atlantic cod (Gadus morhua) in the western Gulf of Maine and their relationship to an index of recruitment success

Atlantic cod, Gadus morhua, harvested in US waters are currently managed as a Gulf of Maine stock and as a stock comprising Georges Bank and southern New England populations. Over the past two and a half decades, success of age‐1 recruitment to the Gulf of Maine stock has varied by more than an order of magnitude. To investigate the hypothesis that this variation is related to variation in the transport of larval cod to nursery areas, we carried out model simulations of the movement of planktonic eggs and larvae spawned within the western Gulf of Maine during spring spawning events of 1995–2005. Results indicate that the retention of spring‐spawned cod, and their transport to areas suitable for early stage juvenile development, is strongly dependent on local wind conditions. Larval cod retention is favored during times of downwelling‐favorable winds and is least likely during times of upwelling‐favorable winds, during which buoyant eggs and early stage larvae tend to be advected offshore to the Western Maine Coastal Current and subsequently carried out of the Gulf of Maine. Model results also indicate that diel vertical migration of later stage larvae enhances the likelihood of retention within the western Gulf of Maine. Consistent with model results is a strong correlation between age‐1 recruitment success to the Gulf of Maine cod stock and the mean northward wind velocity measured in Massachusetts Bay during May. Based on these findings, we propose a wind index for strong recruitment success of age‐1 cod to the Gulf of Maine stock.     

Simulating the dispersion of vertically migrating sprat larvae (Sprattus sprattus (L.)) in the German Bight with a circulation and transport model system

A model system consisting of two three-dimensional circulation models and a three-dimensional transport model is used to simulate the dispersal of sprat larvae (Sprattus sprattus (L.)) in the German Bight. A Northwest European shelf sea model simulates the currents of the North Sea and provides sea surface elevation data for the open boundaries of the German Bight circulation model. The German Bight circulation model has a horizontal grid resolution of 2.75 km and a vertical resolution of 5 m except for the bottom layer. The driving forces of the circulation models are the M2-tide, six-hourly time-dependent wind stress and air pressure fields as well as monthly climatological density fields. The simulated currents serve as input to the transport model. The model area, grid size and the vertical resolution of the transport model are in accordance with the German Bight circulation model. The transport model simulates advection and diffusion using a Monte Carlo method. Furthermore it incorporates a simulation of active vertical movement of the larvae. The vertical migration pattern is dependent on the time of day and size of the larvae and is based on field data from the German Bight. Horizontal larvae distribution data, gained from two out of five field surveys carried out in the German Bight during spring and summer 1991, are used as initial values for the starting points of the tracers in the transport model. Simulated tracer distributions are compared with larval distributions observed during subsequent cruises about 3 weeks later. Discrepancies and similarities between tracer and larval distributions are discussed.