Coupling of an individual-based population dynamic model of Calanus finmarchicus to a circulation model for the Georges Bank region

An individual-based life history and population dynamic model for the winter–spring dominant copepod of the subarctic North Atlantic, Calanus finmarchicus , is coupled with a regional model of advection for the Gulf of Maine and Georges Bank. Large numbers of vectors, each representing individual copepods with elements for age, stage, ovarian status and other population dynamic variables, are carried in a computation through hourly time steps. Each vector is updated at each time step according to development rate and reproductive functions derived from experimental data. Newly spawned eggs are each assigned new vectors as needed. All vectors are subject to random mortality. Thus, both life history progression and population dynamics of C. finmarchicus are represented for the temperatures in the Gulf of Maine–Georges Bank region in the active season. All vectors include elements representing depth, latitude and longitude. This allows coupling of the population dynamics to the tide- and wind-driven Dartmouth model of New England regional circulation. Summary data from the physical model are used to advance vectors from resting-stock locations in Gulf of Maine basins through two generations to sites of readiness for return to rest. Supply of Calanus stock to Georges Bank comes from all of the gulf and from the Scotian Shelf. The top of the bank is stocked from western gulf basins; the North-east Peak is stocked from Georges Basin and the Scotian Shelf. All sources contribute to stock that accumulates in the SCOPEX gyre off the north-west shoulder of Georges Bank, explaining the high abundance recurrently seen in that region. There is some return of resting stock to Wilkinson Basin in the western gulf, but other basins must mostly be restocked from upstream sources to the north-east.     

Juvenile cod (Gadus morhua) mortality and the importance of bottom sediment type to recruitment on Georges Bank

The demersal settlement of pelagic juvenile fish has been considered a critical period in which the final adjustment is made to the size of a year class. Distribution patterns of pelagic and recently settled juvenile cod (Gadus morhua) were examined from nine surveys on Georges Bank during the summer over 5 years, 1984–1989, to relate juvenile survival to the sedimentary environment. Pelagic juveniles were widespread across Georges Bank in June, and by mid‐July they occurred on all bottom types from sand to gravel on eastern Georges Bank. However, by late July‐early August they were mostly abundant on the northeastern edge gravel deposit, which with its complex relief, provides abundant prey and refuge from predators. A bank‐wide estimate of pelagic juvenile abundance in 1986 and 1987 was used to assess mortality of the recently settled juveniles and to evaluate the relative importance of survivors from the northeastern edge gravel area to recruitment of the Georges Bank population. Settlement mortality rates over 1–2 months on the northeastern gravel area ranged from 3 to 8% day^?1, which compared reasonably with other studies. The seasonal abundance of the pelagic juveniles was almost an order of magnitude higher in 1987 than 1986; however, recruitment at age 1 was similar, indicating that a high mortality of the demersal juveniles occurred in 1987. The limited northeastern gravel area on Georges Bank may represent a survival bottleneck depending on the variability in the distribution and abundance of juvenile cod settlement in relation to that of their predators.     

Interannual differences in larval haddock survival: hypothesis testing with a 3D biophysical model of Georges Bank

The ultimate goal of early life studies of fish over the past century has been to better understand recruitment variability. As evident in the Georges Bank haddock (Melanogrammus aeglefinus) population, there is a strong relationship between recruitment success and processes occurring during the planktonic larval stage. This research sought new insights into the mechanisms controlling the recruitment process in fish populations using biological–physical modeling methods together with laboratory and field data sets. We created the first three‐dimensional model of larval haddock on Georges Bank by coupling models of hydrodynamics, lower trophic levels, a single copepod species, and larval haddock. Interactions between feeding, metabolism, growth, vertical behavior, advection, predation, and the physical environment of larval haddock were quantitatively investigated using the coupled models. Particularly, the model was used to compare survival over the larval period and the sources of mortality in 1995 and 1998, 2 years of disparate haddock recruitment. The results of model simulations suggest that the increased egg hatching rates and higher food availability, which reduced starvation and predation, in 1998 contributed to its larger year‐class. Additionally, the inclusion of temperature‐dependent predation rates produced model results that better agreed with observations of the mean hatch date of survivors. The results from this biophysical model imply that food limitation and its related losses to starvation and predation, especially from hatch to 7 mm, may be responsible for interannual variability in recruitment and larval survival outside of the years studied.     

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.     

Annual weather variability and its influence on the population dynamics of Calanus finmarchicus

The reaction of the population of Calanus finmarchicus to relatively extreme annual cycles of weather in the North Sea was investigated by one-dimensional model simulations. A population dynamics model for C. finmarchicus was coupled with a physical and a biological upper-layer model for phosphate, phytoplankton and detritus to simulate the development of the successive stages of C. finmarchicus. Observed annual weather cycles were used to drive the physical water column model, the results of which were then input to the plankton model. The simulations yielded the temporal development of all stages of C. finmarchicus over an annual cycle in terms of numbers and weights. Compared with the results reported for 1984 by Carlotti and Radach (1996; Limnol . Oceanogr . 41: 522–539), it appears that the temporal range for the occurrence of the peak concentration of C. finmarchicus may be 2 weeks in the northern North Sea. The simulated variability is in accordance with observations. Years with more continuous primary production such as the year 1984, which was cloudy and windy during the summer, may then provide the optimum conditions with respect to producing an abundance of zooplankton during the following year.     

Larval lobster (Homarus americanus) distribution and drift in the vicinity of the Gulf of Maine offshore banks and their probable origins

Surveys for lobster larvae in offshore waters of the north‐eastern Gulf of Maine in 1983, 1987 and 1989 confirm that local hatching occurs mainly at depths <100 m over the banks, including Georges and Browns Banks. Detailed studies in the vicinity of Georges Bank in late July of both 1987 and 1989 indicate that the first and second moult stages were located primarily over the bank whereas stages III and IV lobster were collected both over and off the bank. At times stage IV lobster were more abundant off the bank than over it. The condition of stage III and IV lobster, as measured by a lipid index, was better off than over Georges Bank in 1988 and 1989 indicating a possible physiological advantage to being off the bank. In addition, the higher surface temperatures off Georges Bank would shorten larval development time to settlement. To determine the probable hatch sites of stage IV lobster collected off of Browns Bank in 1983 and off of Georges in 1987 and 1989, a 3‐D circulation model of the Gulf of Maine was used to simulate larval lobster drift backwards in time. In all cases, areas off Cape Cod, MA, and off Penobscot Bay, ME were suggested as the source of the larvae, although most of the larval trajectories never reached these near‐shore waters that are well‐known, larval hatching areas. The model‐projected larval release times match most closely the observed inshore hatch off Massachusetts but model uncertainties mean that coastal Maine cannot be ruled out as a source. Georges Bank is also a potential source because the present model does not take into account short‐term wind events, off‐bank eddy transport or the possibility of directed off‐bank larval swimming. Examination of weather records prior to and during our 1988 and 1989 sampling periods indicates that winds were not of sufficient intensity and duration to induce larval transport off Georges Bank. The shedding of eddies from the northern flank of Georges Bank into the Gulf of Maine are a relatively common phenomenon during summer but not enough is known about them to evaluate their contribution to possible cross‐bank transport of lobster larvae. Directed larval swimming is another possible source for the stage IV lobster found near Georges Bank. Plankton distributions across the northern frontal zone of Georges Bank in 1988 were used as proxies for the scarce larval lobsters. The more surface distribution of the microplankton, in particular, supports the possibility that wind and eddy events may be important in the transport of stage III and IV lobsters off of Georges Bank. Further studies are needed to evaluate these possible additional sources of advanced stage lobster larvae found off of the offshore banks.     

A general biophysical model of larval cod (Gadus morhua) growth applied to populations on Georges Bank

Field-derived growth rates (RNA-DNA based) of cod (Gadus morhua) larvae collected on the southern flank of Georges Bank were higher on average in May 1993 than May 1994, despite the apparent higher abundance of potential prey in 1994. A biophysical modeling study is presented here in which factors are examined that may have led to the difference in population mean growth. A one-dimensional physical model, forced by winds and tides, was used to simulate the vertical structure (of currents, temperature field, and turbulent kinetic energy dissipation rate) following a column of water in a Lagrangian sense at a site on the southern flank of Georges Bank over 5-day periods in late May of 1993 and 1994. The biophysical model and observed zooplankton abundance allowed us to explore the vertical structure and temporal (hourly) evolution of feeding and growth for cod larvae in relation to environmental conditions. Our trophodynamic model is improved over previous versions and now includes the effect of light on larval feeding response, as well as the effect of temperature on larval metabolic costs, ingestion, and digestion. Larval prey profiles, comprising four copepod species, were used from a time series of 1/4-m² MOCNESS tows to define the prey field. Data from a collateral time-series of larval gut contents (1-m² MOCNESS tows) was used to define maximum ingestion (satiation level) and prey selection. Model outputs provide depth-dependent estimates of growth, prey biomass ingested, larval length, and larval weight. Water-column growth-rate profiles were made for four size classes of larvae (5, 6, 7 and 9 mm) under the environmental conditions observed in May 1993 and 1994. A weighted-mean growth rate based on the mean vertical distribution of larvae was estimated for each size class. In all cases, when using all available potential prey, the model-derived 1994 growth rates were higher (by 3–6% day⁻¹) than those for 1993. However, simulations in which 7-mm larvae followed the field-derived weighted mean depth over the sampling period, and were limited to their preferred Pseudocalanus prey, resulted in average growth of 12.2% day⁻¹ for 1993 and 9.7% day⁻¹ for 1994. These compared closely to the field growth means of 11.3% day⁻¹ in 1993 and 9.8% day⁻¹ in 1994. Thus, the lower observed growth in May 1994 may have resulted from depth-dependent food limitation and prey-selectivity coupled with the greater metabolic costs induced by the higher temperature that year.     

Optimum temperature and food-limited growth of larval Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) on Georges Bank

We estimated recent growth of Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) larvae collected on the southern flank of Georges Bank in May 1992–94 from the ratio of RNA to DNA (R/D) and water temperature. Growth of both species increased with water temperature to about 7°C and then decreased. The highest growth rates were observed in May 1993 at water temperatures around 7°C. These data confirm an earlier observation of comparable temperature optima for growth of Atlantic cod and haddock larvae in the north‐west Atlantic. Comparisons of field growth rates and temperature optima with data for larvae cultured at high temperatures and prey densities in the laboratory suggest that growth may have been food‐limited at higher temperatures on Georges Bank. Given that 7°C is the long‐term mean water temperature on the southern flank in May and that climate models predict a possible 2–4°C rise in water temperatures for the western North Atlantic, our findings point to a possible adverse effect of global warming on Atlantic cod and haddock.     

Modelling changes in the length–frequency distributions of fish larvae using field estimates of predator abundance and size distributions

The goal of this study is to determine if an individual-based size-dependent model can realistically simulate changes in the length–frequency distributions of several species of fish larvae collected in Conception Bay in 1993 and 1994, using field estimations of growth and predator abundance. We first model the length–frequency distribution of field samples with the best possible estimates of mean growth rate. Then, we add predation mortality given the characteristics of the predator community observed during our surveys, which was composed of macrozooplankton and adult capelin. The larval fish community is generally not affected by predation by macrozooplankton, as the average instantaneous mortality rate predicted by the model was 0.004 day^–1. Fish larvae appear to be more vulnerable to predation by the population of adult capelin. We estimate that an abundance of adult capelin ranging between 0.2 and 1.0 individuals per 1000 m^–3 may have a substantial impact on the larval fish community. The predictions of an individual-based model are directly related to the accuracy of estimates of the mean growth rates of the larval fish cohorts. We find that it is difficult to differentiate size-selective removal of individuals from random selection by analysing changes of the length–frequency distributions of the larval fish community.     

Spatial distribution and overlap between ichthyoplankton and pelagic fish and squids on the southern flank of Georges Bank

We conducted larval and adult fish surveys on the southern flank of Georges Bank during the spring of two years (1990 and 1995) with contrasting physical conditions. We employed canonical correspondence analysis (CCA) to examine the relationships between physical variables and the spatial distribution of pelagic fish and ichthyoplankton. Surface temperature bottom temperature, and vertical stratification were significant factors affecting larval fish distributions, and there were groups of species with similar responses to these variables. There were also consistent relationships between physical variables and pelagic fish and squid abundances and spatial distributions. Pelagic fish and ichthyoplankton with similar responses to hydrographic variables had high spatial overlap, and variation in hydrographic regimes modulated the strength of this interaction. Pelagic fish and squids are potentially important predators of both larval and juvenile fish on Georges Bank. Hydrographic structure modulates the degree of spatial overlap and therefore likely influences the strength of predator–prey interactions.     

An adjoint data assimilation approach to diagnosis of physical and biological controls on Pseudocalanus spp. in the Gulf of Maine–Georges Bank region

The underlying scientific objective here is to determine the mechanisms that control seasonal variations in the abundance of Pseudocalanus spp. in the Georges Bank–Gulf of Maine region. It is postulated that the observed distributions result from the interaction of the population dynamics with the climatological circulation. The problem is posed mathematically as a 2-D advection–diffusion–reaction equation for a scalar variable. Given an initial distribution of animals, we seek the population dynamics source term R ( x , y ) such that integration of the forward model will result in predictions3 that minimize the sum of squares of differences with observed concentrations at a later time. An adjoint data assimilation technique has been designed for these purposes.
This approach has been used to invert for the population dynamics associated with the transition between bimonthly (i.e. for 2 months) climatological Pseudocalanus spp. distributions derived from MARMAP data. Vertically averaged velocity and diffusivity fields diagnosed from hydrodynamical simulations of the climatological flow are specified. Solutions converge rapidly, and the procedure reduces the cost function by an order of magnitude within 50 iterations. The resulting population dynamics vary considerably in space and time, as does the balance between local tendency, physical transport and biological source terms. Generally speaking, the patterns in population dynamics are not inconsistent with current knowledge concerning potential controls such as predation and food limitation. Analysis of the solutions indicates that the Pseudocalanus spp. population centres located in the western Gulf of Maine and on Georges Bank may be self-sustaining, in contrast to prior studies which characterize the former as a source region for the latter.     

Dispersal of black sea bass (Centropristis striata) larvae on the southeast U.S. continental shelf: results of a coupled vertical larval behavior – 3D circulation model

In the marine environment, pelagic dispersal is important for determining the distribution and abundance of populations, as well as providing connections among populations. Estimates of larval dispersal from spawning grounds are important to determining temporal and spatial patterns in recruitment that may have significant influences on the dynamics of the population. We present a case study of the dispersal of Centropristis striata (black sea bass) larvae on the southeast U.S. continental shelf. We use a coupled larval behavior – 3D circulation model to compare the effects of the timing and location of spawning against that of larval vertical migration on larval dispersal. Using the results of field data on larval vertical distributions, we compare the dispersal of virtual 'larvae' which have ontogenetic changes in vertical behavior with that of particles fixed near the surface and near the bottom. Larvae were released at potential spawning sites four times throughout the spawning season (February through May) for 3 yr (2002–04) and tracked for the assumed larval duration (from 27 to 37 days including the egg stage). Results indicate that adult behavior, in the form of spawning time and location, may be more important than larval vertical behavior in determining larval dispersal on the inner- and mid- continental shelves of this region.     

Food organisms and feeding habits of larval and juvenile Japanese flounder Paralichthys olivaceus at Ohama Beach in Hiuchi-Nada, the central Seto Inland Sea, Japan

ABSTRACT:   The abundance of food organisms and feeding habits of larval and juvenile Japanese flounder were examined during the period from May to August in 1999, 2000 and 2001 at the sandy Ohama beach, the central Seto Inland Sea. The food organisms collected with a sledge net consisted of 40 families from 18 orders, dominated by mysids, crangonids and gammarids. The mean densities of mysids, crangonid shrimp ( Crangon spp.), gammarids and fish were 2.74, 6.74, 2.91 and 0.15 individuals/m², respectively. The main prey of the flounder ( n  = 202; range of total length 9.80–75.95 mm) was mysids and small crangonid shrimp (<14 mm in body length). Prey fish availability was low, as the density of fish was low. The small crangonid shrimp was abundant, and the large crangonid shrimp, which could prey on larval flounder, was not abundant. The crangonid shrimp was important not as a predator for the flounder but as prey. The flounder preferred epifaunal mysids, Nipponomysis ornata and Anisomysis ijimai , to sand-burrowing mysids, Iiella oshimai , and avoided crangonid shrimp.     

Analysis of the trophy sport fishery for the speckled peacock bass in the Rio Negro River, Brazil

Abstract  The middle portion of the Rio Negro River in Brazil near the equator supports a popular recreational sport fishery for speckled peacock bass, Cichla temensis (Humboldt). The objective of this study was to determine the effect of fishing mortality on this population. Fish were collected from sport-fishing ( n  = 72) and commercial ( n  = 103) catches and otoliths were aged to estimate longevity, growth and natural mortality. Recreational anglers in this region seek to catch, then release, larger speckled peacock bass; and fish larger than 62 cm standard length (SL) (about 4.5 kg) served as a bench mark to assess the potential impact of subsistence and commercial harvest on the abundance of larger fish in the sport fishery. Time of opaque band formation on otoliths generally coincided with the dry season (November to April); these bands appeared to form once per year, but formation was highly variable. Speckled peacock bass grew to 62 cm SL on average in 6.4 years, but some fish obtained this size in 4–5 years. Maximum age was 9 years, but most fish were less than 7 years. Instantaneous annual natural mortality ( M ) estimated from maximum size, longevity and growth ranged from 0.19 to 0.44. Simulation modelling predicted that exploitation rates of fish >25 cm SL similar to the estimated natural mortality rates would reduce the abundance of fish >62 cm by 67–89% compared with no harvest. Even modest exploitation rates of 5% and 10% would result in approximately 30–50% reduction, respectively, of these larger fish. Abundance of large speckled peacock bass that sustains the sport fishery is susceptible to low rates of exploitation in this remote region of Brazil.     

Distribution and abundance of overwintering Calanus finmarchicus in the Faroe–Shetland Channel

The distribution and abundance of overwintering Calanus finmarchicus in the Faroe–Shetland Channel was studied during December 1994 and January 1995. Maximum abundance of animals in the Channel was approximately 50 000 m⁻², with a peak concentration of 627 m⁻³ at a depth of 930 m. Copepodite stages IV and V accounted for > 98% of the population. A clear association was found between the horizontal and vertical distribution of animals and the Arctic water masses in the bottom of the Faroe–Shetland Channel. The Wyville–Thomson Ridge formed a barrier to the southern distribution of Arctic waters and the abundance of overwintering C. finmarchicus was 25 times lower to the south of the Ridge than to the north. Spatial variability in lipid content and composition indicated that overwintering C. finmarchicus in the southern Norwegian Sea were in poorer condition with respect to wax ester content, and in a more advanced state of emergence from overwintering, than the animals within the Channel. The overwintering stock in the Channel probably originated from the Norwegian Sea or from north of the Faroe Islands. The Faroe–Shetland Channel is an important source of animals advected into the northern North Sea in the spring (March/April). The population abundance in the Faroe–Shetland Channel was estimated to be 4.5 × 10¹⁴ individuals, which is more than adequate to account for the mean concentration of adult stages observed in the northern North Sea in April.     

The demography of Calanus pacificus during winter–spring Californian El Niño conditions, 1992–1993: implications for anchovy?

Samples from the southern California sector of the California Current System were examined to test for changes in abundance, reproduction, recruitment and naupliar survival of the planktonic copepod, Calanus pacificus, coincident with the 1993 Californian El?Niño, relative to 1992 (also El Niño conditions) and to 1989–1991 (defined as `normal'). In 1993, as in 1992, females were rare in both winter and spring, but per capita reproduction was less, food limitation was greater and biomass of chlorophyll was reduced only in winter. Recruitment was more variable than was naupliar survival. Recruitment increased in both El Niño springs, but survival of older naupliar stages decreased. The mesoscale distributions of larval anchovy, relative to eggs and nauplii of Calanus, did not result in efficient use of the reduced supply of this source of food, and the abundance of larval anchovy did not cause measurable variation in the survival of naupliar Calanus.     

Coastal winds and larval fish abundance indicate a recruitment mechanism for southeast Australian estuarine fisheries

Coastal winds transport water masses and larval fish onshore or offshore which may influence estuarine recruitment, yet our understanding of the mechanism underlying this relationship is limited. Here, we combine datasets from a historical database of larval fish off southeast Australia with a high-resolution atmospheric reanalysis model to show that normalised abundance of coastally spawned larvae increased with weak to moderate upwelling favourable winds 14 days prior to sampling. The increase in abundance may reflect increased nutrient and plankton availability for larval fish. Normalised larval abundance decreased following strong upwelling favourable winds but increased after onshore (downwelling favourable) winds, due to wind-driven transport. By combining a commercial estuarine fisheries catch-rate dataset (4 species, 8 estuaries, 10 years) and the high-resolution atmospheric reanalysis model, we show that negative effects of upwelling favourable winds during the spawning period can be detected in lagged estuarine commercial fisheries catch rates (lagged by 2–8 years depending on species' growth rates), potentially representing the same mechanism proposed for larval fish. Upwelling favourable winds in the southeast Australian region have increased since 1850 while onshore winds have decreased, which may have reduced larval recruitment to estuaries. Coastal winds are likely an important factor for estuarine recruitment in the southeast Australian region and future research on the estuarine recruitment of fish should incorporate coastal winds.     

A system for model-based biomass estimation of larvae in intensive cod larvicultures

High and unpredictable mortality rates are observed in the larval rearing of cod (Gadus morhua). As a means of addressing this problem, we present a model-based estimator system which can be used to indirectly measure the larval density through monitoring the live food dynamics and larval growth. The estimator has been evaluated in a conceptual context using a preliminary model formulation, and the observability of the process has been investigated. It was found that the two parameters, live food dynamics and larval growth, contain enough information for the larval density to be estimated under noisy conditions, given the correct model. When the system is applied practically, the estimation error will depend on the measurement and model accuracy; this is especially true with respect to the predictability of the feed intake rate of the fish.     

A Lagrangian ensemble model of Calanus finmarchicus coupled with a 1D ecosystem model

A population dynamics model of Calanus finmarchicus based on Lagrangian particles has been coupled with a 1-D ecosystem model. Each of the particles represents a variable number of copepods which experience the same fate. Therefore all copepods of a single particle represent a cohort and are characterized by a common set of individual properties such as age, development-stage, depth, structural weight (length), lipid pool or food satiation. The physical environment is parameterized by a 1-D-water column with a vertical resolution of 1 m and a maximum depth of 800 m. Copepod food supply is provided by an interactive Eulerian NPZD model where Z represents microzooplankton. The model correctly reproduces both the dynamics of the ecosystem and the life history of the copepods in the Norwegian Sea. Simulated results of trajectories of particles in the water column, and of individual growth and stage development were analysed. Results on seasonal abundance, development time, number of generations, depth profiles, and patterns of diurnal and ontogenic migration are compared with field data from OWS India.