Simulation of mackerel (Scomber scombrus) recruitment with an individual-based model and comparison with field data

An individual‐based model (IBM) for the simulation of year‐to‐year survival during the early life‐history stages of the north‐east Atlantic stock of mackerel (Scomber scombrus) was developed within the EU funded Shelf‐Edge Advection, Mortality and Recruitment (SEAMAR) programme. The IBM included transport, growth and survival and was used to track the passive movement of mackerel eggs, larvae and post‐larvae and determine their distribution and abundance after approximately 2 months of drift. One of the main outputs from the IBM, namely distributions and numbers of surviving post‐larvae, are compared with field data as recruit (age‐0/age‐1 juveniles) distribution and abundance for the years 1998, 1999 and 2000. The juvenile distributions show more inter‐annual and spatial variability than the modelled distributions of survivors; this may be due to the restriction of using the same initial egg distribution for all 3 yr of simulation. The IBM simulations indicate two main recruitment areas for the north‐east Atlantic stock of mackerel, these being Porcupine Bank and the south‐eastern Bay of Biscay. These areas correspond to areas of high juvenile catches, although the juveniles generally have a more widespread distribution than the model simulations. The best agreement between modelled data and field data for distribution (juveniles and model survivors) is for the year 1998. The juvenile catches in different representative nursery areas are totalled to give a field abundance index (FAI). This index is compared with a model survivor index (MSI) which is calculated from the total of survivors for the whole spawning season. The MSI compares favourably with the FAI for 1998 and 1999 but not for 2000; in this year, juvenile catches dropped sharply compared with the previous years but there was no equivalent drop in modelled survivors.     

Occurrence of postmortem myoliquefactive kudoosis in Atlantic mackerel, Scomber scombrus L., from the North Sea

Members of the myxosporean genus Kudoa occur in various marine teleosts worldwide. Several species are of concern to the fishery and aquaculture industries as they may produce unsightly cysts in the fish host's musculature or are associated with postmortem myoliquefaction of the fish muscle, commonly referred to as 'soft flesh'. This study describes the occurrence and effects on a host of a Kudoa species in Atlantic mackerel, Scomber scombrus , from the northern North Sea. Generalized postmortem myoliquefaction associated with Kudoa sp. occurred in 0.8% of the examined fish, i.e. 11 of 1339 mackerel developed 'soft flesh'. There was a significant difference in the prevalence of myoliquefaction between medium sized (400–600 g) and large mackerel (>600 g). The prevalence reached 8.9% in the latter host size group. No subclinical infections of Kudoa sp. were detected when examining fresh muscle ( n  = 103) and blood ( n  = 165) samples for spores using light microscopy. Affected mackerel developed generalized myoliquefaction after 38–56 h post-catch. No inflammatory host response was associated with the presence of plasmodia within single body muscle fibres of 'soft flesh' affected fish. Based on comparison of myxospore dimensions and analysis of the nuclear small subunit (SSU) ribosomal DNA, the present Kudoa species is assigned to Kudoa thyrsites . However, due to the species' apparently very wide geographical distribution and host range, its varying effect on different fish host species, together with the still unknown life cycle of Kudoa spp., the taxonomic status of K. thyrsites appears not to be fully resolved.     

Tracking shifts in Atlantic mackerel (Scomber scombrus) larval habitat suitability on the Northeast U.S. Continental Shelf

Climate change has altered the oceanographic environment and subsequently the habitats of marine species. Fish and invertebrate populations’ responses to habitat include movement with latitude and depth to remain within their fundamental niches. The northwest Atlantic mackerel (Scomber scombrus) population has fluctuated over the last century due in part to changes in the environment. We used species distribution models to understand the influence of the physical (temperature) and biological (zooplankton) environment on mackerel larval abundance, and how such relations have determined larval habitat suitability in the Northeast U.S. Shelf. Atlantic mackerel larval presence and abundance correlated with sea temperature and copepod abundances, suggesting that larval survival may be sensitive to specific temperatures and zooplankton prey. Predicted abundances were spatially interpolated to estimate Atlantic mackerel larval suitable habitat. Metrics for habitat quality indicate that the Mid‐Atlantic Bight has become less suitable over time. Since the 1970s, the proportion of Northeast U.S. Shelf suitable habitat located in the Mid‐Atlantic Bight has decreased, as southern New England and the western Gulf of Maine regions have become more suitable. Habitat suitability within the Northeast U.S. Shelf has shifted northeast: from the Mid‐Atlantic Bight‐southern New England border towards the northeast portion of southern New England. While total Northeast U.S. Shelf habitat suitability has decreased since the 1970s, the decline in the time series trend was not statistically significant. Thus, while select ecoregions have decreased in habitat suitability, larval habitat does not appear to be the only contributor to decreases in the U.S. Atlantic mackerel contingent.     

An individual-based model for the direct conversion of otolith into somatic growth rates

Otolith increment width and larval fish data (length and weight) were used to develop an individual‐based model (IBM) to describe daily resolved growth rates of North Sea herring (Clupea harengus) larvae (Autumn Spawners) caught during International Herring Larvae Surveys in the ICES area IVa from 1990 to 1998. The model combines sagittal otolith readings (core and individual increment measurements), larval standard length and weight data, and solves an over‐determined set of linear system equations for all parameters using the method of least square residuals. The model consists of a matrix, which describes the increment width formation of 119 larvae, a vector containing their length/weight measurements, and a vector describing residuals. The solution vector yields age‐dependent maximum somatic growth rates of herring larvae up to an age of 41 days with sizes ranging from 10 to 25 mm. The observed environmental temperature in which larvae dwelled was relatively uniform. Therefore, measured increment width was individually used to determine daily growth from any single larva in relation to their potential maximum growth under optimal feeding conditions. The results are discussed with respect to the spatial and temporal variability of larval occurrence. Finally, an analysis of error estimation of the larval growth characteristics is presented.     

Environmental control of Northeast Atlantic mackerel (Scomber scombrus) recruitment in the southern Bay of Biscay: case study of failure in the year 2000

We investigate the effect of strong meteorological perturbations in early spring on the success of mackerel (Scomber scombrus) recruitment in the N/NW Iberian area (southern Bay of Biscay) for the period 1999–2008. In 2000, the year of the most pronounced recruitment failure on record, two consecutive multidisciplinary surveys sampled hydrographic conditions and mackerel eggs, larvae and post‐larvae over the main mackerel spawning grounds of the north and northwest coast of the Iberian Peninsula. Analysis of egg and larval abundance and birthdates based on the otoliths of mackerel juveniles caught between July and October 2000 showed that there were no survivors from the early spring spawns, indicating a massive loss of early spawning effort. Moreover, the abundance of 1‐year‐old mackerel estimated from an acoustic survey carried out in 2001 was the lowest observed within the 1999–2008 time series. This low or null survival from the early spawns in 2000 could be due to the meteorological and oceanographic conditions of that spring, in particular two storm events in April after a relatively calm March. The first storm event from the north caused strong local wind in the southern Bay of Biscay but a weak oceanographic response. The second storm event from the southwest was mainly felt west of Galicia and caused a notable increase in shelf currents and a shift of the hydrographical structure along the shelf. Detailed analysis of strong wind pulses in early spring within the historical recruitment record suggests that strong local turbulence generated by high wind speeds and advection of larvae caused by the enhancement of shelf currents can contribute to reduced recruitment. Our observations indicate that, in 2000, both mechanisms were present.     

Environmental factors and recruitment of mackerel, Scomber scombrus L. 1758, along the north-east Atlantic coasts of Europe

Increasingly, scientific publications refer to some of the environmental factors affecting the recruitment in fish species. However, presently, there is little information available concerning the influence of the environment on the recruitment of Scomber scombrus, the North-east Atlantic mackerel (NEAM). In this contribution, North Atlantic Oscillation (NAO), upwelling and turbulent mixing (or `turbulence') series were compared with recruitment estimates, at age-0. The results show that the southern prespawning migration pattern of the Atlantic mackerel is directed towards areas with low turbulent mixing at spawning time, providing a `stable environment' for egg and larval survival. In the southern areas, where the spawning starts, the turbulence conditions of prespawning and spawning periods have the largest influence on the success of recruitment; this is related, possibly, to the more `stable' weather in the subsequent months and for the remainder of the year. In contrast, in the northern areas, the role of turbulence over the entire year becomes increasingly more relevant; this is related, possibly, to the high levels of turbulence during autumn and winter, which may become limiting to the survival of juveniles. About 50% of the variability in the Atlantic mackerel recruitment may be explained by means of environmental variables, such as turbulence. Other variables, such as upwelling and NAO, are only slightly, or not, statistically significant.     

First‐year survival of North East Atlantic mackerel (Scomber scombrus) from 1998 to 2012 appears to be driven by availability of Calanus,a preferred copepod prey

Mackerel (Scomber scombrus) is one of the ecologically and economically most important fish species in the Atlantic. Its recruitment has, for unknown reasons, been exceptional from 1998 to 2012. The majority (75%) of the survivors in the first winter were found north of an oceanographic division at approximately 52°N, despite the fact that mackerel spawns over a wide range of latitudes. Multivariate time series modelling of survivor abundance in the north revealed a significant correlation with the abundance of copepodites (stage I–IV) of Calanus sp. in the spawning season (April to June). The copepodites were a mix of C. helgolandicus (dominating) and C. finmarchicus. The growth of mackerel larvae is known to be positively related to the availability of nauplii and copopodites of preferred prey species, namely, large calanoid copepod species such as Calanus. The statistical relationship between mackerel survivors and abundance of Calanus, therefore, most likely, reflected a causal relationship: high availability of Calanus probably reduced starvation, stage‐specific predation and cannibalism (owing to prey switching). The effects of other abundant, but less preferred zooplankton taxa, (Acartia sp., Branchiopoda spp. and Echinodermata spp. larvae), as well as stock size, temperature and wind‐induced turbulence were not found to be significant. However, stock size was retained in the final model because of a significant interaction with Calanus in oceanic areas west of the North European continental shelf. This was suggested to be a consequence of a density driven expansion of the spawning area that increased the overlap between early life stages of mackerel and food (Calanus) in new areas.     

Availability of horse mackerel (Trachurus trachurus) in the north-eastern North Sea, predicted by the transport of Atlantic water

One of the clearest relationships between a single marine climate variable and fisheries yield is found between the transport of Atlantic water into the North Sea in the winter and catches of horse mackerel (Trachurus trachurus L.) in the northern North Sea 6 months later. A three‐dimensional numerical model, the NORWegian ECOlogical Model system (NORWECOM), has estimated this inflow during the winter from 1976 to 2000, which is strongly correlated with the Norwegian fleet's catch of horse mackerel (r² = 0.70) in the following autumn. It is hypothesized that a large influx of this relatively warm and nutrient rich water during the winter may support an early spawning of zooplankton and high biological production during the spring and summer. This might be the biological reason for a large fraction of the horse mackerel stock migrating into the North Sea during strong inflows.     

Interannual variability of the early life history of walleye pollock near Shelikof Strait as inferred from a spatially explicit, individual-based model

A coupled biophysical model is used to hindcast the early life history of a population of walleye pollock ( Theragra chalcogramma ), to assess possible physical causes of interannual variability in recruitment. The modelling approach combines a primitive equation, rigid'lid hydrodynamic model with a probabilistic, individual-based biological model of growth, development, and mortality. Individuals are tracked through space using daily velocity fields generated from the hydrodynamic model, along with self-directed vertical migrations appropriate to each life stage in the biological model. The hydrodynamic model is driven with wind and runoff time series appropriate to each year. Biological model output compares favourably with observed spatial distributions for specific years. Lloyd's index of patchiness, calculated from model output, was similar to values calculated from field data. Five noncontiguous years were chosen for hindcasts to span a wide range of meteorological conditions (winds, runoff) and recruitment success. Interannual comparisons suggest that two years of above average recruitment (1978 and 1988), and one year of below average recruitment (1991), experienced flow fields which carried many individuals into the Alaskan Stream. At the same time, the vigorous flow fields generated in each of these years carried some individuals onto the shelf area to the south-west of the spawning site. A year with low runoff and weak winds (1989) exhibited weak circulation, with extended retention of larvae near the spawning site. A year with high runoff (1987) was notable for the strength and frequency of mesoscale eddy activity. Eddies appear capable of both enhancing patchiness of early larvae (through retention) and dissipating patchiness of juveniles (through mesoscale mixing). Larvae retained in an eddy feature exhibit a narrower range of sizes than the population outside that feature.     

Effect of environmental variability on body condition and recruitment success of Atlantic Mackerel (Scomber scombrus L.) in the Gulf of St. Lawrence

The objective of this study was to assess the effect of environmental variability on the dynamics of the Atlantic mackerel (Scomber scombrus L.) stock in the Gulf of St. Lawrence (GSL). We first described the dominant modes of physical and biological (zooplankton) variability using Principal Components Analyses of 40 variables. Two principal modes of variability were identified, a long‐term mode (15–20 yr) associated with a warming of the GSL and a second mode describing alternating cold and warm periods at a higher frequency (5–10 yr). A strong link between physical forcing and the dynamics of zooplankton species known to be important for mackerel was shown. Second, a set of Generalized Additive Models (GAM) was developed to explore how these environmental variations could influence mackerel condition (Fulton's K) and recruitment success (R_s). Optimal GAMs including variations in abundance, species composition and phenology of key copepods improved model performance by 40–50% relative to those considering only physical environmental conditions. The results are consistent with the match–mismatch hypothesis and illustrate the key role of zooplankton dynamics in modulating variations in mackerel K and R_s. Finally, this study showed that large variations in R_s could be caused by varying environmental conditions independently of the influence of stock biomass. Our results strongly indicate that the effect of environmental variability should be considered in the implementation of an ecosystem‐based approach to Atlantic mackerel stock management.     

A numerical model of the dispersion of blue whiting larvae, Micromesistiuspoutassou (Risso), in the eastern North Atlantic

A numerical circulation and transport model system was used to simulate the dispersion of larvae of blue whiting, Micromesistius poutassou (Risso) in the eastern North Atlantic. The area of the model extends from the northern Bay of Biscay to the Norwegian Sea and covers the shelf-edge and adjacent waters at a horizontal resolution of around 20 km in 16 vertical layers. Larval input data were based on the long-term mean distribution, abundance and seasonal occurrence of larvae, derived from historical information. The circulation model was run using tidal forcing and climatological density fields as well as both climatological meteorological forcing and actual six-hourly wind stress fields for 1994 and 1995. Transport from the main spawning areas to the west of the British Isles and north of Porcupine Bank was associated with currents along the shelf-edge and in the Rockall Trough. Tracers were either dispersed to the north and north-east along the shelf-edge, extending into the northern North Sea and Norwegian Sea, or were retained in the Rockall Gyre and over Porcupine Bank. A less intense southerly flow from Porcupine Bank was observed both under climatological conditions and in the 1995 simulation, when winds were more variable than in 1994. The results based on the 1995 meteorological conditions showed the most extreme retention of tracers in the Rockall Trough/shelf-edge area west of Scotland and a low penetration of tracers onto the shelf. These results are discussed in relation to the observed distribution of 0-group juveniles and to indices of year-class strength – in particular, in relation to the 1995 year class, which is the highest year-class estimate of blue whiting on record.     

Effects of synthetic kisspeptin peptides and GnRH analogue on oocyte growth and circulating sex steroids in prepubertal female chub mackerel (Scomber japonicus)

In recent years, kisspeptin peptides, encoded by kiss genes have been used to manipulate reproductive processes in farmed animals, including fish. Our previous studies demonstrated that the chub mackerel brain expresses kiss1 and kiss2 and intramuscular injection of synthetic Kiss1 pentadecapeptide (Kiss1‐15) but not Kiss2 dodecapeptide (Kiss2‐12) accelerates spermatogenesis in prepubertal male chub mackerel (Scomber japonicus). In the present study, we evaluated their effects in prepubertal female chub mackerel. The gonadosomatic index (GSI) values of experimental fish did not show any significant changes. Condition factor (CF) values increased significantly in Kiss1‐15 treated fish, in comparison with control and GnRH analogue (GnRHa) injected fish. Histologically, only perinucleolar oocytes were found in all experimental fish. However, Kiss and GnRHa treated fish showed a significant increase in the perinucleolar oocyte diameter, in comparison with the control fish. Gene expression analyses revealed decreased expression of gnrh1 in the telencephalon‐preoptic region of the brain of Kiss2‐12 and GnRHa injected fish, in comparison with control fish. In contrast, GnRHa injected fish exhibited higher levels of fshβ in the pituitary, with no changes in the levels of lhβ among different treatments. Levels of circulating sex steroids, testosterone, and estradiol‐17β were significantly higher in Kiss1‐15 injected fish, in comparison with control fish. These results indicate that synthetic kisspeptin peptides and GnRHa can induce oocyte growth in prepubertal female chub mackerel.     

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.     

Bio-physical modelling of the early life stages of haddock, Melanogrammus aeglefinus, in the North Sea

An individual-based modelling approach was developed to investigate the spatial and temporal patterns in the recruitment processes of North Sea haddock, Melanogrammus aeglefinus . The approach was based on the realization that the survivors to recruitment of an annual cohort are most probably not drawn at random from the initial population of eggs, but represent the fastest-growing individuals. Individual growth rates reflect the unique exposure of each larva to the environment along its drift trajectory. In this context, the environment refers to a wide range of factors affecting growth such as food, turbulence and temperature. A combination of a model of egg production by the adult stock, a particle-tracking scheme, and a model of larval growth and mortality rate was used to simulate the dispersal trajectories, and the survival of haddock larvae spawned at different times and locations on the continental shelf. The particle tracking was driven by flowfields from a climatological implementation of the Hamburg Shelf–Ocean Model (HAMSOM) for the North Sea and NE Atlantic. The system was able to resolve spatial and temporal patterns in the recruitment process and indicated that the surviving population of larvae was drawn from a restricted part of the spawning distribution. The results have the potential to guide the development of future conservation measures in fisheries management.     

Stock assessment of the blue jack mackerel,Trachurus picturatus,in the North‐eastern Atlantic

A total of 49,151 blue jack mackerel, Trachurus picturatus, (Bowdich) was collected in Madeira Island (North‐eastern Atlantic) between 2002 and 2016 to evaluate possible influence of fishing on landings and reproductive parameters. A decreasing trend in the length composition was observed over the study period and length at first maturity decreased by 2.78 cm TL. Maximum yield per recruit decreased from 2002 to 2016 but the corresponding fishing mortality was constant (F_max = 0.4/year). Considering the fishing mortality level in 2016, it is evident that the stock may be exploited beyond its sustainability limit. Amendments of the purse‐seine fishing regulations and implementation of measures to reduce fishing effort are suggested.     

Modelling the retention and survival of Browns Bank haddock larvae using an early life stage model

Browns Bank, the principal spawning ground for haddock on the south-west Nova Scotian shelf, is composed of two distinct production zones: the inshore Bay of Fundy (BoF) region and the offshore south-west Nova (SWN) bank region. Fish growing in the BoF are larger at age than those in the SWN region. Analysis of research vessel (RV) data shows that the majority of age-2 haddock have the size of south-west Nova Scotia fish, suggesting that the majority of surviving fish were retained in SWN. We used an early life stage (ELS) model to address two questions. First, we asked whether the length-at-age difference between Bof and SWN is evident at the larval stage. Using a temperature-based growth model, we found that predicted size differences for late larvae would be less than 0.5 mm. From consideration of the average growth curves to age-2 for the two regions, we showed that this difference was not the seed for the size difference in later life stage. The second question we addressed was how well the ELS model predicted partitioning of late larvae between SWN and the BoF corresponding to the ratio of SWN/BoF age-2 juveniles found in the RV data. We ran the model on a representative number of years between 1973 and 1992, and found a significant correlation between the model simulation and the data (cor=0.71, P =0.047). This indicates that the age-2 size distribution is a reflection of retention and survival occurring during the first two months of life. Model/data discrepancies are explained in terms of differential mortality between the two regions.     

Pacific sand lance of the Port Moller estuary, southeastern Bering Sea: an estuarine^dependent early life history

Three waves of spawning Pacific sand lance (Ammo-dytes hexapterus) entered the Port Moller estuary from mid-January to late May 1990. Each wave laid its eggs on sand in lower Moller Bay with the center of egg distribution about 14 km inside the estuary. After incubation for 45 to 94 d, each cohort of eggs hatched out over a 41- to 63-d period. Larvae moved at a rate of 0.21 knvd“¹ toward a deep fjordlike basin at the head of Herendeen Bay inside the estuary about 20 km southwest of the center of hatch. The basin has the lowest flushing rate of the estuary, and unlike the rest of Port Moller, it is vertically stratified, which allows the development of a spring-summer zooplankton community with greater biomass than any other location in the estuary. Larvae may have moved to the basin to enhance growth or to avoid offshore transport to areas of low food abundance, but we cannot demonstrate a direct link between growth and habitat. We conclude that the Port Moller sand lance stock has an estuarine early life history that evolved in response to the unique physical conditions of the Port Moller estuary–a shallow, well-mixed site with sandy substrate that is suitable for incubation of demersal eggs next to a deep, vertically stratified fjord with a rich zooplankton community that is suitable for rearing of larvae.     

东、黄海日本鲭种群鉴定和划分的研究进展

分布于东、黄海的日本鲭(Scomber japoncus)在我国海洋渔业中具有重要的地位,随着中韩、中日渔业协定的生效,开展日本鲭等重要经济鱼类的种群归属研究显得尤为重要。关于东、黄海日本鲭种群划分,20世纪中日两国学者利用标志放流和渔业调查数据分别进行了研判,但中日间以及国内对其种群划分存在不同见解。21世纪以来,中国一些研究者利用形态框架法和分子遗传学方法对前辈的种群划分进行了验证并提供了一些证据,但分析结果值得商榷。总结相关研究结果,多数中国学者将分布于东、黄海的日本鲭划分为东海西部种群、五岛西部种群和闽南—粤东地方种群,也有中国学者将其划分为东海群系和闽南—粤东地方种群;日本学者将分布于东海不同越冬场日本鲭归属为对马暖流群系,也有日本学者将分布于东、黄海和日本海西部的日本鲭划分为九州西部群系和东海西部群系。针对东、黄海日本鲭种群划分存在的不同观点,今后研究者应同时利用目前较为先进的、相对可操作的鱼类框架形态和分子遗传等判别技术,增加产卵场样本采集覆盖面,以获得判别东、黄海日本鲭种群相对最为科学的证据。     

Effect of low salinity on the expression profile of Na+/K+‐ATPase and the growth of juvenile chub mackerel (Scomber japonicus Houttuyn)

Chub mackerel (Scomber japonicus Houttuyn) is one of the most commercially important scombroid fish used as a food resource. Recently, there has been a demand for efficient rearing methods of this fish for a full‐life cycle aquaculture. In the present study, we evaluated the physiological responses in the juvenile S. japonicus to different ambient salinities. A significantly higher gain of the body mass was observed in the juveniles reared in 24 g/L and 13 g/L seawater than in those reared in natural seawater (34 g/L) within 40 days of the experimental period without affecting mortality. A principal enzyme for osmoregulation, Na⁺/K⁺‐ATPase, was expressed in the ionocytes located in the gill filaments of the juveniles. The number and the cell size of ionocytes and the enzymatic activity of Na⁺/K⁺‐ATPase in the gills decreased within 10 days after the low‐salinity challenge, which implies the reduction of the energy‐consuming active ion secretion under the low‐salinity environment. The physiological capacity for adaptation to low‐salinity seawater in chub mackerel could be basic knowledge to carry out culturing of these fish in coastal sea pens where ambient salinity fluctuates. The improvement of the growth performance by rearing in low‐salinity seawater will contribute to the efficient production of the seed juveniles for aquaculture.     

A spatially explicit, individual-based model to assess the role of estuarine nurseries in the early life history of North Sea herring, Clupea harengus

Herring (Clupea harengus) enter and remain within North Sea estuaries during well‐defined periods of their early life history. The costs and benefits of the migrations between offshore spawning grounds and upper, low‐salinity zones of estuarine nurseries are identified using a dynamic state‐variable model, in which the fitness of an individual is maximized by selecting the most profitable habitat. Spatio‐temporal gradients in temperature, turbidity, food availability and predation risk simulate the environment. We modeled predation as a function of temperature, the optical properties of the ambient water, the time allocation of feeding and the abundance of whiting (Merlangius merlangus). Growth and metabolic costs were assessed using a bioenergetic model. Model runs using real input data for the Scheldt estuary (Belgium, The Netherlands) and the southern North Sea show that estuarine residence results in fitter individuals through a considerable increase in survival probability of age‐0 fish. Young herring pay for their migration into safer estuarine water by foregoing growth opportunities at sea. We suggest that temperature and, in particular, the time lag between estuarine and seawater temperatures, acts as a basic cue for herring to navigate in the heterogeneous space between the offshore spawning grounds at sea and the oligohaline nursery zone in estuaries.