Modelling Atlantic mackerel spawning habitat suitability and its future distribution in the north‐west Atlantic

We investigated the effect of environmental conditions on Atlantic mackerel spawning habitat in the southern Gulf of St. Lawrence (sGSL). Based on generalized additive models, we (i) modelled the optimal spawning habitat of mackerel in the sGSL using daily egg production (DEP) in June, (ii) predicted known and new potential present spawning habitats in the GSL and the north‐west Atlantic, and (iii) assessed how they respond to future climate change. Our findings showed that both mackerel presence–absence and given‐presence DEP were associated with sea surface temperature (10–16.5°C), salinity above 31 and depth < 120 m. Adding zooplankton showed a marked effect on the DEP given‐presence compared to the presence–absence. Predictions of spawning habitats under present (1999–2012) and future scenario (2066–2085) conditions were estimated from the presence–absence model without zooplankton, using physical conditions of the BNAM. Under present conditions, our model predicted well the main spawning habitat in the sGSL and other known secondary spawning habitats in the northern GSL (nGSL), the western and southern Newfoundland, and the north‐west Atlantic coast. Under future conditions, our study suggests that spawning habitats in the sGSL and the nGSL would expand. Our results, therefore, suggest that mackerel could benefit from a warmer GSL, minimizing the potential for a northward migration of the stock due to decreasing suitability of the sGSL as its main spawning ground, and a new but spatially limited potential habitat in Newfoundland coasts. These results can be used to inform stock management and develop adaptive management plans in the context of climate change.     

Quantifying the influence of geography and environment on the northeast Atlantic mackerel spawning distribution

Mackerel (Scomber scombrus) in the northeast Atlantic have shown changes in distribution at certain times of the year, which have impacted their exploitation and management. In this study, mackerel spawning habitat over 21 recent years was characterised using generalised additive modelling, based on spatial egg density data collected every third year during targeted ichthyoplankton surveys. Mackerel spawning distribution was found to depend primarily on geographical variables (coordinates and bottom depth), with preferred spawning locations on the shelf‐edge from the north of the Iberian peninsula to the west of Scotland, with a maximum west of Ireland. Environmental drivers had a lesser influence on egg distribution. Dome shaped relationships were found with temperature and mixed layer depth, with respective optimum at 13°C and around –300 m. The model was used to reconstruct maps of the potential habitat (areas where conditions were suitable, but not necessarily used, for spawning). Little changes were observed over the years in the potential habitat, suggesting that the expansion of the egg distribution (realised habitat) was not triggered by changes in the environmental variables investigated. Little evidence was found for density‐dependent habitat selection. There was a tendency for mackerel to make more use of areas of lesser suitability in years with large stock size (1992 and 2010). This pattern, however, broke down in 2013, when stock size was the highest, as spawning occurred very south and concentrated in the most suitable habitat.     

Dynamic changes in American lobster suitable habitat distribution on the Northeast U.S. Shelf linked to oceanographic conditions

American lobster (Homarus americanus) supports one of the most valuable regional fisheries in the United States, with its abundance and distribution profoundly influenced by environmental conditions. To explain how lobster distribution has changed over time and assess the role of environmental variables on these changes, we used random forest classification and regression tree models to estimate occupancy and biomass in two seasonal periods. The occupancy models were fit to static and dynamic variables, which yielded model fits with AUC scores of 0.80 and 0.78 for spring and fall, respectively. Biomass models were fit with the same data and resulted in models explaining 61% and 63% of the spring and fall biomass variance, respectively. Significant variables scored in the formation of the regression trees were secondary productivity (i.e., zooplankton), bathymetry characteristics, and temperature. American lobster suitable habitat has changed regionally; habitat has increased in the Gulf of Maine and declined in Southern New England. There is also evidence of declining habitat along the inshore margin of the Gulf of Maine, which has been accompanied by a shift in occupancy probability offshore. Habitat suitability results from the random forest models provide insights on the structure and function of lobster habitat and context to understand recent population trends.     

Ichthyoplankton-based spawning dynamics of blue mackerel (Scomber australasicus) in south-eastern Australia: links to the East Australian Current

We describe findings of three ichthyoplankton surveys undertaken along south‐eastern Australia during spring (October 2002, 2003) and winter (July 2004) to examine spawning habitat and dynamics of blue mackerel (Scomber australasicus). Surveys covered ～860 nautical miles between southern Queensland (Qld; 24.6°S) and southern New South Wales (NSW; 41.7°S), and were mainly centred on the outer shelf including the shelf break. Egg identifications were verified applying mtDNA barcoding techniques. Eggs (n = 2971) and larvae (n = 727; 94% preflexion) occurred both in spring and winter, and were confined to 25.0–34.6°S. Greatest abundances (numbers per 10 m²) of eggs (1214–7390) and larvae (437–1172) occurred within 10 nm shoreward from the break in northern NSW. Quotient analyses on egg abundances revealed that spawning is closely linked to a combination of bathymetric and hydrographic factors, with the outer shelf as preferred spawning area, in waters 100–125 m deep with mean temperatures of 19–20°C. Eggs and larvae in spring occurred in waters of the East Australian Current (EAC; 20.6–22.3°C) and mixed (MIX; 18.5–19.8°C) waters, with none occurring further south in the Tasman Sea (TAS; 16.0–17.0°C). Results indicate that at least some of the south‐eastern Australian blue mackerel stock spawns during winter‐spring between southern Qld and northern NSW, and that no spawning takes place south of 34.6°S due to low temperatures (<17°C). Spawning is linked to the EAC intrusion, which also facilitates the southward transport of eggs and larvae. Since spring peak egg abundances came from where the EAC deflects offshore, eggs and larvae are possibly being advected eastwards along this deflection front. This proposition is discussed based on recent data on blue mackerel larvae found apparently entrained along the Tasman Front.     

Covariation in climate, zooplankton biomass and mackerel recruitment in the southern Gulf of St Lawrence

Between 1982 and 1991, an annual survey of stage I egg production of Atlantic mackerel ( Scomber scombrus ) was conducted in June/early July in the southern Gulf of St Lawrence. We investigated the relationship between interannual variability in biomass of zooplankton, determined from the archived survey plankton samples, and mackerel recruitment, estimated from the proportion of three-year-olds in the catch of the commercial fishery. Zooplankton biomass varied by a factor of 2.5, primarily owing to fluctuations in the >1000 μm size fraction. The index of mackerel recruitment fluctuated by a factor of ≈20 and was positively related (linear regression: P  < 0.05; n  = 10) to variations in the zooplankton biomass. Both mackerel recruitment and zooplankton biomass were negatively related (linear regression: P  < 0.05) to RIVSUM, a measure of freshwater discharge from the St Lawrence River system and an index of variability in the region's climate. Three hypotheses are put forward to explain these observations: (1) there is a strong link between interannual variation in abundance of copepod females, which produce prey for mackerel larvae, and larval survival; the exceptional recruitment and subsequent year class resulted from an exceptional production of Calanus finmarchicus nauplii; (2) years of high zooplankton biomass provide better feeding conditions and consequently higher survival of mackerel juveniles; and (3) mackerel recruitment and zooplankton biomass are independently under the control of an underlying physical process, without strong trophic linkage. The first hypothesis is supported by a study of copepod species composition and female abundance conducted for four of the survey years. At the present time, none of these hypotheses can be ruled out.     

Evaluating the utility of the Gulf Stream Index for predicting recruitment of Southern New England‐Mid Atlantic yellowtail flounder

The justification for incorporating environmental effects into fisheries stock assessment models has been investigated and debated for a long time. Recently, a state‐space age‐structured assessment model which includes the stochastic change in the environmental covariate over time and its effect on recruitment was developed for Southern New England‐Mid Atlantic yellowtail flounder (Limanda ferruginea). In this paper, we first investigated the correlations of environmental covariates with Southern New England‐Mid Atlantic yellowtail flounder recruitment deviations. The covariate that was most strongly correlated with the recruitment deviations was then incorporated into the state‐space model and alternative effects on the stock‐recruit relationship were estimated and compared. For the model that performed best as measured by Akaike information criterion, we also compared the estimates and predictions of various population attributes and biological reference points with those from an otherwise identical model without the environmental covariate in the stock‐recruit function. We found that the estimates of population parameters are similar for the two models but the predictions differed substantially. To evaluate which model provided more reliable predictions, we quantitatively compared the prediction skill of the two models by generating two series of retrospective predictions. Comparison of the retrospective prediction pattern suggested that from an average point of view, the environmentally explicit model can provide more accurate near‐term recruitment predictions especially the one year ahead recruitment prediction. However, the accuracy of the near‐term recruitment prediction from the environmentally explicit model was largely determined by the accuracy of the corresponding environment prediction the model provides.     

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.     

Mechanistic links between climate and fisheries along the east coast of the United States: explaining population outbursts of Atlantic croaker (Micropogonias undulatus)

Climate has been linked to variation in marine fish abundance and distribution, but often the mechanistic processes are unknown. Atlantic croaker (Micropogonias undulatus) is a common species in estuarine and coastal areas of the mid‐Atlantic and southeast coasts of the U.S. Previous studies have identified a correlation between Atlantic croaker abundance and winter temperatures in Chesapeake Bay, and have determined thermal tolerances of juveniles. Here we re‐examine the hypothesis that winter temperature variability controls Atlantic croaker population dynamics. Abundance indices were analyzed at four life history stages from three regions along the east coast of the U.S. Correlations suggest that year‐class strength is decoupled from larval supply and is determined by temperature‐linked, overwinter survival of juveniles. Using a relation between air and water temperatures, estuarine water temperature was estimated from 1930 to 2002. Periods of high adult catch corresponded with warm winter water temperatures. Prior studies indicate that winter temperature along the east coast is related to the North Atlantic Oscillation (NAO); variability in catch is also correlated with the NAO, thereby demonstrating a link between Atlantic croaker dynamics, thermal limited overwinter survival, and the larger climate system of the North Atlantic. We hypothesize that the environment drives the large‐scale variability in Atlantic croaker abundance and distribution, but fishing and habitat loss decrease the resiliency of the population to periods of poor environmental conditions and subsequent weak year classes.     

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.     

Predicting the occurrence of Atlantic bluefin tuna (Thunnus thynnus) larvae in the northern Gulf of Mexico: building a classification model from archival data

Although bluefin tuna are found throughout the Atlantic Ocean, spawning in the western Atlantic has been recorded predominantly in the Gulf of Mexico (GOM) in spring. Larval bluefin tuna abundances from the northern GOM are formulated into an index used to tune the adult stock assessment, and the variability of this index is currently high. This study investigated whether some of the variability in larval bluefin tuna abundances was related to environmental conditions, by defining associations between larval bluefin tuna catch locations, and a suite of environmental variables. We hypothesized that certain habitat types, as defined by environmental variables, would be more likely to contain bluefin tuna larvae. Favorable habitat for bluefin tuna larvae was defined using a classification tree approach. Habitat within the Loop Current was generally less favorable, as were warm‐core rings, and cooler waters on the continental shelf. The location and size of favorable habitat was highly variable among years, which was reflected in the locations of larval bluefin tuna catches. The model successfully placed bluefin tuna larvae in favorable habitat with nearly 90% accuracy, but many negative stations were also located within theoretically favorable habitat. The probability of collecting larval bluefin tuna in favorable habitat was nearly twice the probability of collecting bluefin tuna larvae across all habitats (35.5 versus 21.0%). This model is a useful addition to knowledge of larval bluefin tuna distributions; however, the incorporation of variables describing finer‐scale features, such as thermal fronts, may significantly improve the model’s predictive power.     

Trends and change points in surface and bottom thermal environments of the US Northeast Continental Shelf Ecosystem

Temperature is an important factor in defining the habitats of marine resource species. While satellite sensors operationally measure ocean surface temperatures, we depend on in situ measurements to characterize benthic habitats. Ship‐based measurements were interpolated to develop a time series of gridded spring and fall, surface and bottom temperature fields for the US Northeast Shelf. Surface and bottom temperatures have increased over the study period (1968–2018) at rates between 0.18–0.31°C per decade and over a shorter time period (2004–2018) at rates between 0.26–1.49°C per decade. A change point analysis suggests that a warming regime began in the surface waters in 2011 centered on Georges Bank and the Nantucket Shoals; in following years, most of the Northeast Shelf had experienced a shift in surface temperature. A similar analysis of bottom temperature suggests a warming regime began in 2008 in the eastern Gulf of Maine; in following years, change points in temperature occurred further to the west in the Gulf of Maine, finally reaching the Middle Atlantic Bight by 2010. The spatial pattern in bottom water warming is consistent with well‐known oceanographic patterns that advect warming North Atlantic waters into the Gulf of Maine. The varying spatial and temporal progression of warming in the two layers suggests they were actuated by different sets of forcing factors. We then compared these trends and change points to responses of lower and higher trophic level organisms and identified a number of coincident shifts in distribution and biomass of key forage and fisheries species.     

Prevalence and development of intraleucocytic haemogregarines from Northwest and Northeast Atlantic mackerel, Scomber scombrus L.

Abstract. Scomber scombrus on both sides of the North Atlantic were infected with similar intraleucocytic haemogregarines. Detection of parasites in blood smears alone was found to be an unreliable indicator of prevalence. In the Northwest Atlantic, prevalence of blood infections varied annually and seasonally, with an average of 18.4%, whereas tissue imprints showed up to 98.3% prevalence. Northeast Atlantic fish had a 4.0% prevalence in blood smears and 100% in spleen imprints. Age 1 mackerel were the principal hosts of haemogregarines in the Northwest Atlantic. Scanty infections of single and paired parasites were observed primarily in lymphocytes and neutrophils of peripheral blood and tissue imprints. Also present in the blood and tissues of Northwest Atlantic mackerel were intracellular schizonts with up to 20 merozoites, and fish from the same region showed haemogregarines within histological lesions in kidney and spleen. Ultrastructural studies on Northeast Atlantic mackerel indicated the parasite is apicomplexan and morphologically dissimilar to Haematractidium scombri. Its relationship with Goussia clupearum was not determined.     

Connectivity of the bay scallop (Argopecten irradians) in Buzzards Bay,Massachusetts, U.S.A.

The harvest of bay scallops (Argopecten irradians) from Buzzards Bay, Massachusetts, U.S.A. undergoes large interannual fluctuations, varying by more than an order of magnitude in successive years. To investigate the extent to which these fluctuations may be due to yearly variations in the transport of scallop larvae from spawning areas to suitable juvenile habitat (settlement zones), a high‐resolution hydrodynamic model was used to drive an individual‐based model of scallop larval transport. Model results revealed that scallop spawning in Buzzards Bay occurs during a time when nearshore bay currents were principally directed up‐bay in response to a persistent southwesterly sea breeze. This nearshore flow results in the substantial transport of larvae from lower‐bay spawning areas to settlement zones further up‐bay. Averaged over the entire bay, the spawning‐to‐settlement zone connectivity exhibits little interannual variation. However, connectivities between individual spawning and settlement zones vary by up to an order of magnitude. The model results identified spawning areas that have the greatest probability of transporting larvae to juvenile habitat. Because managers may aim to increase scallop populations either locally or broadly, the high‐connectivity spawning areas were divided into: (i) high larval retention and relatively little larval transport to adjoining settlement areas, (ii) both significant larval retention and transport to more distant settlement areas, and (iii) little larval retention but significant transport to distant settlement areas.     

Predation dynamics of mackerel on larval and juvenile anchovy: is capture success linked to prey condition?

We tested whether the predation dynamics of chub mackerel Scomber japonicus and spotted mackerel S. australasicus on young anchovy Engraulis japonicus relates to individual growth characteristics of the prey and could account for the growth-selective survival predicted by recruitment hypotheses. Juvenile and adult mackerel were sampled along with their young anchovy prey field in 2004 (juvenile mackerel and larval anchovy) and 2005 (adult mackerel and juvenile anchovy) off the Pacific coast of Honshu, Japan. The recent 5-day mean growth rate of larval and juvenile survivors and prey found in the stomach of mackerel was estimated from the otolith microstructure. No significant difference was found between the recent growth of larval or juvenile survivors and that of preyed individuals. We conclude that despite a relatively small body size, the high activity level and predation skills displayed by mackerel prevent fast-growing larvae and early juveniles from benefitting in terms of the expected survival advantage over slow-growers. Hence, growth-selective predation mortality of larval fish would depend on the feeding ecology of the predator rather than predator size. Selection for fast growth is more likely to occur under predation pressure from invertebrate organisms and small pelagic fish specialized on zooplankton, such as herring and anchovy.     

Entrainment of larval fish assemblages from the inner shelf into the East Australian Current and into the western Tasman Front

Entrainment and transport of larval fish assemblages by the East Australian Current (EAC) were examined from the coastal waters of northern New South Wales (NSW) to the western Tasman Front, via the separation of the EAC from the coast, during the austral spring of 2004. Shore‐normal transects from the coast to the EAC off northern NSW revealed an inner shelf assemblage of near‐shore families (Clupeidae, Engraulidae, Platycephalidae and Triglidae), an EAC assemblage dominated by Myctophidae and Gonostomatidae, and a broadly distributed assemblage over the continental shelf dominated by Scombridae and Carangidae. Further south and after the EAC had separated from the coast, we observed a western Tasman Front assemblage of inner shelf and shelf families (Clupeidae, Engraulidae, Serranidae, Scombridae, Carangidae, Bothidae and Macroramphosidae). The abundance of these families declined with distance from the coast. Surprisingly, there was no distinctive or abundant larval fish assemblage in the chlorophyll‐ and zooplankton‐enriched waters of the Tasman Sea. Water type properties (temperature‐salinity, T‐S), the larval fish assemblages and family‐specific T‐S signatures revealed the western Tasman Front to be an entrained mix of EAC and coastal water types. We found an abundance of commercially important species including larval sardine (Sardinops sagax, Clupeidae), blue mackerel (Scomber australasicus, Scombridae) and anchovy (Engraulis australis, Engraulidae). The entrainment and transport of larval fish from the northern inner shelf to the western Tasman Front by the EAC reflects similar processes with the Gulf Stream Front and the Kuroshio Extension.     

Turbulent mixing and mesoscale distributions of late-stage fish larvae on the NW Shelf of Western Australia

Light traps were deployed to describe vertical and cross‐shelf distributions of late‐stage larval fishes during five cruises in each of the 1997/98 and 1998/99 summers in the region of the Gulf of Exmouth on the southern North West Shelf of Western Australia. At each light trap station on a cross‐shelf transect we measured water temperature, salinity and chlorophyll a and used vertical plankton tows to estimate zooplankton biomass and copepod abundance. Current meters were deployed on moorings near the transect and the data used to model flows and mixing on the NW Shelf and in the Gulf. The majority of reef, pelagic and baitfish larvae (81, 83 and 66% respectively) were collected at only two stations that marked the boundary between stratified waters offshore and well‐mixed water within the Gulf. Most baitfishes (primarily Spratelloides spp.) were captured by traps deployed near the seabed, while reef fishes (mostly pomacentrids, lethrinids and siganids) and pelagic species (mostly scombrids and carangids) were captured in traps deployed near surface. Catch composition varied between summers with 64% of baitfishes collected in the first summer, while the majority of reef and pelagic fishes (81 and 80% respectively) were captured in the second summer. Modelling of circulation showed that the velocity of tidal currents was enhanced by constriction of flow between NW Cape and South Muiron Island and by shallowing of the shelf. Flood‐tide intrusions of water allowed the thermocline to move up the continental shelf, upwelling cool nutrient‐rich water that was then mixed throughout the entire water column at stations in the mouth of the Gulf. This upwelling and mixing resulted in higher chlorophyll a concentrations and copepod abundances either as a result of local in situ growth or advection/aggregation processes, and may account for the great abundances of late‐stage fish larvae in the mouth of the Gulf.     

不同强度拉尼娜事件下东海鲐栖息地的时空变动

鲐(Scomber japonicus)是一种重要的经济鱼种, 其栖息地环境易受到气候变化的影响。本研究依据 1950— 2015 年 2.5 m、25 m、50 m 水层温度数据以及 Ni?o3.4 指数, 计算东海鲐栖息地适宜性指数(HSI), 对比分析不同强度拉尼娜事件期间鲐栖息地的时空变动。结果表明, 拉尼娜事件期间鲐渔场适宜栖息地面积高于正常气候条件。 弱拉尼娜事件和中强度拉尼娜事件期间, 鲐渔场各水层温度偏低, 适宜的温度面积比例较高, 栖息地适宜性较高; 而强拉尼娜事件期间, 鲐渔场各水层温度偏高, 适宜的温度面积比例较低, 栖息地适宜性较低。相较于强拉尼娜事件, 弱拉尼娜事件和中强度拉尼娜事件期间, 适宜栖息地的经度重心偏东, 纬度重心偏南, 不适宜栖息地的经度重心偏西, 纬度重心偏北。此外, 鲐渔场各水层最适宜温度等值线在强拉尼娜事件下偏西北方向, 推测不同强度拉尼娜下鲐栖息地空间分布差异, 可能是由于各水层最适宜温度显著的月间分布差异所致。研究表明, 不同强度拉尼娜事件对东海鲐栖息地的影响具有显著差异。     

Egg and larval distributions of seven fish species in north-east Atlantic waters

The distribution of egg and larvae of mackerel, horse mackerel, sardine, hake, megrim, blue whiting and anchovy along the European Atlantic waters (south Portugal to Scotland) during 1998 is described. Time of the year, sea surface temperature and bottom depth are used to define the spawning habitat of the different species. Mackerel, horse mackerel, and sardine eggs and larvae presented the widest distribution, whereas megrim and anchovy showed a limited distribution, restricted to the Celtic Sea and the Bay of Biscay respectively. Correspondingly mackerel, horse mackerel and sardine showed the highest aggregation indices. Blue whiting larvae were found at the lowest temperatures, whereas anchovy eggs and larvae were found in the warmest waters. The analysis is a basis for evaluation of ongoing changes in the pelagic ecosystem of the north‐east Atlantic.