The distribution of cod (Qadus morhua) in relation to temperature and oxygen level in the Gulf of St. Lawrence

Bottom temperature and oxygen level were measured in the northern Gulf of St. Lawrence during a ground-fish trawl survey held in August and September 1991. Cumulative relative frequencies of temperature and oxygen level and cumulative relative frequencies of temperature x cod and oxygen X cod were computed with consideration of the random stratified sampling scheme. A total of 84-3% of temperature x cod frequency cumulated from 2^oC to 4^oC, but only 25% of the frequency of temperature alone cumulated from 2^oC to 4^oC. At an oxygen level of 3.4 mg/1, only 2.6% of the oxygen X cod frequency had cumulated, while 24.3% of the cumulative frequency of oxygen alone was already observed at the same oxygen level. A comparison of the cumulative relative frequencies of temperature and of temperature X cod indicates that cod choose intermediate temperatures in the available range. A comparison of the cumulative relative frequencies of oxygen and of oxygen x cod indicate that cod avoid water with an oxygen level below 3.4 mg/1. This oxygen level is known to cause physiological stress in captivity. Data from the stations with oxygen levels ± 3.4 mg/1 were ignored, and the cumulative relative frequency of temperature was recomputed. As a result, the avoidance of cold water by cod was emphasized: 23% of the cumulative frequency of temperature alone was below 2^oC, and 47.8% was above 4^oC, in contrast to 17.2% and 57.5%, respectively, prior to removal of data from hypoxic stations.     

Modelling the advection and diffusion of eggs and larvae of Greenland halibut (Reinhardtius hippoglossoides) in the north-east Arctic

Since the late 1980s there has been considerable uncertainty in recruitment levels of the north‐east Arctic stock of Greenland halibut (Reinhardtius hippoglossoides). The abundance of several year classes, originally considered very low at 0–3 yr age, appeared higher than expected at the age of 6+. This may be due to poor targeting of recruitment surveys of the younger year classes. The present work considers the transport and dispersion of eggs and larvae of Greenland halibut by numerical modelling in order to predict the locations of the initial recruitment grounds. Current fields from a 3D baroclinic hydrodynamic model are fed into a Lagrangian particle‐tracking model developed for the Barents Sea area. The particles are released into the current at the spawning field along the shelf slope from Lofoten to Bear Island (69–75°N). Vertically, the particles can follow a predefined depth‐by‐age curve or be kept at a fixed depth. This model system is used for different years to examine changes in the drift pattern. The results indicate that spawning location, transport depth and inflowing activity to the Barents Sea are important factors influencing the distribution of juveniles.     

Lobster larval transport in the southern Gulf of St. Lawrence

A bio‐physical semi‐Langrangian model was developed to follow the drift and abundance of lobster larvae from hatching to settlement as post‐larvae. The geographic domain encompasses Northumberland Strait and the areas surrounding Prince Edward Island in the southern Gulf of St. Lawrence. The model was run for ten larval seasons, 1983–87 and 1997–2001, representing two periods of egg production. The model was forced with tides, winds, heat fluxes, and marine and freshwater fluxes at open boundaries. Biological inputs were location, date, and density of larvae at hatching, development rates, a time window and a minimum bottom temperature required for successful settlement, and two scenarios of daily mortality. Net drift was west to east but stronger on the north than the south side of Prince Edward Island. The hatch was greater in the second 5‐yr period, but the spatial patterns of settlement were similar. For both mortality scenarios, the same five of the 24 larval source areas were important in providing settlers. Horizontal shear of larval distribution indicates fishing communities were dependent on the hatch from fishing grounds of multiple upstream communities. Variation in daily and annual post‐larval settlement was greatest north of Prince Edward Island. From the 24 source areas there was an eightfold range in the fraction of larvae surviving to settlement, with advection into deep water an important cause of mortality. Four to five possible fishery management areas were identified using multidimensional scaling to group sink areas having shared source areas. We hypothesize leaky discontinuity in connectivity between these groups.     

Distribution and drift pathways of Greenland halibut (Reinhardtius hippoglossoides) during early life stages in the eastern Bering Sea and Aleutian Islands

We describe the spatial distribution and dispersal pathways of Greenland halibut (Reinhardtius hippoglossoides) early life stages based on historical field data from the eastern Bering Sea and adjacent water along the eastern Aleutian Islands. Our results indicate that Greenland halibut from preflexion larvae to newly settled juveniles have a long pelagic duration and are subject to extended drift pathways. Hatching may occur in deep water, below 530 m, and larvae rise in the water column as they grow. Flexion/postflexion larvae are mostly found around the Pribilof Islands over the middle shelf (50–100‐m isobaths) in July, and settling occurs during late summer on the middle shelf near St. Matthew Island. However, given that age‐1 individuals were primarily found on the outer shelf, it appears that Greenland halibut actively move to deeper water with age (or size). The mechanisms of slope–shelf connectivity in preflexion larvae may be related to the Bering Slope Current in the vicinity of both Bering and Pribilof Canyons. This study shows that Greenland halibut early life stages have extensive horizontal ontogenetic migrations in the Bering Sea, and utilize a range of geographic areas over the basin and slope along the Aleutian Islands and in the eastern Bering Sea. Based on these results, it is hypothesized that settlement success and recruitment of Greenland halibut may be influenced by variability in currents and flows of the Bering Sea slope and shelf during their transport.     

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.     

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.     

Movement of adult male snow crab, Chionoecetes opilio, in the southern Gulf of St. Lawrence and eastern Nova Scotia, Canada

A total of 12,755 adult male snow crabs (Chionoecetes opilio) were tagged with spaghetti–Carlin tags in 33 sites throughout the southern Gulf of St. Lawrence (sGSL) and eastern Nova Scotia (eNS) between 1993 and 2003. Of the 1971 tag returns obtained between 1994 and 2004, 1703 had accurate geographical recapture positions. The average distance travelled was 16.7 km for sGSL (maximum 165 km) and 61.5 km for eNS (maximum 368 km). The release site and the number of days at liberty before recapture were significant factors affecting the distance travelled by the crabs. Size had no significant effect on the distance travelled or the direction of movement. The present observations support the sedentary nature previously reported for the sGSL snow crab stock, but not the eNS stock, where the movement of snow crab was more dynamic. In both regions, there were movements between management areas, which indicates that managing the fisheries in discrete units is not warranted.     

Environmental influences on Atlantic bluefin tuna (Thunnus thynnus) catch per unit effort in the southern Gulf of St. Lawrence

The Atlantic bluefin tuna (Thunnus thynnus) population in the western Atlantic supports substantial commercial and recreational fisheries. Despite quota establishment and management under the auspices of the International Commission for the Conservation of Atlantic Tunas, only small increases in population growth have been estimated. In contrast to other western bluefin tuna fisheries indices, contemporary estimates of catch per unit effort (CPUE) in the southern Gulf of St. Lawrence have increased rapidly and are at record highs. This area is characterized by the Cold Intermediate Layer (CIL) that is defined by waters <3°C and located at depths of 30–40 m in September. We investigated the influence of several in situ environmental variables on the bluefin tuna fishery CPUE using delta‐lognormal modelling and relatively extensive and consistent oceanographic survey data, as well as dockside monitoring and mandatory logbook data associated with the fishery. Although there is considerable spatial and temporal variation of water mass characteristics, the amount of available habitat in the southern Gulf of St. Lawrence (assuming a > 3°C thermal ambit) for bluefin tuna has been increasing. The percentage of the water column occupied by the CIL was a significant environmental variable in the standardization of CPUE estimates. There was also a negative relationship between the spatial extents of the CIL and the fishery. Properties of the CIL account for variation in the bluefin tuna CPUE and may be a factor in determining the amount of available feeding habitat for bluefin tuna in the southern Gulf of St. Lawrence.     

黄河口及邻近水域矛尾虾虎鱼资源丰度的时空分布

根据2013年6月、8月、10月以及2014年2月、4月、5月在黄河口及邻近水域获取的6个航次的渔业资源和环境调查数据,研究了黄河口及邻近水域优势种矛尾虾虎鱼(Chaeturichthys stigmatias)资源丰度的时空分布,并利用广义可加模型分析了其与时空和环境因子之间的关系。结果表明:矛尾虾虎鱼主要集中分布于37.80°N–38.20°N,119.00°E–119.40°E的水域。矛尾虾虎鱼资源丰度月变化明显,8月最大为862 g/h,其次6月资源丰度为521 g/h,5月最小。底层水温和底层盐度对矛尾虾虎鱼资源丰度的时空分布有显著影响。黄河口及邻近水域矛尾虾虎鱼分布的底层水温适宜范围为17~22℃,底层盐度适宜范围为18~27。水温、盐度在河口区变化范围较大,矛尾虾虎鱼能较好地适应河口区底层温度、盐度等环境因子的变化,成为河口定居性主要优势种。     

Using otolith microchemistry to reconstruct habitat use of American eels Anguilla rostrata in the St. Lawrence River–Lake Ontario system

Catadromy among freshwater eels is increasingly recognised as being facultative, with some individuals carrying out growth exclusively in brackish or coastal marine waters, or switching between brackish or marine waters and freshwater habitats. In an attempt to reconstruct habitat use of yellow‐stage American eels in a large river‐lake ecosystem, trace element line scans were obtained, using LA‐ICP‐MS, from the otoliths of 110 eels sampled at various locations throughout the St. Lawrence River–Lake Ontario (SLRLO) system. Elemental profiles for strontium (Sr), barium (Ba), manganese (Mn) and magnesium (Mg) enabled us to distinguish three chemical signatures that appear to represent three distinct habitats within the SLRLO. Of these, one was shown to likely correspond to the brackish estuary (high strontium values). The other two signatures, characterised by low strontium but variable concentrations of barium and manganese, may correspond to habitats within the main‐stem St. Lawrence River and one or more of its tributaries. Most (78%) of the switches among habitats occurred within the first four years after recruitment suggesting an increasing likelihood for eels to maintain residence in one habitat as they grow older. This suggests that tributaries may provide important habitats for American eels during the first several years after recruiting to the SLRLO. In addition, our results suggest that a small proportion American eels in the SLRLO can undertake movements on the order of at least 200 km during the early growth stage. This information has important implications for the management and conservation of this species in the system.     

Change in habitat associations and geographic distribution of thorny skate (Amblyraja radiata) in the southern Gulf of St Lawrence: density‐dependent habitat selection or response to environmental change?

We describe changes in the habitat associations and geographic distribution of thorny skate during their feeding season in the southern Gulf of St Lawrence, based on 32 yr of monitoring by a bottom‐trawl survey. In the 1990s, geographic range contracted sharply and distribution shifted into a narrow band of warm deep waters. These changes appeared to reflect altered habitat selection by individual skates, rather than local depletion of the skates that had habitually occupied the vacated areas or a change in the timing of seasonal migrations. Changes in skate distribution coincided with a decline in skate biomass and a cooling of bottom waters. The contraction in the geographic range of skates appeared to be a density‐dependent response, more closely linked to the decline in skate abundance than to the change in environmental conditions. The cause of the shift in habitat associations is less certain. An index of the distribution shift was more strongly correlated with skate biomass than with an index of temperature conditions, and the change in temperature associations is in the direction predicted by density‐dependent bioenergetic considerations. However, the shift into deeper waters is into a depth zone where skate condition is relatively low, contrary to the expectation that fish should be concentrated in optimal habitat at low population size. On the other hand, while the shift into warm deep waters coincided with a cooling of waters at intermediate depths, distribution failed to shift back to the earlier pattern as these waters warmed in the late 1990s.     

Spatial distribution of fishing effort in relation to seafloor habitats in the Kattegat,a GIS analysis

• 1. High‐resolution fishing effort data from the Kattegat, an important fishing ground for Swedish and Danish fishermen, was analysed in a Geographical Information System (GIS). Data were based on position data for individual tows reported by Swedish fishermen during 2001–2003.
• 2. Gear dimension and towing speed data were used to calculate an index for area swept per hour for each main gear type in use in the area. These indexes were multiplied by fishing effort and a grid theme of fishing intensity was created after GIS analysis.
• 3. Around 44% of the entire area was affected by Swedish fisheries during the study period, hence 56% was not affected.
• 4. Effort was highly concentrated in certain areas and 10% of the total area was covered more than twice per year.
• 5. Overlaying the effort data set with habitat maps classified according to the European Nature Information System (EUNIS) showed that the fishing pressure differed between habitats. For deep rocky and muddy habitats, almost the entire area was affected by fisheries during the study period, while both intensity and proportion affected were lower in sandy sediments and gravels.
• 6. Around 55% of the deep rocky habitats were trawled more than twice per year. Of the muddy seafloor areas, 41% were covered more than twice per year and 4% more than once a month by trawls.
• 7. The biological effects of the fishing intensities found were analysed using a database (MarLIN) containing assessments of marine habitat recoverability. All habitats except muds are probably in a nearly or fully recovered condition (as defined by MarLIN). A considerable part of the muddy habitats are in a permanently altered condition owing to fishing disturbance taking place more frequently than the indicated recovery time.
• 8. Danish fishermen report more fishing effort in the area than do Swedes. Owing to differences in reporting fishing positions, Danish fisheries could not be included in the present study. Scaling up the results to include Danish fisheries is discussed.
• 9. The results of the present study could be used to target habitat management goals more precisely, while minimizing the negative impact of restrictions on the fishing sector.
• 10. A prerequisite for performing similar studies is the availability of high‐resolution fishing effort data and high‐resolution benthic habitat maps.

Copyright © 2006 John Wiley & Sons, Ltd.     

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.     

Habitat use of cownose rays (Rhinoptera bonasus) in a highly productive,hypoxic continental shelf ecosystem

Highly productive surface waters and hypoxic (dissolved oxygen, DO ≤ 2.0 mg L^?1) bottom waters develop seasonally on the northwestern Gulf of Mexico continental shelf due to nutrient and freshwater inputs from the Mississippi‐Atchafalaya River system. We investigated the spatial distribution of the cownose ray (Rhinoptera bonasus), a highly mobile, bentho‐pelagic species that is a seasonal resident of the shelf, in relation to surface chlorophyll, bottom‐water hypoxia, and other environmental variables (salinity, temperature, depth). We used synoptic trawl and aerial surveys to investigate ray distributions at both shelfwide (100–1000s km) and local (5–50 km) spatial scales. Shelfwide sampling indicated that rays were associated with regions of high surface chlorophyll and low bottom salinity and DO, conditions characterizing the Mississippi‐Atchafalaya plume region. Local sampling in and around the hypoxic zone indicated that rays preferred habitats where bottom waters were hypoxic but they primarily occupied normoxic (DO > 2.0 mg L^?1) waters above the bottom hypoxic layer. Stomach fullness and diet composition were similar between rays sampled in habitats with hypoxic versus normoxic bottom waters. These results indicate that cownose rays are strongly associated with riverine‐influenced regions of the shelf and preferentially use habitats with hypoxic bottom waters, perhaps for benthic foraging. Collectively, our results highlight the importance of considering the responses of mobile species to enhanced productivity and to hypoxia‐induced habitat degradation, which are both the products of coastal eutrophication.     

Springtime abundance of chaetognaths in the shelf region of the northern Gulf of Alaska, with observations on the vertical distribution and feeding of Sagitta elegans

Chaetognatha were among the most dominant macrozooplankton taxa collected in 6 years of springtime collections, both throughout the water column and near-bottom on the continental shelf in the northern Gulf of Alaska. Three species ( Sagitta elegans, S. scrippsae, and Eukrohnia hamata ) were collected in 248 collections, although S. elegans was the numerically dominant species overall during most cruises. Collections taken in epibenthic sleds generally contained about two orders of magnitude more chaetognaths than those that sampled throughout the water column. Two size modes were apparent in the S. elegans size-frequency distributions which were believed to belong to different cohorts. Growth rate was on the order of 2–3 mm per month. The two cohorts showed asynchronous diel vertical distribution patterns, with the smaller individuals found near the surface during the day whereas the larger individuals were near the surface at night. About 7.5% of the S. elegans examined contained food. Copepods made up the majority (∼89% by number) of the diet of both large and small individuals. Euphausiid juveniles and cirripede larvae were also observed, as well as several incidences of cannibalism by large chaetognaths on smaller individuals. Based on the results of two diel series, this species was observed to feed mainly during the night-time.     

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.     

Distribution patterns of Pacific halibut (Hippoglossus stenolepis) in relation to environmental variables along the continental shelf waters of the US West Coast and southern British Columbia

Knowing how Pacific halibut (Hippoglossus stenolepis) distribute in relation to ocean conditions is of primary importance to halibut managers, as they are tasked with estimating stock size and designing effective monitoring programs amidst a changing climate. This research examined near‐bottom environmental data alongside halibut survey catch data for the years 2006–2009 on the continental shelf of Oregon, Washington, and southern British Columbia. The objectives of the research were to: (1) characterize summer environmental conditions and halibut distribution; (2) explore ranges and possible tolerance thresholds for halibut in relation to temperature, dissolved oxygen (DO), salinity, and pH; and (3) identify the primary environmental factors affecting distribution of halibut and model the observed relationships. Seasonal hypoxia is an annual feature of the study area and results suggest halibut exhibited an apparent DO minimum threshold of 0.9 mL L^?1. Ordinary least squares multiple regression analysis indicated that depth, temperature, and DO were significant variables in predicting halibut distribution, whereas salinity and bottom type were not. Ambiguity in model results led to the use of two additional analytical methods, geographically weighted regression (GWR) and tree regression, to examine regional variation and the overarching structure of halibut distribution. The three models yielded similar results indicating the importance of DO and temperature as variables describing structure. The GWR model yielded the best fit of the three when using DO as a predictor variable, indicating that regional variation is a factor. These results suggest that low, but above‐threshold, DO may be contributing to catchability differences in the survey.     

Bluefin tuna (Thunnus thynnus) distribution in relation to sea surface temperature fronts in the Gulf of Maine (1994–96)

Fishery‐linked aerial surveys for bluefin tuna (Thunnus thynnus) were conducted in the Gulf of Maine (GOM) from July through October, 1994–96. Each year, from 507 to 890 surface schools were detected and their locations examined in relation to oceanographic conditions. Correlations between bluefin tuna presence and environmental variables were explored for sea surface temperature (SST), distance to a SST front, frontal density (relative density of all SST fronts seen in a given 1 km area for 2 weeks prior to each tuna sighting), and bottom depth and slope. Mean SST associated with bluefin schools was 18.1°C (±2.8). Schools were located at a mean distance of 19.7 km (±19.6) from SST fronts, and in water masses with an average frontal density of 28.2 m km^?2 (±35.7). Mean bottom depth of detected schools was 139.0 m (±70.3), and mean bottom slope was 0.7% rise (±0.7). A binomial generalized linear model fit to these variables indicated that bluefin are seen closer to fronts than locations in which no tuna were seen. Using simple and partial Mantel tests, we investigated the spatial correlation between bluefin tuna presence and the environmental variables, controlling for spatial autocorrelation. For each day that schools were sighted, we performed 24 Mantel tests, on a combination of response and predictor variables. The spatial relationship between bluefin tuna and SST fronts was inconsistent. Our analysis identified significant spatial structure in the bluefin school locations that had no significant correlation with any of the measured environmental features, suggesting that other untested features, such as prey density, may be important predictors of bluefin distribution in the GOM.     

Influence of oceanographic conditions on abundance and distribution of post‐larval and juvenile carangid fishes in the northern Gulf of Mexico

Relationships between abundance of post‐larval and juvenile carangid (jacks) fishes and physical oceanographic conditions were examined in the northern Gulf of Mexico (GoM) in 2011 with high freshwater input from the Mississippi River. Generalized additive models (GAMs) were used to explore complex relationships between carangid abundance and physical oceanographic data of sea surface temperature (SST), sea surface height anomaly (SSHA) and salinity. The five most abundant carangid species collected were: Selene setapinnis (34%); Caranx crysos (30%); Caranx hippos (10%); Chloroscombrus chrysurus (9%) and Trachurus lathami (8%). Post‐larval carangids (median standard length [SL] = 10 mm) were less abundant during the spring and early summer, but more abundant during the late summer and fall, suggesting summer to fall spawning for most species. Juvenile carangid (median SL = 23 mm) abundance also increased between the mid‐summer and early fall. Most species showed increased abundance at lower salinities and higher temperatures, suggesting entrainment of post‐larval fishes or feeding aggregations of juveniles at frontal convergence zones between the expansive river plume and dynamic mesoscale eddy water masses. However, responses were species‐ and life‐stage specific, which may indicate fine‐scale habitat partitioning between species. Ordination methods also revealed higher carangid abundances at lower salinities for both post‐larval and juvenile life stages, with species‐ and life‐stage specific responses to SST and SSHA, further suggesting habitat separation between species. Results indicate strong links between physical oceanographic features and carangid distributions in the dynamic northern GoM.