Distribution, age, and growth of young-of-the year greater amberjack (Seriola dumerili) associated with pelagic Sargassum

Patterns of distribution and growth were examined for young-of-the-year (YOY) greater amberjack (Seriola dumerili) associated with pelagic Sargassum in the NW Gulf of Mexico. Seriola dumerili were collected off Galveston, Texas, from May to July over a two-year period (2000 and 2001) in both inshore (<15 nautical miles [nmi]) and offshore zones (15−70 nmi). Relative abundance of YOY S. dumerili (32−210 mm standard length) from purse-seine collections peaked in May and June, and abundance was highest in the offshore zone. Ages of S. dumerili ranged from 39 to 150 days and hatching-date analysis indicated that the majority of spawning events occurred from February to April. Average daily growth rates of YOY S. dumerili for 2000 and 2001 were 1.65 mm/d and 2.00 mm/d, respectively. Intra-annual differences in growth were observed; the late-season (April) cohort experienced the fastest growth in both years. In addition, growth was significantly higher for S. dumerili collected from the offshore zone. Mortality was approximated by using catch-curve analysis, and the predicted instantaneous mortality rate (Z) of YOY S. dumerili was 0.0045 (0.45%/d)

Patterns of Vertical Habitat Use by Atlantic Blue Marlin (Makaira nigricans) in the Gulf of Mexico

We examined data from pop-up archival transmitting (PAT) tags (n = 18) to characterize aspects of vertical habitat use by blue marlin (Makaira nigricans) from the Gulf of Mexico (GOM). Two of these tags were recovered and provided fine-scale information about diving patterns and the relationship between time at depth and temperature. Similar to previous studies, blue marlin in the GOM spent most of their time at the surface and at temperatures within 3° C of surface temperatures. Time at depth was multimodal and the magnitude of the smaller modes was dependent upon the strength and depth of the thermocline. Importantly, time at depth was a complex function of the temperature change relative to the surface, time of day, lunar phase, and water column structure. Temperature change with depth between the western and eastern GOM and the adjacent western Atlantic Ocean was also examined. The depth range (maximum depths varied between 68 and 388 m) varied widely between fish and did not appear to correspond with any particular magnitude of temperature change relative to the surface. Although these data may help to improve stock assessments that are based upon habitat standardizations of CPUE, progress will be limited until the distribution of feeding activity with depth and other aspects of blue marlin behavior in relation to capture probability are elucidated

Utilization of Saltmarsh Shorelines by Newly Settled Sciaenids in a Texas Estuary

Post-settlement patterns of habitat use along saltmarsh shorelines of Galveston Bay, Texas were examined for three sciaenids; spotted seatrout (Cynoscion nebulosus), red drum (Sciaenops ocellatus), and Atlantic croaker (Micropogonias undulatus). Collections were made summer through fall of 1997 and 1998 using a 1.5-m beam trawl hand-towed along the outside edge of salt marshes. Sciaenids were collected from tidal pass, bay, and remote tidal creek areas to assess large-scale (bay-wide) patterns of distribution and abundance. Cynoscion nebulosus were smaller and most numerous at bay stations, with densities peaking in June. Conversely, S. ocellatus were collected in higher numbers, and smaller sizes, at stations near the tidal pass, with peak densities in September. Micropogonias undulatus occurred in high numbers near both the tidal pass and remote tidal creeks, with undetectable size differences among areas and peak densities in November. Densities of C. nebulosus and S. ocellatus were greater at sites nearer the larval supply; patterns were less clear for M. undulatus. Small-scale patterns of habitat use were investigated within remote tidal creeks, with trends in density and size apparent for M. undulatus, while few C. nebulosus and S. ocellatus were collected. Interannual variability in densities occurred within and among stations, although total densities for each species were relatively similar between 1997 and 1998

Genetic evaluation of population structure in white marlin (Kajikia albida): the importance of statistical power

The genetic basis of population structure in white marlin (Kajikia albida) is not well understood. Previous evaluation of genetic population structure in this species utilized a small number of molecular markers to survey genetic variation across opportunistically collected samples of adults, resulting in statistically significant levels of genetic differentiation for some pairwise comparisons and global levels of genetic differentiation that approached statistical significance. This study increased statistical power to improve resolution of genetic population structure in white marlin by surveying a larger number of molecular markers across sample collections of increased size, including collections from additional geographic locations and a robust collection of larvae. Increased statistical power resulted in lower levels of genetic heterogeneity compared with the previous study, and results were consistent with the presence of a single genetic stock of white marlin in the Atlantic Ocean. These results indicate that when statistical power is low, the ability to distinguish noise from a true signal of population structure is compromised. This relationship is especially important for population genetic assessments of marine fishes where genetic differentiation, if it exists, is expected to be low

Taxonomic Richness and Diversity of Larval Fish Assemblages in the Oceanic Gulf of Mexico: Links to Oceanographic Conditions

Biodiversity enhances the productivity and stability of marine ecosystems and provides important ecosystem services. The aim of this study was to characterize larval fish assemblages in pelagic waters of the northern Gulf of Mexico (NGoM) and identify oceanographic conditions associated with areas of increased taxonomic richness (T ) and Shannon diversity (H’). Summer ichthyoplankton surveys were conducted in the NGoM in 2015 and 2016 using neuston net (surface layer; upper 1 m) and oblique bongo net (mixed layer; 0–100 m) tows. Over 17,000 fish larvae were collected over the two-year study, and 99 families of fish larvae were present. Catch composition in the surface layer was relatively similar to the mixed-layer catch, with carangids (jacks), scombrids (mackerels, tunas), and exocoetids (flyingfishes) being numerically dominant, while deep-pelagic species, including myctophids (lanternfishes), gonostomatids (bristlemouths), and sternoptychids (marine hatchetfishes), were present almost exclusively in the mixed layer samples. Generalized additive models were used to evaluate the effect of oceanographic conditions on ichthyoplankton abundance and biodiversity. Salinity and sea surface height (SSH) were the most influential oceanographic conditions, with higher occurrence, higher T , and higher H’ all present in areas of lower salinity, and lower SSH. This study highlights the ecological importance of cyclonic mesoscale features and areas of upwelling as areas of increased biodiversity for larval fishes, and also indicates that the mixed layer in the NGoM is essential habitat for deep-pelagic fishes during the early life interval. F

Predicting habitat suitability for the co-occurrence of an estuarine mesopredator and two top predatory fishes

Determining habitat associations and co-occurrence patterns among species can provide insight into their behavior and shared roles in ecosystem function. This study used a long-term gill-net survey to investigate habitat associations and co-occurrence patterns of three predatory fishes in a large estuarine complex in the northwestern Gulf of Mexico including two top predators alligator gar (Atractosteus spatula) and bull shark (Carcharhinus leucas) and a mesopredator Atlantic stingray (Hypanus sabina). Habitat suitability models were used to assess habitat conditions under which multiple conspecifics or species co-occurred. Environmental drivers of habitat suitability differed within and across species, but salinity and temperature were the most influential variables. Intraspecific co-occurrence of both alligator gars and bull sharks were primarily observed between conspecifics of similar size in low-salinity waters, but intraspecific co-occurrence was higher for alligator gars than for bull sharks. Although sample size was low, intraspecific co-occurrence of Atlantic stingrays was frequent in open-water regions of the estuary and consisted of both juveniles and adults. Interspecific co-occurrence between alligator gars and bull sharks was primarily observed in habitats commonly used by both species, particularly in low-salinity regions of the estuary. Even though there may be confounding effects of sampling gear, interspecific co-occurrence of Atlantic stingrays with either alligator gars or bull sharks was limited, suggesting there was limited overlap in estuarine habitats or environmental conditions used by Atlantic stingrays relative to the two other species investigated. These results clarify the influence of abiotic and biotic variables on the distribution and abundance of these species and suggest potential areas where interactions between them may shape their roles as predators

Distribution, growth, and mortality of sailfish (Istiophorus platypterus) larvae in the northern Gulf of Mexico

Ichthyoplankton surveys were conducted in shelf and slope waters of the northern Gulf of Mexico during the months of May–September in 2005 and 2006 to investigate the potential role of this region as spawning and nursery habitat of sailfish (Istiophorus platypterus). During the two-year study, 2426 sailfish larvae were collected, ranging in size from 2.0 to 24.3 mm standard length. Mean density for all neuston net collections (n=288) combined was 1.5 sailfish per 1000 m2, and maximum density was observed within frontal features created by hydrodynamic convergence (2.3 sailfish per 1000 m2). Sagittal otoliths were extracted from 1330 larvae, and otolith microstructure analysis indicated that the sailfish ranged in age from 4 to 24 days after hatching (mean=10.5 d, standard deviation [SD]=3.2 d). Instantaneous growth coefficients (g) among survey periods (n=5) ranged from 0.113 to 0.127, and growth peaked during July 2005 collections when density within frontal features was highest. Daily instantaneous mortality rates (Z) ranged from 0.228 to 0.381, and Z was indexed to instantaneous weight-specific growth (G) to assess stage-specific production potential of larval cohorts. Ratios of G to Z were greater than 1.0 for all but one cohort examined, indicating that cohorts were gaining biomass during the majority of months investigated. Stage-specific production potential, in combination with catch rates and densities of larvae, indicates that the Gulf of Mexico likely represents important spawning and nursery habitat for sailfish

Changes in Microbial Plankton Assemblages Induced by Mesoscale Oceanographic Features in the Northern Gulf of Mexico

Mesoscale circulation generated by the Loop Current in the Northern Gulf of Mexico (NGOM) delivers growth-limiting nutrients to the microbial plankton of the euphotic zone. Consequences of physicochemically driven community shifts on higher order consumers and subsequent impacts on the biological carbon pump remain poorly understood. This study evaluates microbial plankton <10 μm abundance and community structure across both cyclonic and anti-cyclonic circulation features in the NGOM using flow cytometry (SYBR Green I and autofluorescence parameters). Non-parametric multivariate hierarchical cluster analyses indicated that significant spatial variability in community structure exists such that stations that clustered together were defined as having a specific 'microbial signature' (i.e. statistically homogeneous community structure profiles based on relative abundance of microbial groups). Salinity and a combination of sea surface height anomaly and sea surface temperature were determined by distance based linear modeling to be abiotic predictor variables significantly correlated to changes in microbial signatures. Correlations between increased microbial abundance and availability of nitrogen suggest nitrogen-limitation of microbial plankton in this open ocean area. Regions of combined coastal water entrainment and mesoscale convergence corresponded to increased heterotrophic prokaryote abundance relative to autotrophic plankton. The results provide an initial assessment of how mesoscale circulation potentially influences microbial plankton abundance and community structure in the NGOM

Comparison of Plankton Catch by Three Light-Trap Designs in the Northern Gulf of Mexico

The ichthyoplankton catch and zooplankton biomass estimates of three light-trap designs-cylindrical, quatrefoil, and rectangular-were compared over three consecutive nights at an offshore petroleum platform in the northern Gulf of Mexico. The quatrefoil light trap had higher fish and zooplankton abundance estimates than the other two designs. Categorical analysis of the two abundant fish taxa, Opisthonema oglinum and Anchoa spp., indicated that catch by the quatrefoil and rectangular traps was similar, capturing more larvae than juveniles and more O. oglinum than Anchoa spp. relative to cylindrical trap catch. Across all fish species, the quatrefoil captured a greater percentage of larvae. Other ontogenetic and species-specific differences were noted among the light-trap designs. Samples from vertical plankton tows underestimated larger size classes compared to light-trap catch. Light-trap catch per unit effort (CPUE) declined through the night, especially for the quatrefoil, and increased with depth. In contrast, the percentage of larvae captured across all traps increased through the night and decreased with depth, indicating that CPUE was related mostly to juvenile catch. The percentage of larvae also decreased with increasing water current speed

Spatial, Temporal, and Habitat-Related Variation in Abundance of Pelagic Fishes in the Gulf of Mexico: Potential Implications of the Deepwater Horizon Oil Spill

Time-series data collected over a four-year period were used to characterize patterns of abundance for pelagic fishes in the northern Gulf of Mexico (GoM) before (2007-2009) and after (2010) the Deepwater Horizon oil spill. Four numerically dominant pelagic species (blackfin tuna, blue marlin, dolphinfish, and sailfish) were included in our assessment, and larval density of each species was lower in 2010 than any of the three years prior to the oil spill, although larval abundance in 2010 was often statistically similar to other years surveyed. To assess potential overlap between suitable habitat of pelagic fish larvae and surface oil, generalized additive models (GAMs) were developed to evaluate the influence of ocean conditions on the abundance of larvae from 2007-2009. Explanatory variables from GAMs were then linked to environmental data from 2010 to predict the probability of occurrence for each species. The spatial extent of surface oil overlapped with early life habitat of each species, possibly indicating that the availability of high quality habitat was affected by the DH oil spill. Shifts in the distribution of spawning adults is another factor known to influence the abundance of larvae, and the spatial occurrence of a model pelagic predator (blue marlin) was characterized over the same four-year period using electronic tags. The spatial extent of oil coincided with areas used by adult blue marlin from 2007-2009, and the occurrence of blue marlin in areas impacted by the DH oil spill was lower in 2010 relative to pre-spill years