Environmental Influences on Juvenile Fish Abundances in a River-Dominated Coastal System

We investigated the influence of climatic and environmental factors on variations in juvenile abundances of marine fishes in a river-dominated coastal system of the north-central Gulf of Mexico, where an elevated primary productivity sustains fisheries of high economic importance. Fish were collected monthly with an otter trawl at three stations near Mobile Bay from 1982 to 2007. Fish sizes were used to isolate juvenile stages within the data set, and monthly patterns in juvenile fish abundance and size were then used to identify seasonal peaks for each species. The average numbers of juvenile fish collected during these seasonal peaks in each year were used as indices of annual juvenile abundances and were related to corresponding seasonal averages of selected environmental factors via a combination of principal components analysis and co-inertia analysis. Factors contributing the most to explain interannual variations in juvenile fish abundances were river discharge and water temperature during early spring–early summer, wind speed and North Atlantic Oscillation index during late fall–winter, and atmospheric pressure and wind speed during summer–fall. For example, juvenile abundances of southern kingfish Menticirrhus americanus during summer–fall were positively associated with atmospheric pressure and negatively associated with wind speed during this period. Southern kingfish juvenile abundances during late fall–winter were also negatively associated with wind speed during the same period and were positively associated with river discharge during early spring–early summer. Juvenile abundances of the Atlantic croaker Micropogonias undulatus during early spring–early summer were negatively associated with river discharge and North Atlantic Oscillation during late fall–winter. Overall, the importance of river discharge for many of the species examined emphasizes the major role of watershed processes for marine fisheries production in coastal waters of the north-central Gulf of Mexico

Population Dynamics, Relative Abundance, and Habitat Suitability of Adult Red Drum (Sciaenops ocellatus) in Nearshore Waters of the North-Central Gulf of Mexico

In the Gulf of Mexico, the red drum (Sciaenops ocellatus) is an immensely popular sportfish, yet the Gulf of Mexico stock is currently managed as data-limited in federal waters. The results of the federal stock assessment conducted in 2016 for Gulf of Mexico red drum were not recommended for providing management advice. Consequently, we sought to address data gaps highlighted in the assessment by producing up-to- date overall and sex-specific growth models, standardized indices of relative abundance, and predictions of habitat suitability and by updating estimates of natural mortality. Using a time series for the period of 2006–2018, we assigned ages of 0–36 years to 1178 red drum. A negative binomial generalized linear model including variables for year, depth, surface temperature, dissolved oxygen, and bottom salinity was used to standardize an index of relative abundance. Examination of catch per unit of effort revealed that adult red drum were significantly more abundant in state waters than in federal waters. These findings were explained by habitat suitability models, which were used to identify surface current velocity, surface temperature, and depth as the strongest predictors of relative abundance. The results of our investigation reveal that the adult spawning stock of red drum in the Gulf of Mexico is not fully protected by the catch moratorium in federal waters

Evaluating the Performance of Vertical Longlines to Survey Reef Fish Populations in the Northern Gulf of Mexico

A common critique of many stock assessments is the lack of fishery‐independent abundance indices and age composition data. Such data streams are essential in evaluating population trajectories that are derived largely from harvest and age composition of landings. For example, high scientific uncertainty in the most recent stock assessment of Gulf of Mexico red snapper Lutjanus campechanus resulted from a conflict between trends in fishery‐dependent and fishery‐independent data. Because sample sizes for the latter data were an order of magnitude lower, resolution of the conflicting trends was even more problematic. Recognizing the need for cost‐effective expansion of fishery‐independent data in the region, we evaluated the performance of vertical longline surveys for sampling reef fish within a large artificial reef zone in the northern Gulf of Mexico. Specifically, we (1) determined species composition and the length frequency of red snapper (the dominant species captured) as a function of hook size and bait type within our survey area during 2010; (2) evaluated the effect of different soak times on catch for various hook types (a combination of hook size and bait type); and (3) utilized our results to test the effect of artificial reef type on red snapper CPUE and mean size. During March–November 2010, we conducted 532 vertical longline sets, capturing 1,217 red snapper that ranged from 184 to 827 mm FL. Mean FL of red snapper differed among hook sizes, with 3/0 and 8/0 hooks sampling smaller fish than 11/0 hooks. Soak time trials revealed a significant effect of soak time on CPUE, with peak catch rates observed at 5 min. As habitat area increased, the mean size and CPUE of red snapper increased. We conclude that our vertical longline is an effective gear for sampling red snapper, and we recommend protocols to maximize its utility and standardize its use

Defining Sex-Specific Habitat Suitability for a Northern Gulf of Mexico Shark Assemblage

Understanding the factors that influence species' distributions is crucial for implementing effective management and conservation practices, yet difficult for highly vagile species like sharks. Many shark species demonstrate either spatial and/or temporal sexual segregation, further confounding accurate quantification of habitat suitability. Given the importance of understanding spatiotemporal patterns in the distribution of coastal shark assemblages, we sought to quantify sex-specific abiotic factors that influence seasonal variation in a coastal shark assemblage using data from a long-term fisheries-independent bottom longline program in the northern Gulf of Mexico. Catch data (individuals/100 hooks/hour) were coupled with a suite of potentially predictive variables: surface and bottom values for temperature (degrees C) and salinity (psu), sea surface height (m), three-dimensional surface and bottom current velocity (u, v, w, in m/s), bottom dissolved oxygen (mg/I), depth (m), substrate grain size (mm), daylength (min), and distance from shore (km). Data were analyzed using boosted regression trees (BRT) to describe the relationships between catch data and environmental factors potentially influencing sex-specific species distribution and abundance. Between May 2006 and November 2018, we conducted 1,226 bottom longline sets and caught 13,742 individuals encompassing 67 species. The majority of the animals captured (74%) were elasmobranchs, primarily sharks. Two species from each of the following three categories were selected for further analyses: small coastal sharks (Atlantic sharpnose shark Rhizoprionodon terraenovae and blacknose shark Carcharhinus acronotus), large coastal sharks (blacktip shark C. limbatus and sandbar shark C. plumbeus), and shelf-associated sharks (smoothhound sharks, Mustelus spp. and scalloped hammerhead Sphyrna lewini). Depth and distance from shore were the strongest predictors of distribution and relative abundance, followed by longitude and bottom salinity; other factors (e.g., temperature, daylength, substrate grain size) were less predictive. For the six species examined, predictive factors were often the same for males and females, although the range of preferred values varied. Surprisingly, the importance of these predictors varied little across seasons. Collectively, our findings demonstrate that sexual segregation is the norm for sharks in the north-central Gulf of Mexico. Long-term fishery-independent monitoring to further quantify these sex-based differences in habitat use should be prioritized, particularly in light of impending climate change