Diets of Atlantic Sharpnose Shark (Rhizoprionodon terraenovae) and Bonnethead (Sphyrna tiburo) in the northern Gulf of Mexico

Diets of two coastal sharks, Atlantic Sharpnose Shark (Rhizoprionodon terraenovae) and Bonnethead (Sphyrna tiburo), were examined along the Texas and Alabama coasts in the northern Gulf of Mexico (GOM). Atlantic Sharpnose Sharks were collected from the northwest (n= 209) and northcentral (n= 245) GOM regions while Bonnetheads were collected from two locations within the northwest GOM (Galveston, Texas, n= 164; Matagorda, Texas, n= 79). Dietary analysis was conducted using stomach contents identified to the lowest taxonomic level, which were quantified using the index of relative importance (IRI) and non-parametric statistical analyses. Atlantic Sharpnose Sharks were revealed to be primarily piscivorous, with an overall %IRI of 79.76% for teleost fishes. Bonnetheads were shown to prey primarily on crustaceans (90.94% IRI), mainly crabs (22.06% IRI). Diets for Atlantic Sharpnose Sharks and Bonnetheads were evaluated by region and ontogeny, where variations by ontogeny were examined based on length at 50% maturity (L50) values, delineating mature from immature individuals. Atlantic Sharpnose Sharks and Bonnetheads showed a decrease in dietary prey species richness from juveniles to adults using %IRI. Regional dietary differences existed with Atlantic Sharpnose Sharks from the northwest GOM consuming more crustaceans than conspecifics from the northcentral GOM. Bonnetheads collected from Galveston, TX consumed more crab than Bonnetheads from Matagorda, TX, while Bonnetheads from Matagorda, TX displayed a diet with higher prey species richness. Our results highlight differences in diets of two common shark species at both local and regional spatial scales

Age Composition and Distribution of Red Drum (Sciaenops ocellatus) in Offshore Waters of the North Central Gulf of Mexico: An Evaluation of a Stock Under a Federal Harvest Moratorium

Because of a lack of fishery- dependent data, assessment of the recovery of fish stocks that undergo the most aggressive form of management, namely harvest moratoriums, remains a challenge. Large schools of red drum (Sclaenops ocellatus) were common along the northern Gulf of Mexico until the late 1980s when increased fishing effort quickly depleted the stock. After 24 years of harvest moratorium on red drum in federal waters, the stock is in need of reassessment; however, fishery dependent data are not available in federal waters and fishery-independent data are limited. We document the distribution, age composition, growth, and condition of red drum in coastal waters of the north central Gulf of Mexico, using data collected from a nearshore, randomized, bottom longline survey. Age composition of the fishery-independent catch indicates low mortality offish age 6 and above and confirms the effectiveness of the federal fishing moratorium. Bottom longline surveys may be a cost-effective method for developing fishery-independent indices for red drum provided additional effort can be added to nearshore waters (\u3c20 m depth). As with most stocks under harvest bans, effective monitoring of the recovery of red drum will require the development of fishery-independent indices. With limited economic incentive to evaluate non-exploited stocks, the most cost-effective approach to developing such monitoring is expansion of existing fishery independent surveys. We examine this possibility for red drum in the Gulf of Mexico and recommend the bottom longline survey conducted by the National Marine Fisheries Service expand effort in nearshore areas to allow for the development of long-term abundance indices for red drum

Investigating Life History Differences Between Finetooth Sharks, Carcharhinus isodon, in the Northern Gulf of Mexico and the Western North Atlantic Ocean

The life history of the finetooth shark, Carcharhinus isodon, off South Carolina was studied by determining age, growth, and size and age at maturity. These data were compared to a recent study describing the same parameters for finetooth sharks in the northern Gulf of Mexico. Cervical vertebrae were extracted from 168 specimens (71 males and 97 females), ranging in size from 376 to 1,262 mm fork length (FL), and prepared for age analysis using standard techniques. Sex-specific von Bertalanffy growth models were generated and yielded the following growth equations: Lt = 1,311 mm FL (1 - e-0.19(t - (-2.17))) for females and Lt = 1,151 mm FL (1 - e-0.33(t - (-1.43))) for males. The oldest female and male aged were 12.4 yr and 10.4 yr, respectively. Median length where 50% of the population was mature was 1,021 mm FL for females, corresponding to an age of 6.3 yr and 1,015 mm FL for males, corresponding to an age of 5.0 yr. Finetooth sharks in the western North Atlantic Ocean had higher observed ages and there was a significant difference in size at age between neonate finetooth sharks in the western North Atlantic Ocean and the northern Gulf of Mexico; however, there were no significant differences among von Bertalanffy growth function parameters between regions examined. Results indicate lower amounts of regional variation in life history parameters for finetooth sharks when compared to other small coastal sharks

Estimating Exploitation Rates in the Alabama Red Snapper Fishery Using a High-Reward Tag–Recapture Approach

Accurate estimates of exploitation are essential to managing an exploited fishery. However, these estimates are often dependent on the area and vulnerable sizes of fish considered in a study. High-reward tagging studies offer a simple and direct approach to estimating exploitation rates at these various scales and in examining how model parameters impact exploitation rate estimates. These methods can ultimately provide a better understanding of the spatial dynamics of exploitation at smaller local and regional scales within a fishery—a measure often needed for more site-attached species, such as the Red Snapper Lutjanus campechanus. We used this approach to tag 724 Red Snapper during 2016 in the Alabama Artificial Reef Zone within the northern Gulf of Mexico to estimate recreational exploitation rates in Alabama waters. We fitted a series of tag return models, analyzed using maximum likelihood, to examine how release depth, movement between depth strata, fish length, and the rate at which anglers released fish impacted estimates of exploitation rate under a range of assumed natural and tagging mortality rates. Our model results suggested higher fishing mortality in the shallower depth stratum than in the deep stratum, constant movement rates with release depth, and constant release rates across fish lengths. Exploitation rate for the aggregate tagged population across the entire sample area was estimated at 0.14. Exploitation rates estimated for each depth stratum were 0.20 (shallow stratum

Distributions of Sharks across a Continental Shelf in the Northern Gulf of Mexico

Declines in shark populations have sparked researchers and fishery managers to investigate more prudent approaches to the conservation of these fish. As managers strive to improve data collection for stock assessment, fisheries-independent surveys have expanded to include data-deficient areas such as coastal regions. To that end, a catch series from a nearshore survey off Alabama was combined with data from a concurrent offshore survey with identical methodology to examine the depth use of sharks across the continental shelf (2–366 m). The combined data set contained 22 species of sharks collected from 1995 to 2008: 21 species in the offshore data set (1995–2008) and 12 species in the nearshore data set (2006–2008). Depth was a significant factor determining species’ distributions, primarily for Atlantic sharpnose Rhizoprionodon terraenovae, blacknose Carcharhinus acronotus, and blacktip C. limbatus sharks. Blacknose sharks had the highest catch per unit effort (CPUE) in the middepth stratum (10–30 m), blacktip sharks had consistently higher CPUE in the shallow depth stratum (,10 m), and Atlantic sharpnose sharks showed high abundance throughout both the shallow and mid-depth strata. Length frequency and sex ratio analyses suggest that Atlantic sharpnose and blacknose sharks are using waters greater than 30 m deep for parturition, whereas adult blacktip sharks are probably using shallow waters for parturition. Our abundance patterns illustrate a continuum of depth use across the inner continental shelf. Surveys that do not encompass the entirety of this ecosystem fail to accurately characterize the distributions of these important predators

Tiger Sharks Eat Songbirds: Reply

In response to our recent paper (Drymon et al. 2019), Yosef (2019) questions the mechanism proposed to explain interactions between tiger sharks (Galeocerdo cuvier) and migratory songbirds, while offering an alternative mechanism based on a single observation. We appreciate the comments from Yosef and the opportunity to respond

Gulf-Wide Decreases in the Size of Large Coastal Sharks Documented by Generations of Fishermen

Large sharks are top predators in most coastal and marine ecosystems throughout the world, and evidence of their reduced prominence in marine ecosystems has been a serious concern for fisheries and ecosystem management. Unfortunately, quantitative data to document the extent, timing, and consequences of changes in shark populations are scarce, thwarting examination of long-term (decadal, century) trends, and reconstructions based on incomplete data sets have been the subject of debate. Absence of quantitative descriptors of past ecological conditions is a generic problem facing many fields of science but is particularly troublesome for fisheries scientists who must develop specific targets for restoration. We were able to use quantitative measurements of shark sizes collected annually and independently of any scientific survey by thousands of recreational fishermen over the last century to document decreases in the size of large sharks from the northern Gulf of Mexico. Based on records from fishing rodeos in three U.S. coastal states, the size (weight or length) of large sharks captured by fishermen decreased by 50–70% during the 20 years after the 1980s. The pattern is largely driven by reductions in the occurrence and sizes of Tiger Sharks Galeocerdo cuvier and Bull Sharks Carcharhinus leucas and to a lesser extent Hammerheads Sphyrna spp. This decrease occurred despite increasing fishing effort and advances in technology, but it is coincident with the capitalization of the U.S. commercial shark long-line fishery in the GOM

Red Snapper Distribution on Natural Habitats and Artificial Structures in the Northern Gulf of Mexico

In 2011, an intensive, multiple-gear, fishery-independent survey was carried out in the northern Gulf of Mexico (GOM) to collect comprehensive age and length information on Red Snapper Lutjanus campechanus. Based on this synoptic survey, we produced a spatial map of Red Snapper relative abundance that integrates both gear selectivity effects and ontogenetically varying habitat usage. Our methodology generated a spatial map of Red Snapper at a 10-km2 grid resolution that is consistent with existing knowledge of the species: Red Snapper occurred in relatively high abundances at depths of 50–90 m along the coasts of Texas and Louisiana and in smaller, patchy “hot spots” at a variety of depths along the Alabama coast and the west Florida shelf. Red Snapper biomass and fecundity estimates were higher for the northwestern GOM than for the northeastern GOM, as the latter area contained mostly smaller, younger individuals. The existence of similar surveys on petroleum platforms and artificial reefs also enabled us to calculate their relative contribution to Red Snapper distribution compared with that of natural habitats.We estimated that for the youngest ageclasses, catch rates were approximately 20 times higher on artificial structures than on natural reefs. Despite the high catch rates observed on artificial structures, they represent only a small fraction of the total area in the northern GOM; thus, we estimated that they held less than 14%of Red Snapper abundance. Because artificial structures—particularly petroleum platforms—attract mostly the youngest individuals, their contribution was even lower in terms of total population biomass (7.8%) or spawning potential (6.4%). Our estimates of Red Snapper relative abundance, biomass, and spawning potential can be used to design spatial management strategies or as inputs to spatial modeling techniques

Age, Growth, and Mortality of Atlantic Tripletail in the North-Central Gulf of Mexico

In the southeastern USA and the Gulf of Mexico (GOM), Atlantic Tripletail Lobotes surinamensis are increasingly targeted by recreational anglers, indicating that stock status should be assessed. A critical need for such assessments is age-specific data; however, previous studies have drawn conflicting conclusions regarding the most appropriate structure for aging. Moreover, growth parameters and mortality rates for GOM Atlantic Tripletail are unknown. Therefore, the goals of this study were to (1) evaluate sagittal otoliths and first dorsal spines as aging structures; (2) model combined and sex-specific growth; and (3) estimate mortality rates for GOM Atlantic Tripletail. From 2012 to 2019, Atlantic Tripletail (N = 230, including a near-record-size specimen) were collected from the north-central GOM via hook and line and were aged using otoliths and first dorsal spines. Total length ranged from 212 to 940 mm, and age ranged from 0.07 to 5.27 years. Otoliths produced higher percent agreement (95.0%) and lower average percent error (3.0%) between readers compared to spines (82.9% and 6.5%, respectively). The von Bertalanffy growth parameters differed slightly between the otolith-based data (mean asymptotic length [L∞] = 762.42 mm, Brody growth rate coefficient [k] = 0.69 year−1, and hypothetical age at which length equals zero [t0] = −0.58 year) and spine-based data (L∞ = 718.83 mm, k = 0.79 year−1, and t0 = −0.56 year). For both otolith- and spine-based sex-specific data, the best-fitting version of the von Bertalanffy growth function permitted L∞ to vary by sex. Chapman– Robson estimates of instantaneous total mortality rate and total annual mortality rate were 1.15 and 68.66%, respectively. Based on empirical, life history-based methods, the instantaneous natural mortality rate was estimated at 0.75–0.97 and the instantaneous fishing mortality rate was estimated at 0.18–0.45, suggesting low levels of exploitation. These growth parameters and mortality estimates will provide information for future stock assessments, thereby ensuring sustainability of the GOM stock of Atlantic Tripletail