William B. Driggers

Contemporary population structure and post-glacial genetic demography in a migratory marine species, the blacknose shark, Carcharhinus acronotus

Patterns of population structure and historical genetic demography of blacknose sharks in the western North Atlantic Ocean were assessed using variation in nuclear‐encoded microsatellites and sequences of mitochondrial (mt)DNA. Significant heterogeneity and/or inferred barriers to gene flow, based on microsatellites and/or mtDNA, revealed the occurrence of five genetic populations localized to five geographic regions: the southeastern U.S Atlantic coast, the eastern Gulf of Mexico, the western Gulf of Mexico, Bay of Campeche in the southern Gulf of Mexico and the Bahamas. Pairwise estimates of genetic divergence between sharks in the Bahamas and those in all other localities were more than an order of magnitude higher than between pairwise comparisons involving the other localities. Demographic modelling indicated that sharks in all five regions diverged after the last glacial maximum and, except for the Bahamas, experienced post‐glacial, population expansion. The patterns of genetic variation also suggest that the southern Gulf of Mexico may have served as a glacial refuge and source for the expansion. Results of the study demonstrate that barriers to gene flow and historical genetic demography contributed to contemporary patterns of population structure in a coastal migratory species living in an otherwise continuous marine habitat. The results also indicate that for many marine species, failure to properly characterize barriers in terms of levels of contemporary gene flow could in part be due to inferences based solely on equilibrium assumptions. This could lead to erroneous conclusions regarding levels of connectivity in species of conservation concern.

Profiling plasma steroid hormones: a non-lethal approach for the study of skate reproductive biology and its potential use in conservation management

Information regarding sexual maturity and reproductive cycles in skates has largely been based on gross morphological changes within the reproductive tract. While this information has proved valuable in obtaining life history information, it also necessitates sacrificing the skates to obtain this data. In contrast, few studies have used circulating steroid hormones to establish when these batoids become reproductively capable or for the determination of reproductive cyclicity. This study summarizes our current knowledge of hormonal analyses in determining skate reproductive status and offers information that suggests analysis of circulating steroid hormone concentrations provide a means to determine size at sexual maturity and asses reproductive cycles without the need to sacrifice the skate.

Size and age estimates at sexual maturity for the little skate Leucoraja erinacea from the western Gulf of Maine, U.S.A.

Size and age estimates at sexual maturity were determined for 162 male and 273 female little skates Leucoraja erinacea collected from the western Gulf of Maine. Maturity ogives suggest that 50% maturity in females occurs at age 9.5 years and 480 mm total length (LT), whereas 50% maturity in males occurs at a slightly younger age of 7.7 years and smaller size of 460 mm LT. Age estimates were made from 389 L. erinacea ranging in size from 93 to 570 mm LT. The index of average per cent error and age-bias plots indicated that the ageing methods were precise and non-biased. Additionally, annual periodicity of band formation was validated with oxytetracycline in eight individuals (three males and five females) ranging in age from 3 to 12 years. In conclusion, results from this study indicate that L. erinacea exhibits characteristics that make other elasmobranch populations highly susceptible to overexploitation.

Validated age, growth and maturity of the bonnethead Sphyrna tiburo in the western North Atlantic Ocean

The age, growth and maturity of bonnetheads Sphyrna tiburo inhabiting the estuarine and coastal waters of the western North Atlantic Ocean (WNA) from Onslow Bay, North Carolina, south to West Palm Beach, Florida, were examined. Vertebrae were collected and aged from 329 females and 217 males ranging in size from 262 to 1043 mm and 245 to 825 mm fork length, LF , respectively. Sex-specific von Bertalanffy growth curves were fitted to length-at-age data. Female von Bertalanffy parameters were L∞  = 1036 mm LF , k = 0·18, t0  = -1·64 and L0  = 272 mm LF . Males reached a smaller theoretical asymptotic length and had a higher growth coefficient (L∞  = 782 mm LF , k = 0·29, t0  = -1·43 and L0  = 266 mm LF ). Maximum observed age was 17·9 years for females and 16·0 years for males. Annual deposition of growth increments was verified by marginal increment analysis and validated for age classes 2·5+ to 10·5+ years through recapture of 13 oxytetracycline-injected specimens at liberty in the wild for 1-4 years. Length (LF50 ) and age (A50 ) at 50% maturity were 819 mm and 6·7 years for females, and 618 mm and 3·9 years for males. Both female and male S. tiburo in the WNA had a significantly higher maximum observed age, LF50 , A50 and L∞ , and a significantly lower k and estimated L0 than evident in the Gulf of Mexico (GOM). These significant differences in life-history parameters, as well as evidence from tagging and genetic studies, suggest that S. tiburo in the WNA and GOM should be considered separate stocks.

Variability in the Reproductive Cycle of Finetooth Sharks, Carcharhinus isodon, in the Northern Gulf of Mexico

Abstract From 2005 through 2008, seven mature female Finetooth Sharks, Carcharhinus isodon, were collected in the central northern Gulf of Mexico between April and June, a time coinciding with parturition and ovulation for the species. Five specimens displayed states of pregnancy and ovarian development consistent with a biennial reproductive cycle. Two individuals had near-term pups and vitellogenic oocytes, a condition indicative of an annual reproductive cycle. These observations are the first report of annual reproduction in Finetooth Sharks and represent the first documented case of intraspecific divergence in the reproductive cycles for any elasmobranch within a discrete area.

Variability in the Reproductive Biology of the Atlantic Sharpnose Shark in the Gulf of Mexico

Abstract The reproductive biology of the Atlantic Sharpnose Shark Rhizoprionodon terraenovae in the Gulf of Mexico was investigated by examining 1,306 specimens (693 females, 613 males) collected from the Florida Keys to waters off Brownsville, Texas. The results of this study confirm the annual reproductive cycle established for this species; however, there was a significant amount of variability within the cycle. Ovulatory and postovulatory females were present from March to October, indicating that mating and ovulation were occurring over a more protracted period than previously described (e.g., May to July). The occurrence of postpartum females from April to September, the varying sizes of the embryos across several months, and the occurrence of mature spermatozoa in the testes of adults from March to November also corroborate the evidence of reproductive plasticity in this species. This observed variability in the reproductive cycle indicates that the Gulf of Mexico Atlantic Sharpnose Shark population is not completely synchronous in regards to mating, ovulation, and parturition, as a portion of the population is demonstrating reproductive asynchrony. Although the cause of this asynchrony remains unclear, it may be related to the environmental conditions of the Gulf of Mexico, which could provide water temperatures that are optimal for the reproduction of this species through much of the year (i.e., March to October), resulting in a protracted reproductive cycle. Given the results of the current study, the reproductive cycles of other carcharhinid species in this region should be examined in more detail to determine whether there is asynchrony in them as well, as this phenomenon could impact future management strategies.

Observations on the Distribution of Ichthyoplankton within the Saco River Estuary System

Abstract Determining planktonic larval species composition and abundance data can serve to elucidate local patterns of distribution, determine an areas importance as a nursery ground, and help clarify broad-scale trends of adult distribution and spawning ranges. Although the Saco River and estuary is the fourth largest waterway system in Maine, this ecosystem has remained relatively unstudied over the last thirty years, and research describing the temporal ichthyoplankton composition and distribution is virtually absent. The present study examined the structure of the ichthyoplankton community and determined the temporal and spatial variation in species diversity and abundance within the Saco Bay estuary system. Weekly sampling trips during the months of June, July, and August in 2007 revealed ten species of ichthyoplankton present in the study area. Ulvaria subbifurcata (Radiated Shanny) and Tautogolabrus adsperus (Cunner) dominated the abundance data followed by Hippoglossoides platessoides (American Plaice) and Myoxocephalus octodecimspinosus (Longhorn Sculpin).

Long-term assessment of whale shark population demography and connectivity using photo-identification in the Western Atlantic Ocean

The predictable occurrence of whale sharks, Rhincodon typus, has been well documented in several areas. However, information relating to their migratory patterns, residency times and connectivity across broad spatial scales is limited. In the present study photo-identification data is used to describe whale shark population structure and connectivity among known aggregation sites within the Western Central Atlantic Ocean (WCA). From 1999 to 2015, 1,361 individuals were identified from four distinct areas: the Yucatan Peninsula, Mexico (n = 1,115); Honduras (n = 146); northern Gulf of Mexico, United States (n = 112), and Belize (n = 49). Seasonal patterns in whale shark occurrence were evident with encounters occurring in the western Caribbean Sea earlier in the year than in the GOM. There was also a significant sex bias with 2.6 times more males present than females. Seventy sharks were observed in more than one area and the highest degree of connectivity occurred among three aggregation sites along the Mesoamerican Reef. Despite this, the majority of resightings occurred in the area where the respective sharks were first identified. This was true for the WCA as a whole, with the exception of Belize. Site fidelity was highest in Mexico. Maximum likelihood modelling resulted in a population estimate of 2,167 (95% c.i. 1585.21–2909.86) sharks throughout the entire region. This study is the first attempt to provide a broad, regional population estimate using photo-identification data from multiple whale shark aggregations. Our aim is to provide population metrics, along with the description of region-scale connectivity, that will help guide conservation action in the WCA. At a global level, rapidly growing photographic databases are allowing for researchers to look beyond the description of single aggregation sites and into the ocean-scale ecology of this pelagic species.

Estimation of Hook Selectivity of Red Snapper and Vermilion Snapper from Fishery-Independent Surveys of Natural Reefs in the Northern Gulf of Mexico

Abstract Implementation of circle hook regulations in the Gulf of Mexico will impact the length structure and age structure of the snapper-grouper fishery catch as well as demographic data for stock assessments; therefore, an understanding of circle hook selectivity patterns is critical. Indirect selectivity analysis of the vertical-line catch of Red Snapper Lutjanus campechanus and Vermilion Snapper Rhomboplites aurorubens showed that for both species, there were significant differences in mean FL among hook sizes, broad length frequency distributions, and wide selectivity curves. Although the results suggest that hook size regulations could be a useful management strategy for targeting desired size-classes of these snappers, the broadly overlapping length distributions indicate that undersized catch would not be eliminated. Selectivity curves generated from the different families of distributions produced equally good fit to the data and provided a basis for evaluating various selectivity curves when the size structure of the sampled population is unknown. If the size structure of the population is known, then the use of direct selectivity methods is recommended. Right-skewed distributions generally fit the data best, suggesting that hook regulations are likely to be more effective if the desired goal is to reduce the amount of undersized catch by eliminating small hook sizes. Conversely, elimination of large hooks appears to be less likely to reduce the catch of larger size-classes because mouth gape is likely the primary limiting factor and small hooks can catch large fish. Catch rates were significantly different among hook sizes; thus, regulations based on hook size could impact fishing effort and change the dynamics of how the snapper-grouper fishery is prosecuted. Tradeoffs between moderate improvements in size-class targeting, changes in effort, and various components of fishing mortality (i.e., catch and regulatory discards) will require further investigation through simulation modeling or field experimentation.

Development of a Nonlethal and Minimally Invasive Protocol to Study Elasmobranch Reproduction

Abstract An understanding of basic reproductive biology is essential for successful species-specific management of elasmobranch fishes (sharks, skates, and rays). Such information is often gained through gross dissection or other lethal techniques, which are not appropriate for threatened and endangered species. Previous work on other vertebrates suggested that sex steroid hormones can be extracted from muscle tissues to identify reproductive status. Collecting for muscle biopsy is quick and minimally invasive and can be done without removing an animal from the water. Thus, the objective of the current study was to determine the efficacy of using muscle steroid hormones to assess the reproductive biology of elasmobranch fishes. The results suggest that concentrations of muscle progesterone, testosterone, and estradiol can be successfully quantified to study reproduction by radioimmunoassay. Additionally, there were significant correlations between the plasma and muscle estradiol concentrations in Spiny Dogfish Squalus acanthias and the progesterone, testosterone, and estradiol concentrations in Atlantic Sharpnose Sharks Rhizoprionodon terraenovae. The present investigation thus demonstrates that skeletal muscle is a nonlethally harvestable tissue that is well suited for studying the reproductive biology of elasmobranchs.