The Status of the United States Population of Night Shark, Carcharhinus signatus

Night sharks, Carcharhinus signatus, are an oceanic species generally occurring in outer continental shelf waters in the western North Atlantic Ocean including the Caribbean Sea and Gulf of Mexico. Although not targeted, night sharks make up a segment of the shark bycatch in the pelagic longline fishery. Historically, night sharks comprised a significant proportion of the artisanal Cuban shark fishery but today they are rarely caught. Although information from some fisheries has shown a decline in catches of night sharks, it is unclear whether this decline is due to changes in fishing tactics, market, or species identification. Despite the uncertainty in the decline, the night shark is currently listed as a species of concern due to alleged declines in abundance resulting from fishing effort, i.e. overutilization. To assess their relevance to the species of concern list, we collated available information on the night shark to provide an analysis of its status. Night shark landings were likely both over- and under-reported and thus probably did not reflect all commercial and recreational catches, and overall they have limited relevance to the current status of the species. Average size information has not changed considerably since the 1980’s based on information from the pelagic longline fishery when corrected for gear bias. Analysis of biological information indicates night sharks have intrinsic rates of increase (r) about 10% yr–1 and have moderate rebound potential and an intermediate generation time compared to other sharks. An analysis of trends in relative abundance from four data sources gave conflicting results, with one series in decline, two series increasing, and one series relatively flat. Based on the analysis of all currently available information, we believe the night shark does not qualify as a species of concern but should be retained on the prohibited species list as a precautionary approach to management until a more comprehensive stock assessment can be conducted

Characteristics of Shark Bycatch Observed on Pelagic Longlines off the Southeastern United States, 1992–2000

Data collected by fisheries observers aboard U.S. pelagic longline vessels were examined to quantify and describe elasmobranch bycatch off the southeastern U.S. coast (lat. 22°–35°N, long. 71°–82°W). From 1992 to 2000, 961 individual longline hauls were observed, during which 4,612 elasmobranchs (15% of the total catch) were documented. Of the 22 elasmobranch species observed, silky sharks, Carcharhinus falciformis, were numerically dominant (31.4% of the elasmobranch catch). The catch status of the animals (alive or dead) when the gear was retrieved varied widely depending on the species, with high mortalities seen for the commonly caught silky and night, C. signatus, sharks and low mortalities for rays (Dasyatidae and Mobulidae), blue, Prionace glauca; and tiger, Galeocerdo cuvier; sharks. Discard percentages also varied, ranging from low discards (27.6%) for shortfin mako, Isurus oxyrinchus, to high discards for blue (99.8%), tiger (98.5%), and rays (100%). Mean fork lengths indicated the majority of the observed by-catch — regardless of species — was immature, and significant quarterly variation in fork length was found for several species including silky; dusky, C. obscurus; night; scalloped hammerhead, Sphyrna lewini; oceanic whitetip, C. longimanus; and sandbar, C. plumbeus; sharks. While sex ratios overall were relatively even, blue, tiger, and scalloped hammerhead shark catches were heavily dominated by females. Bootstrap methods were used to generate yearly mean catch rates (catch per unit effort) and 95% confidence limits; catch rates were generally variable for most species, although regression analysis indicated significant trends for night, oceanic whitetip, and sandbar sharks. Analysis of variance indicated significant catch rate differences among quarters for silky, dusky, night, blue, oceanic whitetip, sandbar, and shortfin mako sharks

Validity, Identification, and Distribution of the Roundscale Spearfish, Tetrapturus georgii (Teleostei: Istiophoridae): Morphological and Molecular Evidence

The roundscale spearfish, Tetrapturus georgii Lowe, 1840, is known only from four specimens from the Mediterranean and eastern North Atlantic. Additional specimens have not been identified since 1961, making the validity and distribution of this species unclear. Analysis of 16 billfish specimens from the western North Atlantic on the basis of scale morphology, anus position, and mitochondrial DNA confirms the validity of this species and extends its distribution. Mid-lateral scales are soft, notably rounded anteriorly, and bear 2–3 points distinct from those of the sympatric longbill spearfish (Tetrapturus pfluegeri Robins and de Sylva, 1963) and white marlin (Tetrapturus albidus Poey, 1860). Position of anus relative to first anal fin and a related morphometric ratio (distance from anus to first anal fin origin: height of first anal fin) are intermediate between T. pfluegeri and T. albidus. These characteristics match those described by Robins (1974) from the four eastern North Atlantic specimens of T. georgii. The mitochondrial ND4L, ND4, and cyt b gene sequences strongly support reciprocal monophyly of the western North Atlantic specimens relative to other Atlantic istiophorids. The difficulty in distinguishing between morphologically similar T. georgii and T. albidus in the field and the previously unrecognized presence of T. georgii in the western North Atlantic has implications for stock assessments of T. albidus, a species that is severely overfished

Manuscript 1: Elasmobranch Bycatch Observed on Pelagic Longlines Off the Southeastern U.S. Coast, 1992-1997

Data collected by fisheries observers aboard U.S. pelagic longline vessels were examined to quantify and describe elasmobranch bycatch off the southeastern U.S. coast (22° - 35° N, 71° - 82° W). From 1992 to 1997, 608 sets or gear deployments were observed which resulted in the capture of 2649 elasmobranchs (17.5 percent of the total catch). Of the 21 elasmobranch species observed, silky sharks, Carcharhinus falciformis, were numerically dominant (34.4%) followed by dusky, C. obscurus (16.3%), blue, Prionace glauca (8.9%), unidentified sharks (6.4%), tiger, Galeocerdo cuvier (5.9%), and scalloped hammerhead, Sphyrna lewini (5.7%). The catch status of the animal (alive or dead) when the gear was retrieved varied widely depending on the species, with high mortalities seen for the commonly caught silky and dusky sharks and low mortalities for rays, blue sharks, and tiger sharks. Discard percentages depended mostly on the marketability of the species, ranging from low discards (20.4 %) for shortfin mako, Isurus oxyrhinchus, to high discards for blue (99.5 %), tiger (96.7 %), and rays (100 %). Percentages of live and dead discards also varied considerably among species. Analysis of variance indicated significant catch rate differences among seasons for silky, dusky, blue, oceanic whitetip (C. longimanus), sandbar (C. plumbeus), and shortfin mako sharks. Mean fork lengths indicated the majority of the observed bycatch, regardless of species, was immature, and significant seasonal variation in fork length was found for several species including silky, dusky, blue, night (C. signatus), and oceanic whitetip sharks. While sex ratios overall were relatively even, blue and tiger shark catches were dominated by females. Only silky sharks showed significant seasonal sex ratio variation, with a higher proportion of males observed in the winter. Bootstrap methods were used to generate yearly mean catch rates (catch per unit effort) and 95 % confidence limits. Catch rates were in turn used to generate an overall estimate of the numbers of elasmobranchs discarded dead and alive during 1992-1997. These estimates predict 38,775 (18,780 and 63,751, lower and upper 95 % confidence limits) and 31,859 (13,837 and 56,712, lower and upper 95 % confidence limits) sharks were discarded alive and dead, respectively, during this time period. Possible implications of the results of this study for shark fishery management are discussed for the most common bycatch species

Manuscript II: A Demographic Analysis of the Silky Shark, Carcharhinus falciformis, Population Off the Southeastern United States

The best available biological information for the silky shark, Carcharhinus falciformis, was used to produce a demographic analysis for the silky shark population off the southeastern U.S. Monte Carlo simulation was used to vary natural mortality (M), fecundity (mx), age-at-maturity (tmat) and longevity (tmax) to account for some of the uncertainty in these vital rates. Demographic analyses were run 1000 times to allow estimation of 95% confidence intervals. Under the scenario of natural mortality only, results indicated the silky shark population would grow at a rate of 4.5 %/year (R0 = 2.038, G = 16.223, and r = 0.044). Adding the latest available fishing mortality estimate (F) for large coastal sharks resulted in a population decreasing at a rate of 4.8%/year. Finally, to incorporate the hypothetical effect of gear selectivity on fishing mortality, length frequencies for silky sharks caught on longlines off the southeastern U.S. were used to estimate gear selection at various ages. Demographic analyses that consider hypothetical gear selection, when compared to analyses using constant across-age mortality models, suggest that gear selectivity and the resulting fishing mortality-at-age may be an important influence on demographic model output, and should be considered where possible when using demographic analyses as management tools

Characteristics of Shark Bycatch in the Pelagic Longline Fishery off the Southeastern US, 1992-1997

This important and exciting title represents the first authoritative volume focussed on pelagic (open ocean) sharks as a group. Virtually every pelagic shark expert in the world has contributed to this landmark publication which includes the latest data and knowledge on pelagic shark biology, fisheries, management, and conservation. Pelagic sharks face unprecedented levels of exploitation in all the world\u27s oceans through both direct fisheries and by-catch, and effective management for these species is contingent upon solid science and data, which this book brings together for the first time. All those involved in shark biology will need to have a copy of this book.https://nsuworks.nova.edu/occ_facbooks/1045/thumbnail.jp

Effects of Species Misidentification on Population Assessment of Overfished White Marlin Tetrapturus albidus and Roundscale Spearfish T. georgii

The white marlin Tetrapturus albidus (Istiophoridae) is considered to be among the most overexploited species under international management jurisdiction in the Atlantic Ocean, resulting in diverse stakeholder concerns. Efforts have been made to add it to the US Endangered Species List. Its population status has become uncertain with the realization that: (1) longstanding misidentifications of white marlin have occurred with the recently recognized, morphologically similar and sympatric roundscale spearfish T. georgii; and (2) the 2 species have unknowingly been assessed and managed as a species group. We show that roundscale spearfish currently constitute a substantial proportion (~27%) of the overall ‘white marlin’ catch in the western North Atlantic, with high spatial variation within this region. Consequently, the accuracy of current biological knowledge on white marlin, some of which has formed the input for past population status modeling, is compromised by ‘contamination’ due to misidentification. Population assessment simulations in the western North Atlantic incorporating the proportion of roundscale spearfish (27%) were run; these indicated that historical changes in the ratio of the 2 species, as well as differences in the population growth rates between T. albidus and T. georgii, affect simulation results. Our findings suggest misidentifications between the species may have affected the accuracy of past T. albidus population assessments in the western North Atlantic, which therefore need re-visiting to permit improved management and recovery of this species. New collection of life history data for T. albidus and T. georgii is also recommended to corroborate the results of historical studies