Relative Abundance of Pelagic Sharks in the Western North Atlantic Ocean, Including the Gulf of Mexico and Caribbean Sea

Abstract

Little information exists on the status of pelagic shark populations in the Atlantic Ocean, especially in the Gulf of Mexico and Caribbean Sea. We derived indices of relative abundance for pelagic sharks based on mandatory logbooks and observer reports from a scientific observer program of the United States (US) pelagic longline fleet. Time series data from the pelagic longline logbook program (1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005) and the pelagic longline observer program (1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005) were standardized with Generalized Linear Model (GLM) procedures. Declines in relative abundance for the 6 pelagic shark species or genera examined in the logbook data analysis ranged from 43% for make sharks, Isurus spp., to 88% for blue sharks, Prionoce g/auca, whereas declines in relative abundance obtained from the observer data analysis were less accentuated than those in the logbook data analysis, with the trend being positive for night sharks, Carcharhinus signatus, and thresher sharks, Alopias spp. There was no significant change in the fork length at capture over the time period considered for blue sharks, shortfin makes Isurus oxyrinchus, or night sharks. The trends obtained must be viewed cautiously given recognized shortcomings, especially of the logbook dataset, and the highly migratory nature of pelagic sharks, which requires a more comprehensive evaluation of trends throughout their range.RESUMEN Existe poca informaci6n sobre el estado de las poblaciones de tiburones pel~gicos en el Oc~o AtImtico, especialmente en el Golfo de M6xico y el Mar Caribe. Derivarnos Indices de abundancia relativa para tiburones pel~gicos a partir de cuademos de biili:ora e informes de observadores de un prograrna de observadores cientlficos de la flota palangrera peJagica de los Estados Unidos (BE.UU.). Las series temporales del prograrna de cuademos de bit&:ora de palangre peJagico (1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005) y del prograrna de observadores de palangre peJagico (1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005) se estandarizaron por medio de Modelos Lineales Generalizados (GLMs). Las reducciones en abundancia relativa de las seis especies 0 generos de tiburones peJB.gicos que se examinaron variaron entre el 43% para IsuTUS spp. y el 88% para Prionace glauca, mientras que los descensos en abundancia relativa obtenidos del anaIisis de los datos de observadores fueron menos acentuados que los del anaIisis de los datos de los cuadernos de bit4cora, con Carcharhinus signatus y Alopias spp. mostrando tendencias positivas. No se encontraron cambios significativos en la longitud furcal de las capturas durante el periodo analizado para P. g/auca, Isurus oxyrinchus 0 C. signatus. Las tendencias obtenidas deben interpretarse con cautela debido a distintos problema...

Complex Movements, Philopatry and Expanded Depth Range of a Severely Threatened Pelagic Shark, the Oceanic Whitetip (Carcharhinus longimanus) in the Western North Atlantic

“…This is attributable to several features of these tracks: (1) individuals remained within 500 km of the tagging site during May and part of June (2) limited displacement (<500 km from Cat Island) of 22.2% of tagged individuals in subsequent months and (3) the return of the individuals that left the EEZ after 60–160 days (philopatry). Pelagic longlining was prohibited by The Bahamas in the 1990 s, which may have helped conserve oceanic whitetips in this area even as they declined elsewhere in the North Atlantic [4]. The Bahamas subsequently reinforced this measure with a commercial trade ban on all sharks in 2011.…”

“…Oceanic whitetips were historically abundant, replacing primarily temperate-dwelling blue sharks ( Prionace glauca ) as the numerically dominant pelagic shark at lower latitudes [2]. Several studies have shown substantial population declines in oceanic whitetips, most likely related to mortality associated with the global shark fin trade [4]–[6]. This species is now listed as “Critically Endangered” in the Northwest Atlantic and “Vulnerable” globally by the International Union for the Conservation of Nature (IUCN) [7].…”

Shark bycatch and depredation in the U.S. Atlantic pelagic longline fishery

“…However, the reduction in U.S. commercial PLL fishing effort in recent years has presumably reduced the relative scale of impacts by this fishery. Excluding 1991 for which data was absent, the average annual catch weight of blue sharks from 1990 to 1999 was more than 50-fold higher in recreational (hook) than in U.S. commercial PLL fisheries (annual range of 9-340 mt [annual mean value of 114 mt] for recreational; 0.12-8.00 mt [annual mean value of 2 mt] for commercial) (Cortés 2002). So in fact, targeted or incidental capture of sharks in U.S. recreational (hook) fisheries may have posed a more significant threat to certain shark populations even before fishing effort by the U.S. domestic PLL fleet decreased to it current levels, and presumably continues to eclipse commercial catch rates of certain pelagic shark species.…”

Reassessing the value of nursery areas to shark conservation and management

“…Indeed, many shark populations have declined with respect to their pre-exploited biomass (Baum & Myers 2004;Shepherd & Myers 2005;Cortés et al 2007), raising concern for their conservation and requiring the development of management planning.…”