Molecular resolution of marine turtle stock composition in fishery bycatch: a case study in the Mediterranean

Abstract

Based on an extensive sampling regime from both nesting populations and bycatch, frequency analyses of mitochondrial (mt) DNA control region haplotypes in the Mediterranean were used to assess the genetic structure and stock composition of the loggerhead sea turtle, Caretta caretta, in different marine fisheries. The analyses show the following. (i) In drifting longline fisheries working in Mediterranean pelagic habitats 53-55% of turtles caught originated from the Mediterranean stock; (ii) In bottom-trawl fisheries all turtle bycatch is derived from this regional stock; (iii) This regional stock contribution to fishery bycatch suggests that the population size of the Mediterranean loggerhead nesting population is significantly larger than previously thought. This is consistent with a recent holistic estimate based on the discovery of a large rookery in Libya. (iv) Present impact of fishery-related mortality on the Mediterranean nesting population is probably incompatible with its long-term conservation. Sea turtle conservation regulations are urgently needed for the Mediterranean fisheries. (v) The significant divergence of mtDNA haplotype frequencies of the Turkish loggerhead colonies define this nesting population as a particularly important management unit. Large immature and adult stages from this management unit seem to be harvested predominantly by Egyptian fisheries. (vi) Combined with other data, our findings suggest that all the nesting populations in the Mediterranean should be considered as management units sharing immature pelagic habitats throughout the Mediterranean (and possibly the eastern Atlantic), with distinct and more localized benthic feeding habitats in the eastern basin used by large immatures and adults. (vii) Between the strict oceanic pelagic and the benthic stages, immature turtles appear to live through an intermediate neritic stage, in which they switch between pelagic and benthic foods.

Movement patterns of marine turtles in the Mediterranean Sea: a review

“…However, individuals tagged in the Atlantic have been found in the eastern Mediterranean as well, as far as the Sicily Channel (Bolten et al 1992) and the north Ionian Sea (Manzella et al 1988). Genetic studies too have indicated high proportions of Atlantic individuals in the oceanic zone of the Sicily Channel (Laurent et al 1998) as well as their occurrence in the neritic zone of the Tunisian continental shelf (Figure 5), though at a lower proportion (Casale et al 2008b). Current data suggests that Atlantic turtles do not enter the Adriatic or do so in very low numbers (Giovannotti et al 2010;Garofalo et al 2013).…”

“…The modal size of loggerheads found on the Atlantic coast of northern Europe is about 23 cm CCL (20.5 cm straight carapace length, Hays & Marsh 1997; converted according to Bjorndal et al 2000), suggesting that Atlantic turtles enter the Mediterranean at this size or larger. The smallest turtle found in the eastern Mediterranean and carrying an mtDNA haplotype endemic to the western Atlantic was in the size range 30-34 cm CCL (Laurent et al 1998). Genetic markers have shown that Atlantic loggerheads occur in high numbers, especially in the oceanic zone of the westernmost part of the Mediterranean basin, closer to the Strait of Gibraltar, through which they enter from the Atlantic ( Figure 5) (Carreras et al 2006).…”

“…The Mediterranean loggerhead population shows a genetic divergence from the Atlantic populations (Bowen & Karl 2007), and a population substructure (Laurent et al 1998;Carreras et al 2007;Clusa et al 2013), indicating the presence of independent management units for conservation. Loggerhead turtles are carnivorous: for the first period of life they mainly feed upon pelagic prey at the sea surface, then they increasingly shift towards benthic prey (Bjorndal 1997), although they also feed through the whole water column (e.g.…”

Kemp's Ridley Sea Turtle Saga and Setback: Novel Analyses of Cumulative Hatchlings Released and Time-Lagged Annual Nests in Tamaulipas, Mexico

“…Juvenile loggerheads leading a pelagic existence grow at rates of 1-2 inches (2.9-5.4 cm) per year (Bjorndal et al 2003;Snover 2002) over a period as long as 7-12 years ) before moving to more coastal habitats. Recent studies have suggested that not all loggerhead sea turtles follow the model of circumnavigating the North Atlantic Gyre as pelagic juveniles, followed by permanent settlement into benthic environments (Bolten and Witherington 2003;Laurent et al 1998). These studies suggest some turtles may either remain in the pelagic habitat in the North Atlantic longer than hypothesized or move back and forth between pelagic and coastal habitats interchangeably (Witzell 2002).…”

Twelve new polymorphic microsatellite markers from the loggerhead sea turtle (Caretta caretta) and cross-species amplification on other marine turtle species

“…is well established, most surveys have analysed the population structure revealed by mitochondrial DNA (mtDNA) (Encalada et al 1998;Laurent et al 1998;Kaska 2000;Bowen et al 2004;Carreras et al 2006). Nuclear markers such as microsatellites have allowed the identification of conservation and management units, the differences in the migratory behaviour of males and females, and the mating behaviour of several marine turtle species (Crim et al 2002;Bowen et al 2005;Jensen et al 2006).…”