Mitochondrial DNA D-loop variation and implications for stock structure of the four-wing flyingfish, Hirundichthys affinis, in the central Western Atlantic

The four-wing flyingfish, Hirundichthys affinis, is an epipelagic, open-water species found throughout the tropical (central) Atlantic. In the central western Atlantic it supports commercially important fisheries in three geographically separate areas: the eastern Caribbean islands; the southern Netherlands Antilles; and northeast Brazil, although the resource remains unmanaged in all three areas and the stock structure unresolved. Previous tagging studies of H. affinis, in the central western Atlantic indicated very little about the movements of flyingfish off northeastern Brazil, but did show that flyingfish move freely between the eastern Caribbean islands, although they do not appear to travel as far as the southern Netherlands Antilles. However, tagging studies only address geographical movement of fish and cannot discriminate among genetically discrete stocks. In this study, we use mtDNA markers (restriction fragment length polymorphisms [RFLPs] of the D-loop region) to examine the genetic variation and its implication for the stock structure of flyingfish across the central western Atlantic. A total of 360 flyingfish were sampled from the commercial fisheries in the three geographical areas between January and August 1995. Sixty fish were taken from each of the two spawning populations in Barbados and 60 from the spawning populations in each of Dominica and Tobago (in the eastern Caribbean); 60 from Curacao (in the southern Netherlands Antilles); and 60 from Caicara (in Rio Grande do Norte, northeast Brazil). The mtDNA D-loop region was amplified using the polymerase chain reaction (PCR) method and digested to produce RFLPs using five restriction enzymes. Extensive genetic diversity was observed, with distinct composite mitotypes being detected for each of the three geographical areas, indicating a lack of gene flow between these areas and the existence of at least three unit stocks of H. affinis in the central western Atlantic. The results of cluster analyses of composite mitotype sequence divergence and population sequence divergence, and parsimony analysis of composite mitotypes were entirely consistent with a 3-stock model. Furthermore, genetic heterogeneity was detected among eastern Caribbean populations indicating restricted gene flow even within a sub-region and emphasizing the need for more detailed studies of flyingfish spawning behaviour. These results contrast with the typically low levels of genetic variation reported for oceanic pelagic species, and for other marine species (reef fish, spiny lobster, queen conch) in the Caribbean and indicate that major ocean current patterns are not good predictors of gene flow for all species. The implications for management of the flyingfish resource in the central western Atlantic are that three independent stock assessments and management strategies would be appropriate; and that management of the eastern Caribbean stock will need to be at a regional level since the stock is shared between the different island states, while management of the southern Netherlands Antilles stock and the northeast Brazil stock could be at a national level.