Microsatellite Analyses of Blacktip Reef Sharks (Carcharhinus melanopterus) in a Fragmented Environment Show Structured Clusters

Abstract

The population dynamics of shark species are generally poorly described because highly mobile marine life is challenging to investigate. Here we investigate the genetic population structure of the blacktip reef shark (Carcharhinus melanopterus) in French Polynesia. Five demes were sampled from five islands with different inter-island distances (50–1500 km). Whether dispersal occurs between islands frequently enough to prevent moderate genetic structure is unknown. We used 11 microsatellites loci from 165 individuals and a strong genetic structure was found among demes with both F-statistics and Bayesian approaches. This differentiation is correlated with the geographic distance between islands. It is likely that the genetic structure seen is the result of all or some combination of the following: low gene flow, time since divergence, small effective population sizes, and the standard issues with the extent to which mutation models actually fit reality. We suggest low levels of gene flow as at least a partial explanation of the level of genetic structure seen among the sampled blacktip demes. This explanation is consistent with the ecological traits of blacktip reef sharks, and that the suitable habitat for blacktips in French Polynesia is highly fragmented. Evidence for spatial genetic structure of the blacktip demes we studied highlights that similar species may have populations with as yet undetected or underestimated structure. Shark biology and the market for their fins make them highly vulnerable and many species are in rapid decline. Our results add weight to the case that total bans on shark fishing are a better conservation approach for sharks than marine protected area networks.

Blacktip reef sharks, Carcharhinus melanopterus, have high genetic structure and varying demographic histories in their Indo‐Pacific range

“…, ; Mourier & Planes ; Vignaud et al . ), Aldabra Atoll (Stevens ), Palmyra Atoll (Papastamatiou et al . ,b, ), the Great Barrier Reef (Chin et al .…”

“…Previous studies of the population genetics of blacktip reef sharks have been on a single population (Mourier & Planes ) or at a regional scale (Vignaud et al . ), so the global genetic structure and extent of mixing among reefs in the Indo‐Pacific have been unknown up to this point.…”

Genetic Network and Breeding Patterns of a Sicklefin Lemon Shark (Negaprion acutidens) Population in the Society Islands, French Polynesia

“…This low genetic variability has encouraged sicklefin lemon sharks to develop different strategies to avoid mating between close relatives with evidence of migrations across islands and atolls. The highly isolated and fragmented environment of French Polynesia [26] may limit encounter rate of unrelated mates despite dispersal and migrations across the Society Islands. These results encourage for long-term monitoring to survey the population response to increasing anthropogenic factors [22] despite the economic importance of these sharks in the local tourism industry [24].…”

“…These life history characteristics and reproductive behaviour are less evident for Negaprion acutidens in French Polynesia as: (1) a few males appear to dominate paternity (Figure 3) (2), some males are resident to an island and do not roam like other species do [22], and (3) although evidence for dispersal was found, individuals within the entire Archipelago remain closely related (Figure 1). Another reason that may limit the effectiveness of inbreeding avoidance may be the fragmented habitat in this region which tends to isolate populations [26] and may reduce the opportunity to find unrelated mates.…”

“…Our study was conducted in French Polynesia, a fragmented system of small islands and atolls separated by deep ocean (depth ≥ 2 000 m), which is expected to increase the effective isolation of reef-associated populations [26]. We monitored a population of sicklefin lemon sharks ( Negaprion acutidens ) visiting a recreational diving site during a 5-year period in Moorea (17° S, 149° W; Figure S1).…”