Breeding patterns of shark populations in Moorea Island
Where do female sharks give birth?
The collapse of shark populations has resulted from the difficulties of shark recruitment to keep up with the over-exploitation they are experiencing worldwide (high fishing pressure and habitat degradation). Sharks are particularly vulnerable to overexploitation due to their reproductive characteristics (e.g., low fecundity and late age at maturity) which are more similar to that of mammals than teleost fishes. There is little information available on mating systems, reproductive mechanisms, genetic basis of parentage in sharks and population turnover.
RECRUITMENT, TURNOVER AND SELF-RECRUITMENT OF SHARK POPULATIONS IN MOOREA ISLAND
Knowledge of genetic structure of population is important for implementation of conservation plans. Unlike coral fish, reef sharks give birth to few well developeed pups that will grow alone in shark nurseries. Only adult individuals have the ability of dispersal by migrating to reproduce with other populations. Where coastal sharks are giving birth, as well as whether or not they show philopatry to specific nursery sites has important implications for the spatial scale of their management and conservation.
Direct estimates of breeding patterns and connectivity in reef-associated sharks are essential to understand their population dynamic and adapt or optimize the conservation strategies for these endangered marine predators. While there have been some attempts to investigate breeding patterns of reef sharks from litter reconstruction by molecular analyses, direct fine scale dispersal conducted by females during parturition as well as connectivity between islands has never been assessed. Here we used microsatellite DNA markers and a likelihood-based parentage analysis to assess the breeding patterns of female blacktip reef sharks in a Polynesian island.
The use of genetic tools such as microsatellite markers and parentage analysis allows me to analyse the recruitment of shark populations as well as the population structure by studying parentage links between individuals of the population. These analyses allows me infering self-recruitment rate and percentage of inbreeding in these shark populations and in the end investigating their degree of genetic vulnerability. Thus the spatial scale of gene flow, the connectivity and the self-recruitment rate can be assess for both species sicklefin lemon sharks and blackfin reef sharks in Moorea. This kind of analysis requires sampling most of the individual of the population and their recruits.
The aim of this study is to apply parentage analysis based on hyper-variable microsatellite DNA markers to investigate processes of recruitment and connectivity led by female sharks as well as mating patterns. The approach is based on the identification of offspring produced by genotyped adults from the island. Providing the location of adults and juveniles is known or can be assumed due to the restricted home range of adults and the sedentary behaviour of juveniles within their nursery, the dispersal process conducted by females to give birth in specific locations can be investigate. These data represent the first direct estimates of the breeding patterns of females and local connectivity of sharks.
•Skin sampling on free living sharks (with a tipped modified spear gun or caught by fishing)
•Development of microsatellite markers for C. melanopterus and N. acutidens
•Parentage analysis to determine mating patterns, reproductive migrations and nursery utilization
Biopsy techniques used for skin samples:
For genetic analysis, we need to collect pea-sized tissue samples containing DNA from the shark. To collect these samples, we use different techniques depending on the shark species we want to sample and its size.
Adult lemon sharks (from 2 to 3 metres total length) were sampled using a biopsy tip mounted on the end of a spear gun, as shown in the film bellow. No sharks are harmed with this technique and are not sampled more than once.
Juvenile sharks (sicklefin lemon sharks as well as blackfin reef sharks) are fished at dawn in the shallow waters of their nurseries. For that, I use gillnets and handlines with barbless hooks. When caught, sharks are brought back to the shore to measure their length, determine the sex and cut a fin clip. They are then released in the water. Some adult blacktip reef sharks are also caugth inside the lagoon with this technique.
Adult blackfin reef sharks are also fished from a boat in different places of the Island with a fishing rod and are immobilised along the boat to measure shark length, determine the sex and cut a fin clip before releasing them.
Visual monitoring of pregnancy
First we can follow the reproductive cycle through the monitoring of pregnancy as shown by an increase in size of the abdomen region in females (see below).
Female migration to their nursery to give birth
From parentage analysis, we assigned juveniles sampled in their nursery between 2008 and 2010 to their parents within the studied population.
As a result, most females gave birth at their home island but some migrated to specific nursery areas outside the area they are attached to, sometimes going to another island 50 km away across deep ocean. Our analysis also revealed that females migrated to the same nursery for every birthing event. Many offspring showed a high level of inbreeding indicating an overall reduced population size, restricted movements and dispersal, or specific mating behaviour. Females represent the vectors that transport the genes at nursery grounds, and their fidelity should thus define reproductive units. As females seem to be philopatric, males could be the ones dispersing genes between populations. These results highlight the need to conserve coastal zones where female reef sharks seem to exhibit philopatry during the breeding season.
Figure 1: Migrations of female blacktip reef sharks from their site they are attached to, to their nursery area in Moorea. Note that some are going to the same nursery on multiple years to give birth. This was demontrated using genetic parentage analysis that assigned juveniles back to their parents within the population.
Figure 2: Reproductive migrations between Moorea and Tetiaroa, 2 islands separated by about 50 km and deep water (>2000 m).
Similarly female sickefin lemon sharks showed reproductive philopatry to different nursery of Moorea and Tetiaroa over a 2-year life reproductive cycle (Mourier et al. 2013_PLoS One).
This research was published in:
Mourier, J.& Planes, S. (in press). Direct genetic evidence for reproductive philopatry and associated fine-scale migrations in female blacktip reef sharks (Carcharhinus melanopterus) in French Polynesia. Molecular Ecology, 22(1), 201-214.