Recruitment, reproductive system and social organisation of shark populations in Moorea Island
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. Understand if population are genetically isolated is important for conservation, because if isolated populations they could loose genetic variability affecting their evolution. In past studies, fish populations were believed to be demographically “closed” and composed of meta-populations linked by larval dispersal. Recently, some studies suggest that marine populations would be relatively “closed”. Unlike coral fish, reef sharks give birth to few pups autonomous that will grow alone in shark nurseries. Only adult individuals have the ability of dispersal by migrating to reproduce with other populations.
At present, no studies have investigated the recruitment, the turnover and the self-recruitment rate of shark populations.
The use of genetic tools such as microsatellite markers and parentage analysis will permit me to analyse the recruitment of shark populations in Moorea Island by studying parentage links between individuals of the population. This will provide me to investigate the supplying of adult population (recruitment) by young sharks and to understand if these populations are more “open” (supplied by individuals from other populations) or “closed” (supplied by individuals coming from the reproduction of sharks from Moorea). These analyses will permit me to infer self-recruitment rate and percentage of inbreeding of these shark populations and indeed investigate their degree of genetic vulnerability. Thus the spatial scale of gene flow, the connectivity and the self-recruitment rate will be determine for both species sicklefin lemon sharks and blackfin reef sharks in Moorea. This kind of analysis will require sampling most of the individual of the population and their recruits.
The aims are to determine if the recruitment in shark populations in Moorea Island, taken here as a model for Polynesian Islands, is sufficient for the sustainability of shark populations and to understand if the populations are open or closed.
Methods:
• 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 population turnover
GENETIC ASSESSMENT OF SHARK MATING SYSTEMS
Sharks are particularly vulnerable to overexploitation (mostly high fishing pressure and habitat degradation) due to their reproductive characteristics such as low fecundity and late age at maturity which are more similar to that of mammals than teleost fishes. Despite the fact that reproductive capacity is such an important aspect of animal conservation management, little is known about mating systems, reproductive mechanisms, and genetic basis of parentage in sharks.
Employing microsatellite markers, I will try to infer mating system parameters such as polyandry, philopatry and breeding-site fidelity in Moorea Island’s shark populations.
SOCIAL STRUCTURE AND ORGANIZATION OF SHARK GROUPS/ POPULATIONS
Recently research about social structure in animal populations has advanced considerably. This has revealed that organisms such as fish, which were at first thought to be socially primitive, have complex and structured social networks. Sharks are often observed in groups, such as grey reef sharks (Carcharhinus amblyrhynchos) that aggregate in groups of female and scalloped hammerheads (Sphyrna lewini) that swim in polarized schools. Their relative brain-body ratios are comparable to those of mammals, suggesting that they might be capable of complex cognitive behaviours. Sharks have already been observed to form dominance hierarchies and have been shown to be capable of learning.
But very little is known about the overall organization, structure, and complexity of the group/population of sharks.
My objectives are to investigate the organisation of shark aggregations, determining whether social interactions are characterized by non-random partner selection in groups. Since reef sharks are giving birth in nursery area and let juveniles growing autonomously without taking care of them, the question is to investigate if young sharks recruit and join their parents to form family structured groups.
By genetically sampling groups of sharks, I will use microsatellite markers and relatedness analysis to determine if shark groups are composed of close relatives.
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 long) were sampled using a biopsy tip mounted on the end of a spear gun, as shown in the film. This technique is also tested on adult blackfin reef sharks that are smaller (from 1 to 1.5 metres). 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 inside the lagoon on shore in nursery area. For that, I use small longlines deployed perpendicularly from the shore with about 5 hooks. When caught, sharks are brought back to the shore to measure length, determine sex and do a biopsy on one of the fins. After they are released in the water. Some adult blackfin 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 to do a biopsy before release them to the deep. In the next month, blackfin reef sharks will also be tagged by coded coloured spaghetti tags for a population size estimation based on capture-tag-recapture method.
I will later analyze the DNA in a molecular laboratory in Perpignan, France.
