The role that an individual shark plays within its social network not only serves to maintain cohesion within the population, but it can enhance the network’s resilience to environmental and anthropogenic disturbances. Researchers from Centre de Recherche Insulaire et Observatoire de l’Environnement (CRIOBE USR3278, PSL Research University: EPHE – CNRS – UPVD) recently published a study in the journal Biology Letters that examined the social network of a population of blacktip reef sharks (Carcharhinus melanopterus) in French Polynesia (Figure 1). With support from a researcher at Macquarie University in Australia, the team demonstrated that the interactions between sharks within the social network made the network robust to shark removal. This study also suggested that sharks were able to quickly learn from capture questioning the use of capture-recapture model frameworks for these species.
Figure 1: Blacktip reef sharks networking in the lagoon of Moorea (Photo credit: Lauric Thiault)
In a previous study published in the journal Animal Behaviour (Mourier et al. 2012), the team discovered that blacktip reef sharks could form preferred associations in the wild. ‘Sharks have social networks too! Even though their social structure is not as complex as that of most mammals, sharks can display an affinity for one another’ explains Dr. Johann Mourier, the principal investigator of the study.
To study the social network of sharks, Mourier used the patterns found on the dorsal fins (Figure 2) – which are unique to the individual and can be likened to fingerprints in humans – to identify each individual shark. Such patterns allow researchers to track individuals in space and through time.
Figure 2: The patterns on a shark’s dorsal fin are likened to a human’s fingerprint, and allow researchers to track individual sharks in space and time.
‘Through a global analysis of sharks present together, we were able to build their social network using social science-based methods’, explains Mourier. ‘Capture-release-recapture surveys were simultaneously conducted after each dive to calculate individual probabilities of being captured’.
Investigating the blacktip reef shark’s social network has revealed that not all individuals are equal in terms of maintaining the connectivity between social clusters. Researchers ran simulations, where individual sharks were removed from the network, and what they found was that up to 50% of the population could be removed before the entire network collapsed. In addition to this surprising result, Mourier also noticed that the sharks he had already caught were now nearly impossible to catch as they were avoiding his fishing gear. ‘A shark’s ability to learn from previous negative experiences results in a network that is more robust to removal from fishing.
Finally, results from this study highlight that extreme care must be taken before applying capture-release-recapture models for shark population studies because individual sharks do not have equal probability of being caught.
This original work highlights the complexity of relationships between individuals in the wild and has implications for shark conservation, where it is now imperative that we consider the relative contribution of an individual to the overall resilience of the population.
Mourier, J., Brown, C. and S. Planes. 2017. Learning and robustness to catch-and-release fishing in a shark social network. Biol. Lett. 2017 13 20160824; DOI: 10.1098/rsbl.2016.0824. Published 15 March 2017
Johann MOURIER (Perpignan, France)