Not just a FAD: protecting silky sharks in fishing hotspots
Fish aggregating devices (or FADs) are used by tuna fishers globally to attract shoals of their target species. Unfortunately, highly threatened and social silky sharks also form large aggregations – often near these floating attractions in the ocean. Bryan wants to know what’s driving these aggregations to form near FADs. He hopes that by uncovering this information, this project will help inform fisheries managers on suitable ways to mitigate silky sharks being killed near FADs when they are caught as tuna bycatch.
Bryan Keller
Animals have fascinated me since I was young. I remember gazing at sharks whenever I visited aquariums and I would sometimes ask questions that had yet to be answered. This motivated me to carry out research that would provide answers to my questions, as well as scientific insights into our friends in the sea. After earning my BSc from the University of Arizona I was able to begin my own research.
For my MSc I studied the social behaviour of lemon sharks at the Bimini Biological Field Station and then earned my PhD from Florida State University, where I studied...
Silky sharks and Fish Aggregating Devices (FADs): Visitation dynamics, social behavior and implications for fisheries
To assess how environmental parameters and the social behaviour of the silky shark affect aggregation dynamics and habitat use near fish aggregation devices (FADs); and to provide fisheries managers with these data to better inform mitigation measures, which could lead to a decrease in bycatch of the species globally.
The silky shark has been identified as one of the shark species most vulnerable to overfishing and the bycatch of silky sharks near FADs has resulted in population declines around the world. The recognition of this issue is timely in view of the fact that the use of FADs is increasing rapidly. Our research will contribute to preventing the extirpation of silky sharks from fishing hotspots and will assist the global recovery of the species.
The silky shark is found worldwide, typically in tropical oceanic waters near continents or islands. Like many elasmobranch species, it grows slowly, reaches maturity late and has relatively few young. It is also highly social and often forms large aggregations, many of which unfortunately occur around FADs, devices that are used by fishers around the world to target tuna species. This association with FADs and the species’ conservative reproductive traits make the silky shark one of the shark species most vulnerable to overfishing.
Silky sharks represent more than 90% of shark bycatch in the fisheries that use FADs. The goal of our work is to conduct research that will help to reduce the global bycatch of the species. Many scientists have studied silky sharks and FAD use. We know that the sharks exhibit strong ties to these floating objects and that diel cycles contribute to this association. Given the strong social nature of silky sharks, other research has highlighted the importance of including social behaviour in studies of FAD use.
Our project addresses this scientific gap by including social behaviour in aggregation and habitat use models. We will use acoustic telemetry, with an innovative analytical model, to study the social structure of silky sharks and assess how environmental parameters and this sociality affect aggregation dynamics and habitat use. We hope to provide habitat managers with more robust information for protecting silky sharks. For example, we could find that juvenile silky sharks are more likely to aggregate near FADs at different times compared to mature adults. The overall goal of this research is to provide habitat managers with the data necessary to produce fisheries management plans that would reduce the global bycatch of the species.
- To better understand the social factors affecting aggregations of silky sharks at FADs. For example, are the aggregations structured by size or sex? Are aggregations at FADs different from aggregations in other habitats? The data acquired will enable social factors to be incorporated into the development of a predictive model (see below).
- To develop a model to predict aggregation dynamics for FAD use, incorporating ontogeny, seasonality, environmental parameters and social factors to refine this index; and to use the model to predict FAD use based on these biotic and abiotic factors.
- To use our findings from the above to inform fisheries managers so that they can potentially implement mitigation measures to reduce the bycatch of silky sharks. Our findings will be submitted to regional fisheries management organisations and conservation bodies such as CITES to assist in the species’ recovery. We will work with international fisheries management groups to ensure that the conservation effect of our research is maximised.