Dragonfly team member Philipp Neubauer worked with The Pacific Community (SPC) to more accurately assess the status of silky sharks in the Pacific Ocean.
Silky shark
Named for their smooth skin, silky sharks are found in tropical waters around the world. They typically grow to about 2.5 metres length, and one of their main food sources is tuna.
Because tuna are also a sought-after fisheries species, silky sharks get bycaught in tuna fisheries. The sharks have also been valued for their fins, and to a lesser extent, their meat, skin, and jaws. Previously, when captured as bycatch, the sharks were retained for human consumption; often only the fins were used, and the remainder of the shark was discarded (a practice termed “finning”). For this reason, silky sharks have historically been subject to substantial fishing pressure.
Like sharks in general, their slow growth, long gestation period, and small litter sizes make silky sharks vulnerable to fishing impacts, and slow to recover from depletion. These population characteristics, in combination with bycatch mortality in tuna fisheries, mean that silky shark populations in the Pacific Ocean have become severely depleted. They are now classed as a “vulnerable” species by the International Union for Conservation of Nature.
Tuna-fishing management
Tuna-fishing operations in the western and central Pacific Ocean require careful regulation from the Western and Central Pacific Fisheries Commission (https://www.wcpfc.int). Over half of the world’s tuna catch comes from this region, where it is an important revenue source for developing Pacific communities, and worth billions of US dollars to the global fishing industry.
Due to unsustainable silky shark bycatch in tuna fishing in the past, the WCPFC has introduced a number of regulatory policies aimed at reducing the impact of commercial fishing operations on shark populations. A global ban on drift nets came into place in 1992, followed by the introduction of closure periods for the use of FAD (fish aggregation devices that aggregate tuna for purse-seine fishing) in the 2010s. More recently, a non-retention policy for silky shark was introduced in 2013. This “non-retention” management intervention has been a significant change, with the potential to help recover silky shark populations.
To understand the impact of these policies on silky shark numbers, Dragonfly and the SPC used a mix of historical data and innovative modelling to estimate trends in the silky shark population size.
“While completely bycatch-free fisheries are almost impossible to achieve, it’s really important to this region of the Pacific to reduce the bycatch of silky sharks down to the minimum,” Philipp says.
“It is important that we maintain regulations that improve conservation measures in our oceans, and that we monitor how the regulations are working, to ensure that tuna fishing can remain a sustainable operation for all associated species.”
The use of longlines and FADs for tuna fishing means that silky sharks can often still be caught as bycatch. Particularly juvenile silky sharks are vulnerable to captures as they tend to congregate around tuna schools, and seek shelter below or around floating objects.
The WCPFC hopes that modifications such as the use of circle hooks, different baits, and adherence to non-retention policies will reduce fishing impacts on silky sharks, and help populations start to recover. Nevertheless, tracking population recovery requires scientists to make use of available data, trying to estimate population trends.
Stock assessment
Stock assessment is the science of combining different types of data, including biological information and data from fisheries records (logbooks) and onboard observers; these data are then used in population models to estimate the current numbers or biomass of the stock and trends over time.
Generally, there are fewer data available for bycatch than for target species, making these assessments challenging for bycaught sharks. SPC has been collaborating with Dragonfly scientists to help address the difficulties of assessing the stocks of silky shark and other shark species in the Pacific Ocean.
“Understanding shark populations requires more than just data—it demands a deep, nuanced approach to modelling and analysis,” Philipp notes.
To account for the uncertainties in the understanding of silky shark biology and the data limitations, Dragonfly applied a multi-model inference approach. The initial process for this approach was to create hundreds or even thousands of different models that account for the range of possible assumptions and uncertainties related to biology, data, and other model settings. These initial model ensembles were then reduced to a core set of models that were used for the analyses to provide management advice on the probable size of the populations and long-term trends.
“The challenge with shark conservation is not just the data we have, but the data we lack,” says Philipp. “The flexibility of a multi-model approach creates the ability for preliminary assessments, incorporation of diverse data sources, and the exploration of various hypotheses.”
“Our goal is to continue pushing the boundaries of our understanding, providing clear information for shark conservation, and the most accurate picture of silky shark populations possible, even in the face of sparse information.”
The latest stock assessment shows silky shark numbers are trending upwards. These positive results, presented to the WCPFC Scientific Committee in Manila in August 2024 by the SPC and Dragonfly, suggest that WCPFC’s management measures are working.
Project team
Read more
- Technical report for this project
- Our assessment of oceanic whitetip shark
- Our risk assessment for whale shark
Silky shark photograph CC BY Alex Chernikh
Our role at SPC involves carrying out stock assessments on tuna, sharks and billfish, in order to provide management advice to the WCPFC. The team at Dragonfly have worked with us for eight or nine years, specifically focused on shark assessments. They have worked really hard to not just deliver the assessments but to develop and improve the methods. We’ve really appreciated the added value they have brought to every assessment.
It’s amazing to not only sit at the table with data science experts like the Dragonfly team, but also to see their proficiency in delivering comprehensive assessments that incorporate new approaches and include uncertainties in such a statistically robust way.
Sharks are vital to the health of marine ecosystems, and alongside our focus on tuna stock, we are committed to developing more robust approaches to assessing shark stocks to provide better management advice.
Dragonfly have been instrumental in helping us and the WCPFC to not only better understand the shark stocks, but also to assess whether the management measures are effective in the way they are intended to be.
Paul Hamer
Principal Fisheries Scientist
The Pacific Community .