Silky shark populations in the Pacific may be recovering

Silky shark conservation

An ongoing research partnership between Dragonfly Data Science and The Pacific Community (SPC) is playing a critical role in assessing the status of several vulnerable shark species in the Pacific Ocean.

Most recently, Dragonfly Data Science and SPC have been working together to assess the population of silky sharks in the western and central Pacific Ocean.

“From our recent assessment, we’re really pleased to see positive signs that the silky shark population is recovering and we have some robust science to support this,” Dragonfly director and fisheries scientist, Dr Philipp Neubauer, says.

“The fishing mortality of silky shark went from critical levels—in other words, close to levels that would have eventually led to a population collapse—to levels that have allowed the silky shark population to rebuild, which is an astonishing turnaround for a shark species.”

Named for their smooth skin, silky sharks are typically found in tropical open ocean waters. They grow to be about 2.5 metres in 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 are also valued for their fins, and to a lesser extent, their meat, skin, and jaws. For this reason, silky sharks have also historically been subject to substantial fishing pressure.

Although most of the current silky shark catches are bycatch in tuna fisheries, their 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.

“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.”

Tuna-fishing operations are managed by the Western and Central Pacific Fisheries Commission (WCPFC). Over half of the world’s tuna catch comes from this region, worth about 6 billion US dollars to the global fishing sector every year. 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.

Principal Fisheries Scientist (Stock Assessment and Modelling) at SPC, Paul Hamer, says that estimating shark numbers can be a challenging and complex task due to limited historical data and the general poor understanding of the biology of most oceanic sharks.

“Sharks are vital to the health of marine ecosystems, but unlike more comprehensively-studied species such as tuna, historically, we know very little about shark catches, basic biology, movement patterns and population structure.”

“Improving this knowledge about silky sharks is essential, not only for improving our advice to the WCPFC on their conservation efforts, but also for understanding more about their vulnerability to not only fishing but also climate change and related changes to oceanic ecosystems and food chains.”

Multi-model inference

Generally, there are fewer data available for bycatch than for target species, making stock assessments for bycaught sharks challenging.

“Understanding shark populations requires more than just data—it demands a deep, nuanced approach to modeling and analysis,” Philipp says.

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.”

Read more

This research was presented at a recent meeting of the Scientific committee of the Western Central Pacific Fisheries Commission (WCPFC) in Manila. You can read the technical report here.

Silky shark photograph CC BY Alex Chernikh

Get in touch

To find out about shark bycatch, fisheries assessments, or multi-model inference, feel welcome to get in touch with Philipp Neubauer (philipp@dragonfly.co.nz).