As climate change intensifies across the global ocean, fish stocks are shifting their distributions in search of suitable environmental conditions. As fish do not recognize political boundaries, they move to where the environment best supports their growth, reproduction and survival. While much attention has been given to fish stocks migrating between neighbouring EEZs, less focus has been paid to straddling fish stocks that move between EEZs and the high seas. A new study published today in Science Advances sheds light on this critical yet understudied consequence of climate change.

“Climate change is driving shifts in marine species distributions, with many moving towards higher latitudes, deeper waters, or along local environmental gradients, such as localized currents and upwelling systems,” said Dr. Juliano Palacios Abrantes, the paper’s first author and a postdoctoral fellow at the UBC’s Institute for the Oceans and Fisheries (IOF). “The potential consequences of these shifts, include changes to fisheries management and governance, sustainability of stock, economic and food security, among others.”

“We identified 347 stocks across 67 species, including both highly migratory and less mobile species, that straddle the areas between regional EEZs and the high seas and their potential shift in distribution due to climate change, and the results were significant,” Palacios Abrantes stated.

Thirty-seven (37) per cent of the identified stocks are projected to shift their distribution between EEZs and the high seas by 2030, rising to more than 50 per cent by 2050. “Those are substantial figures, and the shifts are expected regardless of whether we follow a high- or low-emissions climate change scenario,” said Dr. William Cheung, professor and Director in the IOF and senior author on the paper. “Even currently well-managed fish stocks within national waters may become vulnerable as they cross into areas governed by different, and sometimes weaker, regional fisheries management frameworks. This poses serious challenges for sustainability, equity, and cooperation in ocean governance.”

A majority (57 percent) of the shifting stocks are large pelagic species, and include commercially valuable species, such as tuna and billfishes, which are shared across all regions. Among the top 10 most frequently shared species are the silky shark (Carcharhinus falciformis), blue shark (Prionace glauca), wahoo (Acanthocybium solandri), skipjack and yellowfish tuna, which are shared across Australasia, the central Indo-Pacific and the temperate North Atlantic.

However, many shifting stocks are not highly-migratory. In some regions, like the EEZs of South America, less mobile but still straddling species, experience significant shifts in distribution, including Chilean jack mackerel (Trachurus murphyi), which spans the Pacific Ocean high seas and the EEZs of Chile and Peru, and Patagonian toothfish (Dissostichus eleginoides), which ranges across subantarctic waters of the southern ocean including the EEZs of Argentina, Uruguay, and the Malvinas Islands (Falkland Islands). “This does not mean that these fish stocks will be less impacted,” said Cheung. “Rather, their challenges will be driven more by shifts within regional waters.”

The study also finds that an increasing number of economically important stocks such as herring, flatfish, and American plaice are expected to experience shifts within EEZs, while others – such as Japanese flying squid – are projected to move from EEZs into the high seas.

“Access to fishing on the high seas is limited. Only a handful of wealthy countries have the resources to fish there,” said Colette Wabnitz, co-author and Research Associate at the IOF “As fish stocks move into these areas, island and coastal nations could lose important revenue from foreign fishing agreements, and communities may face greater food insecurity and the loss of livelihoods as their access to resources declines.”

“Designing precautionary and adaptive management approaches in response to shifting fish stocks is politically challenging,” said Cheung. “To be effective, regional fisheries management organizations (RFMOs) must undergo transformative changes, including reforming membership rules to strengthen principles of equity, inclusiveness and sustainability, allocating quota informed by the best available science that incorporates climate change, and greater and more equitable investment in regional scientific capacity.”

“Ongoing negotiations at diverse international fora, including those under the Agreement on Biodiversity of Areas Beyond National Jurisdictions (BBNJ) and RFMOs among others, will need to pay close attention to these trends,” said Palacios Abrantes. “Without proactive governance, the shifting of straddling fish stocks could exacerbate existing inequities and undermine efforts to manage fisheries sustainably.”