For anyone paying attention, it's no secret there's a lot of weird stuff going on in the oceans right now. We've got a monster El Nino looming in the Pacific. Ocean acidification is prompting hand wringing among oyster lovers. Migrating fish populations have caused tensions between countries over fishing rights. And fishermen say they're seeing unusual patterns in fish stocks they haven't seen before.
Researchers now have more grim news to add to the mix. An analysis published Monday in the Proceedings of the National Academy of Sciences finds that the ability of fish populations to reproduce and replenish themselves is declining across the globe.
"This, as far as we know, is the first global-scale study that documents the actual productivity of fish stocks is in decline," says lead author Gregory L. Britten, a doctoral student at the University of California, Irvine.
Britten and some fellow researchers looked at data from a global database of 262 commercial fish stocks in dozens of large marine ecosystems across the globe. They say they've identified a pattern of decline in juvenile fish (young fish that have not yet reached reproductive age) that is closely tied to a decline in the amount of phytoplankton, or microalgae, in the water.
"We think it is a lack of food availability for these small fish," says Britten. "When fish are young, their primary food is phytoplankton and microscopic animals. If they don't find food in a matter of days, they can die."
The worst news comes from the North Atlantic, where the vast majority of species, including Atlantic cod, European and American plaice, and sole are declining. In this case, Britten says historically heavy fishing may also play a role. Large fish, able to produce the biggest, most robust eggs, are harvested from the water. At the same time, documented declines of phytoplankton made it much more difficult for those fish stocks to bounce back when they did reproduce, despite aggressive fishery management efforts, says Britten.
When the researchers looked at plankton and fish reproduction declines in individual ecosystems, the results varied. In the North Pacific — for example, the Gulf of Alaska — there were no significant declines. But in other regions of the world, like Australia and South America, it was clear that the lack of phytoplankton was the strongest driver in diminishing fish populations.
"When you averaged globally, there was a decline," says Britten. "Decline in phytoplankton was a factor in all species. It was a consistent variable."
And it's directly linked to climate change: Change in ocean temperature affects the phytoplankton population, which is impacting fish stocks, he says.
Food sources for fish in their larval stage were also a focus of research published earlier this summer by Rebecca Asch, now a post-doctoral research associate at Princeton University. Asch studied data from 1951 to 2008 on 43 species of fish collected off the Southern California coast and found that many fish have changed the season when they spawn. When fish spawned too early or too late in the season, there can be less plankton available to them, shrinking their chance of survival. She calls it a "mismatch" between when the fish spawn and when seasonal plankton blooms.
Knowing just how vulnerable our fisheries are to potential climate change is on the radar of NOAA Fisheries. The agency has put together a Fish Stock Climate Vulnerability Assessment report expected to be released in early 2016. And like many things associated with climate change, there will be winners and losers.
Jon Hare is the oceanography branch chief for NOAA Fisheries' Northeast Fisheries Science Center and a lead researcher on the agency's assessment. He says they looked at 82 fish and invertebrate species in the Northeast. About half of the species, including Atlantic cod, were determined to be negatively impacted by climate change in the Northeast U.S. Approximately 20 percent of the species are likely to be positively impacted—like the Atlantic croaker. The remainder species were considered neutral.
Similar assessments are underway in the California Current and the Bering Sea, and eventually in all of the nation's large marine ecosystems.
"This is where the idea of ecosystem-based management comes in. It's not only fishing that is impacting these resources," says Hare. "We need to take a more holistic view of these resources and include that in our management."
Britten says the fact that productivity of a fishery can change should be an eye-opener for fisheries management.
"It's no longer just pull back on fishing and watch the stock rebound. It's also a question of monitoring and understanding the ability of stocks to rebound, and that's what we demonstrated in this study. The rebound potential is affected as well," says Britten.
Clare Leschin-Hoar is a journalist based in San Diego who covers food policy and sustainability issues.