Climate change spells trouble for some fish populations

FishingBoat_CommercialIt’s no secret there’s a lot of weird stuff going on in the oceans right now. For example, there is a monster El Nino looming in the Pacific, ocean acidification is causing stress 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.

In addition, researchers now have even more grim news to add to the mix as an analysis published 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,” lead author Gregory L. Britten, said in a recent article.

Britten and 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,” Britten continued. “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 explains in the article that 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.

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,” Britten added. “Decline in phytoplankton was a factor in all species. It was a consistent variable.” And as the article states, it’s directly linked to climate change as the change in ocean temperature affects the phytoplankton population, which is impacting fish stocks.

Food sources for fish in their larval stage were also a focus of research published earlier this summer by Rebecca Asch. 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.

Jon Hare of NOAA Fisheries’ Northeast Fisheries Science Center said in the article they have 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% of the species are likely to be positively impacted however, such as 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.”

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