Climate Change Disrupting Fish Habitats

While he was a graduate student at Scripps Institution of Oceanography at UC San Diego, Chih-Hao Hsieh and Scripps professor George Sugihara began asking questions about how climate change and warming ocean waters might be influencing marine life.

They began answering some of those questions based on data from the California Cooperative Oceanic Fisheries Investigations (CalCOFI), a program based at Scripps that has monitored the marine environment of the California Current for nearly 60 years. They started to see patterns emerge as they evaluated the sensitivity of fish habitats in response to climate-driven ocean warming.

The study progressed steadily, but it was taking time away from Hsieh’s core thesis work. The project needed to be delayed.

Flash forward a year. Hsieh, with a Scripps Oceanography doctoral degree under his belt, was hired as a professor at the Institute of Oceanography at National Taiwan University. Shortly thereafter, Hsieh dug back into the data and with several colleagues completed the analysis, which was recently published in the journal Global Change Biology.

The resulting work, the first broad study of its kind, describes how the effects of climate change are being felt across a wide ensemble of sea life. The climate-induced changes span from migration pattern alterations to key population shifts.

“This is the first evidence in the ocean that climate change can have dramatic effects on large-scale fisheries ecosystems,” said Sugihara. “These are some very interesting consequences that people haven’t really thought about. These warming events could actually cause a constellation of species that normally don’t interact to begin to interact and that could have potentially large effects on what we think ought to be the natural ecosystem. This is akin to the action of invasive species but on a very large scale.”

Sugihara said large-scale studies on ecosystem responses to climate change have been done in terrestrial systems, such as for birds and plants. In addition, anecdotal studies have been suggested for a few isolated marine species, such as Bearing sea pollock, for which changes in distribution could be due to fishing pressure. Hsieh’s study, however, is the first to show the effect in the marine environment using non-fishery-based data on a large scale and for a large ensemble of species.

To arrive at their results, the scientists studied quantities of larvae for 34 fish groups. Numbers and geographic locations of fish larvae — a quantity known as “biomass”—are indicative of the abundance of fish species. They compared that information with physical measurements, including water temperature.

Among their findings, the researchers describe a boost in the population of 25 fish groups as the water temperature shifted from cold to warm temperatures over recent decades.

They also found that fish species that typically migrate vertically in the marine water column shifted geographically northward to colder waters, a change that wasn’t seen in fish that don’t migrate as such in the water column. The authors speculate this is because the upper layers of the water column warmed considerably more than deeper levels, leaving the bottom dwellers less impacted. Migrating species would have sensed the warming more readily and moved in response.

The researchers also discovered that groups that typically reside in the far open ocean shifted closer to shore as the temperature increased and species that normally reside in coastal areas moved even closer to shore.

“These sensitivities to climate can cause different fish species to start interacting,” said Sugihara. “It’s almost like seeing ocean invaders come into the coast and these ad hoc mixed ecosystems could potentially have large ecological and commercial consequences down the road. They could further destabilize fish stocks, making them more variable and less predictable, adding risk to the already risk-prone fishing industry.”

A related study led by Hsieh that recently appeared in the Canadian Journal of Fisheries and Aquatic Sciences showed that fished or exploited species are more sensitive to environmental changes.

“Open-ocean fishes that were rarely studied due to their low economic values may in fact provide important clues signifying how marine organisms are responding to climate variations,” said Hsieh. “The interactions found between oceanic and shallow water coastal species also imply that anthropogenic disturbances, for example fishing, could have profound indirect effects on other components of the marine ecosystem.”

In addition to Hsieh and Sugihara, coauthors of the Global Change Biology paper include Hey Jin Kim (Scripps Oceanography and Monterey Bay Aquarium Research Institute), William Watson (Southwest Fisheries Science Center), and Emanuele Di Lorenzo, aScripps graduate now at Georgia Institute of Technology.

The study was funded by The National Science Foundation CAMEO (Comparative Analysis of Marine Ecosystem Organization) program, the National Marine Fisheries Service, National Science Council’s (Taiwan) Long-term Observation and Research of the East China Sea, and National Taiwan Ocean University’s Center for Marine Bioscience and Biotechnology.

—Mario C. Aguilera