Please select your home edition
Edition
Noble Marine 2022 SW - LEADERBOARD

Understanding ocean changes and climate just got harder

by NOAA Fisheries 29 Mar 2020 13:05 UTC
Understanding ocean changes and climate just got harder © NOAA Fisheries

A new study shows that two important indicators for understanding and predicting the effects of climate variability on eastern North Pacific marine ecosystems are less reliable than they were historically. This finding has important implications for fisheries and ecosystem management from Alaska to California.

Until recently, oceanographers and fishery biologists summarized and understood complex and long-term relationships between regional fish stock productivity and ocean climate patterns using the Pacific Decadal Oscillation index (PDO) and the North Pacific Gyre Oscillation (NPGO).

The PDO accurately described the climate pattern north of 38 degrees N, and the NPGO described the pattern S of 38 degrees N. These two indices captured climate patterns that dominated North Pacific ecosystems for much of the 20th century.

"We were able use these indices to describe changes in collective environmental features including sea surface temperature, sea surface height, sea level pressure, salinity, chlorophyll-a, and ocean nutrients. They also helped us understand and predict changes in the productivity of species like plankton and fish that are sensitive to long-term weather patterns without having to understand specific relationships between individual species and their environment," said Mike Litzow, NOAA fisheries oceanographer and study lead. "We could count on these indices as reliable tools to summarize this broad suite of climate-biology-relationships."

But all of that changed beginning around 1988-89.

At that time, a shift in North Pacific climate changed the way that the atmosphere affected ocean conditions. In particular, the Aleutian Low, a semi-permanent low pressure system that is one of the main drivers of the PDO pattern, became less active.

Historically, the Aleutian Low had a major influence on ocean variables like temperature, wind mixing of the water column, and ocean currents that were related to the PDO. After 1988-89, these relationships grew weaker so that the PDO was no longer as good a predictor of overall ocean conditions. Similar changes in atmospheric drivers of the NPGO also occurred.

Of most relevance to fishery management is that, starting in the late 1980s, the relationship between these two major climate indicators and fish stock production weakened.

"In other words, the PDO and the NPGO were less effective tools for helping us understand and predict biological productivity in the ocean," added NOAA Fisheries Mary Hunsicker, a coauthor on the study.

History of PDO and NPGO as effective tools for influencing and predicting changes in marine ecosystems

Numerous studies have shown that PDO and NPGO patterns have had considerable influence on climate-sensitive natural resources in the Pacific and over parts of North America in the 20th Century prior to the late 1980s.

The years 1925, 1947, and 1977 were labeled regime shifts due to the large scale shift in the structure of marine ecosystems as a result of a shift in the climate. In these years, a "warm" phase of the climate (indicated by a positive PDO) favored high production for Alaska fish stocks and low production for Washington, Oregon and California stocks. The "cool" phase of the climate had the opposite effect on Alaska and west coast stocks.

Scientists have linked PDO and NPGO fluctuations to changes in seabird populations, marine mammal abundance and distribution, plankton communities, and even the bloom time of lilac and honeysuckle bushes in spring in the western United States. The NPGO was shown to be particularly important for predicting salmon survival for West Coast salmon.

Predicting salmon productivity in Alaska using the PDO

When the PDO was in a "warm" phase, like during 1925-1945 and from 1977 through at least the late 1980s, Alaska salmon production was high. However, in years when the PDO was in a cool regime, (like 1947-1976) Alaska salmon production was low.

"In the 20th Century we could reliably predict trends in salmon abundance because there was a clear link between interdecadal changes in North Pacific climate and salmon stocks," said Hunsicker. "This was a simpler approach than understanding the complex ecosystem relationships affecting salmon production."

Just by looking at this one indicator scientists could predict whether salmon production was going to be high or low in a particular year or a series of years.

PDO links to Pacific groundfish productivity and ecosystem transitions

The PDO pattern has also been shown to correlate to groundfish stock production. A strong one year jump in sablefish recruitment (number of young fish that survive to maturity) in the Northeast Pacific coincided with the 1976-77 regime shift. Pacific halibut recruitment also shifted similar to the interdecadal phases of the PDO. Like Alaska salmon, halibut flourish during warm phases of the PDO. Several flatfish species (arrowtooth flounder, Greenland turbot) have exhibited PDO-like recruitment histories.

According to Litzow, the PDO and NPGO also have been powerful tools for capturing climate shifts that have resulted in major reorganizations in biological communities over vast oceanic regions.

Perhaps the most notable among these occurred in the Gulf of Alaska. Prior to the mid-1970s, the Gulf of Alaska ecosystem was dominated by lower trophic level forage species including capelin, shrimp and sand lance. The ecosystem supported highly productive shrimp and crab fisheries.

A shift in climate coupled with high exploitation rates by international and domestic fleets lead to a decline in shrimp and crab abundance in the Gulf of Alaska. The ecological community structure changed. Higher trophic level groundfish including cod-like fish and flatfish, most notably walleye pollock and Pacific cod, became the dominant species in the Gulf of Alaska ecosystem. This was captured by the PDO.

The new norm

For many years, the PDO and NPGO served as useful indices for not only understanding climate variability but also predicting fish productivity and distribution patterns. Scientists, resource managers, and fishermen used them to guide sustainable resource management and business planning.

"However, with continued warming of the world's oceans, these tools are becoming less and less reliable for summarizing and understanding physical and biological relationships," stressed Litzow.

From the Bering Sea to southern California, individual climate and biological variables are changing. Ocean temperatures are changing and so are wind patterns that drive ocean currents and ocean mixing. As a result, correlations between the PDO and NPGO indices and regional climate and biological variables that have previously been strong, are now weaker or have disappeared completely.

"Our challenge is to better understand the mechanisms linking physical properties of the North Pacific atmosphere-ocean system to important ecosystem variables, including but not limited to salmon abundances. With that kind of understanding, we would be that much better equipped to know what we need to monitor for the near term and for future projections," said Nick Bond, Joint Institute for the Atmosphere and the Ocean, University of Washington.

The scientific team emphasized the need for new predictive tools to replace the PDO and NPGO indices to understand climate variability and important ecological responses as old patterns no longer hold true.

Related Articles

Gray Whale population abundance
Eastern North Pacific Gray Whale population increases after observed decline To understand how the eastern North Paci?c gray whale population is responding to changes in the environment following its recovery from low numbers due to commercial whaling, we study changes in abundance over time. Posted on 5 Apr
New research reveals diversity of Killer Whales
Long viewed as one worldwide species, killer whale diversity now merits more Scientists have resolved one of the outstanding questions about one of the world's most recognizable creatures, identifying two well-known killer whales in the North Pacific Ocean as separate species. Posted on 31 Mar
Where the Leatherbacks Roam
Leatherbacks commonly swim from the South and Mid-Atlantic Bights during the warmer months Scientists find evidence of critical feeding grounds for endangered leatherback turtles along the U.S. Atlantic coast by studying movement behavior with satellite tags. Posted on 30 Mar
Meet Makana
One of the first Hawaiian Monk Seal Pups of 2024 Hawai'i Marine Animal Response partnered with Kahuku Elementary School to name the first Hawaiian monk seal pup of O'ahu in 2024. Posted on 23 Mar
Marine heatwaves reshape ecosystem
Heatwaves are becoming more frequent and intense in our oceans A new study highlights marine heatwaves' complex and cascading effects on marine ecosystems. While some species may benefit from these changes, others are likely to struggle. Posted on 20 Mar
California Current ecosystem shows resilience
It is facing a strong 2024 El Niño event The 2023-2024 California Current Ecosystem Status Report shows an abundance of forage fish and a productive system fueled by upwelling. Posted on 19 Mar
Some research takes a lifetime
Researchers keep track of Northern Elephant Seals using flipper tags Long-term research under Marine Mammal Protection Act scientific research permits provides insight into northern elephant seal moms and pups. Posted on 17 Mar
Making strides in marine mammal research
NOAA Fisheries and partners celebrate the 50th anniversary of the Endangered Species Act 2023 marked the 50th anniversary of the Endangered Species Act. Under this law, NOAA Fisheries is responsible for the conservation and recovery of more than 160 endangered and threatened marine species—including many marine mammals. Posted on 12 Feb
NOAA Fisheries and BOEM release joint strategy
Part of a larger interagency effort to promote recovery of endangered species Today, NOAA Fisheries and the Bureau of Ocean Energy Management (BOEM) released a final joint strategy to protect and promote the recovery of endangered North Atlantic right whales while responsibly developing offshore wind energy. Posted on 26 Jan
Documenting the Elusive North Pacific Right Whale
Dive in with the NOAA Fisheries Podcast North Atlantic right whales have justly gotten a lot of attention and news coverage due to their dwindling numbers and sightings along the busy East Coast. Posted on 13 Jan
Henri-Lloyd - For the ObsessedStoneways Marine 2021 - FOOTERHyde Sails 2022 One Design FOOTER