New evidence suggests that abnormal ocean currents occasionally cause pelagic red crabs to emerge outside their natural range

For decades, people have wondered why pelagic red crabs – also called tuna crabs – sometimes wash ashore by the millions on the west coast of the United States. New research shows that atypical currents, rather than abnormal temperatures, are likely to pull them out of their home area off Baja California.

In addition to the discovery, the scientists also created a seawater flow index that could help researchers and managers identify abnormal current years.

The new study, published July 1 in Limnology and Oceanography, began after lead author Megan Cimino drove past a stranded pelagic red crab on her way to her Monterey office in 2018. Cimino, a biological oceanographer with the National Oceanic and Atmospheric Administration (NOAA) and UC Santa Cruz through the Institute of Marine Sciences Fisheries Collaborative Program, had seen another stranding near her offspring in Southern California a few years earlier.

“At the time, I had no idea what a red crab was, what was going on, why they would be there,” she said. “But it was very clear that something else was going on in the ocean – something unusual.”

She brought the question to her colleagues and the laboratory decided to look into the mechanisms behind the seemingly random appearances.

The group spent months collecting data on the crabs and their recorded range. They searched oceanographic research studies, video data from remote-controlled vehicles, citizen science programs, and even online media like Twitter.

Integrating the different types of data proved difficult, but eventually the team had a clear idea of ​​the range and strandings of the species from 1950 to 2019.

When comparing this data with marine conditions such as temperature and currents, the scientists found that the occurrence of red crabs outside of their normal range correlated with the amount of seawater flowing from Baja California to central California. The finding supports strong currents as a key indicator of crab presence versus the other main hypothesis that warm water caused by ocean heat waves and El Niño events is causing the occurrence.

To study the currents, researchers used a regional ocean model of the California Current System developed by researchers from the ocean modeling group at UC Santa Cruz.

“What you’re doing is putting a tracer – you can think of it like a dye – in a certain part of the ocean and then running the model backwards in time to see where it’s coming from,” said Michael Jacox. a physical oceanographer with dual membership with NOAA and UC Santa Cruz.

Based on these tracer experiments, the team created the “Southern Source Water Index” (SSWI), which shows how much water comes off the central California coast south of the US-Mexico border.

“It is this waterway that produces some of these unusual species,” said Ryan Rykaczewski, a fishing oceanographer at NOAA and the University of Hawai’i at Mānoa. “It’s not just the pelagic red crabs, even if these are the most striking species we see on the coast.”

The red crabs attract public interest and are an important source of food for many other species. These factors made it a good subject to study, but they’re not the only thing that the currents create. They represent a larger phenomenon that researchers can understand better with the help of the SSWI.

“The index could serve as a kind of early warning system for what the state of the sea is like this year and whether we are expecting southern species in northern regions,” said Cimino. “This can help us plan, manage, and meet expectations for by-catches or various fisheries.”

As climate change increases the variability of marine conditions, the locations of the species will shift. Knowing where to look for specific organisms helps researchers make more accurate observations and population estimates.

“We can go back and look at this spring water index and maybe use it as a predictive tool for how the composition of coastal species will change,” said Rykaczewski. “And that could help us with ecosystem management.”

The way currents shift is an often overlooked piece of the puzzle when it comes to understanding climate change. Scientists are now in the process of testing whether the southern spring water index is sensitive to it.

“We think a lot about changes such as temperature and oxygen, but changes in the contribution of water from different locations in the wider North Pacific are also very important for understanding climate change,” said Rykaczewski.

The pelagic red crab movement is just one example of the practical application of such studies.

“I think it’s really very important that when we think about climate change, we don’t just think that warm temperatures mean some kind of reaction, and we’re really trying to look into the mechanisms,” said Jacox.

With the case study of red crabs and the creation of the southern spring water index, the researchers now have another tool at their disposal.

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Co-authors, along with cited researchers, are Steven Bograd, Stephanie Brodie, Gemma Carroll and Elliot Hazen at NOAA and UC Santa Cruz, and Bertha Lavaniegos at the Centro de Investigación Cientifica y Educación Superior de Ensenada in Baja California, Mark Morales at UC Santa Cruz and Erin Satterthwaite from NOAA, UC Santa Barbara, and Colorado State University.

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