Despite the ecological and economic importance of anchovies (anchovies) offshore California, the ebb and flow of their populations has been a mystery for decades. New research reveals a correlation between the length of the food chain that supports juvenile anchovies and the boom and bust cycles of California's anchovy populations. Shorter food chains are associated with population growth, providing implications for fisheries management and conservation efforts.
New research from Scripps University and NOAA scientists finds that ecological correlations help explain the boom and bust in California's anchovy populations. If these correlations are confirmed in further studies, they could one day help inform management of California's anchovy fisheries and improve conservation efforts.
The role of anchovies in marine life
The American anchovy (Engraulismordax) is California's most spectacular marine life - including pods of sea lions, pods of dolphins, a vital food source for the lucrative tuna fishery and pods of whales. But one of the hallmarks of California's offshore anchovy populations is boom and bust cycles that can last for more than a decade. These ups and downs reverberate throughout ocean ecosystems, sometimes causing sea lion pups to starve and sometimes causing brown pelicans to starve to the point of abandoning their chicks.
What exactly causes these booms and busts remains elusive, despite decades of scientific research, particularly by the CalCOFI research project, a collaboration between UC San Diego's Scripps Institution of Oceanography, the National Oceanic and Atmospheric Administration (NOAA), and the California Fish and Wildlife Service. The program surveys marine ecosystems up and down the California coast and is one of the largest and longest ocean monitoring programs in the world.
Key findings of the study
The research, published in Nature Communications on December 5, was funded by the National Oceanic and Atmospheric Administration (NOAA) and the National Science Foundation. Researchers analyzed 45 years of anchovy juveniles collected during CalCOFI surveys and found that the length of the food chain that supports juvenile fish is closely related to the rise and fall of anchovy populations.
Specifically, the shorter the food chain, the greater the number; the longer the food chain, the smaller the number. The larval food chain is shorter, with fewer steps for one animal to eat another between the photosynthetic phytoplankton at the bottom of the food chain that collects the sun's energy and the larvae (which eat mostly zooplankton).
One possible explanation for this correlation is that shorter food chains are more efficient, so more energy from the bottom of the chain reaches the anchovy larvae, said Rasmus Swalethorp, lead author of the study and associate project scientist at Scripps University. This is because organisms in different parts of the food chain are known to lose energy every time they eat each other, Svalerhop said.
"It's like the energy loss that occurs when electricity travels from a power plant to our homes - the longer the distance, the more energy is lost along the way. The same goes for going from one level of the food chain to the next - the more steps, the less energy reaches the anchovy larvae. The juvenile fish may be eating the exact same food, but when the food chain is lengthened, it may mean that the food is not as abundant, or that the same food does not contain as much energy."
Therefore, shorter food chains may support more individual anchovy larvae.
Swalethorp began the research behind this paper in 2014, hoping to take advantage of the CalCOFI sampling program to better understand the ebb and flow mechanisms of this important player in the California Current ecosystem.
"The ocean is a very large place and our ability to representatively sample it is very limited," Svalertop said. "CalCOFI is the most comprehensive survey of marine ecosystems on Earth, and it represents our best chance at understanding these larger ecological mechanisms."
Specifically, the researchers wanted to test the idea that a key determinant of how many anchovies survive danger as juveniles in a given year is the structure of the food chain in which these larvae participate. To do this, the researchers used stable nitrogen isotope analysis to determine the food chain length of 207 approximately three-week-old anchovy larvae collected by the CalCOFI program between 1960 and 2005.
In 2020, the research team published a paper detailing this method of estimating the length of the food chain of chemically preserved fish, which is based on the idea that when one organism eats another, the eaten organism leaves a chemical signature in the tissues of its consumer. In this case, the analysis doesn't reveal the exact identity of who eats whom, but it can be used to infer how many links there are in the food chain between phytoplankton and anchovy larvae.
The analysis showed that shorter juvenile food chains tended to appear one or two years before an anchovy population boom, while longer juvenile food chains were associated with lower anchovy populations one to two years later. Furthermore, changes in food chain length persist through most booms and busts.
As for how and why food chain lengths increase or decrease from year to year, Swarethorp offers some possible explanations.
"Anchovy larvae are very vulnerable to starvation, and their survival really depends on how efficiently energy reaches them," Svaletop said. "If the food chain is short and efficient, that might help more juveniles survive, which could help drive a boom cycle over the next year or two."
Because the current study was unable to identify individual species within the longer and shorter food chains found, the study cannot explain why longer food chains are associated with anchovy depression or vice versa. Whatever the reason for this correlation, the Juvenile Food Chain Index—an annual measurement of juvenile food chain length using stable nitrogen isotopes—has the potential to become a useful tool for estimating anchovy population trends in the near future, but more research is needed to explore its potential.
Juvenile food chain length appeared to be an important driver during the study period, but there are other important drivers and their relative importance is likely to vary over time and space. It would be tempting to extend the study's time series beyond the present in the future, as the current analysis does not cover the years after 2015, when anchovy populations surged again offshore California. He added that the team is starting to explore the complex questions of who is eating whom as the food chain lengthens and what triggers these changes at the bottom of the food chain.
Beyond that, the true test of the correlation's explanatory power comes when researchers try to apply it to other regions and other fish species, such as the Peruvian anchovy Engraulisringens, from the largest single fishery in the world.
Reference "Anchovy booms and busts linked to nutritional shifts in juvenile fish diets," December 5, 2023, "Nature Communications."
DOI:10.1038/s41467-023-42966-0
Compiled source: ScitechDaily