The crash of California’s sardine fishery that shuttered the Cannery Row made famous by John Steinbeck could have happened even without the pressure brought on by voracious fishing fleets, but at a slower rate and with faster recovery.
That is one conclusion of new research from Scripps Institution of Oceanography, UC San Diego, which sought to tease apart the roles that climate and human influences have played on populations of these economically crucial species throughout history. Neither of those two forces entirely explains what causes the fluctuations, said Martin Lindegren, the lead author of a study published this month in the journal Proceedings of the National Academy of Sciences (PNAS).
“The main outcome [of the study] in terms of understanding the mechanisms of the fluctuations is that it’s a synergy between internal dynamics within the population, things like maturity, growth, and mortality, and external factors such as climate that create the unique fluctuations of sardine and anchovy,” Lindegren said.
Populations of sardine and anchovy in the northeastern Pacific Ocean and other areas around the world known as upwelling regions that are fed by nutrients from deeper waters undergo periodic fluctuations over time scales spanning from several decades to centuries. The fluctuations of these two fish species are inversely correlated, so that when the sardine population is at its highest, the anchovy population is often very low. The causes of these mysterious fluctuations have long been controversial, as both climate and fishing have strong effects on fish populations, and it is difficult to separate the influences of both. In the paper led by Lindegren, a postdoctoral scholar in the lab of Scripps biological oceanographer David Checkley, researchers used a combination of population models and historical data to unravel the underlying causes of these fluctuations.
The study focused on the sardine and anchovy in the California Current, a current in the Pacific Ocean that moves south along the west coast of North America from British Columbia to Baja California. Accurate modeling of sardine and anchovy fluctuations required understanding the natural life history of the fish, the conditions of their environment, the impact of fishing, and, crucially, how these effects interact.
The model consisted of “cohort” models of the sardine and anchovy as well as a climate module simulating sea surface temperature (SST).
The cohort models follow a group of fish born at the same time through their life cycle. The accuracy of the model was verified by “hindcasting” or predicting past population dynamics using the model, and comparing the results to historical records. The historical fluctuations of sardine and anchovy in the California Current are well-characterized thanks to a strong natural record left behind in sediment. Over the past thousands of years, scales of sardine and anchovy have been preserved in sediment layers in the Santa Barbara Basin, located off the coast of Southern California. The population data derived from these sediment records were compared with model simulations driven by a temperature “proxy” used to reconstruct the climate experienced by the sardine and anchovy over the past 350 years.
A temperature proxy is a preserved natural record of past climate, which can be used in place of direct measurements. In this case, the proxy was based on variability in tree rings that depends on the environmental conditions (including temperature) experienced by a tree during its lifetime.
“We evaluated several climate proxies, and found the tree-ring record in Southern California/Baja California to be the most accurate,” said Lindegren. “California has a lot of old trees.”
The model was able to accurately reproduce sardine-anchovy fluctuations from 1661 to the present. It showed that the sardine and anchovy fluctuations were not controlled solely by climate, as had been previously suggested, or by the human impact of fishing.
Although sardine and anchovy do have opposite responses to temperature—sardine populations do better with warmer temperatures, and anchovy with cooler ones—the population fluctuations could not be attributed only to climate.
“If it were only climate, the populations would simply track the climate, perhaps with a lag of one or two years, but that’s not the case.” Lindegren said.
Lindegren and his co-authors also evaluated the effect of fishing on the dynamics, and used the model to determine the cause and possible inevitability of the famous sardine “crash” of the 1940s that wiped out Monterey, California’s Cannery Row packing houses and the livelihoods of scores of commercial fishermen. They found that fishing can increase the rate of decline of a fish population, and decrease the eventual minimum size of that population. This means that the overall pattern will stay the same, but “with a lot of fishing the population will show more variability, and perhaps be less predictable,” said Lindegren.
To better understand the collapse of the Pacific sardine population in the 1940s, the research team evaluated several different scenarios that included various combinations of sea surface temperatures and fishing rates. The researchers found that the recruitment conditions for the sardine were bad at this time, and even without fishing the probability of a crash was about 20 percent. But the enormous exploitation rate – about 50 percent of the sardine population was being caught each year before the collapse – made the rate of collapse much faster than it would have been otherwise, and the rate of recovery slower.
The study was able to reproduce the history of Pacific sardine and anchovy, but predicting the future of these populations and their response to climate change is still uncertain. The simplest effect of a warming planet on the ocean is that global sea surface temperatures will likely increase, seemingly making conditions more favorable for sardines. In reality, the future is unlikely to be that simple. The warming surface temperatures will undoubtedly change the dynamics of ocean currents and upwelling in ways that are still unknown, making predictions of the effect of climate change on these population dynamics very difficult.
“That’s another challenge,” Lindegren said.
– Mallory Pickett is a master’s student in the lab of chemical oceanographer Andreas Andersson at Scripps Institution of Oceanography, UC San Diego