Citation: Salvatteci, R., Schneider, R. R., Galbraith, E., Field, D., Blanz, T., Bauersachs, T., Crosta, X., Martinez, P., Echevin, V., Scholz, F., & Bertrand, A. (2021). Smaller fish species in a warm and oxygen-poor Humboldt Current system. Science, 375(6576). https://doi.org/10.1126/science.abj0270
The Humboldt Current off the coast of Peru is one of the world’s most productive fishing grounds. Roughly fifteen percent of the global fish catch comes from this region of the ocean, and the fish caught are mostly pelagic (open ocean) species, such as anchovy. The region’s extremely high productivity is a result of the current itself; the Humboldt Current flows from south to north, and brings cold, low-salinity water from the Antarctic up toward the tropics. It also drives the upwelling of nutrient-rich water from the deep ocean to shallower depths, which feeds the phytoplankton at the base of the food web and, as a result, the plentiful supply of fish.
However, the anchovy catch in the Humboldt Current has been steadily declining in the last several decades, primarily due to overfishing and climate change. A team of scientists from two German research centers – the Institute of Geosciences and the GEOMAR Center for Ocean Research – designed an experiment to isolate the effect of climate change, specifically.
Sediment Cores Contain Paleoclimate Clues
First, the team collected sediment cores from the seafloor off the coast of Peru. The cores were roughly 15 meters long, and contained sediments from 141,000 years ago (at the bottom of the core) all the way to today (at the top of the core). After the cores were collected, the team measured a variety of chemical parameters in the cores to get a record of how temperature, oxygen concentrations, and primary production (the total amount of biomass produced from inorganic molecules like carbon dioxide) have changed throughout the last 141,000 years. This is called a paleoclimate reconstruction. The team also examined the fossilized fish vertebrae present throughout the length of the sediment cores and recorded the species they found.
Eemian Conditions Mirror Those of Today
Through these measurements, the research team determined that climate changes taking place around 125,000 years ago (a time period called the Eemian) were very similar to the climate changes taking place today (the Holocene). In fact, Eemian conditions recorded by the sediment cores are the same as “projected conditions” in the Humboldt Current in the year 2100.
Note: these “projected conditions” are the results of the International Panel on Climate Change’s RCP8.5 (Representative Concentration Pathway 8.5) trajectory, in which greenhouse gas emissions continue to rise throughout the 20th century, rather than decrease. RCP8.5 is considered a worst-case scenario, and given current trends, this climate future is “very unlikely,” though still possible.
While Eemian-like conditions may not come to pass in 2100, it is still worthwhile to understand the effects of such climate changes on the Humboldt Current ecosystem. The higher sea surface temperatures and lower oxygen concentrations of the Eemian led fish communities to be dominated by much smaller goby-like fishes (60% of the vertebrae found) and mesopelagic fishes. These fish are highly adapted to living in oxygen-deficient water, or in water which rapidly cycles between oxygen-rich and oxygen-deficient. Importantly, anchovy abundances in the Eemian were miniscule compared to their abundances today.
This relationship between environmental conditions and fish abundances suggests that there’s an environmentally-triggered tipping point at which anchovies will cease to dominate the Humboldt Current.
Implications for Fisheries
The ripple effects of a species turnover in the Humboldt Current would not only be a shock to the local food web. It would also be relevant to global fisheries. Anchovies are present in a shocking number of food items, including Worcestershire sauce, fish sauce, and Caesar salad dressing, in addition to their use as fishmeal. Gobies and mesopelagic fishes are far less palatable and more difficult to catch, and thus far less useful as a foodstock.
Already, a shift to several warm water species like V. lucetia (a mesopelagic fish) has been observed. The research team cautions that unless changes are made to halt global warming, the warming and de-oxygenation of the Humboldt Current may be inevitable. Such changes may lead to irreparable damage to the anchovy fishery, and all who rely on it.
I’m a PhD candidate in Earth System Science at Stanford University, and I study how microbes in deep ocean sediments produce and consume greenhouse gases. I’m a native of the landlocked state of Minnesota, so I’ve always been fascinated by the ocean. When I’m not in the lab, I love to race triathlons, forward “The Onion” articles to friends and family, and hike with my hound dog Banjo.