Increasing acidity in the world’s oceans will cause massive organ failure in yellowfin tuna larvae, according to a disturbing new study published in the Journal of Experimental Marine Biology and Ecology. Carbon dioxide in the ocean is changing the chemical conditions under which life has evolved.
This particular tuna is shaped like a torpedo and has dark metallic blue backs, yellow sides, and a silver belly. They have very long anal and dorsal fins and finlets that are bright yellow. The lifespan of a yellowfin can be six or seven years. Highly migratory, they are found throughout the Pacific, Atlantic and Indian Oceans. This fish is prized in the raw sashimi market and overfishing has impacted the Yellowfin Tuna , but populations are considered somewhat abundant. So far the world’s fisheries have managed the population so that it does not result in the same fate as the threatened bluefin tuna. “The bluefin tuna, which has been endangered for several years and has the misfortune to be prized by Japanese sushi lovers, has suffered a catastrophic decline in stocks in the Northern Pacific Ocean, of more than 96%, according to research published.” It is estimated that 90 percent of all large predatory fish—including tuna, sharks, swordfish, cod and halibut—are gone.
John R. Platt of ScientificAmerican reports in his blog about the new threat to the yellowfin tuna: ocean acidification.
For this study, researchers from the University of California at Santa Barbara, the Inter-American Tropical Tuna Commission and other organizations collected yellowfin larvae from a commercial aquaculture bloodstock which is normally exposed to pH levels between 8.27 and 7.74. That’s slightly less acidic then neutral water, which has a pH of 7, but also less acidic than many natural conditions. The larvae were taken then taken to a lab and exposed to waters with four different levels of carbon dioxide, which changed the pH. The first tank, considered the control, had a pH of 8.1. The second had a pH of 7.6, which matches global warming projections for the year 2100, while the third had a pH of 7.3, matching projections for the year 2300. A fourth pH level of 6.9 was considered the “lowest projection for the Pacific Ocean.”
All of that acid added up. The researchers found that it caused damage to the liver, kidney, pancreas, muscle tissue and eyes of the yellowfin larvae—all within a week of exposure. Their growth rates also suffered, ranging from 20 to 41 percent
What does all of that mean? Well, based on the damage to the eyes alone, the researchers concluded that the larvae would have had a mortality rate of between 50 and 100 percent. Even if they survived past those odds, the damage to their kidneys and other organs would have caused all kinds of health conditions later in life, putting them even further at risk.
Dina Navon of OceanBites expands on the ocean acidification study:
Essentially, as carbon dioxide builds up in the atmosphere, some of that excess CO2 gets dissolved in the oceans when particles from the atmosphere interact with the vast surface area of the world’s oceans. As CO2 dissolves, it reacts with water to form several chemical compounds that lower the pH of the water, thereby creating more acidic seawater. We’ve seen many posts here at Oceanbites about the impacts of acidification on marine organisms, especially on those which build their bodies using calcium. Calcified organisms depend on a particular pH in order to recruit calcium carbonate – the chemistry of the water is very important to them! However, most organisms can actually only tolerate a fairly narrow range in pH, making ocean acidification a threat not only to calcified organisms but also to many other species. In the current study, researchers found that acidic pH led to increased death rates and deformities in larval yellowfin tuna.
Frommel and colleagues took newly-fertilized eggs from an established stock population of yellowfin tuna held at a laboratory on the Pacific Ocean side of Panama. One of the nice things about working on development in fish is that, under the right conditions, most species will breed almost continuously, providing the embryonic material needed to perform large-scale experiments. This isn’t the case for some other species that developmental biologists like to work with, like bats, which breed much less frequently. The fertilized tuna eggs were split into four groups and reared in water with the following pH levels:
- the current pH, pH 8.1
- a “near future” projection for the year 2100, pH 7.6
- a “far future” projection for the year 2300, pH 7.3
- a lowest projection for the Pacific, pH 6.9
snip
Larvae from each treatment were sampled after seven and nine days and assessed for the level of cellular damage to several organs (including the liver, pancreas, kidneys, and eyes), as well as their feeding success (measured by the number of prey items in their gut).
The researchers found significant tissue damage in larval tuna exposed to moderate and low pH (Fig 3). Damage was particularly severe in the pancreas and eye tissues. The survival rate of the larval fish dropped significantly at more acidic pHs (Fig 4). There was a distinct drop in survival rate even at a moderately acidic pH of 7.3. Fish in extremely acidic conditions died more and grew less. Fish reared under acidic pHs had emptier stomachs, although this trend was not statistically significant (Fig 5).
NOAA’s PMEL program explains the grim role that natural upwelling is now playing by bringing more acidic water to the surface.
Water in the North Pacific is naturally rich in CO2 because the deep water has been out of contact with the atmosphere for a very long time as a result of global ocean circulation patterns. While the water masses travel along the “oceanic conveyer belt,” they accumulate CO2 through natural respiration processes that break down sinking organic matter. Along the U.S. West Coast, winds blow from north to south during spring and summer months, displacing surface water offshore as a result of Earth’s rotation. Deeper water rich in CO2 and nutrients and depleted in O2 upwells to the surface nearshore to replace the displaced surface water.
These characteristics of the upwelling water masses have consequences for both the carbon cycle and ocean acidification along the West Coast. The nutrients in the upwelled water stimulate intense primary production in the nearshore areas where upwelled water reaches the euphotic zone. In addition, upwelling brings more acidic water with lower carbonate saturation states to the surface, which may have deleterious consequences for marine organisms. The interplay of ocean acidification and natural carbon cycle processes make the coastal ocean along the U.S. Pacific coastline an interesting and complex region to do research.