Science

Ocean Acidification in the Northeast: Cod Can’t Catch a Break!

Ocean acidification affects larval cod, and the Gulf of Maine may be particularly vulnerable. Photo credit: Julie Skadal

Ocean acidification, known to scientists as “the other CO₂ problem”, may cause changes to our marine fauna—including severely depleted cod—that we have only begun to wrap our heads around.

Ocean acidification is caused by the rise of atmospheric carbon dioxide from the burning of fossil fuels. As this excess carbon dioxide dissolves in ocean water, carbonic acid is formed, increasing the concentration of hydrogen (H+) ions, and lowering pH. These chemical reactions also consume carbonate ions, an essential building block for many shell-building organisms, which use various forms of calcium carbonate to create their shells or skeletons. Collectively, these changes in ocean chemistry are known as ocean acidification.

Global ocean pH has already dropped 0.1 units, from 8.2 to 8.1 since the Industrial era. Though this seems like a very small change, pH is measured on a logarithmic scale, and represents a percent change in hydrogen ion concentration. A change of 1 unit pH thus represents a change ten-times as large in hydrogen ion concentration. Thus, the ocean’s pH change in the past century actually represents a 30% increase in acidity, and is projected to fall another 0.3-0.5 units– up to a 300% increase in acidity.

Scientists have begun to study the effects of elevated CO₂ levels on certain species to try and gauge how acidifying waters will affect marine-dwelling organisms. Reactions to acidified conditions vary widely. The news is good and bad, but mostly bad.  A few corals and other marine species can survive elevated CO₂, but most seem unable to adapt quickly enough to keep up with the changing ocean. Acidification is especially hard on very young shelled organisms, on certain phytoplankton, and on reef-building corals.   Many species exhibit reduced ability to build their shells, or even dissolution of shell material in experimental conditions.

One study on Atlantic cod investigated physiological and histological traits of larval fishes under elevated CO₂ conditions, determining that ocean acidification may provide an additional stressor for cod larvae. The study showed that larvae under higher CO₂ levels actually exhibited higher growth rates than control groups, by utilizing energy that would normally be allocated to organ development during critical growth stages. This re-allocation away from organ development resulted in severe tissue damage in the liver, pancreas, kidney, gut and eye of larvae at 32 days post-hatch. The degree of damage significantly increased with CO₂ concentration, and was mostly categorized as damage that would lead to “functional impairment or loss of the organ” (Frommel et al. 2011).

Cod recruitment—the number of juvenile cod reaching maturity—is currently at a record low in New England, and the effects of ocean acidification on larval cod are likely to exacerbate this trend in the future. Cod populations are already at a fraction of healthy, sustainable levels. While decades of overfishing and habitat destruction have caused this problem, ocean acidification may set up another barrier to recovery.

A recent coastal carbon study found that the northeastern coastal waters of the U.S. have relatively low buffering capacity with respect to ocean acidification effects, due to already-low pH. This means that the Gulf of Maine and its fisheries may be among the most severely affected by acidifying waters. Good science and planning will be necessary to help coastal ecosystems and economies adapt to this worrying trend.

Most importantly, we must remember that cod and other marine species of immediate importance to us are dealing with multiple challenges at a time when a history of overfishing has left them unable to replenish themselves.  Dr. Les Kaufman, Professor of Biology with the Boston University Marine Program and a Marine Conservation Fellow at Conservation International, argues that our responses to these ecological challenges must be multi-dimensional. “With climate change, the seas are warming, acidifying, and growing more violent all at the same time.  Multiple challenges that drain population strength want multiple solutions that build it back up, including providing untrawled seabottom that feeds and shelters baby cod plus no-take areas and fishing methods that spare the highly productive cod older than eleven or twelve years.” NOAA’s own climate change policy recommends habitat protection and fishing impact reduction as appropriate policy responses to the effects of ocean acidification.

We must stop pouring CO₂ into the atmosphere and ocean, but without these parallel common sense fishery management measures, our fisheries have little hope of surviving the challenge of climate change.


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