New Study Suggests Ocean Acidification May Directly Harm Fish

Fossil fuel combustion, and with it the release of heat-trapping carbon dioxide (CO₂), is still growing globally. Beyond climate change, this is also causing the world’s “other CO2 problem”, ocean acidification, i.e., the formation of carbonic acid when CO₂ from the atmosphere enters seawater. Studies have already demonstrated a multitude of negative effects of elevated CO₂ conditions for many groups of marine organisms such as corals, plankton, shellfish and sea urchins, including a decrease in the abundance of the carbonate ion (CO₃^2-), on which many marine creature rely to form their calcareous shells and skeletons. While impacts on these taxa might reasonably be assumed to eventually affect fish, to date scientists have assumed marine fish themselves were immune to the direct effects of ocean acidification.

However, work by two SoMAS faculty, recently published online in the journal Nature Climate Change, demonstrates that “the-fish-are-okay” belief ignores an important knowledge gap – the possible effects of CO₂on the early development of fish eggs and larvae. Co-authors of the study, Christopher Gobler and Hannes Baumann, are professors at the Stony Brook University School of Marine and Atmospheric Science (SoMAS). A third author, Dr. Stephanie Talmage, is a recent doctoral graduate of that program. Together, they represent one of several international teams working to better understand the effects of CO2 on fish.

The present study by Gobler and Baumann is the first to show that elevated CO₂ levels significantly decreased survival and growth rates in eggs and larvae of a fish. The researchers reared newly fertilized eggs of a common estuarine fish, the inland silverside (Menidia beryllina), under different CO₂ levels predicted for future oceans (current: ~400 ppm , mid-century: ~600 ppm, end-of-century: ~1,000 ppm). They found that egg and larval stages of these fish were highly sensitive to CO₂. On average, survival rates in the first week after hatching declined by over 70% under elevated (1,000 ppm) compared to current day CO₂ conditions. In addition, surviving larvae were 18% smaller in the high than in the low CO₂ group. The experiment was fully replicated and repeated five separate times with consistent results.

Professor Gobler said: “We knew from the study of other ocean animals, such as scallops and clams, that the earliest life stages such as larvae are most sensitive to CO₂ and thus targeted the same life stage during our investigation of fish.”

Initial concern about ocean acidification focused on calcifiers, like this deepwater coral

The study joins a growing body of evidence suggesting that fish will both directly and indirectly be affected by ocean acidification, perhaps leading to less productive commercial fish stocks. Dr. Brad Warren, Science Director of Sustainable Fisheries Partnerships, an international NGO dedicated to sustainable seafood and marine/freshwater conservation, commented: “This study is a shot across the bow and shows that some important fish stocks may be eroded by high CO₂levels. And keep in mind, as estuarine fish, inland silversides are likely to be adapted to higher levels of CO₂ than many fish found in the open ocean, where chemistry is much more stable. This suggests that many commercially harvested marine fish stocks may be vulnerable too. Pelagic spawners, such as albacore, bigeye, yellowfin and bluefin tuna, whose larvae are not adapted to acidified waters, could be particularly vulnerable.”

However, the authors also caution that our understanding is still too limited for generalizations. Fish are a highly diverse group of animals and species will likely react differently to increasing CO₂ levels.

Dr. Baumann stated, “In light of the broad implications of our findings, we believe that now is the time to comprehensively investigate fish early-life CO₂ sensitivity, not just in one but in a wide range of species. We also have to address the general potential of marine organisms to adapt to the CO₂ levels projected for future oceans.”

Baumann, Gobler and their graduate students will be broadening their investigation into the effect of CO₂ on the eggs and larvae of fishes with the arrival of a $650,000, three-year grant from the National Sciences Foundation (Will Rising CO₂ Levels in the Ocean Affect Growth and Survival of Marine Fish Early Life Stages?).

Atlantic silversides, one of the species to be used to investigate the direct effects of CO2 on development of fish eggs and larvae.

The new study will use three model fish species: M. beryllina, M. menidia (Atlantic silversides) and Cyprinodon variegates (sheepshead minnow). The work with M. beryllina will investigate the responses of populations of this species from differing latitudes. These species/populations are ecologically important due to their intermediate trophic position, they have comparable life histories to commercial marine fish, they offer differences in genetic growth capacity and presumed sensitivity, and they are highly amenable to laboratory experimentation. The new project will include assaying the possible synergistic effects involving varying CO₂ levels, temperature and feeding conditions. Repeating this approach through several generations will demonstrate the extent to which CO₂ resistance may evolve through natural selection. Collectively, this study will make significant advances toward understanding how ocean acidification may challenge the world’s most valuable marine resource, fish. The new work will start in early 2012.