Ocean acidification has been recognized as one of the major changes to ocean ecosystems from rising greenhouse gas emissions. As atmospheric carbon dioxide (CO2) levels have increased, oceans have absorbed approximately one-third of the total human-generated CO2 emissions (NRC 2010). Higher oceanic levels of absorbed CO2 have caused a shift in ocean chemistry, lowering ocean pH and, as a result, decreasing the available concentration of carbonate ions in the water (Cooley and Doney 2009). There has already been an observed 30% increase in the acidity of the ocean’s surface waters, with pH declines from 8.2 to 8.1 (Caldeira and Wickett 2003). The pH is likely to decline by an additional 0.3 to 0.4 units (to ~7.8) during the 21st century as oceans absorb even more atmospheric CO2 (Feely et al. 2012).

Certain regions, such as Alaska and the West Coast, will experience stronger acidification impacts due to cold waters with relatively high levels of CO2 (Feely et al. 2012; Walsh et al. 2014). In tropical coral reef regions, such as Hawaii and the Caribbean, acidification will limit reef formation and further exacerbate degradation due to warming temperatures and bleaching (NRC 2010; IWG 2014). These effects may be compounded by elevated concentrations of nutrients from runoff (Kelly et al. 2011), which can lead to phytoplankton blooms and a short-term rise in pH. However, when the bloom ends and these organisms decompose, seawater will decrease in oxygen and the pH will also decline (Feely et al. 2012; Doney et al. 2014).

The magnitude of acidification will also vary greatly along the coasts and be influenced by other processes, such as upwelling, pollutant runoff, and soil erosion (Kelly et al. 2011; Duarte et al. 2013). For example, in coastal Washington, strong upwelling patterns cause cold surface waters with a naturally low pH, which will be compounded by the effects of atmospheric CO2 absorption (Barton et al. 2012; Feely et al. 2012).

Ocean acidification has the potential to cause many direct and indirect impacts on fisheries. In general, marine species exhibit decreased calcification, growth, survival, and abundance in acidified waters, although the magnitude of these effects is not universal across all species or species assemblages (Kroeker et al. 2013). Acidification will cause declines of pteropods – the basis of the marine food chain – that use calcium carbonate in the formation of their exoskeletons. Calciferous zooplankton are a common prey source for many fish populations. Declines in their production could lead to limited availability of preferred prey and resulting declines in fish populations and fishery productivity (e.g., Johnson 2012; Keener et al. 2012; Chapin et al. 2014), such as pink salmon (O. gorbuscha) (Chapin et al. 2014) and sablefish (also known as black cod [Anoplopoma fimbria]) fisheries (Sydeman and Thompson 2014).

Shellfish populations that depend on carbonate ions to form their shells will be particularly vulnerable to ocean acidification, and are likely to experience declines in development, spawning, growth and survival (e.g., Cooley and Doney 2009; IWG 2014). Crustaceans, such as lobsters, have exhibited slower growth rates and thinner shell formation in acidic waters, which makes them more susceptible to predation (Arnold et al. 2009). Dungeness crabs (Cancer magister) have also demonstrated decreased juvenile survival in acidic waters in laboratory experiments (Sydeman and Thompson 2014). Other vulnerable fisheries include red crab (Chaceon quinquedens), scallop (Placopecten magellanicus), soft-shell clam (Mya arenaria), and blue crab (C. sapidus) populations (Najjar et al. 2010; NEFMC 2014).

Commercially important shellfish fisheries, such as the aquaculture operations (e.g., oyster hatcheries) in the Pacific Northwest, have already experienced higher than historic rates of mortality and lower rates of productivity (Barton et al. 2012; WA State Blue Ribbon Panel on Ocean Acidification 2012) due to acidic waters. Shellfish industries in other regions of the United States are also at risk, including Alaska (Johnson 2012) and the Southeast (Ning et al. 2003; Needham et al. 2012; Anderson et al. 2013). For example, in the Gulf of Mexico, aquaculture of small bait, food fish, and shellfish (e.g., shrimp, oysters, crawfish) is a valuable regional industry (Twilley et al. 2001) and likely to experience similar declines (Anderson et al. 2013).

More acidic waters will likely also drive changes in species physiology and ecology. For example, acidification has been linked to limited sperm mobility of sea urchins (Reuter et al. 2010), and reduced oxygen capacity and decreased activity levels in the Humboldt squid (D. gigas) (Rosa and Seibel 2008). Fish may require increased energy expenditure to regulate internal blood and tissue pH levels in acidic waters (Ishimatsu, Hayashi, and Kikkawa 2008; Heuer and Grosell 2014). Some reef species such as clownfish (Amphiprion percula) may even become more susceptible to predation as decreased pH levels have been associated with damaged calcareous otoliths and therefore alterations in auditory sensory behavior (Simpson et al. 2011), as well as inhibited olfactory cues (Dixson, Munday, and Jones 2010). Jellyfish may be able to tolerate acidified ocean waters and thrive as other species decline (Winans and Purcell 2010). Jellyfish blooms cause several problems for fisheries as jellyfish outcompete smaller fish and consume eggs, clog fishing nets, and kill fish in aquaculture pens (Purcell, Uye, and Lo 2007).

Although there is some uncertainty regarding the response of fish species to ocean acidification, fish populations will be affected by loss of coral reef (Hoegh-Guldberg et al. 2007) and deep-water coral habitat (Turley, Roberts, and Guinotte 2007). Ocean acidification will decrease rates of coral reef formation and reefs may erode quickly, further hindering the ability of reefs to withstand compounding non-climate stressors, such as pollution and habitat degradation (Leong et al. 2014). Reef fish that use corals for spawning, foraging, and protection are likely to experience declines as corals degrade (Leong et al. 2014). For example, some of the most commercially valuable species in the Caribbean such as spiny lobster (Panulirus argus) and conch (Lobatus gigas) are at risk from the loss of critical coral habitat (Mahon 2002; ValdésPizzini et al. 2010). In addition, rockfish (Sebastes spp.) along the West Coast may experience declines due to loss of their deep-water coral habitat from acidification (WA Blue Ribbon Panel on Ocean Acidification 2012).