Global temperatures averaged over land and ocean surfaces increased about 0.85°C (1.53°F) from 1880 to 2012 (IPCC 2013). Temperatures in the United States have increased by almost 1.1°C (2°F) in some areas, and the last decade was the warmest on record (Walsh et al. 2014). Average sea surface temperature increases of close to 0.5°C (1°F) over the past century have also been observed (IPCC 2013; NOAA 2015b). Temperatures will continue to increase over the coming century, with the potential for increases of 1.1-2.2°C (2-4°F) in the coming decades and up to 2.8°C (5°F) by 2100 (Walsh et al. 2014); exact amounts will depend on future greenhouse gas emissions levels (IPCC 2013; Church et al. 2013). Interannual and interdecadal variability, such as the El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO), could cause even greater ocean warming during some periods. Increased air temperatures are also increasing the rate and extent of sea ice melt, which may create new habitat and areas for fish and fishing. Melting sea ice will also increase freshwater runoff and could lead to decreased salinity levels. The Gulf of Alaska, for example, has already experienced decreased salinity due to melting sea ice (Royer and Grosch 2006).

Warming oceans will lead to a variety of impacts on fisheries, both positive and negative. These effects range from altered prey availability and species range shifts to habitat degradation and/or expansion (e.g., Brander 2007; Daw et al. 2009; Link et al. 2010). Increasing ocean temperatures will lead to phenological shifts (e.g., changes in larval duration, metabolic rate, timing of spawning and growth) and changes in the body size of some species (e.g., Brander et al. 2007; O’Connor et al. 2007). These effects may trigger large-scale redistributions of fish catch (Cheung et al. 2010) and potential localized extinctions (Cheung et al. 2009; Holmyard 2014).

Rising temperatures are expected to cause changes in the timing of phytoplankton and zooplankton production, potentially leading to declines in prey availability for fish larvae if warmer temperatures do not also prompt earlier fish spawning (Brodeur et al. 2008; Auth et al. 2011; NEFMC 2014). In the Bering Sea, for instance, phytoplankton bloom timing influences recruitment levels of walleye pollock (Gadus chalcogrammus). Higher temperatures are likely to cause earlier phytoplankton blooms, which support copepod eggs or copepod nauplii, the major food source for larval walleye pollock. A potential seasonal mismatch between phytoplankton bloom timing and therefore when food is available for larval walleye pollock may lead to decreased survival and recruitment (Johnson 2012).

Temperature plays a key role in fish physiology, development, and abundance. Due to warmer temperatures, many organisms will require increased metabolic rates and consume more oxygen, causing declines in dissolved oxygen levels (Najjar et al. 2000). Warming oceans are likely to lead to decreased reproduction, disruptions in spawning activity, and recruitment for many fish species (Najjar et al. 2000; Guidry and Mackenzie 2011). In areas such as the Chesapeake Bay, some species (e.g., blue crab [Callinectes sapidus], oysters) may benefit from warmer temperatures due to increases in overwintering survival (Najjar et al. 2010). Some species may also experience increased growth, productivity, and abundance, such as the Atlantic croaker (Micropogonias undulatus), which is expected to exhibit a 60-100% increase in biomass by 2100 (Hare et al. 2010), as well as sardines (Sardinops spp.) (Sydeman and Thompson 2014). Rising ocean temperatures may also lead to changes in migration timing, such as earlier shoreward migration of lobsters (Homarus americanus) (Mills et al. 2013), and altered migration timing of Pacific salmon (Oncorhynchus spp.) to streams and rivers (Beamish et al. 2010) and the ocean (ISAB 2007).

Temperature also influences species distribution. Some species live near the upper end of their thermal tolerance limit; examples include reef fish species (Guidry and Mackenzie 2011) and the black sea bass (Centropristis striata) (Najjar et al. 2010). Temperature changes may drive expansion and/or contraction of species ranges (Ning et al. 2003; Leong et al. 2014). In Chesapeake Bay, warmer-water species such as southern flounder (Paralichthys lethostigma) and tarpon (Megalops atlanticus) are likely to benefit from warming, while species at the upper end of their thermal tolerance may migrate northward or deeper and experience reduced production (Najjar et al. 2010). Warming waters may cause the northward shift of some commonly fished species, as well the appearance of southern species that compete with native stocks (e.g., tuna, sharks, and mackerel) (Johnson 2012). Coldwater species such as red hake (Urophycis chuss) have already shifted from the Mid-Atlantic Bight north to the Gulf of Maine (EAP 2009; Pinsky and Fogarty 2012). Of the 82 fish and invertebrate species analyzed in the Northeast Fish and Shellfish Climate Vulnerability Assessment (NOAA 2016a), over 60% exhibit high to very high potential for future changes in distribution, including shifts into and out of the Northeast shelf ecosystem (Hare et al. 2016). Other species projected to shift northward to more suitable thermal habitat include sockeye salmon (O. nerka), albacore tuna (Thunnus alalunga), bigeye tuna (T. obesus), yellowfin tuna (T. albacares), dolphinfish (Coryphaena hippurus), and swordfish (Xiphias gladius) (Roessig et al. 2004; Keener et al. 2012; PRCCC 2013; Chapin et al. 2014; Sydeman and Thompson 2014; Leong et al. 2014), which may create opportunities for increased catch and revenue and expanded fishing operations (Sumaila et al. 2011). However, increasing temperatures are generally expected to cause declines in overall catch resulting from population shifts towards higher latitudes and deeper waters when possible throughout the remainder of the United States, particularly at the southern end of species’ ranges (Cheung et al. 2009; Nye et al. 2009) and in tropical areas (Holmyard 2014). In the Gulf of Maine, warming waters are responsible for a collapsing cod fishery, where waters have warmed 99% faster than other areas in the country even with significant reductions in catch quotas (Pershing et al. 2015).

Climate-driven redistribution of traditionally fished commercially valuable stocks will be one of the greatest effects felt by commercial, recreational, and subsistence fishermen. Pinsky and Fogarty (2012) found that commercial fishing effort in the Northeast has tended to shift northward as species shift, though there were lags in the timing and amount of these shifts. Fishing effort has also been documented to shift northward for the Atlantic surfclam (Spisula solidissima) (McCay et al. 2011). Due to potential range shifts, there may be a need for flexible permitting structures that allow commercial fishermen to shift between or target multiple species (e.g., Link et al. 2010; Mills et al. 2013). For species such as silver hake (Merluccius bilinearis) that have a distinct northern and southern stock, northward shifts of the southern stock may lead to merging of two previously distinct stocks that were managed separately (Link et al. 2010), which will require a re-evaluation of management approaches. Range shifts will decrease recreational and subsistence access to popular and traditionally fished species, which could result in economic losses for recreational operators (Roessig et al. 2004), and require targeting of new species (Doney et al. 2014). Switching to new species could be a cultural challenge and may require changing fishing gear, a costly expense for small-scale fishermen (Roessig et al. 2004).

Changes in air and sea temperatures may have positive and negative effects on fish habitat. In the Arctic, sea ice cover has been declining at a rate of approximately three percent per decade, and the six lowest sea ice years on record were observed between 2007 and 2012 (AFSC 2012); Arctic sea ice is expected to disappear completely by mid-century (Chapin et al. 2014). Melting sea ice caused by warming air temperatures could create new habitat for fish stocks such as cod (Gadus spp.), Pacific herring (Clupea pallasii), and walleye pollock, allowing access to fishing areas that were previously inaccessible (Cheung et al. 2010; Chapin et al. 2014). Increasing temperatures may also result in earlier and decreased freezing of shoreline habitats in the Chesapeake Bay that are suitable locations for oyster reefs and aquaculture facilities (Najjar et al. 2010). However, rapid freshwater runoff from melting sea ice and glaciers may also flood and degrade critical nursery, foraging, and refuge habitat for salmon (Johnson 2012; Chapin et al. 2014; CA Fish Passage Forum 2014). In addition, warming temperatures will lead to increased frequency and severity of coral bleaching events (Guidry and Mackenzie 2011; Keener et al 2012).

Higher water temperatures will prompt increased water column stratification, causing hypoxic, or low oxygen, conditions. Combined with excess nutrient input, these hypoxic conditions can increase in the extent of areas such as the Gulf of Mexico “dead zone,” and may disrupt fish endocrine systems (Thomas et al. 2007). Increased water column stratification will also lead to decreases in vertical mixing and is likely to prompt overall declines in net primary productivity, which could have cascading trophic impacts and limit food availability for some fish and shellfish populations (Behrenfield et al. 2006). Increasing ocean temperatures stimulate harmful algal blooms, which may increase the incidence and persistence of toxic dinoflagellate, such as the one that causes ciguatera fish poisoning (Tester et al. 2010; Carter et al. 2014). Finally, increased temperatures in mid-latitude waters are likely to cause increased evaporation and increasingly saline seas (Rhein et al. 2013). Increased salinity could lead to increased instances of pathogens that thrive in higher salinity conditions, such as oyster diseases and predatory snails, resulting in increased mortality of Chesapeake Bay oyster populations (Najjar et al. 2000).

Warming water temperatures may also promote the survival of toxic pests and invasive competitors in some regions (Twilley et al. 2001; Roessig et al. 2004; Johnson 2012; Anderson et al. 2013; Sanford et al. 2014; Doney et al. 2014) that may stress native fish populations. Several examples already exist, including the European green crab (Carcinus maenas), which has negatively impacted clam populations in California and Maine (Doney et al. 2014); the Atlantic oyster drill (Urosalpinx cinerea), which is threatening Olympia oysters along West Coast estuaries (Sanford et al. 2014); and invasive tunicates in Alaska that have overtaken shellfish habitat (Johnson 2012). Warming waters may also increase the prevalence of some predators, such as Humboldt squid (Dosidicus gigas) that prey on native fish populations in California (Sydeman and Thompson 2014).

The response of fish populations to climate change will also be influenced by human responses. For instance, as a stock shifts northward driven by temperature changes, fishing activities may 5 shift northward with the stock, which would allow continued access to the stock; a potential increase in capital costs as a result of a shift in operations could lead to an overall decrease in net revenue (Sumaila et al. 2011; Pinsky and Fogarty 2012). In some cases, fishing may become more concentrated in a smaller area, placing increased pressure on stocks (McCay et al. 2011; Pinsky and Fogarty 2012).