In addition to higher air and sea temperatures due to human activities, oceans experience variability in temperature and circulation from natural oscillations such as ENSO and PDO events. During an ENSO event, decreased winds lead to a weakening of upwelling and warmer waters in the Pacific (e.g., Hansen et al. 2006; Lehodey et al. 2006). There has already been an observed increase in the intensity of ENSO events; the 1982-83 and 1997-98 El Niño events were the strongest observed in the past century (Hansen et al. 2006) and the 2015-16 event tied the 1998 El Niño as the strongest on record (NOAA 2016b).

Although the links between climate change and natural environmental cycles are not certain, it is expected that the frequency and intensity of ENSO events will increase as climate patterns (e.g., increased temperatures, decreased winds) shift (Barange and Perry 2009; Yeh et al. 2009). In addition to shifts in ENSO events, ocean circulation in general, such as thermohaline circulation and the formation of North Atlantic Deep Water that drives ocean circulation patterns, may weaken and result in greater water column stratification (Rahmstorf 2006).

Shifts in ocean circulation and the intensity and frequency of ENSO events have the potential to alter the structure and productivity of many fisheries. Warmer ENSO events have been linked to declines in net primary productivity due to increased water column stratification and suppressed vertical mixing that limits the availability of nutrients in the upper layer of water upon which phytoplankton depend (Behrenfeld et al. 2006). In California, previous ENSO events have led to large increases in sea surface temperature (2-3°C [3.6-5.4°F]), decreases in primary and secondary productivity, and limited prey availability for fish populations (Sydeman and Thompson 2014).

Changes in ocean circulation will also lead to changes in 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; Keener et al. 2012). For example, changes in upwelling strength and ENSO and PDO cycling have also been shown to affect the timing and composition of fish larvae in the California Current ecosystem (Brodeur et al. 2008; Auth et al. 2011), highlighting the potential for increased shifts and variability in fish populations and changes in dominant species driven by changes in ocean circulation.

Additionally, many fish stocks have been shown to fluctuate at interannual and interdecadal timescales with natural environmental variability (Lehodey et al. 2006). Equatorial tuna populations, for instance, respond strongly to shifts in temperature and ocean conditions (e.g., ENSO events) (Keener et al. 2012); their migration routes and distribution may shift eastward as primary production decreases and upwelling patterns weaken with more intense and prolonged ENSO events (Guidry and Mackenzie 2011; Leong et al. 2014).

Species and habitats experiencing other stresses may be more susceptible to changes in ocean circulation. For example, changing ocean conditions (e.g., higher sea temperature, changes in timing of upwelling, limited prey) may lead to decreased marine survival rates for salmon, thus compounding the effects of degraded freshwater systems (ISAB 2007; Schindler et al. 2008; McIlgorm et al. 2010). ENSO events have also been linked to mass coral bleaching incidents around the world along with cascading impacts on fish populations (Glynn et al. 2001; Glynn et al. 2014; Reynolds et al. 2014). Changing ocean circulation patterns may also disturb the ecological connectivity of coral reef populations, further limiting the ability of corals to recover after bleaching events (Keener et al. 2012).