~ Precipitation

Precipitation patterns vary greatly across the United States, as do projected future changes. In the past century, the country has experienced an overall increase in precipitation, including an increase in extreme precipitation events over the past three to five decades (Walsh et al. 2014). Although it is difficult to predict future precipitation patterns, it is generally expected that precipitation in the northern United States will increase and become heavier, particularly in the winter (Horton et al. 2014), while the southwestern region, including California, will experience drier conditions (Walsh et al. 2014). In the Gulf of Mexico, periods of intense rainfall are likely to alternate with periods of intense drought (Twilley et al. 2001), while the Pacific Northwest is likely to receive generally higher and more intense precipitation, particularly in the winter and spring (McIlgorm et al. 2010).

Altered precipitation patterns may drive shifts in anadromous and estuarine fish species composition in coastal areas due to changes in salinity (Roessig et al. 2004; Gregg et al. 2011). Salmon and other anadromous stocks, for instance, may experience altered reproductive timing and success due to seasonal shifts in the timing and intensity of drought and flood periods (e.g., Snover et al. 2005; ISAB 2007). Periods of drought may lead to higher saline conditions, resulting in reduced suitable habitat for fish (Twilley et al. 2001). For example, brown (Farfantepenaeus aztecus) and white shrimp (Litopenaeus setiferus) that use estuaries along the Atlantic coast and Gulf of Mexico as nursery habitat may experience declines due to increasing saline conditions driven by drought and warmer temperatures (Ning et al. 2003). Higher levels of rainfall could lead to increased inundation or degradation of important coastal habitats, such as tidal marshes, mangroves, and shallow coral reefs that fish use for protection, spawning, and rearing of juveniles (Najjar et al. 2010; Carter et al. 2014); more frequent and intense storms may likewise damage port and fishing infrastructure (Daw et al. 2009).

Intensification of precipitation is also likely to compound other climatic stressors. For example, escalations in freshwater runoff may cause increased water stratification in estuaries and coastal waters, reducing primary productivity rates and nutrient upwelling (Kennedy et al. 2002). Increased stratification, when combined with higher rates of nutrient runoff, will likely reduce deep-water oxygen and expand hypoxic zones in areas such as Chesapeake Bay and the Gulf of Mexico (Najjar et al. 2000; Twilley et al. 2001). Other studies project that more frequent upwelling-favorable winds may lead to increased coastal upwelling in a changing climate (Bakun et al. 2015), particularly at high latitudes (Wang et al. 2015). A long-term increase in upwelling associated with stronger upwelling-favorable winds was documented over 27 years along the central California coast, although this may not be indicative of a long-term trend (García-Reyers and Largier 2010). Increased upwelling, when combined with potential increases in nutrient runoff after precipitation events, may result in increased primary production.

Table 2: Potential impacts of variable precipitation on fisheries.

Observed Changes

  • Changes in precipitation patterns vary across the United States, with an average increase in precipitation since 1900
  • Observed increased frequency of extreme precipitation events in last 3 to 5 decades

Projected Future Changes

  • Northern U.S. likely to experience more frequent and intense precipitation
  • Southwest likely to see drier conditions
  • Shifts between intense rainfall and intense drought in Gulf of Mexico
  • Pacific Northwest likely to experience higher average and more concentrated precipitation

Potential Impacts on Fisheries

  • Shifts in species composition of anadromous and estuarine species (e.g., salmon) attributable to salinity changes in coastal areas
  • Altered reproductive timing and success of anadromous species due to timing of seasonal droughts and flooding
  • Inundation of coastal fish habitat (e.g., mangroves, tidal marshes, shallow corals)

Key Compounding Factors & Impacts

  • Salinity: Seasonal variation likely based on relative rates of precipitation and evaporation, as well as stratification.
  • Freshwater runoff and expansion of low oxygen zones: Increased freshwater input may lead to greater water column stratification. Combined with the potential for higher nutrient pollution runoff and increased localized phytoplankton blooms, these compounding factors will exacerbate hypoxic areas by limiting mixing of water and replenishment of oxygen to deep waters.