Updated: Oct 27, 2020
Climate change is impacting the health and biological integrity of marine and estuarine waters throughout the United States, and globally. Rising average air and water temperatures, more frequent and extreme weather events, and steadily rising sea levels are changing baseline environmental conditions, and may alter the distributions and life history patterns of marine/estuarine organisms, including fish, invertebrates, sea birds, sea turtles and marine mammals. The magnitude of these ecological changes is expected to increase in the future, with important implications for strategic, effective management of marine and coastal resources, including sustainable fisheries and swimmable waters. One especially widespread (global) indicator of the effects of climate change (specifically increased sea surface temperatures) on marine resources is the increasing magnitude of change in the distribution of marine and estuarine fish species. (Roessig et al. 2004, Nye et al. 2009, Koenigstein et al. 2016). For example:
Along the southeastern Brazilian coast, up to 50 fish species have altered their distribution patterns in recent decades, with subtropical species contracting their range in the north, and tropical species expanding their ranges to the south. (Araujo et al. 2018);
A general shift northward in fish migration patterns has been documented in European estuaries, from Portugal to Scotland, from the mid-1970s to present (Nicolas et al. 2011);
Within the Tagus Estuary, along the Portuguese Atlantic coast, an increase in sea surface temperatures from 1978 to 2006 was correlated with greater abundance of sub-topical species and a decline in temperate fish species (Vinagre et al. 2009); and
Tropical fish species are increasing in abundance within temperate estuaries along the coast of South Africa, and future projected changes in the intensity and periodicity of precipitation and river flow is anticipated to strongly affect coastal fish populations in this region (James et al. 2013).
However, discerning climate-driven changes in marine fish distributions is challenging – the signal from climatic effects may be confounded by other factors such as forage availability, changes in inshore habitat structure and commercial overharvesting. In addition, marine fish populations can undergo cyclic patterns of abundance associated with multi-decadal natural changes in oceanic currents, such as the North Atlantic Oscillation (NAO), the Pacific Decadal Oscillation (PDO), and the El Nino-Southern Oscillation (ENSO) (Crozier and Hutchings 2014). Even under nearly constant environmental conditions, fish distributions are not static. Fish populations occupy optimal habitats under low abundances, but also disperse into less optimal habitats at high abundances (Sinclair 1988, MacCall 1990). This means that species that are only rarely or periodically seen in temperate estuaries may be driven there in response to higher densities/competition for resources in more tropical waters and not necessarily because of favorable temperatures.
Many aquatic and marine species are sensitive to temperatures just a few degrees higher than those they are generally adapted to in nature (Kennedy et al. 2002). Oceanic warming simultaneously reduces the total amount of dissolved oxygen that can be held in water and increases demand for oxygen in cold blooded aquatic animals. Even modest increases in ocean temperatures may affect growth/metabolism, determine behavior and alter distribution patterns. The Intergovernmental Panel on Climate Change (IPCC 2014) has documented an average global temperature increase among land and ocean surfaces of 0.85 °C (1.53 °F) between 1880 and 2012. The upper ocean (0 to 75 m) has, on average, warmed by 0.11 °C (0.20 °F) every decade since the early 1970s.
Increased surface water temperature, along with changing patterns of precipitation and riverine hydrology may alter the timing and magnitude of phytoplankton production in estuaries, favoring production by species known to form harmful algal blooms (HABs) (Pyke et al. 2008)—such as the notorious “red tides” currently occupying a large expanse of the southwestern Florida coastline, resulting in massive fish kills, and respiratory distress to humans on beaches. Toxic effects of HABs vary; some forms may exhibit toxicity to fish and aquatic biota even at low cell concentrations, while others may be essentially non-toxic but present a nuisance through high biomass production – they interfere with grazing by zooplankton and alter patterns of nutrient supply and elemental recycling (Gobler et al. 2017).
Along the U.S. Atlantic coast, warm-temperate fish species fish assemblages may benefit from climate changes that are impacting cooler-water species, by expansion of their range to more northern estuaries. One of the most compelling examples of this phenomenon is Narragansett Bay, Rhode Island. Nye et al. (2009) documented changes in the abundance and latitudinal distribution for several bottom-dwelling species, which were historically abundant and characteristic of the Narragansett Bay winter fish community, including red hake (Urophycis chuss), and silver hake (Merluccius bilinearis). Simultaneously, the abundance of warm water species that migrate into the Bay during summer such as butterfish (Peprilus triacanthus) and scup (Stenotomus chrysops) increased. These changes coincided with a 90% decline in winter flounder (Pseudopleuronectes americanus) abundance in the Bay (Oviatt 2004, Jefferies et al. 2011). Winter flounder spawn in estuaries at temperatures ranging from 1 to 10 °C, with optimal spawning conditions at 2 to 5 °C. The evolution of cold water spawning in winter flounder is a mechanism for avoiding predation on newly emerged/metamorphosing larvae, principally by sand shrimp (Crangon septemspinosa). Winter flounder eggs hatch when sand shrimp have historically been absent or dormant in the Bay. However, as winter water temperatures increased, sand shrimp remained active and consumed flounder larvae (Taylor and Collie 2003). Warmer waters are also associated with greater egg mortality rates, reduced larval growth rates, and diminished larval condition (Keller and Klein-McPhee 2000). Winter flounder have historically exhibited long-term cyclical abundance patterns; however, abundance peaks have diminished in recent decades.