20-24 November 2017


NOTE: This document is a repeat of that appearing as a Supplementary Information File for last week.

Variations in the physical and chemical properties of ocean waters occur over a range of temporal and spatial scales. In some cases, changes in ocean properties are accompanied by changes in dominant species. The best known of such relationships is the response of the Peruvian anchoveta fishery to physical changes in the eastern tropical Pacific associated with El Niño. In fact, a sharp decline in anchovy population first signaled physical changes in the tropical Pacific as an El Niño event evolved. Changes in species dominance during El Niño typically last for 12 to 18 months. Today, scientists are investigating other changes in dominant species in response to longer-term (two to three decades) fluctuations in physical properties of the ocean.

Recent discoveries by researchers at the Monterey Bay Aquarium Research Institute (CA) and similar research centers in Mexico and Peru have unveiled an interesting correspondence between changes in physical and chemical properties of the eastern Pacific Ocean and biological regime shifts between anchovies and sardines over periods of decades. Records of the harvest of these two fish species are available for a century or more and are bolstered by information on the populations of their prey (i.e., zooplankton) and predators (e.g., salmon, sea birds). Even changes in the production of sea bird guano, deposited in layers, provide an even longer-term (albeit proxy) record of biological regime shifts. Shifts in dominance between anchovies and sardines appear to be related to changes in Pacific basin-wide sea surface temperatures, upwelling, CO2 flux from the ocean to atmosphere, and even mean global air temperature. While the correspondence between these elements may differ in timing and sometimes the sign is not exactly what would be initially expected, the correlation raises interesting questions for the scientific community and underscores the importance of an ecosystem approach in the study of population dynamics.

A better understood relationship between decadal-scale changes in ocean physical properties and species dominance involves the Pacific Decadal Oscillation (PDO). PDO is a long-lived ENSO-like climate variation over the North Pacific. PDO is better known in the eastern North Pacific and along the West Coast of North America, probably because of more intensive study of those areas. As noted in Chapter 11 of the textbook, the PDO goes through warm phases and cold phases that persist for two to three decades. Coastal waters from the Pacific Northwest north to Alaska are higher than usual during a warm phase and lower than usual during a cold phase. Sardines dominate during a PDO warm phase and anchovies dominate during a PDO cold phase. During a warm phase, the warm surface waters in the eastern Pacific cut off coastal upwelling. Nutrient supply and plankton populations decrease followed by a decline in the anchovy population. Eventually sardines take over in the warmer waters and their populations soar. Their large numbers during PDO warm phases at times spurred the development of significant fishery operations until cooler waters returned and the numbers of sardines plummeted. Just such a rise in sardine populations in the 1930s and 1940s followed by their sudden decline in the late 1940s in Monterey, CA served as a backdrop of the John Steinbeck novel Cannery Row.

Biological shifts are also associated with much shorter-term oscillations in sea surface temperature and more localized upwelling and mixing regimes associated with eddies (rings) that break off the Gulf Stream and the Gulf of Mexico Loop Current. The circulation in eddies alters the temperature and nutrient supply thereby impacting the local marine ecosystem over a period of about four months to a year.

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Prepared by James A. Brey, Ph.D., email
© Copyright, 2017, The American Meteorological Society.