Oceanic-Atmospheric Modes of Variability and Their Effect On River Flow and Blue Crab (Callinectes sapidus) Abundance in the Northcentral Gulf of Mexico
Oceanic-atmospheric modes of variability occur on interdecadal, multidecadal, decadal, and interannual timescales and their influence on climate around the world has been confirmed. The present study investigates Mississippi River and Pascagoula River flows in response to the influence of one or more of the four oceanic-atmospheric modes of variability: the Pacific Decadal Oscillation (PDO), the Atlantic Multidecadal Oscillation (AMO), the North Atlantic Oscillation (NAO), and the El Nino Southern Oscillation (ENSO). These modes of variability are present in phases: PDO warm (PDOw) and cold (PDOc), AMO warm (AMOw) and cold (AMOc), NAO positive (NAOp) and negative (NAOn), and ENSO warm (ENSOw), neutral (ENSOn), and cold (ENSOc). High Mississippi River mean flow was associated with the PDOw, AMOc, and NAOp phases, with low river flow linked to their opposite phases. High Pascagoula River mean flow was related to the AMOc and NAOp phases, with low river flow linked to their opposite phases. Pascagoula River flow was significantly higher during the ENSOw than ENSOc events, within PDOw/AMOw/NAOp and NAOn phase. Blue crab data on abundance were taken from fishery-independent trawl survey programs conducted by the Gulf Coast Research Laboratory, Ocean Springs, Mississippi and the Louisiana Department of Wildlife and Fisheries in coastal waters of Mississippi and Louisiana, respectively. Four long-term climatic phases (PDOc/AMOc/NAOn, PDOc/AMOc/NAOp, PDOw/AMOc/NAOp, and PDOw/AMOw/NAOp and NAOn) overlapped with four distinct periods of annual blue crab abundance that were identified using hierarchical agglomerative clustering and non-metric, non-parametric multidimensional scaling techniques. The following abundance periods were delineated: period I (1967-1970), period II (1971-1980), period III (1981-1998), and period IV (1999-2004). For all but three years (1991, 1995, 2005) the overall abundance of blue crabs fell into chronological sequences under climatic phases. A single year (1990) did not group with any of the four abundance periods. Periods II and III were characterized by high numbers of crabs and increased river flow, whereas Period IV was distinguished by low numbers of crabs and decreased river flow. Years of lowest abundance (period IV) occurred at a time of unprecedented change in habitat associated with catastrophic storms, the cumulative consequences of man-made alterations to coastal wetlands, and an unfavorable climatic regime. Whether a shift to a more favorable climatic regime would increase abundance is unknown. Blue crab abundance was related to long-term hydrological conditions across the Mississippi River and Pascagoula River basins with 23% of the variability explained by oceanic-atmospheric modes of variability (AMO, NAO), salinity, and frequency of southeast winds. These factors may favor blue crab productivity by increasing marsh edge habitat, decreasing predation, and facilitating shoreward transport of megalopae. The importance of biotic factors associated with quality of habitat as refuge has been emphasized by recent studies. Because climate operates on an ever-changing coastal environment and because of the inability to quantify sources of natural mortality of young crabs, prediction of blue crab abundance is difficult when current knowledge is coupled solely with the influence of climatic factors.