Long‐term change in the abundances of northern Gulf of Mexico scyphomedusae <i>Chrysaora</i> sp. and <i>Aurelia</i> spp. with links to climate variability

<jats:p>Variability in northern Gulf of Mexico <jats:italic>Chrysaora</jats:italic> sp. and <jats:italic>Aurelia</jats:italic> spp. abundance and distribution over time and in relation to climate forces operating at large and regional scales was analyzed using two multi‐decadal data sets derived from trawl data collected during fisheries‐independent surveys conducted each summer (1987–2007) and fall (1985–2007). Populations exhibited remarkable increases and decreases in size during the past two decades. During peak years, an estimated mean (± 1 SD) total 4.4 (1.5) billion <jats:italic>Chrysaora</jats:italic> sp. and 9.6 (0.9) billion <jats:italic>Aurelia</jats:italic> spp. occurred during summer (May–July) and fall (October–November), respectively. Earlier reported increases in jellyfish abundance have not continued; however, the frequency of <jats:italic>Chrysaora</jats:italic> sp. in trawls has increased with time. Jellyfish “hot spots” are waters southwest of Mobile Bay, west of the Mississippi River outflow, and near the Louisiana‐Texas border. “Cold spots” occurred in waters > 40 m in depth. Annual variations in <jats:italic>Chrysaora</jats:italic> sp. and <jats:italic>Aurelia</jats:italic> spp. abundance were temporally coherent, suggesting a common set of environmental drivers. Jellyfish abundance was positively related to the El Niño Southern Oscillation, Atlantic Multi‐decadal Oscillation, and Pacific Decadal Oscillation and was negatively related to the Great Plains Lower Level Jet. Jellyfish were also more abundant when regional sea surface temperatures were cooler than average in spring and warmer than average in summer‐early fall. Increased jellyfish production occurred during years when the northern Gulf experienced wet and warm winters, dry and cool springs, and wet and warm summers and when the Mississippi River watershed experienced dry winters, wet springs, and wet summers.</jats:p>