A passive acoustic survey using a random stratified design detected spawning aggregations of sand seatrout Cynoscion arenarius over 2 sequential spawning seasons (2004 and 2005) in Tampa Bay, Florida. In 2005, an intense Karenia brevis red tide at ichthyotoxic concentrations entered Tampa Bay 3 mo after the spawning season began. The bloom persisted through the end of the spawning season and was temporally associated with significant changes in the spatial distribution of spawning aggregations. Red tide was most prevalent and concentrated within the lower portion of Tampa Bay and extended to a lesser degree into the middle bay, but remained absent from the upper bay. While the percentage of sand seatrout aggregations in the middle bay zone did not significantly change from 2004 to 2005, aggregations in the non-impacted area of the upper bay increased, and the red tide-exposed lower bay experienced a significant and pronounced decline. These significant bay-wide changes in the sand seatrout spawning population coincided with the red tide event, most notably the considerable decline in the lower bay, but the broad spatial distribution of the aggregations in concert with certain reproductive and life history characteristics may buffer the population from long-term effects. Typically, it is difficult to assess the effect of disturbances on marine fish populations due to complexities in measuring the extent of the perturbation and the magnitude of the loss to the population. Our spatially explicit sampling design further enabled us to demonstrate the widespread effects of red tide on fisheries and provides an important tool for assessing the extent of loss to a spawning fish population.

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