| Summary: | The harmful algal bloom species, <i>Karenia brevis</i>, forms annual, often intense blooms in the Gulf of Mexico, particularly along the west Florida shelf. Though the ability of <i>K. brevis</i> blooms to cause mass mortalities in juvenile fish are well documented, the direct effect of bloom concentrations on larval fish has not been studied extensively. To better understand the potential effect of <i>K. brevis</i> on larval fish survival, laboratory spawned red porgy (<i>Pagrus pagrus</i>) larvae from 4–26 days post-hatch were exposed to concentrations of <i>K. brevis</i> observed in the field for either 24 or 48 h. This species is representative of fish which spawn in regions of the Gulf of Mexico and whose larvae are epipelagic and may encounter <i>K. brevis</i> blooms. In this study, three different <i>K. brevis</i> strains varying in the amount of brevetoxin produced were tested. Larval survivorship was found to be inversely proportional to the amount of brevetoxin produced by each strain. The EC<sub>50</sub> value from the combined 24 h experiments was ~163,000 <i>K. brevis</i> cells L<sup>−</sup><sup>1</sup>, which corresponds to cell concentrations found in moderately dense blooms. Larval mortality also increased substantially in the 48 h versus 24 h exposure treatments. These findings indicate <i>K. brevis</i> blooms have the potential to contribute to natural mortality of fish larvae and further reduce inter-annual recruitment of fishery species whose stocks in the Gulf of Mexico may already be depleted.
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