WHAT IS GOING ON WITH Perkinsus marinus IN THE GULF OF MEXICO?  

The NOAA Status and Trends Mussel Watch Program sampled nearly every major coastal lagoon and estuary in the Gulf of Mexico from 1986 through 2010. This contribution examines trends in the principal oyster disease, Dermo, oyster length, and oyster sex ratio over this time series. Perkinsus marinus prevalence and mean and median weighted prevalence declined significantly over the time series in the eastern, western, northern, and southern Gulf. In each case, values varied cyclically, approximating 4 years, from 1986 to 2001 and then declined in a stepwise fashion in 2002, remaining lower over the rest of the time series. The length of the largest individuals declined in three of four regions.  Visually, this decline was interrupted periodically over the 1986-2001 time frame, but the size of the largest individuals dropped distinctly in 2002 and remained lower than in preceding years through 2010 in all but the southern Gulf. The fraction of the large animals collected by Mussel Watch that were female declined significantly over the time series in all four regions. Over much of the time series prior to 2006, for all Gulf regions, these large animals tended to be 70-80% female, whereas beginning in 2006, they tended to be less than 60% female and often well below 50% female. The hypothesis posed is that Dermo increased in virulence circa 2000, such that more of the heavily infected animals died prior to collection, thereby lowering Dermo median infection intensity. This additional mortality, being concentrated in the larger animals, produced a decline in the average length of the largest animals thereby shifting their sex ratio dramatically towards males. By inference, fecundity declined due to the loss of the largest females and the decline in the total number of females. These trends occurred throughout the Gulf of Mexico.