Sunday, May 17, 2009

Report on Seafan Nudibranch Outbreak 2005

Tritonia Report Thad Murdoch
Population outbreak of the Tritonia hamnerorum nudibranch, an obligate grazer of Gorgonia ventalina sea fans, across Bermuda’s northern reefs in 2005


EXECUTIVE SUMMARY
On July 14th 2005 a large population of the nudibranch Tritonia hamnerorum was found to be at infestation levels on a collection of patch reefs on the Devil’s Flat reef complex. This species was never before recorded in Bermuda.

• The nudibranch numbered in the many hundreds on some sea fan colonies, and was feeding on the sea fans, which caused damage and death to many of these important sessile animals.

• Surveys carried out over the course of the summer determined that the outbreak was patchily located over a ~20 km2 area along the Northern rim and outer patch reefs of the North Lagoon, but not observed nearer to shore, nor along the Western, Southern nor Eastern rim reefs.

• Highest levels of the outbreak occurred at the buoyed Marine Protected Area at Eastern Blue Cut (EBC), and across Devil’s Flat (DF), causing mortality to roughly half of the sea fans on some areas.

• Re-assessment on November 18th, 2005 at the EBC and DF outbreak sites found that the populations were no longer present, presumably due to the colder temperatures prevalent as winter approached.

• The BBP team will watch out for another outbreak of the nudibranch in the early summer of 2006, when sea water temperatures increase again.

• Population assessments of sea fans indicate that these soft corals are most abundant in shallow outer reef areas, which may account for the high densities of the nudibranchs in the same area.