Accumulation and distribution of skin fluke Neobenedenia girellae eggs on a culture cage

• Neobenedenia girellae eggs distributed mainly on shallow areas of culture cage.
• Egg number decreased with depth but second highest was observed at deepest depth.
• The highest egg accumulation on culture cage was 805.8 eggs/cm2/day.
• Estimated daily egg accumulation for entire cage is more than 53 million egg/day.

Skin fluke causes chronic problems in marine finfish aquaculture. Their eggs possess a long filamentous appendage that allows them to easily attach to culture cages. Fish farmers periodically change and clean cage nets to remove eggs from the culture sites. However, there is very little information about the accumulation rate and distribution of skin fluke eggs on culture cages. We conducted an experiment at a culture site to assess the accumulation of eggs of the Neobenedenia girellae skin fluke at different culture cage depths. In all three trials, we observed the highest egg accumulation in shallow areas, and the egg number decreased with the depth; however, the second highest accumulation was observed at the deepest depth. These distribution patterns likely reflect the behavior of fish within the cage. The highest accumulation rate was 161.2 eggs/cm2/day, with an estimated daily accumulation of more than 10 million eggs on the entire cage (excluding the cage floor). Such information is useful for optimizing and managing cage net changing and also for the development of a new method of removing/killing parasite eggs.