This was the key message delivered by Sintef Researcher Johanne Arff, at a sea louse seminar in Norway last week.
“We were surprised that so many producers failed to investigate gill health ahead of delousing with hydrogen peroxide, as we found that gill problems increase mortality using H2O2,” she said.
Arff has led a project that resulted in the report "Delousing with hydrogen peroxide and environmental factors." The project was put in place following reports of unexplained mortality associated with H2O2 treatments in the autumn of 2013.
Researchers found harmful algae and jellyfish in many of the morts submitted for testing, and wondered if there could be a correlation – therefore initiated a data collection and analysis of existing data on the topic from various databases.
“We know that gill damage is a contraindication for the use of H2O2. Phytoplankton stress the fish, especially when they appear in high densities for long periods, such as in the fjords where algal blooms can last for weeks. Some algae produce toxins while others can cause physical gill damage. It may also well be that certain algae produce toxins when they are exposed to H2O2,” said Arff.
“We found that there was little collection of algal samples, so little is known about algae status before delousing. We also found that gill health is not regularly examined except in those areas with AGD issues. It's something to think about when gill injuries cited as a contraindication for treatment with H2O2” she pointed out.
The researcher also said that it was difficult to find good data on delousing operations.
“Data is stored in many different databases, as well as in handwritten notes in files. The procedures for such documentation also varies greatly from company to company. Therefore, a standardization of – and interaction between – databases is important to be able to extract good empirical data, because there are already a lot of good data to draw on,” she pointed out.