Impact of hydrogen peroxide on hatching ability of egg strings from salmon lice (Lepeophtheirus salmonis) in a field treatment and in a laboratory study with ascending concentrations

• Sub-therapeutic concentrations of H2O2 inhibit development of salmon lice larvae.
• No exposed egg string hatched after a field treatment with 1750 mg L− 1 H2O2.
• Discharge of residual water from H2O2 treatment in well boat areas is not necessary.
• No infective copepodids developed after exposure to > 800 mg L− 1 H2O2.

Well boat treatments with hydrogen peroxide are used to control sea lice infections on farmed salmonids in many salmon producing countries. A study was performed to investigate the hatching ability of salmon lice egg strings following exposure to hydrogen peroxide in both a field treatment and a laboratory experiment. Egg strings were collected prior to and following exposure to the test compound and subsequently hatched in a laboratory. In the laboratory experiment, the inhibiting concentration of hydrogen peroxide on egg string hatching was investigated by exposures to various concentrations. No hatching was observed after the field treatment with 1750 mg L− 1 for 31–32 min at 6.4 °C. Even low concentrations proved to affect the hatching when egg strings were exposed for 36 ± 3 min at 8.0 °C. No infective larvae were observed after exposure to concentrations above 470 mg L− 1 in the laboratory study. Thus, the current practice of discharging residual well boat hydrogen peroxide treatment water 3 km from the treatment site seems unnecessary and reduces on-site treatment capacity.