Trends in sea lice infestations on chum and pink salmon in the Broughton Archipelago remain unchanged despite removal of finfish aquaculture

Abstract

To better understand the relative contributions of sea lice Lepeophtheirus salmonis and Caligus clemensi from farmed and non-farmed sources, infestations with sea lice are described on juvenile chum salmon Oncorhynchus keta and pink salmon O. gorbuscha from the Broughton Archipelago (BA) in coastal British Columbia, Canada, during a period of declining salmon aquaculture presence. A total of 2868 salmon were collected by beach seine from 14 sites between 2016 and 2024 and examined for sea lice infestation by microscope. During this time, production of Atlantic salmon in open netpens in the BA declined from a high of 21 645 metric tonnes (t) in 17 facilities in 2019 to 614 t in 2 facilities in 2024. The annual prevalence of all sea lice on chum salmon ranged from 53.7 % in 2022 to 12.5 % in 2023 and on pink salmon from 62.9 % in 2022 to 7.3% in 2023. In 2024, the prevalence of L. salmonis and C. clemensi on both salmon species increased and was similar to or higher than annual prevalence values measured between 2016 and 2021, indicating the importance of natural reservoirs as sources of sea lice infestation in the BA.