| Summary: | Recently, salmon aquaculture companies in British Columbia, Canada, have experienced significant fish losses resulting in tens to hundreds of millions of dollars in damages due to gill disorders and mouth lesions. Hydroids, the colonial stage of some cnidarians, are the most likely problematic species. Field studies were conducted to examine biofoulant composition, gill health, and the interactions between water parameters, biofoulants, and gill health. In 2020, biofouling was observed at two fish farm sites in the Broughton Archipelago from April 20 to October 30 by suspending 30x30 cm net patches at five depths (1, 5, 10, 15, and 20 m). Net patches remained in the water for 1-3 weeks between pen cleanings (via power washing). After collection, the biofoulants were identified and counted, with hydroids removed and weighed separately. In addition, tow samples were collected weekly to identify any free-swimming stinging-capable species. Biofoulant compositions were mainly composed of Mollusca (mostly Mytilus sp.) and Arthropods (mostly Harpacticoids), hydroids were mostly composed of Obelia sp., and tow samples were composed of mostly medusa-form Obelia sp. GLMMs were built to examine the relationships between the water parameters and the biofoulant species counts, hydroid biomass, and tow sample counts. Both sites saw nearly every parameter significantly associated with biofoulant counts, with the effects stronger at Wicklow Point. Similarly, nearly all parameters were associated with hydroid biomass, however the effects were stronger at Doctor Islets. Only two (ammonia and nitrate levels) and one (ammonia) parameters were associated with the counts of sting-capable species in the tow samples from Doctor Islets and Wicklow Point, respectively. CLMMs were built to examine the relationships between gill health, biofoulant counts and biomass, and water parameters. Iron, nitrate levels, and pH were significantly associated with gill health at Doctor Islets, and temperature, pH, and dissolved oxygen were significant ...
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