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UBC Theses and Dissertations

Water circulation, dissolved oxygen, and ammonia concentrations in fish-net cages Gormican, Stephen Joseph


Fish farming in the protected waters of British Columbia is relatively new, but has undergone a phenomenal growth in the last ten years. Little investigation has been reported with respect to conditions within the net-cages employed in growing salmon at fish farms. In particular, the role of water quality and water exchange has not been examined in relation to local mariculture husbandry practices and hydrography. The first part of this study compared water quality and water flow in two locations, one in Jervis Inlet with a deep entrance sill and the- other in Sechelt Inlet which has a shallow entrance sill. Marked variations in hydrography occurred between the two sites as a result of the differences in sill depth. An internal wave generated at the Sechelt Inlet sill caused daily fluctuations in stratification and hence water properties within the net-cages. No such variations were observed at the Jervis Inlet site. In the second part of this study, water quality and water flow was_ measured in various locations in and near a raft of 24 net-cages. Generally, it was found that within the raft, water flow was diminished in those cages located downstream of the predominate flow direction. However, local topography was thought to have caused marked variation in water quality and water exchange patterns in two of the cages. Ammonia concentrations were not observed to exceed reported sublethal concentrations at any time, over a 25 h period, at any of the depths sampled, within the net-cages. Dissolved oxygen concentrations did, at some depths and times, approach values at which some stress may be felt due to low oxygen. Linear regressions between water quality and water speed were not found to be significant in most cases. The coefficient of determinations were low, indicating that current speed accounted for less than 27% of the variation in water quality.

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