Field and flume tank experiments investigating the efficacy of a bubble curtain to keep harmful jellyfish out of finfish pens

Abstract

In recent years, salmon aquaculture has struggled to mitigate the impact of harmful jellyfish species in northern Europe and beyond. Typically, large aggregations of jellyfish are carried by currents through salmon pens, leading to unsustainable mortality levels in some years. One potential solution that has been regularly proposed is the creation of a ‘bubble curtain’ around fish pens. A bubble curtain is created by releasing compressed air from a perforated tube at depth, forming a plume of bubbles that entrain the water to create a vertical current. This study investigated the efficacy of a bubble curtain as a jellyfish barrier at several aquaculture locations in Ireland and in a controlled flume tank. Field tests on bubble curtains provided mixed results: a high air flow 8 m linear bubble curtain set at 5 m depth effectively deflected large compass jellyfish (Chrysaora hysoscella), whereas, a low air flow ca. 800 m circular bubble curtain set at 5 m depth, around a salmon farm, did not significantly impact the abundance of small hydromedusae on either side of the bubble curtain. Flume tank experiments demonstrated that increased wave height and increased air flow increased jellyfish transport through the curtain and the lateral movement was consistent with Stokes drift, i.e. an elliptical motion that includes forward movement with each wave. These results suggest that sites with relatively high wave energy may be unsuitable for bubble curtain use and that the variable size and shape of jellyfish may be an important factor in jellyfish and bubble curtain interactions.