Harmful algal blooms (HABs) have caused extensive losses at fish mariculture facilities throughout the world. When first experiencing major fish losses, farmers are often ill prepared and are unable to react appropriately or quickly. After recurring HAB-caused fish kills, fish farmers or governments usually begin developing monitoring and mitigation plans of varying complexity and effectiveness.

Experience has shown that there is no single best mitigation method. The simplest, low technology measures are the most desirable, as they the easiest to implement and monitor in the demanding marine environment. Selection of the most suitable techniques should be based on site-specific conditions including the size and volume of culture facilities, water depth and current conditions, vertical distribution of the HAB species, and cause of fish mortality (e.g., neurotoxins, mechanical damage to gills or environmental hypoxia). Towing net-pens away from HABs is a preferred method, if a suitable refuge area is available, but there are significant risks. Isolating net-pen fish from HABs with perimeter skirts or displacing HAB cells from the pens with coarse-bubble aeration or airlift pumping has sometimes been successful. Mariculturists have tried other methods with varying success including sinking of cages, deep pens, pre-emptive harvest, cessation of fish feeding (to lower oxygen use) and culture site relocation. New types of sinkable-offshore pens and enclosed pens with pumped seawater may offer advantages, but are relatively expensive. A promising low-cost technique of apparently little environmental impact is the application of certain types of clay to remove HABs from net-pens by flocculation. I am studying the effect of clay application on benthic invertebrate communities near net pens and the physiological effects of clay on cultured fish.