Review of options for creating and maintaining oxygen refuges for fish during destratification-driven hypoxia in rivers

Climate change, river regulation and water extraction create the conditions where destratification-driven hypoxia will become more common in rivers. Preventing this and the fish deaths that can result requires options that prevent stratification and create oxygen refuges for fish. Here we discuss aeration and mixing approaches that may help prevent fish deaths when flow-related measures are not available. The options were evaluated based on efficacy, environmental risk and readiness to be deployed cost-effectively. The options either promote mixing, aeration or both. Bubble diffusers and paddle wheels used commonly in aquaculture are unlikely to aerate already hypoxic pools. However, if deployed before stratification occurs, they may promote mixing and maintain aeration. In comparison, pumps with Venturi tubes or ultrafine oxygen bubble condensers both mix and aerate, making them suitable for use once hypoxic events are underway. Water jets are low cost and could be deployed quickly. Dosing reaches with calcium peroxide may be useful for emergency aeration, but requires further safety and efficacy testing. Flow management that maintains fish refuges and storage reserves during drought is the best way to guard against fish deaths, but if storage releases are not available, there are options for creating and maintaining oxygen refuges to minimise ecosystem damage.