Anthropogenic sound emitted into the sea has the potential to impact marine mammal by masking relevant sound, causing behavioural changes and, in extreme cases, induce auditory injury or death. It is often required to monitor an operational area of offshore projects for the presence of marine mammals in order to reduce the risk of potential impact. This kind of monitoring is traditionally conducted by trained visual observers, which scan the sea surface for marine mammals. This method, however, is restricted to daylight hours and fair weather conditions. The talk presents a review and evaluation of monitoring methods that could now, or in the near future, be used for marine mammal monitoring for mitigation purposes in periods of low visibility during which the effectiveness of visual observers is reduced.