Fish stress and mortality can be significant problems in both capture and culture operations. In addition to inexpensive and real-time metrics, ones that are simple to use are also desirable for measuring fish stress and predicting mortality. Current methods to define stress rely on expensive, laboratory-based measurements of changes in fish pathology such as disease, necropsy and histology, in physiology such as plasma cortisol, lactate, glucose and ions and in complex behaviour determined from swimming, feeding and predator evasion. All of these methods are often not rigorously linked to fitness outcomes. An alternative is to observe reflex impairment as a direct sign of stress which can be easily and rapidly measured in free swimming or restrained fish responding to peripheral stimuli such as gravity, light, sound and touch. Reflex impairment is correlated with stress and mortality outcomes, eliminating the need for prolonged holding or monitoring of fish. A few examples of reflexes that may be impaired include orientation, startle responses, fin erection, body flex upon restraint, operculum and mouth clamping or gaping, gag response and vestibular–ocular response. Reflex impairment combines the effects of stressors and their interactions and is not dependent on fish size, motivation states and acclimation which make it a consistent sign of stress across a wide range of stressor types and fish ages. Use of reflex impairment to measure stress and predict mortality would significantly improve monitoring of fish health and welfare in many types of field operations such as commercial and recreational fishing, aquaculture, live transport, stock enhancement and tagging.