1. A seasonal reduction in water flow, due both to prevailing weather conditions and anthropogenic disturbance, is a prominent feature of the habitat in many riverine systems, yet the response of many aquatic vertebrates, such as juvenile Atlantic salmon, to low flows is not well understood. However, in accordance with general fitness optimization theory, it might be predicted that salmon will emigrate from shallow areas to seek refuge in pools as water levels decrease to critically low levels (fitness approaches zero). To test this prediction, we directly measured the movement response of individual Atlantic salmon (Salmo salar L.) (74–109 mm) to drought in near-natural mesocosms.
2. In five separate trials during summer, groups of salmon were introduced into enclosed 30-m long sections of stream, each comprising a central 10-m long region of shallow riffle habitat bordered upstream and downstream by 10-m long regions of deeper water. After fish had settled into home ranges, those fish inhabiting deep areas were removed and the water flow was later decreased to zero over 2 days, so that the riffles were nearly dry. The movements of each individual fish within the enclosures were monitored remotely and continuously using a passive integrated transponder (PIT) tracking system.
3. Of the total of 33 fish with home ranges that included only riffle habitat, 14 moved into deep water at some point during drought, but only seven of these fish (0–50% between trials) established new home ranges that included deep areas. The others returned to regions of riffle during the drought. None of the eight fish that definitely had not sampled in pool habitat prior to settling on riffle emigrated during drought.
4. The optimal response of salmon parr to moderate natural drought appears not to be fixed, but for many individual fish it may be to stay in shallow riffle areas. The optimal response of salmon parr to extreme natural drought and anthropogenic de-watering will, in many cases, be to move. An increase within the population of traits that promote an ideal response to natural drought will therefore make it more vulnerable to severe drought and anthropogenic de-watering and vice versa.
5. The absence of emigration by salmon from shallow areas during acute drought can be reconciled with fitness optimization theory if, when natural drought progresses, fish become stranded in shallow areas some considerable time before the point at which they die.