Eurasian otters (Lutra lutra) are known to make extensive use of reservoirs in the Iberian Peninsula, where they forage preferentially on small-size fish (10–20 cm). We hypothesized that the usual consumption of small-size fish by otters in reservoirs is not due to prey size preference, but rather to a higher level of difficulty in capturing larger prey. We studied otter diet in a reservoir that experienced an abrupt drop in stored water caused by an unusually severe drought. We compared relative prey size, hunting success, and diving times between the year of the drought (2017) and two standard rainfall years (2015 and 2016). Otters ate a similar proportion of small and large fish during the drought instead of predominantly eating small fish. Mean diving time during the drought was similar to that of the standard climatic years, indicating a similar physiological cost of capture between small and large fish. Otters had higher hunting success in the drought year (89%) than in the standard years (63%) regardless of prey size. This suggests a higher level of catchability of both fish size classes during the drought as the water level was lower. Results suggest that the usual consumption of small-sized fish by otters in reservoirs could not be related to preference or relative abundance but rather to the difficulty of capturing large-size fish when water levels are high.