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.

中文翻译：

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秋季严重干旱期间水库中水獭的饮食变化

众所周知，欧亚水獭 (Lutra lutra) 广泛利用伊比利亚半岛的水库，在那里它们优先捕食小型鱼类（10-20 厘米）。我们假设水獭在水库中通常食用小型鱼不是由于对猎物大小的偏好，而是由于捕获更大猎物的难度更高。我们在一个水库中研究了水獭的饮食，该水库因异常严重的干旱而导致储存的水量突然下降。我们比较了干旱年（2017 年）和两个标准降雨年（2015 年和 2016 年）之间的相对猎物大小、狩猎成功率和潜水时间。在干旱期间，水獭吃的小鱼和大鱼的比例相似，而不是主要吃小鱼。干旱期间的平均潜水时间与标准气候年相似，表明小鱼和大鱼之间的捕获生理成本相似。无论猎物大小如何，水獭在干旱年份（89%）的狩猎成功率都高于标准年份（63%）。这表明在干旱期间，由于水位较低，两种鱼的捕捞能力水平较高。结果表明，水库中水獭对小型鱼的通常消费与偏好或相对丰度无关，而是与水位高时捕获大型鱼的难度有关。