Popular Science: Otters vs Anglers: Do they catch fish of the same species and sizes?
The Eurasian otter (Lutra lutra) is one of the most important fish consumers in European freshwater ecosystems. Their numbers dramatically declined during the 20th century, mainly due to water pollution, loss of their natural habitat and poaching. Luckily, the otter population has been growing again in the last few decades, which brings conflicts between anglers on one side and the ecologists and society on the other. The anglers claim that the otters cause major losses to their farmed and stocked fish, ecologists point out the importance of otters at the top of the aquatic ecosystem hierarchy and the rest of us simply like otters for their cute and amicable appearance.
This conflict between otters and anglers was tackled by Roman Lyach and Martin Čech from the Institute for Environmental Studies of our faculty. They followed the excrement tracks of one to three otters along the Chotýšanka stream in Central Bohemia and analysed their diet based on the collected samples. At the same time, they gained information about the catches of anglers and numbers of stocked salmonids in Chotýšanka in previous months.
The species of fish, amphibians and crayfish were identified from the otter excrements based on typical traits in their bone fragments, scales and other structures and the length and weight of the consumed fish was calculated according to the length of the bones. The results showed that fish formed 85% of otter diet, with the gudgeon Gobio gobio being the most frequent prey. The majority of eaten fish was smaller than 10 cm, weighed less than 10 grams and only 10% of the consumed fish was of higher commercial value for the anglers. The average weight of the fish caught by the anglers, on the other hand, was 290 grams, with anglers also preferring other species. Otters targeted stocked salmonids far less frequently than was expected.
It was shown that the differences between the catches of otters and anglers are high. The otters mostly target different species of fish of lower weight than anglers prefer, even though sometimes they even catch salmonids of higher fishing value. Similar research in the future on different types of streams with stocked fish might help broaden the knowledge in the otters vs anglers conflict.