University of Guilan Caspian Journal of Environmental Sciences 1735-3033 24 3 2026 07 01 Habitat use, seasonal distribution, and dietary ecology of the Eurasian otter, Lutra lutra in Shoulam River, Guilan Province, Iran 893 903 9598 10.22124/cjes.2026.9598 EN Mohammad-Javad Adati Shoulami Department of Environmental Sciences, Natural Resources Faculty, University of Guilan, Iran Saeid Naderi Department of Environmental Sciences, Natural Resources Faculty, University of Guilan, Iran Amir Zadnemat Department of Environmental Sciences, Natural Resources Faculty, University of Guilan, Iran Saeid Eshagh Nimvari Department of Environmental Sciences, Natural Resources Faculty, University of Guilan, Iran Journal Article 2026 05 11 The Eurasian otter, <em>Lutra lutra</em> serves as a keystone species and apex predator in freshwater ecosystems, playing a vital role in ecological stability. However, local populations face increasing threats from habitat fragmentation and human-wildlife conflicts. This study investigated the presence, seasonal distribution patterns, habitat preferences, and dietary habits of the Eurasian otter along Shoulam River in Guilan Province, Iran, over a one-year period (2021–2022). Given the species’ elusive nature, indirect field surveys were conducted to record presence signs. A total of 254 signs (111 spraints, 141 footprints, and 2 smears) were identified across 149 geographical points. Habitat selection was evaluated using 11 environmental variables, and diet was assessed through spraint analysis. Data were analyzed using non-parametric tests and Principal Component Analysis (PCA). The Kruskal-Wallis test indicated significant seasonal variations in the otter activity (<em>p</em> < 0.001). A distinct divergence emerged between spatial occurrence and marking intensity: while the widest spatial distribution was recorded in autumn, the highest frequency of marking signs occurred in winter (87 signs) and the lowest in summer (41 signs). PCA extracted five components explaining 68.03% of the variance, showing a significant preference for habitats with mature tree cover, low understory density, finer substrates (particle size < 1 cm, such as sand and mud), water depth between 0.5–1 m, and low anthropogenic litter. Diet analysis yielded 487 prey items, with fish being the dominant prey category (Feeding Index [FI] = 0.414), followed by birds (FI = 0.244). The statistical association with fine riverbed substrates indicates that marking behavior is closely linked to foraging grounds, where otters utilize available muddy or sandy banks for sprainting while hunting. The study reveals a high degree of dietary plasticity and opportunistic foraging behavior, characterized by seasonal shifts such as increased amphibian consumption in spring. The discrepancy between spatial spread in autumn and intensive marking in winter suggests a behavioral shift from exploration to territorial defense. Conservation strategies in the Caspian basin should prioritize the protection of riparian vegetation corridors and the mitigation of anthropogenic disturbances to ensure the survival of this near-threatened species.<br /><strong> </strong> https://cjes.guilan.ac.ir/article_9598_07ef0b39dbc25565e671db5ae691b701.pdf