Assessment of Fish Resources and Diversity in Dianchi Lake Following a Fishing Ban: A Combined Approach using Netting and Hydroacoustic Methods

Freshwater fish face severe threats from multiple environmental stressors and anthropogenic disturbances including overfishing, where fishing ban serve as crucial measures for fish population recovery and biodiversity conservation. Currently, the structural changes in fish communities following fishing ban remain poorly understood due to a lack of systematic monitoring data. This study assessed fish composition, spatial distribution, and diversity in Dianchi Lake one-year post-fishing ban using net fishing and hydroacoustic methods. The net fishing results show that a total of 19 fish species were collected, with only three being native species (i.e., Anabarilius alburnops, Carassius auratus, and Misgurnus anguillicaudatus), while dominant species were nonnative (e.g., Neosalanx taihuensis, Hypophthalmichthys molitrix, and Cultrichthys erythropterus). Based on hydroacoustic assessment, fish were unevenly distributed in horizontal space, with the highest densities in the northern region (Area I: 0.33 ± 0.24 ind./m3), followed by the southern region (Area III: 0.15 ± 0.24 ind./m3) and the central region (Area II: 0.09 ± 0.11 ind./m3). Vertically, highest fish abundance occurred at depths of 2–3 m (35.3%), followed by 3–4 m (27.1%). Hydroacoustic estimates revealed there were 4.57 × 108 fish individuals, with their body sizes predominantly smaller than 10 cm. The community exhibited low diversity, with indices as follows: Shannon–Wiener (H ՙ = 1.31), Margalef (Dm = 1.77), and Pielou’s evenness (J' = 0.51). The study demonstrates Dianchi’s fish community is predominantly nonnative, posing a high risk of competitive exclusion for native species. Simultaneously, marked spatial heterogeneity, severe community miniaturization and low diversity indicate weak community stability. The combined hydroacoustic and net fishing approach enables a comprehensive assessment of Dianchi’s fish community status. During fishing ban, measures should strengthen native fish stock enhancement and habitat protection while implementing targeted management of nonnative species to promote ecosystem health restoration.