Composition of Commercial Gillnet Catches in a Groundwater-Fed Lake: Mesh-Specific Patterns and High Representation of Non-Indigenous Carassius gibelio

Small inland lakes support substantial freshwater biodiversity, but independent fish surveys may be constrained by specialised equipment, trained personnel, seasonal replication, and sustained funding. This study evaluated whether repeated observations of routine commercial gillnet catches could characterise the gillnet-catchable fish subcommunity of Çaltıcak (Taşkısığı) Lake, a shallow, approximately 90-ha groundwater-fed lake in Sakarya Province, Türkiye, and support surveillance of the non-indigenous Carassius gibelio. Fish were obtained over 17 sampling nights from 12 February to 3 June 2026 using five separate single-mesh monofilament gillnets (40, 50, 60, 75, and 80 mm mesh-bar length). Total design-based nominal mounted-area effort was 22,451.7086 m2-night, and catch was expressed as area-standardised numerical and biomass catch per unit effort (NPUE and BPUE). A total of 863 individuals from eight species were examined. Blicca bjoerkna, C. gibelio, and Scardinius erythrophthalmus were the most numerically represented species. C. gibelio constituted 26.42% of the examined catch and had a strongly female-biased catch composition (95.2% female). Smaller-bodied cyprinids occurred mainly in 40–50 mm meshes, whereas Cyprinus carpio and Silurus glanis occurred mainly in 80 and 60 mm meshes, respectively; Esox lucius was an exception, occurring mainly in 40–50 mm meshes. The combined catch corresponded to 0.038438 individuals m−2 night−1 and 0.015831 kg m−2 night−1. Approximate overnight soak duration varied seasonally from 9 to 16 h, but exact set and retrieval clock times were not recorded. Deployment coordinates were also not recorded, and sampling was confined to routine commercial fishing zones during February–June. Accordingly, the findings apply only to the gillnet-catchable fish subcommunity and must not be interpreted as a lake-wide fish-community or stock estimate. Repeated monitoring with georeferenced deployment and exact effort records would strengthen this catch-based baseline. © 2026 by the author.