Distribution of Carassius, Cyprinus, and Gnathopogon eggs in a natural reed zone of Lake Biwa

The distribution of cyprinid eggs was investigated in a natural reed zone of Lake Biwa, Japan, through the field collection of eggs and DNA identification. In total, 304 eggs (15 egg populations) were collected on 11 May, 2018, from 3 vegetation areas in a 80-m-long, 20-m-wide survey strip: 53 eggs (two egg populations) from the farthest offshore area dominated by willow trees (Area A); 170 eggs (8 egg populations) from the most inland area dominated by emergent plants, such as reeds (Area B); and 81 eggs (5 egg populations) from an open water area with floating herbaceous islands (Area C). DNA genus identification of 125 randomly selected eggs, with approximately 8 eggs per egg population, identified 108 Carassius, 15 Cyprinus, and 2 Gnathopogon eggs: eggs of the last 2 genera were obtained only from Area A. Subsequent DNA species/subspecies identification of the 108 Carassius eggs (obtained only from Areas B and C) identified 53 eggs as Nigoro-buna (Carassius buergeri grandoculis), 21 as Gengoro-buna (Carassius cuvieri), 2 as Gin-buna (Carassius sp.), and the remaining 32 eggs as non-Gengoro-buna (the analysis failed to distinguish between Nigoro-buna and Gin-buna). The ratio of Gengoro-buna to all Carassius eggs was statistically higher in Area C than in Area B. Cluster and non-metric multidimensional scaling (NMDS) analyses revealed that the egg populations were divided into three groups based on the similarity of species composition, with Groups 1, 2, and 3 characterized by Cyprinus, Nigoro-buna+Gin-buna, and Gengoro-buna, respectively. Groups 1, 2, and 3, corresponded well to Areas A, B, and C in terms of collection locations. Pairwise permutational multivariate analysis of variance (PERMANOVA) among vegetation areas revealed that the species composition of egg populations differed significantly between Areas B and C where Carassius eggs were collected. These results indicate that the main spawning cyprinid fishes differ depending on the vegetation area. The locations where many Nigoro-buna+Gin-buna (probably mostly Nigoro-buna) eggs were collected tended to have low dissolved oxygen (DO) and pH values.