Effects of artificial structures on fish fauna in small streams running through a rural area of Japan

Abstract

Small streams in rural areas are commonly known as habitats for freshwater fish, but have lacked attention from conservation sectors and local governments. We established 30 survey points in three tributaries (Kawamata, Ogawa, and Ujie) of a medium-sized river, the Koise River, that flows into Lake Kasumigaura, Ibaraki Prefecture, Japan. We captured fish using an electrofisher and recorded environmental features from 2010 to 2011, in winter, spring, summer, and autumn, once for each. A total of 119 surveys caught 27,086 fish of 18 species. Of these, Rhodeus ocellatus ocellatus, Gnathopogon elongatus, and Nipponocypris spp. were alien species. Conversely, six species, Lethenteron reissneri, Tachysurus tokiensis, Oryzias latipes, Tanakia lanceolata, Misgurnus anguillicaudatus, and Cotton pollux, were species of conservation concern listed on the Red List of the Ministry of the Environment, Japan. There were five seasonal weirs, which retain water only during the irrigation period, and four fixed weirs, which do not change shape throughout the year, in Kawamata; additionally, there were two seasonal weirs in Ogawa, and one seasonal and eight fixed weirs in Ujie. The density of weirs, the ratio of concrete bottoms, and the vegetation coverage all indicated that Ogawa had the best natural conditions and Ujie the worst. The number of species per survey and the Shannon-Wiener index of diversity for each survey point were highest in Ogawa, and lowest in Ujie. The numbers of Opsariichthys platypus, G. elongatus, T. tokiensis, and Pseudorasbora parva captured per survey were higher at survey points where there was no weir on the close downstream side than at those where there was a weir, especially a fixed weir. These results suggest that cross-river artificial structures, especially fixed weirs, have a negative impact on many freshwater fish species. When employing weirs to utilise water from small streams for irrigation in plains, their negative impacts on freshwater fish should be reduced by using, for example, a seasonal weir with a small gap.