Spatial and temporal patterns in the density of largemouth bass and bluegill in pond-associated lotic habitats

MIYATA Hiroshi, KUNIMOTO Masahirio, INOUE Mikio

doi:10.3825/ece.10.117

Exotic largemouth bass (Micropterus salmoides) and bluegill (Lepomis macrochirus), both of which were introduced from North America, are now widespread throughout Japan, often dominating in irrigation ponds and reservoirs. The rapid expansions of their distributions beyond watersheds are obviously due to human agency; however, their expansions within a watershed may be, more or less, attributable to natural dispersal, which has rarely been investigated. This study presents circumstantial evidence of their dispersal from artificial ponds to irrigation ditches. In the Shigenobu River watershed (Ehime Prefecture, southwestern Japan), 21 study sites were established in river pools and irrigation ditches associated with 6 ponds inhabited by the two species. In each site, population densities of both species were monitored from the spring of 2002 to the autumn of 2003. The monitoring showed that (1) both species attained high densities (maximum density, 8.0 and 12.1 individuals/m² for largemouth bass and bluegill, respectively) in ditches just downstream of ponds in summer, (2) most of them were small in size (<10 cm in fork length), (3) at every study site, their densities were temporarily variable and zero densities were often recorded (i.e., neither of the two species was found), and (4) the two exotic species comprised considerable percentages of fish assemblages in the ditches (41.9 % in the sum total of estimated number of fishes). These results, in combination with a fact that most study ditches had potential barriers to upstream migration (i.e., falls of 0.8-2m in height), suggest that individuals of both species in the ditches were immigrants from the ponds.

Spatial and temporal patterns in the density of largemouth bass and bluegill in pond-associated lotic habitats

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