Did a drastic change in fish species from kokanee to pond smelt decrease the secchi disc transparency in the oligotrophic Lake Towada, Japan?
Takamura, Noriko; Mikami, Hajime; Mizutani, Hitoshi; Nagasaki, Katsuyasu
published: Mar 5, 1999
ArtNo. ESP141014403005, Price: 29.00 €
This study reports on a recent apparent species replacement of kokanee by pond smelt in Lake Towada, and attempts to determine whether this shift in fish community dominance is also responsible for parallel shifts in the zooplankton community structure and water clarity of this lake. Principle component analysis of 18 years data for the fish catch, zooplankton density and transparency showed that the total annual catch of kokanee, and the densities of Acanthodiaptomus pacificus and Daphnia longispina as well as the transparency contributed positively to the first principal component, whereas the total annual catch of pond smelt and the densities of Bosmina longirostris, Keratella quadrata and Filinia terminalis showed negative values on it. Therefore the replacement of kokanee by pond smelt caused a shift of zooplankton community structure from a Daphnia-Acanthodiaptomus/i> community to a Bosmina-rotifer community, which probably led to a decrease of secchi disc transparency. Lake TP and TN concentrations did not increase during the time water clarity decreased, suggesting increased nutrient loading was not the cause of the decline in water clarity. The kokanee showed a feeding shift from D. longispina and A. pacificus to chironomids, amphipods, terrestrial insects and fish under the dominance of B. longirostris and rotifers, when both the total annual catch of kokanee and the annual mean weight of female kokanee were low. Therefore these foods were insufficient to allow the kokanee to grow, while the pond smelt were able to eat B. longirostris as well as D. longispina and A. pacificus. The collapse of the kokanee catch and the increase of pond smelt after the introduction might have been due to both the competition of the two fish for the same food resources and their slightly different feeding habits.