2014 Volume 8 Pages 1-15
In lake Izunuma-Uchinuma where invasive largemouth bass have caused serious environmental problems, various removal methods of the bass have been developed and introduced to control the bass population. The preliminary investigation revealed that abundant recruited bass fry showed diet shifts to piscivory at a small size (>20 mm FL (fork length)) and preyed vigorously on larvae of indigenous fish species. Therefore, it is important to prevent the reproduction of largemouth bass in order to conserve the indigenous fish fauna. To prevent the reproduction of bass, we were focused on the development of artificial spawning nest (ASN) that were able to be eliminated with the eggs of largemouth bass after their spawning in the nest was complete. Various prototypes of ASNs were devised and tested in the lake. In the tests, largemouth bass spawned most frequently in a type of ASN that was a cluster of gravel on a plastic tray surrounded by net screens on three lateral sides. Using this type of ASN, the control project of the largemouth bass population reduction in the lake started with volunteers in 2004. During this project, we needed to eliminate the eggs on the ASNs frequently (twice a week), because the spawned eggs in the ASN hatch and drop off a few days after the spawning. Under natural condition, hatched largemouth bass stay in a substrate for several days until they swim up into the water column. Then, we developed an improved ASN which could stock the hatched larvae of the bass in it. Another tray with fine mesh was attached to the bottom of the ASN and an attachment of a mesh tray under the ASN made it possible to stock the hatched bass for more than a week in the ASN. Therefore, the introduction of the improved ASN (double tray type ASN) reduced the collecting work to once a week and increased the number of eliminated eggs. Moreover, three additional tests were conducted to develop maintenance-free types of ASN. However, no successful result was obtained from these three test types of ASNs. Therefore, the double trays type of ASN is better in terms of cost and benefit. The double tray type of ASN has been adopted for the control project in the lake at the present.
