The spatiotemporal dynamics of invasive three-spined sticklebacks in a large, deep lake and possible options for stock reduction

Abstract

A combination of fishing and surveying methods (trawling, gillnetting, electrofishing and hydroacoustics) were used to obtain insight into the spatiotemporal dynamics of invasive three-spined sticklebacks (Gasterosteus aculeatus) in the pelagic zone of Upper Lake Constance. The resulting information is highly pertinent to the development of management strategies to reduce the negative impact of sticklebacks on the pelagic ecosystem of the lake, and especially on the commercially important native pelagic whitefish (Coregonus wartmanni). The results indicate that sticklebacks are very mobile and opportunistic with respect to habitat selection, making extensive shifts between benthic and pelagic habitats. The greatest abundances of sticklebacks in the pelagic zone were identified in late summer, shortly after their spawning, when densities exceeding 10,000 individuals per hectare were recorded. A second peak occurred in winter, during the spawning season of pelagic whitefish. Data from gillnetting indicate high stickleback densities in the littoral zone of the lake during the spawning season of benthic whitefish (Coregonus macrophthalmus). Thus, damaging predation of pelagic and benthic whitefish eggs and larvae by sticklebacks seems very likely. Electrofishing surveys in tributary rivers revealed that these were used irregularly as migration routes and are therefore unlikely to represent a significant habitat for stickleback reproduction. Depth distribution of sticklebacks in the pelagic zone changed throughout the year with highest densities at depths of 9–12 m in spring, summer and autumn, while in winter densities were highest at 15–18 m. The most efficient option for capturing sticklebacks is trawling during September, at depths of 9−12 m, where very large catches were recorded without significant bycatch of non-target species. This method of control would require special fishing vessels not currently deployed on the lake and its economic feasibility will require careful assessment.