Sampling methods of the wels catfish (Silurus glanis) in freshwater lakes : management of a vulnerable species
Sammanfattning: Sweden hosts the northernmost natural population of wels catfish (Silurus glanis) in the world. The populations of the species decreased severely in the 20th century, mainly due to different sources of anthropogenic impact. In recent years, efforts have been made to rein-troduce the species to previous inhabited areas, restore degraded habitats and to increase the low genetic variation among the populations. Studies and general recommendations concerning sampling techniques and monitoring of S. glanis in temperate lakes are lacking, especially concerning conservation measures. In the current study, the species has been sampled with acoustic camera and electro-fishing, and the results are compared with re-sults from the standardized fishing with fyke nets in the lake. A has also been reviewed to account for other methods that might be viable for sampling S.glanis. Fyke nets sampled lower numbers of catfish of intermediate size, compared to the fish ob-served by the sonar, which were both more numerous and larger. The acoustic camera was found to be efficient for detecting adult catfish in their natural habitat, with the possible ap-plication of making population approximations. The fyke nets sampled few individuas for the given effort, but are still the best available sampling method that provides sampled cat-fish physically. The electrofishing boat did not manage to sample a single catfish and the method was found to be inefficient in the habitat of this lake. Juveniles of S. glanis were underestimated in the samples caught by the gear used in this study, and no larvae were sampled. According to literature, light traps and minnow traps do have some promising properties for sampling these individuals, and future studies should address their viability.
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