Long-term responses of a stream-dwelling grayling population torestrictive fishing regulations

摘要

We used snorkel surveys and catch data to assess long-term effects of a catch and release (C and R) fishery imposed in 1989 on a population of stream- dwelling European grayling (Thymallus thymallus L.) in northern Sweden. From 1991 and onwards, catch (fish size 30 cm) increased from less than two fish per day to more than seven in year 1999. A parallel increase in the grayling population size was indicated in snorkel count survey data from 1996 to 2000. The number of trophy-sized grayling (>50 cm) in the catch data increased dramatically between 1994 and 1997 and was still increasing in 2001. However, according to the snorkel surveys, recruitment declined from 1998 and onwards, resulting in a decrease in the total catch in 2000 and 2001. According to seasonal snorkel surveys and tagging data, the grayling population showed residency in summer but extensive movements between seasons. We argue that it might take up to 10 years or even longer until the full effects of catch and release regulations could be observed and that such populations of grayling must be managed in a water system perspective including also winter habitats.