Living in the edge: the fate of individually marked pike (Esox lucius) stocked in a hyper-eutrophic lake with frequent winter hypoxia

摘要

Eutrophication of lakes has increased the risk of fish kills during winter under ice cover. If such a lake is isolated, the fish assemblage consists of only oxygen-tolerant species without predatory species. Here, our main aim was to analyze the fate of 99 marked pike (Esox lucius) stocked to a pikeless lake that is known to have low oxygen concentrations under ice cover. These pike (total lengths 33.5-76.9 cm; weights 260-3050 g) were stocked in May 2008 and were followed 7years. The first marked pike was caught in August 2008 and the last one in September 2012. Between these dates, 21 pike were caught at least once, five pike were caught twice, and two pike were caught three times. Relative condition was based on the length-weight relationship. Both length and weight affected significantly the recapture probability, whereas relative condition had no effect. The growth of marked pike was estimated based on length or weight increments in two separate models with either starting length or weight together with the number of days between observations. In these models, the number of days had positive, and size, either length or weight, had a negative relationship with growth. According to marginal r(2), length and the number of days explained 65.5% of variation in length increments, whereas the marginal r(2) for weight and the number of days was 49.3% for weight increments. The length increment model was used to compare pike growth in Lake Savijarvi with other Finnish lakes. The comparison showed that during the first four growing seasons after stocking, the pike growth in Lake Savijarvi was faster than in the other lakes studied.