The influence of population density on growth of whitefish ( Coregonus lavaretus L.) in four prealpine lakes

Abstract

In the early 1990s whitefish ( Coregonus lavaretus L.) of Ammersee, a prealpine bavarian lake, showed extremely poor growth. The average weight of 3+ whitefish was lower than 150 g in 1994. But in 1994 the trend changed. Growth of whitefish doubled between 1994 and 1999. The average stomach contents also showed a significant increase in the course of recent years. Parallel to the weight increase a distinct decrease in population density could be seen, monitored with a SIMRAD EY 500 echo sounder. In the first half of 1999, there were about 350 fish per hectar (F ha −1) in Ammersee. That was only about a fifth of the number at the end of 1995. In 1996/97 fish density in a nearby prealpine lake, oligotrophic Walchensee, was clearly lower with fewer than 350 F ha −1 than in Ammersee (mean value 1996: approx. 850 F ha −1, 1997: approx. 770 F ha −1). Despite the lower trophic status, Walchensee whitefish showed clearly better growth: the average weight of 3+ whitefish was 308 g. At the same time the weight of 3+ whitefish of prealpine Kochelsee was 260 g on average. There was an extended food resource for Kochelsee whitefish with the additional possibility of feeding on benthic prey. In comparison to Ammersee this could explain the significantly better growth of Kochelsee whitefish despite an even higher fish density (approx. 1300 F ha −1 in 1996/97). Chiemsee, another prealpine lake, is very similar to Ammersee in relation to fundamental limnological parameters and was studied in 1993 and 1994. In 1994 fish density of Chiemsee was at a maximum of approx. a fifth of that of Ammersee, and whitefish growth was significantly better. 3+ whitefish weighed above 300 g, about double the weight of Ammersee whitefish at that time. A comparison of mean stomach contents of whitefish in both lakes points to intraspecific food competition in Ammersee. Those results indicate a causal connection between whitefish density and growth.