The Urban Waste Water Treatment Directive: Monitoring the vendace populations of Bassenthwaite Lake and Derwent Water, 2006

1. The vendace (Coregonus albula) populations of Bassenthwaite Lake and Derwent Water were monitored by hydroacoustics and gill netting in 2006. This constituted the twelfth year of such monitoring at Bassenthwaite Lake, and the ninth year at Derwent Water. 2. In Bassenthwaite Lake, the population density of all fish increased from a geometric mean of 4.9 fish ha-1 (lower and upper 95% confidence limits of 1.8 and 13.3 fish ha-1) in May to 1158.4 fish ha-1 (lower and upper 95% confidence limits of 747.8 and 1794.3 fish ha-1) in September. However, the complete absence of vendace from gill-net samples (see below) suggested that few, if any, of these fish are likely to be vendace and thus the population may be close to, or even at , local extinction. 3. In Bassenthwaite Lake, gill netting produced a sample of 513 fish comprising 357 perch (Perca fluviatilis), 23 roach (Rutilus rutilus), 131 ruffe (Gymnocephalus cernuus) and 2 brown trout (Salmo trutta). Introduced roach and particularly ruffe thus continue to be major components of the fish communities of most sites within the lake, although introduced dace (Leuciscus leuciscus) have not increased to the same extent and were not recorded in the present survey. 4. The status of the vendace population of Bassenthwaite Lake in 2006 is considered to be at best extremely poor and, at worst, the population may even be extinct. 5. In Derwent Water, the population density of all fish decreased from a geometric mean of 98.0 fish ha-1 (lower and upper 95% confidence limits of 47.9 and 200.5 fish ha-1) in May to 54.9 fish ha-1 (lower and upper 95% confidence limits of 33.1 and 90.9 fish ha-1) in September. The composition of gill-net samples suggested that tentative equivalent figures specific to post-juvenile vendace were 27.3 fish ha-1 (lower and upper 95% confidence limits of 10.1 and 73.6 fish ha-1) in May and 15.1 fish ha-1 (lower and upper 95% confidence limits of 7.8 and 29.3 fish ha-1) in September. 6. In Derwent Water, gill netting produced a sample of 201 fish comprising 87 perch, 3 pike (Esox lucius), 45 roach, 57 ruffe, 1 brown trout and 8 vendace. Introduced roach and ruffe thus continue to be important components of the fish communities of most sites within the lake. Introduced dace have not increased to the same extent and were not recorded in the present survey. Biological parameters of the vendace population have remained relatively stable in recent years, with evidence of continued recruitment and recovery from a 2003 low point in the normal variation in population density exhibited by this species. 7. The status of the vendace population of Derwent Water in 2006 is considered to be acceptable, although there is concern over observed increases in the abundance of introduced populations of roach and ruffe. A potential threat from introduced New Zealand pygmy weed (Crassula helmsii) first raised in 2004 appears to have reduced.