The influence of introduced ponto–caspian scuds and opossum shrimps on perch (perca fluviatilis) diet and growth rate in the lakes of lithuania

SUMMARY The influence of the introduced Ponto-Caspian scuds and opossum shrimps on perch (Perca fluviatilis) diet and growth rate in the lakes of Lithuania The purpose of this study was to examine perch (Perca fluviatilis) feeding habits and diet differences between two types of lakes that differ in having and lacking abundant littoral populations of introduced Ponto-Caspian opossum shrimps and scuds. The growth of the perch has been performed in order to answer whether introduced crustaceans really improve fish food basis in lakes. 1104 individuals were used to perform their growth rate analysis. 861 individuals were used to clear their diet. Fish and littoral benthic animal’s samples were collected in 2004 – 2006 summer and fall seasons in 10 Lithuanian lakes (Dusia, Daugai, Asveja, Žeimenys, Šakarvų, Baluošai, Nedingis, Lavysas, Plateliai and Beržoras). Analyses show that perches consume introduced Ponto-Caspian crustaceans from their first year. Kruskal-Walis ANOVA test show significant general lake effect on consuming rate of both scuds and opossum shrimps when comparing between three types of lakes that differ in littoral scuds and opossum shrimps population size. There was bigger consuming rate in lakes with larger crustacean’s populations in four different age groups of perch. We found significant correlations between relative biomass of littoral scuds in the benthic communities and in the perch stomach in three different age groups of perch. However we failed to find this correlation in third year old age group of perch. Multiple regressions showed that age of perch (A) and relative biomasses of littoral scuds in the benthic communities (BB) have significant influence on relative biomass of scuds in the perch stomach (BP): BP = 0,42005 BB + 3,97621 A – 5,33209. We failed to find significant correlation between benthic animal’s diversity index and feeding niche breadth of perch in studied lakes. There were no significant differences of feeding niche breadth of perch in three lake groups having or lacking abundant populations of introduced crustaceans in different age of perch also. We found that perch feeding niche breadth dependent from perch age. Perch age has significant influence on perch feeding breadth (two-way ANOVA p < 0,05). However, there were no significant differences in feeding niche breadth between different age groups of perch in studied lakes. The feeding selectivity of young perch showed that those lakes that are settled by abundant populations of introduced crustaceans were not similar. We found significant difference in first three years growth of perch between Daugai and Baluošai lakes. These lakes are similar by their abiotic conditions but separate in having and lacking abundant populations of introduced Ponto-Caspian crustaceans. The better growth was in Baluošai Lake that lack abundant littoral scuds and opossum shrimps populations. Thus there was no evidence to confirm that large populations of introduced crustaceans have positive effect on juvenile perch growth rate. In conclusion, our analysis shows that abundant introduced crustacean’s populations do not significant changing feeding niche breadth and feeding selectivity of young perch. Moreover these abundant invasive populations have no positive effect on young perch growth rate