This is the summary report referred to in my last post which you cant access directly it seems. Are angler baits and invasive crayfish important components of barbel diet in English rivers? Study completed at Queen Mary University of London and Bournemouth University. The aim of the study was to assess how angler baits - primarily fishmeal-based pellets - and invasive signal crayfish influence the diet composition of barbel in English rivers. Previous studies have suggested that barbel, whilst being omnivorous, mainly feed on benthic macro-invertebrates, with small fish sometimes taken. It was completed on four rivers: the Rivers Kennet and Lee, where signal crayfish were present, and the Rivers Teme and Hampshire Avon, where they are largely absent (or at least in very low abundance). Fieldwork was completed in summer 2013, when samples of macro-invertebrates and crayfish were collected from each river (as appropriate), along with scales from angler-caught barbel. To assess diet composition of barbel, stomach contents analysis was not used. This was because the method tends to be destructive to the fish and can require large sample sizes in order for patterns to emerge. Instead, a method known as stable isotope analysis was used. This uses material, such as fish muscle, fin clips and/ or scales within mass spectrometry to determine their ratios of 12C:13C and 14N:15N. Allied with the same data from their likely food resources (i.e. macro-invertebrates, fish-meal pellets, small fish, crayfish) then statistical models can predict the likely diet composition of the barbel, with associated levels of confidence, on the basis of the differences in the isotopic values between the species. It is an increasingly common method used in ecology. All of the barbel analysed were 42 to 80 cm in length, i.e. no small barbel were used. In general, fishmeal pellets generally made a large contribution to their diet (up to 59 % in each river). In the Kennet and Lee, crayfish were also an important resource (up to 30 % of population diet). Importantly, the proportion of diet derived from macro-invertebrates (excluding crayfish) was substantially lower. Further modelling revealed considerable variability in barbel diet within each river. In the Avon and Teme, where there were no crayfish, up to 79 % of barbel diet was based on pellets. In the Kennet and Lee, where crayfish were present, they effectively replaced and so reduced the contribution of pellets to individual fish diet. In summary, the evidence from the stable isotope analyses revealed that from three of the four rivers (Teme, Kennet, Avon), the barbel were heavily reliant (>50%) upon angler-introduced baits, with the River Lee showing that invasive crayfish can also be an important dietary component.
Posted on: Sun, 26 Oct 2014 19:24:44 +0000
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