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Bugs in the Cray

Following on from the water quality data analysis - here is some summarised information that TINTT has pulled together using data kindly supplied by the EA. These data are drawn from sites that are being considered for habitat restoration (with a possible long-term view of restoring native trout populations). First of all, it is apparent that there are examples of a number of invertebrate families that are happily living in the Cray that would not be able to exist in polluted environments. You can particularly see the good populations of Gammarus (freshwater shrimp). These shrimp are a valuable find because they have lifecycles that can take two years or more to complete. In addition, because they can't fly, they are not very good at migrating upstream over weirs to colonise new areas (i.e. they can only really re-colonise from upstream populations if they are wiped out). Taken together - their presence in healthy numbers indicates that water quality tends to be good enough to enable them to complete their long lifecycles. They are also a good "year round" supply of protein for fish.
Another interesting pattern is the obvious cycle of Blue Winged Olive (Ephemerellidae) nymphs as they grow to a size that can be captured by the 2-mm sampling net mesh (and are visible to the naked eye!) towards the summer months - and then after mass hatches in summer - decline to become invisible to the samplers during winter (when they are present as eggs or very small nymphs). The apparent decline of Rhyacophila (a predatory caseless caddis) seems to be replaced by an initial increase in the net spinning Hydropsyche (filter-feeding caddis).
Finally - for two sites that are of particular interest in terms of the potential for nearby habitat restoration - these are averaged "biotic index" scores over several years. The big bars are the mean (average) values and the little legs indicate the variability of the figures around that mean (error bars are one "Standard deviation" - which indicates the spread that would capture about 95% of all the individual values that make up the average). BMWP scores use a ranking system based upon the sensitivity of each family to organic pollution. These scores vary between 1 completely tolerant) and 10 (exceptionally sensitive to organic pollution). The score for each individual family that is present in a sample is summed to give the BMWP. The higher the score - the cleaner the water. The average score of each family (ASPT) gives an indication of how many "sensitive" families are present - the closer this value is to 10 the better (although it is never possible that this is the case - because even the cleanest sites are home to tolerant as well as sensitive bugs). Finally, the Number of Taxa is simply the number of different BMWP families that are present in the sample. Overall - the organisms that are present here do not indicate that there is a serious problem with the water quality in these sections of the Cray. They are fairly typical of a stream that is somewhat impacted by urban effluents (compared to a completely pristine rural chalkstream). However, there is no evidence that the scale of impact on the water quality would prevent the survival of salmonid fish at this stage.


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