Alien fish

While cutting my way through dense carr at the top of Radipole (to access and fell willows growing into and damming the river), I noticed a number of small fish left stranded by last weeks retreating flood water. I was able to identify them as sunbleak a species first identified on Radipole in the winter of 2005 when we found a number of examples stranded in the reed stubble after completing our conservation reed cut. This pre-empted a mass die off of sunbleak which washed through into Weymouth harbour.

Rather than the sinister 'pollution incident' that such a scene flag up in ones mind, further investigation revealed that sunbleak can experience 'lemming' years when the population explodes and consequently dies back en mass - presumably due to a lack of availabile food.

The previous sunbleak boom year corresponded with a bumper year for great-crested grebe breeding success and an unprecedented increase in tern foraging over Radipole.

The picture isn't all rosy though as they are not an indigenous species to these shores, (they are believed to have been released by an aquarium hobbyist in the 1980’s) and because of their rapid growth to maturity, (they only reach 10 or 11cm total length) they can out-compete the fry of native cyprinids such as the roach and rudd so critical to the biodiversity of our wetlands as they all feed on the same aquatic micro fauna.

A grim (if predictable) irony is that the sunbleak is declining in its native European range because of a parasite carried by the top mouthed gudgeon – another introduced non-native invader from Asia - just one of the myriad examples of the perils posed to native ecosystems by alien invaders.