I work with and manage trees for a living and in all seriousness this is an horrible pest that has come over from abroad.
As has already been said this time of year the caterpillar thrives on oak tree leaves and foliage, they have loads of small hairs that can easily break off and become airborn, these can cause serious irrtation to the skin and breathing problem for people with asthma. If you are fishing and suspect that this pest is around i would report it to the fishery owner.
There is also a simliar pest called the brown tail moth, which again causes skin irritation and they can be easily identified by silken woven tents on branches etc, with the caterpillar inside.
one bit of adive DONT touch these caterpillars stay well away.
If anyone wants more info then PM me.