At this time of the year the crayfish traps return very few crayfish, no doubt a reflection on the low temperatures and a period of semi-dormancy in these damaging crustaceans. The other day, for example, there were just two crayfish, one in each of the two traps. Also in one of the traps were three fish, a Three-spined Stickleback and two Bullheads.
Despite their unattractive appearance, dorsally flattened and with large head and mouth, I have a soft spot for these fish. This stems from childhood days spent catching them by hand in the local streams around where I grew up. As a child I knew them by the name of ‘Miller’s Thumb’, a local name which references the similarity of their appearance to that of the flattened, swollen thumb of a corn miller, caused by the countless testing of flour texture during the milling process. I had always assumed that this was a common and widespread fish but it turns out that this is no longer the case and it has been flagged as having an unfavourable conservation status across its European range, which extends from the edge of the Arctic Circle south to southern France.
What the Bullhead lacks in physical beauty it more than makes up for in terms of interesting behaviour. Not only can it survive in waters chilled to very low temperatures, hence reaching the Arctic Circle, it has most unusual breeding behaviour. The males are strongly territorial, defending a ‘nest’ that is typically a hollow placed under a stone or amid some woody debris, and look after the eggs and resulting young. The males produce acoustic ‘knocking’ sounds that are used to advertise ownership of their territory and, quite possibly, to attract a suitable mate. Research suggests that females can determine the size (and hence suitability) of a male by the nature of his sounds. Larger males are preferred because they are better able to defend the nest hollow against intruders and because they can cope better with the rigours of single parenthood, which can see males lose 20% of their body weight.
Favoured males may attract multiple females, each adding her load of sticky eggs to the egg mass stuck to the roof of the nest hollow. Females inhabiting lowland streams may spawn several times each year, while those in colder upland streams may just manage the one. Then it is over to the male to protect the eggs and see that they develop, defending them from other males and predators like trout and Signal Crayfish. It is great to see them in this section of the river but I do wonder how much damage the introduced crayfish have done to their population.