Bullhead bonanza (or not, as it turns out)

My biology class at school was always a little dull. We never really went on field trips, unless it was for a vitally important piece of coursework of course, but generally found ourselves stuck inside a large beige laboratory. So imagine my surprise when my younger sister bursts in exclaiming that her class had been out stream dipping on a stream I had never heard of, and that most importantly, they had caught some bullheads, miller’s thumbs, Cottus gobio, whatever you want to call them. This is our only freshwater sculpin, (a cottid as was pointed out earlier today by an anonymous reader) and it was not a species I had seen before. I set off the next day armed with a net, buckets and a friend to help catch them.

It took us a few minutes to find where they were hiding, and after a while we had our first bullhead. Admittedly, it was only about 10mm long, but still, it was a bullhead. More searching revealed a couple of juveniles, around 40mm long. But then we stumbled along a dead individual. And another. And then another. This was strange. Bullheads are long lived for small fish, and the chances of seeing these dead specimens all dying from old age was disproven due to the huge variation in size, from babies right up to huge 120mm adult males. By the end of the day, we had caught 6 bullheads of a range of sizes and seen others alive (plus a few young 3-spined sticklebacks and an adult female), but seen more dead ones in a state of decay. We found no dead sticklebacks, which is odd, and anyone thinking that my sisters class from the other day were to blame, they can’t have been because we went quite far upstream to catch ours whereas they went downstream.

We also found a putrid dead badger upstream with a huge pile of maggots on top. I chose not to tell my sister this...

I went down there again today briefly just to get some photos, and saw four dead specimens, and 3 live, one of which was certainly the large adult we caught last time. Does anyone have any ideas what could have caused this?