Fry on fry predation

Today we completed some more surveys in sites stocked last year. We did three sites in the Burn of Aldernie (Maggieknockater Burn) the lower of which was below the A95 culvert and accessible to migratory fish. Here the density of fry was high, mainly trout fry (193 in a site 20m long by 3.2m wide) but there were also quite a few salmon fry (29 in the same site). As I was processing the catch I noted one of the trout fry had a bulge in its stomach and something protruding from its mouth. Thinking it was a parasite I put it back into the bucket whilst measuring the rest of the fish. There was a huge range in the size of the trout fry from 27mm to over 60mm . On picking up the bulging fry I managed to pull the protruding item from its mouth. It turned out to be a smaller trout fry.

48mm trout fry with half digested 27mm trout fry removed from is gullet. On realising it was a meal I felt a bit of remorse for the greedy larger fry, Too bad for the little guy but the density of fry on this site will reduce much further before the end of the year.

48mm trout fry with half digested 27mm trout fry removed from is gullet. On realising it was a meal that I had just removed I felt a bit of remorse for the greedy fellow.  Too bad for the little guy but the density of fry on this site will reduce much further before the end of the year. The prey length was more than 50% of the predator length, although only a fraction of its weight. Some of the smaller trout fry in the site were in poor condition with low body weight and ragged fins; many of these will suffer a similar fate to the one pictured above. These fry aren’t wasted, the nutrients will help nourish the more dominant faster growing fish. With such a high density of fish the competition is intense with only the fittest surviving. That doesn’t always mean the biggest, some small fish will survive in the marginal habitat during the first year before putting on a growth spurt in the second year.

 

 

The lower Burn of Aldernie survey site, more suited with f than parr with shallow depths.

The lower Burn of Aldernie survey site, more suited to fry than parr with shallow depths.

 

Over 230 fish in this bucket; the catch from 20m of the Burn of Aldernie

Over 230 fish in this bucket; the catch from 20m of the Burn of Aldernie. Experience from many three run surveys shows that only 50-60% of the fry are typically captured in the first run; there were probably as many fry again still in the survey site.

 

 

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