Despite the importance of fish as a food supply for many birds and as an indicator of water quality, remarkably little is known about the fish of Brent Reservoir, primarily because angling is forbidden. Most of our knowledge comes from reports of two occasions (1994–95, 2001–02) when the reservoir was completely drained for engineering work and the fish were caught and temporarily stored elsewhere.
The first drain-down harvested 3,000kg of fish. This apparently suggests poor water health, since a bigger catch might have been expected. The vast bulk of the catch were Roach, and only two other species (Carp and Pike) were in numbers large enough to be deemed statistically significant. However, the catching technique meant that some small species and some young fish would have escaped netting.
The 1994–95 operation exposed the reservoir bed to the air for three months, during which heavy frost frequently occurred. This is believed to have helped the breakdown of silt deposits, leading to an increase of fertility with regards to aquatic fauna and flora. The benefits of this were shown when the reservoir was drained down again in December 2001, because after seven years the stock yield was 70 per cent higher, despite obvious signs of heavy predation, particularly by Cormorant.
- Common Bream Abramis brama
Bream was a “statistically insignificant” species in the 1994 catch, represented by a single individual. However, it was better represented in the 2001 catch.
- Goldfish Carassius auratus
In 1994 this was also a “statistically insignificant” species, represented by a single individual. The 2001 catch yielded a number of goldfish (and other ornamentals), obviously released into the reservoir by members of the public.
- Crucian Carp Carassius carassius
This species was represented by a single individual in the 1994 catch and was deemed “statistically insignificant”. It was not recorded at all in the 2001 catch.
- Common Carp Cyprinus carpio
Common Carp was one of only three species deemed statistically significant in 1994. In 2001, some individuals over 9kg were recorded, but there were only small numbers of fish between 250g and 750g, and these tended to show severe damage by Cormorants, suggesting that numbers of younger fish could have been much higher. However, the species had spawned and achieved a mild success rate. Among the catch was one large white Koi Carp, presumably dumped in the reservoir by a member of the public.
- Roach Rutilus rutilus
Roach accounted for 95 per cent by weight of the total 1994 catch. In 2001 it again dominated the catch, with all age groups present and healthy.
- Tench Tinca tinca
Although Tench was a “statistically insignificant” species in the 1994 catch, it seems to have done better by 2001, having spawned and achieved a mild success rate. However, as with Carp, the small numbers of fish between 250g and 750g showed signs of severe damage by Cormorant attack. So again the levels of younger fish could have been higher.
- Northern Pike Esox lucius
Pike was one of only three species deemed statistically significant in 1994. In 2001, Pike were “into double figures”.
- Three-spined Stickleback Gasterosteus aculeatus
Another “statistically insignificant” species in the 1994 catch, the Stickleback is not mentioned in the report of the 2001 fish rescue — perhaps again because the catching technique allowed the smallest species to escape netting.
- European Perch Perca fluviatilis
Perch was a “statistically insignificant” species in the 1994 catch. It was still present in 2001.
Colmans J. (2002) “Fish”, in Birds of Brent Reservoir: The Natural History of the Welsh Harp (eds L. Batten, R. Beddard, J. Colmans, A. Self). London: Welsh Harp Conservation Group.
Couchman, M. (undated). The Drain Down: Brent Reservoir — A case study. Online publication. Available here (accessed 28 Nov 2016).