Action: Use in-water devices to reduce fish loss from ponds
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- A before-and-after study from the USA found a 95% reduction in the number of double-crested cormorant Phalacrocorax auritus at two ponds in a fish farm following the installation of underwater ropes.
- A replicated study at a fish farm in Australia found that hanging gill nets in ponds did not decrease the number of cormorants swimming in ponds.
Cormorants Phalacrocorax spp. hunt by pursuing fish underwater. Putting nets or other obstacles underwater may reduce hunting effectiveness and so reduce aquaculture losses.
Supporting evidence from individual studies
A before-and-after study at a channel catfish Ictalurus punctatus farm in Mississippi, USA, in January-April 1992 (Mott et al. 1995) found a 95% reduction in double-crested cormorant Phalacrocorax auritus on two ponds following the installation of parallel lengths of 9.5 mm yellow polyethylene rope with foam floats, 6.1 m apart (0.8-2.2 birds/min/day before installation vs. 0.03-0.08 afterwards). Eleven helium balloons also appeared useful in frightening cormorants habituated to ropes. During the week prior to addition 0.29 cormorants/min/day entered, whereas in the week after 0.02 entered. A 4.6 ha pond was monitored for 1,019 minutes before rope installation and 2,418 minutes after.
A replicated study at four silver perch Bidyanus bidyanus rearing ponds in New South Wales, Australia, found that hanging of gill nets and harassment patrols proved ineffective in deterring cormorants (great Phalacrocorax carbo, little black P. sulcirostris and little pied P. melanoleucos) from fishing in the ponds (Rowland 1995). Gill nets (large diameter mesh enabling young fish to swim through) were hung vertically in the water and harassment patrols (people walking around the ponds) were undertaken. Three ponds of 0.1 ha were stocked with fry (50,000-60,000 fish/ha) and one of 0.3 ha stocked with fingerlings (8,000/ha). Survival rates of fry in the three smaller ponds was 0.3%, 0.8% and 3.5% (average 1.5%), and survival of fingerlings 0.9%.
- Mott D.F., Flynt R.D. & King J.O. (1995) An evaluation of floating ropes for reducing cormorant damage at catfish ponds. Proceedings of the Eastern Wildlife Damage Control Conference, 6, 93-97
- Rowland S.J. (1995) Predation of Bidyanus bidyanus (Teraponidae) in ponds by cormorants. The Progressive Fish-Culturist, 57, 248-249