Action: Disturb birds at roosts
Key messagesRead our guidance on Key messages before continuing
- One controlled study from the USA investigated the effects of harassment on fish predation, and found there were fewer double-crested cormorants Phalacrocorax auritus on, and fewer fish were taken from, fish ponds near roosts which were harassed, compared with undisturbed roosts.
- A review found that there was a reduction in the number of cormorants foraging near roosts after night-time disturbance. Four studies, two replicated, from the USA and Israel, found that cormorants moved away from roosts where they were disturbed at night. One study found that this effect was only temporary.
Predation by birds at aquaculture facilities (e.g. fish ponds, raceways and shellfish farms) can cause significant commercial loss (Draulans 1987). With increasing protective wildlife legislation, demand for non-lethal, environmentally safe methods of bird exclusion and scaring have increased. Most fish farmers now rely primarily on non-lethal techniques to accomplish control (i.e. to reduce abundance or exclude fish-eating birds in and around the vicinity of fish farms). Control efforts may be optimized by compiling evidence relating to deterrent or exclusion device efficacy, taking into account costs, practicality of use and the possibility of developing integrated strategies (i.e. combining more than one deterrent method). Avian deterrents can be categorised as auditory, visual, chemical, exclusion, habitat modification and lethal (Bishop et al. 2003). Lethal deterrents are not considered here (except where, very rarely, used as part of an integrated approach).
Bishop, J., McKay, H., Parrott, D. & Allan, J. (2003) Review of international research literature regarding the effectiveness of auditory bird scaring techniques and potential alternatives. Department for Environment Food and Rural Affairs.
Draulans, D. (1987) The effectiveness of attempts to reduce predation by fish-eating birds: a review. Biological Conservation, 41, 219–232.
Supporting evidence from individual studies
A 1995 review assessed effectiveness of techniques used to prevent double-crested cormorant Phalacrocorax auritus predation at aquaculture facilities in the Mississippi delta region, USA (Mott & Boyd 1995), and concluded that disturbing birds at their roosts was more effective than scaring birds from fish farms during the day, with one study finding a 75-90% reduction in cormorant numbers foraging in the area after disturbance.
A controlled experiment over three winters in the vicinity of channel catfish Ictalurus punctatus rearing ponds in the Mississippi delta region, USA (Mott et al. 1998), found that the number of double-crested cormorants Phalacrocorax auritus on or near fish ponds was reduced by approximately 70% following a winter of extensive harassment at roosts. Catfish farmers within an area of intensive roost harassment also reported a reduction in fish predation by cormorants. Significantly fewer cormorants used intensely harassed night roosts than less intensely harassed or roosts that were not harassed. Pyrotechnics (designed to scare birds) were fired at roosting cormorants and those flying towards the roost during the two hours before sunset. Cormorants were counted on or near ponds and comparisons made between intensely harassed, less intensely harassed and non-harassed roost sites.
Replicated before-and-after trials in January-March 2003 at double-crested cormorant Phalacrocorax auritus night roosts (with 2,500 to 34,000 individuals) near catfish farms in Mississippi and Alabama, USA (Glahn et al. 2001), found that hand-held lasers reduced cormorant numbers by 94% to 100%. The time required to achieve success varied (the most effective was 16 min to achieve 100% success at one roost; the least effective 113 min to achieve 94% success at another), but cormorants typically abandoned roosts after three nights of harassment. Six trials (at six sites) were conducted using a Desman™ Laser and five using a Laser Dissuader™. From sunset to 1 hour after sunset (on one to three consecutive evenings), a laser beam was directed at roosting cormorants, from 100-1,000 m distant. Birds were counted before and after treatment. (Note: Laboratory trials found no ocular damage to cormorants exposed to the Desman Laser at distances to the minimum of 1 m tested).
A replicated, controlled study in 1997 near channel catfish Ictalurus punctatus ponds in the Mississippi delta, USA (Tobin et al. 2002), found that harassed double-crested cormorants Phalacrocorax auritus flew farther to their next roost than birds not harassed the previous night. Only 11% of harassed birds returned to the same roost within 48 hours, compared with 81% return to non-harassed roosts. Harassment shifted birds away from areas of catfish farm concentrations, but effects were temporary. Farmers undertook co-ordinated night-roost harassment patrols and prior to patrols, 50 cormorants were radio-tagged and their movements monitored from January through March 1997.
A before-and-after study in Bet She’an Valley, Israel (Nemtzov 2005), found that pygmy cormorants Phalacrocorax pygmeus relocated away from fish farms and bred successfully in nearby wetlands after harassment at all roosting sites in the valley using gas cannons and pyrotechnics in the winters of 1999/2000 and 2000/01. Between 1998 and 2004, number of cormorant nests in the area increased from 60 to 110 (peaking at around 155 in 2001).
- Mott D.F. & Boyd F.L. (1995) A review of techniques for preventing cormorant depredations at aquaculture facilities in the southeastern United States. Colonial Waterbirds, 176-180
- Mott D.F., Glahn J.F., Smith P.L., Reinhold D.S., Bruce K.J. & Sloan C.A. (1998) An evaluation of winter roost harassment for dispersing double-crested cormorants away from catfish production areas in Mississippi. Wildlife Society Bulletin, 26, 584-591
- Glahn J.F., Ellis G., Fiornelli P. & Dorr B. (2001) Evaluation of moderate- and low- power lasers for dispersing double-crested cormorants from their night roosts. Proceedings of the Eastern Wildlife Damage Management Conference, 9, 34-45
- Tobin M.E., King D.T., Dorr B.S., Werner S.J. & Reinhold D.S. (2002) Effect of roost harassment on cormorant movements and roosting in the delta region of Mississippi. Waterbirds, 25, 44-51
- Nemtzov S.C. (2005) Relocation of pygmy cormorants Phalacrocorax pygmeus using scare tactics to reduce conflict with fish farmers in the Bet She'an Valley, Israel. Conservation Evidence, 2, 3-5