Reducing effects of trawl fishing on seabirds by limiting foraging opportunities provided by fishery waste

Abstract

By-catch of seabirds on trawl-fishing gear has been reported worldwide, and is exacerbated by the discharge of fisheries waste. We compared the attraction of seabirds to three forms of fishery waste—unprocessed discharge (offal, fish discards), hashed discharge (smaller chunks passed through a hasher pump) and cutter pump discharge (waste passed through the hasher and a cutter pump to further reduce particle size)—to identify the discharge form that most effectively reduced the risk of seabird by-catch. Seabird responses measured within specified areas astern of the vessel were the abundance of: large albatrosses (Diomedea spp.), small albatrosses and giant-petrels (Thalassarche spp.; Southern Giant-Petrels, Macronectes giganteus; and Northern Giant-Petrels, M. halli), Cape Petrels (Daption capense) and all other procellariid species. Seabirds on the water were less numerous during cutter pump and hashed discharge relative to unprocessed discharge (except small albatrosses—cutter treatment). Also, in some cases, the total number of birds decreased, relative to unprocessed discharge treatments (but not small and large albatrosses—cutter treatment). Particle size may be less important for reducing abundances than temporal discharge patterns, which affected how birds tracked the discharge stream. Manipulating discharge characteristics can reduce seabird attraction to fishing vessels. However, the risk of by-catch remained lowest when no discharging occurred.