Summary
Blue-dyed bait is a seabird bycatch mitigation technique which is assumed to reduce the contrast between the bait and the ocean background making baits visually cryptic to seabirds. We tested this assumption in two ways. First, using measured spectral profiles of blue-dyed baits (fish and squid) and modelled spectral profiles of the ocean under set conditions, we assessed how cryptic baits may be to wedge-tailed shearwaters (Puffinus pacificus) based on the known visual characteristics of this species. Results showed that no baits were perfectly cryptic against the background ocean, but blue-dyed squid were relatively cryptic both in terms of chromatic and achromatic contrasts. However, data indicated blue-dyed fish bait were more apparent due to their ventral surface being brighter than the ocean. Second, during at-sea trials blue-dyed and non-dyed baits that were simultaneously presented submerged on a longline or as surface presentations. During 26 longline sets which presented squid only, a 68% reduction in interactions with blue-dyed squid was observed compared to non-dyed squid. During surface presentations only three to eight percent of blue-dyed squid baits were struck over the duration of the study compared with 75% to 98 % of non-dyed squid bait. In contrast, approximately 48% of all blue-dyed fish baits presented in the first two days of trials received strikes from seabirds but this increased to 90% over the last three days. These results indicate blue-dyed squid bait may decrease seabird bycatch on commercial longline fishing vessels whereas blue-dyed fish bait did not significantly reduce seabird interactions over the entire duration of this study.