Interactions between coastal longline mussel farms and seabirds in Aotearoa/New Zealand: evidence of species-specific attraction and localised depletion

Suspended longline aquaculture is widely used to grow shellfish in shallow coastal waters worldwide, yet few studies have quantified its effects on seabirds that commonly inhabit the same habitats. This study evaluated interactions between mussel farms and seabirds in the Marlborough Sounds, New Zealand, between 2019 and 2025. A total of 18 bird species were recorded from 205 mussel farms sampled over the six-year period, averaging 3.6 species per farm (SE = 0.14) and a mean density of 1.92 individuals ha −1 (SE = 0.17). Black-backed gull was the most numerically abundant and frequently observed species (0.60 individuals ha −1 , SE = 0.1; present at 75% of farms), followed by spotted shag, pied shag, little shag, and white-fronted tern. Most birds observed at farms were roosting (75.2%), with the remainder foraging, swimming, or flying over farm structures. A comparison of bird numbers at an additional 43 mussel farms and equivalent-sized on-water sites at three distances (near: median 57 m; far: median 624 m; distant: median 3.7 km), sampled in spring 2025, showed that mean species richness, total density, and density of the five most frequently occurring species were significantly higher at farms than at near-, far-, or distant-field sites. At shore transects, all species except black-backed gull showed no distributional pattern relative to farms. However, black-backed gull, whereas observed significantly less frequently at farm-adjacent transects than at near- and distant-field transects, indicating localised depletion of shoreline habitat driven by attraction to farms. These findings demonstrate that longline mussel farms strongly attract seabirds, primarily for roosting, while causing measurable, species-specific depletion of adjacent natural habitat for one species, a pattern with implications for coastal aquaculture management and seabird conservation.