Understanding common dolphin and Australasian gannet feeding associations from nutritional and ethological perspectives

Abstract

Abstract Prey detection and subsequent capture is considered a major hypothesis to explain feeding associations between common dolphins and Australasian gannets. However, a current lack of insight on nutritional strategies with respect to foraging behaviours of both species has until now, prevented any detailed understanding of this conspecific relationship. Here we combine stomach content analysis (SCA), nutritional composition of prey, a multidimensional nutritional niche framework (MNNF) and videography to provide a holistic dietary, nutritional, and behavioural assessment of the feeding association between dolphins and gannets in the Hauraki Gulf, New Zealand. Dolphins consumed ten prey species, including grey mullet (Mugil cephalus) as the most representative by wet mass (33.4%). Gannets preyed upon six species, with pilchards (Sardinops pilchardus) contributing most of the diet by wet mass (32.4%) to their diet. Both predators jointly preyed upon pilchard, jack mackerel (Trachurus spp.), arrow squid (genus Nototodarus), and anchovy (Engraulis australis). Accordingly, the MNNF revealed a moderate overlap in the prey composition niche (0.42) and realized nutritional niche (0.52) between dolphins and gannets. This suggests that both predators coexist in a similar nutritional space, while simultaneously reducing interspecific competition and maximizing the success of both encountering and exploiting patchily distributed prey. Behavioural analysis further indicated that dolphin and gannets feeding associations are likely to be mutually beneficial, with a carouselling foraging strategy and larger pod sizes of dolphins, influencing the diving altitude of gannets. Our approach provides a new, more holistic understanding of this iconic foraging relationship, which until now has been poorly understood.