The dolphin tap: assessing the ecosystem role of spinner dolphins in supplying nutrients to coral reefs in Maui Nui, Hawai’i

Introduction Considering the ecological functioning of small cetaceans is important for ecosystem-based management and conservation, including their potential role in transporting limiting nutrients across habitats. Spinner dolphins ( Stenella longirostris longirostris ) in Hawai’i forage nocturnally on mesopelagic prey offshore and, during the day, rest and avoid predators inshore. These predictable behavioral and spatial use patterns in the Maui Nui region suggest that spinner dolphins may transfer pelagic nutrients to inshore habitats, including shallow coral reefs - a mechanism we refer to as “the dolphin tap”. Methods To assess the role of spinner dolphins as nutrient vectors, we quantified spinner dolphin spatial overlap with inshore and coral reef habitats in Maui Nui using vessel-based survey data collected from 2013–2022. We estimated nutrient deposition using standard metabolic models, spinner dolphin distribution and temporal overlap of dolphins with coral reef habitats. Results and discussion We determined spinner dolphin distribution from 51 encounters. We estimated that an individual spinner dolphin deposited 0.10 kg N day -1 (SD = 0.02) into the overall marine environment. Dolphins overlapped with coral reef habitat during 25 encounters and 28% (SD = 36%) of total sighting time. Using daytime-only observations, we estimated an individual spinner dolphin deposited between 0.01 kg N day -1 (SD = 0.02) and 0.02 kg N day -1 (SD = 0.03) over coral reefs, depending on the extent of nighttime deposition. Individual-level annual deposition values were extended to group (mean = 65.40 individuals, SD = 45.24) and population (594 individuals) levels to quantify nutrient deposition in the overall marine environment and to coral reef habitats. This naturally occurring nutrient input from pelagic foraging grounds to inshore habitats may enhance productivity and promote coral reef resilience and health. Our findings provide baseline estimates of nutrient deposition by spinner dolphins in Maui Nui, yet additional research and monitoring are needed to better understand the nutrient dynamics. As Maui Nui’s coral reefs experience stress from warming oceans, this dolphin-mediated subsidy may become increasingly important for sustaining coral reef function. Protecting spinner dolphins is therefore essential to maintaining “the dolphin tap” nutrient pathway and supporting the health of Hawai’i’s coral reef ecosystems.