The spinner dolphin (Stenella longirostris) is a small, highly social marine mammal found throughout tropical and subtropical oceans globally. This species is recognized for its acrobatic leaps and characteristic spinning behavior when breaching the surface. Living primarily in deep, offshore waters, the spinner dolphin has a specialized feeding ecology driven by the daily migration of its prey.
Primary Prey Items
The diet of the spinner dolphin is broadly carnivorous, consisting mainly of small, soft-bodied organisms that inhabit the mid-water column. Their primary food source is mesopelagic fish, which are species that live at depths but migrate toward the surface at night. A substantial portion of this fish diet is composed of lanternfish, or myctophids.
Beyond small fish, the dolphins also consume a variety of cephalopods, including different species of squid and small octopus. Small crustaceans, such as sergestid shrimp, also supplement the diet, contributing to a specific menu of easily digestible prey. The prey items are typically less than 0.002% of the dolphin’s body mass, necessitating highly efficient foraging strategies to meet their energetic requirements.
Hunting Strategy and Timing
Spinner dolphins are almost exclusively nocturnal feeders, resting in sheltered, shallow waters during the day and moving offshore at dusk to begin their hunt. This timing is directly related to the diel vertical migration of their prey, an event where vast quantities of organisms move vertically in the water column over a 24-hour cycle. The dolphins follow this movement to the Deep Scattering Layer (DSL), a dense aggregation of marine life that ascends hundreds of meters toward the surface after sunset.
To reach their targets, the dolphins execute deep foraging dives, often descending to depths between 200 and 400 meters to intercept the rising DSL. Their hunting success is enhanced by cooperative behavior within their large social groups. Dolphins work together to herd schools of fish and squid, compressing the prey into dense, manageable patches. This coordinated effort maximizes the caloric return for the energy spent diving, with groups sometimes forming a cylinder-like ring around the tightly packed prey before individual pairs break off to make a catch.
Geographic and Environmental Diet Shifts
The exact composition of the spinner dolphin’s diet is not uniform across its extensive global range and can vary based on local availability and environmental conditions. Different populations have adapted their foraging to the specific resources available in their region. For instance, the dwarf spinner dolphin found in Southeast Asia is known to occasionally feed on benthic, or bottom-dwelling, organisms associated with coral reefs, a departure from the typical mesopelagic diet.
In the Eastern Tropical Pacific, the diet composition can fluctuate in response to large-scale oceanographic events like the El Niño Southern Oscillation (ENSO). These cycles influence ocean productivity and the distribution of prey, which can force dolphins to shift their foraging areas or rely on different species to maintain their energy intake. Studies using stable isotope analysis suggest that environmental changes can cause shifts in the food web structure, impacting the quality and type of prey available to the dolphins. The morphological variations seen in regional subspecies also reflect adaptations to distinct local ecological pressures and prey fields.