Diving behavior, foraging strategies, and energetics of female Steller sea lions during early lactation

Lactating Steller sea lions (SSL; Eumetopias jubatus) are income breeders, which balance the energetic cost of provisioning a pup with alternating periods of foraging at sea and nursing onshore during a 12–36 month dependency period. Foraging dives may be benthic or epipelagic, and the diverse diet varies seasonally and geographically. The objective of this study was to use animal-borne video and data recorders (VDRs) to record prey captures and characterize the three-dimensional movements, dive performance, and foraging strategies of female SSL while rearing a pup on Lovushki Island, located in the Kuril Island Archipelago in Far Eastern Russia. Female SSL made short foraging trips (8.7 h) and remained relatively close to the rookery (< 15 km) with an average swim speed of 1.5 m sec⁻¹ and metabolic rate of 3.15 W kg⁻¹, which was 1.8-fold greater than their estimated resting metabolic rate. We identified two types of dives representing shallow (Type 3) and deep (Type 4) benthic foraging, primarily on Atka mackerel (Pleurogrammus monopterygius) at night when they rest on the seafloor. The estimated Field Metabolic Rate (FMR; 2.31 W kg⁻¹) for a mean 31.7 h foraging-nursing cycle was only 1.3-fold greater than their estimated resting metabolic rate, primarily because the females spent 73% of their time onshore nursing the pup. The short foraging trips near the rookery and low FMR indicated abundant prey. On average, female SSL captured 35.4 fish during a foraging trip, which represented an estimated 20.8 kg of prey. This was sufficient to remain in energy balance and provision a pup during early lactation. However, a 10–20% reduction in the size or abundance of prey would result in a 38–76% reduction in milk production, respectively, which would cause reduced growth or death of the pup. These calculations indicate the thin margin for energy balance in SSL during early lactation. Hence, any reduction in the size or abundance of prey could have serious consequences for reproductive success and fitness.