Date Published: February 5, 2019
Publisher: Public Library of Science
Author(s): Yu Akiyama, Tomonari Akamatsu, Marianne H. Rasmussen, Maria R. Iversen, Takashi Iwata, Yusuke Goto, Kagari Aoki, Katsufumi Sato, Z. Daniel Deng.
Central place foraging theory (CPF) has been used to predict the optimal patch residence time for air-breathing marine predators in response to patch quality. Humpback whales (Megaptera novaeangliae) forage on densely aggregated prey, which may induce drastic change in prey density in a single feeding event. Thus, the decision whether to leave or stay after each feeding event in a single dive in response to this drastic change, should have a significant effect on prey exploitation efficiency. However, whether humpback whales show adaptive behavior in response to the diminishing prey density in a single dive has been technically difficult to test. Here, we studied the foraging behavior of humpback whales in response to change in prey density in a single dive and calculated the efficiency of each foraging dive using a model based on CPF approach. Using animal-borne accelerometers and video loggers attached to whales, foraging behavior and change in relative prey density in front of the whales were successfully quantified. Results showed diminishing rate of energy intake in consecutive feeding events, and humpback whales efficiently fed by bringing the rate of energy intake close to maximum in a single dive cycle. This video-based method also enabled us to detect the presence of other animals around the tagged whales, showing an interesting trend in behavioral changes where feeding duration was shorter when other animals were present. Our results have introduced a new potential to quantitatively investigate the effect of other animals on free-ranging top predators in the context of optimal foraging theory.
Predators should modify their foraging behavior to efficiently exploit prey whose density and availability dynamically changes over time. Since the study of optimal foraging began in 1966 [1, 2], various theories have been developed to predict the foraging decision of animals. These theories in general assumes that, as animal forage in a small-scale patch, the density of prey in the patch decreases over time, thus the rate of energy intake diminishes (diminishing return). Under this assumption, the optimal forging theory predicts the timing when the animal should stop feeding and leave the patch to maximize the energy intake (or certain currency) per unit time with minimum cost [3–5]. While many empirical tests in laboratories or carefully controlled field experiments have been performed supporting these theories , such studies were mostly restricted to terrestrial or captive animals that move in a relatively small area where visual observation can be conducted.
Assessing the predator–prey interactions of free-ranging diving marine predators is challenging. When studying the predator–prey interactions of rorquals, ship-mounted echo-sounders were commonly used to measure the distribution and abundance of prey near tagged whales in previous studies [12, 17, 21, 22, 39–41]. The advantage of this method is that it maps the prey distribution in the feeding grounds of whales at wider and deeper ranges over a long period of time. Recent study of humpback whales using a ship-mounted echo-sounder revealed that the foraging decisions of humpback whales are driven by both prey depth and density. Humpback whales maximized the energy intake over time by mainly feeding at a shallow depth to minimize their diving and search cost and to increase overall feeding rate . Their study provided new and interesting insights on the ecological decision-making of foraging humpback whales.