Ultra-High Foraging Rates of Harbor Porpoises Make Them Vulnerable to Anthropogenic Disturbance

Abstract

The question of how individuals acquire and allocate resources to maximize fitness is central in evolutionary ecology. Basic information on prey selection, search effort, and capture rates are critical for understanding a predator's role in its ecosystem and for predicting its response to natural and anthropogenic disturbance. Yet, for most marine species, foraging interactions cannot be observed directly. The high costs of thermoregulation in water require that small marine mammals have elevated energy intakes compared to similar-sized terrestrial mammals [1]. The combination of high food requirements and their position at the apex of most marine food webs may make small marine mammals particularly vulnerable to changes within the ecosystem [2-4], but the lack of detailed information about their foraging behavior often precludes an informed conservation effort. Here, we use high-resolution movement and prey echo recording tags on five wild harbor porpoises to examine foraging interactions in one of the most metabolically challenged cetacean species. We report that porpoises forage nearly continuously day and night, attempting to capture up to 550 small (3-10 cm) fish prey per hour with a remarkable prey capture success rate of >90%. Porpoises therefore target fish that are smaller than those of commercial interest, but must forage almost continually to meet their metabolic demands with such small prey, leaving little margin for compensation. Thus, for these "aquatic shrews," even a moderate level of anthropogenic disturbance in the busy shallow waters they share with humans may have severe fitness consequences at individual and population levels.

Discrimination of fast click series produced by tagged Risso's dolphins (Grampus griseus) for echolocation or communication

“…They use sound to communicate and echolocate, emitting directional pulses of high-frequency sound and listening for echoes to build an acoustic scene of prey and landmarks using an active echolocation sense (Madsen and Surlykke, 2013). Our understanding of the echolocation behavior of toothed whales is gradually increasing (Miller et al, 2004;Madsen et al, 2005;Wisniewska et al, 2016;Clausen et al, 2011), but is limited by the challenges of studying the acoustic behavior of free-ranging marine animals that often vocalize at depth and out of sight. Many toothed whales have sophisticated communication systems, but study of their vocal behavior is hampered by problems in identifying which animal makes a sound and which animals respond to these calls.…”

“…A buzz represents the terminal phase of the biosonar-based foraging process, which basically consists of echo-guided search, approach and capture phases (Griffin, 1958). When closing in on targets during buzzes, echolocating toothed whales consistently reduce the inter-click interval (ICI) and output levels of clicks (Morozov et al, 1972;Au, 1993;Miller et al, 1995;Johnson et al, 2006;DeRuiter et al, 2009;Wisniewska et al, 2016). Moreover, buzzes are routinely associated with increased maneuvering and/or changes in the body acceleration rate ('jerk') Miller et al, 2004;Aguilar Soto et al, 2011), likely resulting from fast movements in pursuit of prey.…”

“…Digital tags (DTAGs) (Johnson and Tyack, 2003) that record audio and movement data provide a unique capability to collect acoustic and behavioral information simultaneously from a vocalizing individual, which improves our ability to infer when echolocation or communication takes place (Johnson et al, 2009;Wisniewska et al, 2016). A major challenge in tagging studies of whale vocal behavior is to identify which sounds are produced by the tagged individual and which are produced by nearby conspecifics.…”

Amazon river dolphins (Inia geoffrensis) modify biosonar output level and directivity during prey interception in the wild

“…Invariably, researchers therefore face the trade-off between potential loss of ecological validity in controlled, captive settings versus a lack of control and little power to see in studies of wild animals (Au, 1993;Madsen and Surlykke, 2013). Acoustic tags on animals in the wild have helped bridge that gap over the last decade by providing detailed information of relative output changes in toothed whale biosonars during search, approach and capture of prey (Madsen et al, 2002;Johnson et al, 2004;Wisniewska et al, 2016). Yet, such tags do not provide information about the source parameters of the emitted clicks that in part define the biosonar system performance.…”

“…Tags allow researchers to obtain information on individual echolocation clicks over many hours under circumstances where array recordings are often unattainable (Madsen et al, 2002;Johnson et al, 2004) and, depending on tag placement and the species tagged, returning echoes from actively pursued prey may be recorded (Johnson et al, 2004;Arranz et al, 2011;Wisniewska et al, 2016). However, as echolocation clicks are highly directional (Au, 1993;Koblitz et al, 2012), tag recordings provide a highly distorted perspective on biosonar clicks from the tagged animal (Au et al, 2012) and therefore only allow for relative adjustments of source parameters .…”