Abstract

…Recently, Wisniewska et al. (2016) used high-resolution digital acoustic tags (DTAGs) to monitor the foraging behavior of five harbor porpoises in Danish waters. This approach provided detailed observations on the size and quantity of targeted prey, as well as estimates of how efficiently porpoises capture their prey. Porpoises trapped in a pound net were tagged and monitored for periods between 15 and 23 h. After their release, these porpoises showed extreme feeding behaviors. Foraging was almost continuous, spanning day and night at an “ultra-high” rate (Wisniewska et al. 2016). Prey encounter rates varied between 0 and 200 prey items per hour during the day and between 50 and 550 at night. The porpoises targeted primarily small fish (3–10 cm long) and captured these with a success rate of >90%. These observations were interpreted by Wisniewska et al. (2016) as confirmation that porpoises are “living in the fast lane” (Kanwisher and Sundnes 1965, Read and Hohn 1995, Lockyer 2007). The authors concluded that porpoises are living on an energetic knife-edge, with little margin to compensate for any stochasticity in food availability due to anthropogenic or environmental disturbances. Furthermore, the authors concluded that porpoises did not compete with commercial fisheries given the small fishes consumed.

We agree with Wisniewska et al. (2016) that these small, warm-blooded mammals have one of the highest costs of living among cetaceans, and they are certainly more sensitive than other species to variability in the quality and quantity of ingested prey (Spitz et al. 2012). However, we argue here that the observations reported by Wisniewska et al. (2016) may offer a biased and extreme view of porpoise biology due to (1) the small sample size used in this study (five individuals), (2) the biased age structure of porpoises examined (four juveniles and one adult), (3) the circumstance of this monitoring (i.e., after the animals had been trapped in a pound net for 24 h, prior to release), and (4) the short period of monitoring after tagging (between 15 and 23 h)…