Abstract

…There is, unfortunately, no unbiased way to assess the prey preference and dietary intake of free-ranging marine mammals like harbor porpoises. Although the traditional approach involving stomach content analysis of stranded and bycaught individuals provides important information, animals must either end up on a beach (e.g., due to illness or navigation error) or in a net (e.g., potentially due to a preference for the prey targeted by the fishery) in order to be sampled. In our paper, we took a novel and complementary approach involving analysis of echo information from prey targeted by instrumented porpoises as they hunt freely. As a result, we are reliant on animals incidentally live caught in commercial pound nets to be temporarily restrained for tagging, resulting in a small sample size comprising mostly young individuals. Although we would of course have preferred a broader sample, this does not lessen the significance of our results. Specifically, even if the “ultra-high” foraging rates demonstrated in our paper are only typical of young animals, the resulting higher vulnerability to disturbance will still give rise to a bottleneck effect: all animals are young at some point in their lives. Moreover, animals of 2 yr and younger constitute a significant proportion of the porpoise population (Lockyer and Kinze 2003).

This high proportion of young porpoises, perhaps combined with their inexperience, may explain why this age class prevails in pound nets. Unfortunately, very few of our suction cup tag deployments on adult porpoises have extended beyond a few hours without considerable sliding or detachment of the tag. However, data from an adult female of 170 cm, tagged since our paper was drafted, revealed buzz rates ranging from 35 to 140 buzzes per hour with an average of 73 buzzes per hour over the 13 h deployment, similar to the 86 buzzes per hour that we reported for another adult female in Wisniewska et al. (2016) (Table 1). While the buzz rates of these adults are on average lower than for juveniles (125 per hour), they, nonetheless, appear to target some 1,500–2,000 small fish per day (Table 1). Thus, although our adult sample size is small, Hoekendijk et al.’s concern that high feeding rates are only found in juvenile porpoises does not seem to be supported by our data.