Over the last two years, there’s been a very interesting exploration of how successfully harbour porpoises forage and what the implications are for their vulnerability or ability to tolerate disturbance. Here we’re going to take a bit of a ‘deep dive’ to explore what it all means.
We did a blog post back in 2016, which you can read here, about a really interesting and novel study conducted by a group of researchers at Aarhus University attached sound recording tags (DTAGs) to harbour porpoises. Since then, there has been a new paper commenting on the Aarhus study published and a response from the Aarhus authors. We’ve put the papers on our website to make sure folks can access them (see below!). These studies all provide interesting jigsaw pieces in this research area, so let’s dissect it a bit more here. Grab a hot drink and some rations and read on!
The original 2016 study found that from the tag deployments (which lasted 15-23 hours) that porpoises were very active throughout the day and night, foraging almost constantly. The authors observed “prey encounter rates of 0–200/hour during the day and 50–550/hour after dusk” and that the success rate of feeding attempts were exceptionally high with between 91 – 97% of capture attempts being successful (i.e. out of 550 prey captures in one hour, porpoises would successfully catch prey 500-533 times).
The authors also noted that:
Our results show that, like shrews, porpoises must feed nearly continuously to support their high metabolic demands, leaving very little margin to compensate for changes in their environment. Failure to acquire sufficient energy when operating on an energetic knife-edge may have rapid and severe fitness consequences, giving them low resilience to disturbance: individual porpoises have been reported to starve to death in less than a week. The effects of frequent anthropogenic disturbance and changes in the marine ecosystem on the foraging efficiency of porpoises and other small marine mammals in cold water should therefore be of prime importance in future research.
We preached some caution in this interpretation at the time. Certainly porpoises have limited fat stores and so feeding a lot each day to maintain their metabolic demands (especially when pregnant) is key. The data presented in the 2016 study demonstrated that animals forage a lot and are very successful in capturing prey (all animals over 91%). The study seems to assume that porpoises are vulnerable and uses the high foraging rates they observed to justify that claim. But it’s important to understand how well-adapted porpoises might be to their environment. We know that they feed on a wide array of prey items (see here and here and here for a taste!) and so at least potentially capable of switching between prey items to capitalise on prey encounters and the Wisniewska et al. 2016 study observed ultra-high foraging rates and excellent prey capture rates.
Fast-forward to early January 2018, when a group of researchers led by Hoekendijk, wrote a response to this study (“Resilience of harbor porpoises to anthropogenic disturbance: Must they really feed continuously?”) which was published in Marine Mammal Science. The comment praised the Wisniewska et al 2016 study for it’s novel use of acoustic tags to estimate foraging rates and prey capture rates. They also supported the view that porpoises, as small and warm blooded mammals will have high costs of living (i.e. need to forage frequently) and that they will be sensitive to availability of prey. However they also suggested the original 2016 study:
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).
Specifically, they highlighted that because the tagged animals were mostly young sub adults (estimated between 0.5 and 2.2 years old) they were likely to be feeding on smaller, less-energetically rich prey and highlighted: “search effort, rates of encounter, and prey capture almost certainly differ between adults and juveniles, as well as with space and time.” and critically that as animals get older they move on to larger prey items:
The consumption of larger, high-calorie prey provides greater rates of energy intake and releases individuals from the need to forage continuously, leaving more time for other activities (e.g. resting, mating, traveling; as seen in the ﬁeld). Such an energetic cushion makes porpoises more resilient to anthropogenic disturbances and other environmental changes. Thus, we argue that it is highly unlikely that porpoises could maintain an ultrahigh foraging and capture rates as reported by Wisniewska et al. (2016) because it leaves so little time for other important behaviors.
Of course it’s important to consider that if animals are disturbed and lose foraging opportunities, they may have to compensate and lose time for resting, mating and travelling. It’s unclear the nutritional state of the animals tagged (and held in nets for up to 24 hours before tagging – see below) but given the ultra-high foraging rates following release the results from the 2016 study could also suggest that porpoises have an ability to respond to short term reductions in food intake, implying a resilience to disturbance. As Hoekendjik et al (2018) argue, this could help explain why porpoises are such an abundant and successful species. The recent SCANS III study results indicate that that the North Sea is relatively stable (i.e. neither increasing nor decreasing) between 1994 and 2017 (though it should be considered there have been only three SCANS surveys during this period).
Following the Hoekendijk comment paper, the Aarhus team responsible for the 2016 study, once again led by Wisniewska, published their response and additional data analysed (analysed since the original study was published). It’s really great to see this kind of discussion advancing our knowledge of this topic. The additional data was from two more tagged animals, one adult female and another sub-adult. They observed that both the adults tagged had lower buzz (foraging) rates than juveniles (adult: 79.5 buzz/hr; juveniles: 125.4 buzz/hr). While the sample size is limited, it’s an interesting observation and might match-up well with our understanding of the different quantity and quality (i.e. calories) of prey that different age animals feed upon (i.e. juveniles feeding on smaller prey like gobies in higher numbers than adults who might favour larger sized and more energy rich herring and cod) . It’s certainly worth exploring this further – as the quality of the prey (i.e. how much energy you can get from eating something) differs between prey and is a critical part of this puzzle.
Another key learning from the response was with respect to the nutritional state of the animals (i.e. did they have full stomachs when tagged or were they ‘hungry’?). Wisniewska, et al 2018 indicated that: “Porpoises likely swim into pound nets following prey, and there is always ﬁsh in the nets where the animals are trapped. We do not know to what extent porpoises feed while in the pound net.”
There still seems to be an implicit assumption in both the Wisniewska-led papers that porpoises are vulnerable to disturbance. This term is open to different interpretations. Of course this might be the case and there are many studies showing that they change their behaviour following exposure to (for example) underwater noise. But we need to carefully consider what being ‘vulnerable to disturbance’ means because there are many kinds or levels of disturbance. As highlighted above porpoises are abundant in the North Sea and so it’s important that we question and probe these kinds of assumptions. Could it be that animals are generalists and capable (due to being very successful foragers) of switching between different prey types and patches? Do animals respond to noise by moving away because there is little value in staying in a potentially stressful situation if they can move to a different area and prey? Does being disturbed mean that animals lose foraging opportunities and are thus truly ‘vulnerable’ or do animals have the capability to compensate for lost foraging opportunities?
We do not have the definitive answers currently, but these papers, responses and subsequent discussions and research are absolutely critical to challenging preconceptions about marine mammals and to help advance science.
We can’t recommend highly enough that anyone who is interested in harbour porpoises, whether they are scientists, industry or managers, read these three papers – as they provide really important jigsaw pieces and essential contextual information that help guide our views on how porpoises in the North Sea behave and the health of the population.
Wisniewska,etal,2016 – Ultra-High Foraging Rates of Harbor Porpoises Make Them Vulnerable to Anthropogenic Disturbance. Current Biology.
Hoekendijk et al. 2017 – Resilience of harbor porpoises to anthropogenic disturbance: Must they really feed continuously? Marine Mammal Science
Wisniewska,etal,2018 – Response to “Resilience of harbor porpoises to anthropogenic disturbance: Must they really feed continuously?”
For more information about our work exploring the Population Consequences of Disturbance: PCoD, go here.