This year has seen the publication of a series of papers which provide really important jigsaw pieces in how harbour porpoises exist on a day to day basis, and how they might be affected by being disturbed by underwater noise (e.g. from pile driving, seismic geophysical surveys or shipping). In recent months, a number of papers were published providing more pieces of the puzzle (and some fresh perspective on what the puzzle picture is!). Let’s talk through those papers and what they might mean!

 

How does noise affect feeding ability?

A research team led by Dr. Ron Kastelein of SEAMARCO in The Netherlands, carried out a series of experiments with captive harbour porpoises (a 4 year old male and 7 year old female). A first question was to explore how being exposed to the sounds of pile-driving might impact animals ability to catch food (either stopping feeding, or being less successful in capture). The team observed that the female tested was generally less adept at catching fish than the male – even in the quiet environment. When noise was introduced it was found that as noise exposure increased (a single strike sound exposure level – SELss), the number of capture attempts by the female that were terminated before capture increased, though failure rate was consistent across trials. For the younger male, noise exposure appeared to have little effect with the animal achieving high success rates regardless of the tested noise exposures (up to 143 dB SELss).

Reproduced from Kastelein, et al. 2019. Success (S), Failures (F) and Terminations (T) of feeding attempts to different single strike SEL exposures from pile-driving playbacks.

The authors note:

The two study animals differed in their fish-catching success rate at all noise levels, including under quiet conditions.[…] Individual differences in responses to sound, termination rates, and fish-catching success (even in ambient conditions) may complicate the quantification of the impacts of pile driving sounds on harbor porpoises.

 

How does fasting affect the animal’s condition?

To determine what the possible consequences of reduced foraging success could be on the animals health, the same individuals were the subject of another experiment. This time animals fasted for 24 hours (in separate experiments in each of the four seasons) and the changes in body mass and condition was monitored. Past studies indicated that there were seasonal fluctuations in blubber mass animals are more lean in the summer to cope with the higher water temperatures. and putting on blubber to a greater extent for the winter months (when water temperatures are much lower). In all seasons, the effect of fasting for 24h was that body mass reduced by ~3-6%. This tells us that in cases where animals cannot find suitable prey, the overall condition of the animal is reduced.

 

How much can porpoises eat?

The next logical question, once understanding that animals may lose condition if they cannot find food for 24 hours, is ‘how much food can they regularly eat?’. Therefore, following fasts lasting between 2 and 24 hours, the food intake of four porpoises was monitored. Whilst a typical feed (for these captive animals) was ~20% of their daily energy requirements, following fasting the animals routinely ate over 35% of their requirements in a single bout of feeding, with many consuming between 60-90% of their energy needs in a single feed! Animals were also ready to feed again only two hours later, indicating animals can consume large amounts, digest it quickly and be ready to feed again. This might shed light on how the species can cope with the patchy nature of their prey species, where animals may have to travel between, or search for suitable food patches (losing weight during this time, before feeding readily).

 

What does counting calories teach us?

All of these exciting findings lead us to a paper by Cormac Booth. With concern that porpoises are thought to be vulnerable to starvation when disturbed due to their need for regular food. This study used data from novel tags put on wild animals which recorded information about how often and how much they were trying to catch fish. By combining these data with reviews of what fish porpoises in the area would eat and how much energy they contained, it was possible to explore whether or not animals obtained enough energy (another study estimated how much energy the animals expended while the tags were on the animal). In all but the worst case scenarios, harbor porpoises got the required energy due to their broad diet (consisting of many medium and high energy fish) and high feeding rates.  If animals can find suitable prey, porpoises may be capable of recovering from some disturbances.

Reproduced from Booth 2019. See caption.

 

So can animals find suitable prey?

Even with these advances in our knowledge, a key question that remains is “But what if porpoises simply can’t find enough suitable prey”. Of course this is a fair and important question. We’ve been working with Janneke Ransijn and Dr Sophie Smout at the University of St Andrews to try to shed light on exactly this question. This research team, supported by Kelly Macleod at Joint Nature Conservation Committee, developed a ‘calorific map’ of energy available to harbour porpoises (specifically, a map of porpoise prey species distribution in terms of the calories they represent to porpoises – i.e. megajoules per km2). Harbour porpoises are abundant in the North Sea and their diet consists of a variety of prey species. Energy maps were produced for Atlantic cod, whiting, European sprat, Atlantic herring and sandeels. The modelled prey distribution maps fit well with previously described spatial patterns for the fish species and large amounts of prey energy are predicted to be available to harbour porpoises. However, we can’t say how well the the energy predicted correlates to the actual available energy for porpoises given the role of other marine predators and the fishing industry present in the North Sea.

From Ransijn, et al 2019. Map shows energy available per km^2 for five prey species combined.

This research topic requires further research but provides a first glimpse of how harbour porpoises populations have become, and continue to be, the most common marine mammal species in the North Sea. These studies can also add important knowledge to assessments of the affects of noise disturbance and are critical to be considered in the wider picture of stressors that could impact these and other marine mammal populations.

Check out the papers and report to learn more and to understand the important details at the links below: