A really interesting new study was published last week, which used data collected from tags deployed on harbour porpoises – in order to explore how the animals find food.
Researchers in Denmark, between 2012 and 2014 put 5 small tags that record movement and sounds made by the animal (and in the environment) called DTAGs on wild harbour porpoises in the Kattegat and Belt Seas around Denmark. The data collected were then analysed to explore how often (and on what prey) harbour porpoises are feeding. The study was published last week in Current Biology.
The team found that from the tag deployments (which lasted 15-23 hours) that porpoises were very active throughout the day, 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.
This is a really cool finding and an important piece of the bigger puzzle of understanding the biology of these animals. The authors provide a really cool video which shows the dive pattern of one of the animals – which we’ve posted below.
The authors make note 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.
Certainly porpoises are likely to need to feed nearly continuously to maintain their metabolic demands – and this study demonstrates that very clearly. It’s really unclear at this stage how anthropogenic disturbance affects these animals and how specifically it affects their ability to forage successfully. It should be noted that the study area is an area of extremely high shipping density – with large and small vessels consistently transiting through the region. In addition, the Anholt offshore wind farm was constructed in the region in 2012. Therefore, while we don’t know about the residency patterns of animals – it is likely animals that do spend time in the region are likely to be often be exposed to these disturbance sources when present.
Nonetheless we certainly agree that better understanding these issues is a research area of prime importance – and getting the direct link between disturbance events and lost foraging (and how that affects vital rates and/or animals health) is a crucial step. Currently in the Interim PCoD model this is captured by expert elicitation.
The study found that animals were between 91-97% successful in their feeding attempts, so even in such a potentially pressure-filled environment, animals are adept at capturing prey.
Another interesting point in the methodology is that animals are captured in fishing nets ahead of being tagged.
Upon discovering a porpoise in the net, the fishermen closed the mouth of the net to prevent the animal from escaping. Tagging personnel arrived within 24 hours.
It’s unclear whether the high levels of foraging activity are related to animals attempting to compensate for lost feeding opportunity whilst being held in nets for up to 24 hours. It is speculation at best, but it is possible that the foraging observed in the tag data represents animals foraging heavily to make up for lost time (assuming animals weren’t able to forage during the time they spend in nets) – we’re going to contact the authors to make sure we understand how the study was conducted.
The above points not withstanding, this remains a really interesting study and provides some crucial knowledge on the ecology of harbour porpoises.