A little less than 45,000 square km of cavernous stony reef at Læsø Trindel in Kattegat has quickly become a preferred dining place for porpoises. Since the reef was re-established in the summer of 2008, at which occasion 100,000 tons of Norwegian quarry stones of varying sizes were dumped at the reef, scientists from DCE -Danish Centre for Environment and Energy, Aarhus University, have registered an increase in the nightly porpoise activity.

The porpoise activity at Læsø Trindel and at a smaller reef ten kilometres away has been registered since 2006 during the summer months by means of so-called acoustic data loggers which capture the ultrasounds of the animals. The porpoises use these ultrasounds to echolocate prey and to navigate. They calculate close and remote distances by means of the reflected echoes from for instance fish or fixed structures. Echolocation is the porpoises' most important way of identifying out-of-sight objects.

Marine biologist Lonnie Mikkelsen has analysed the data and found that since the re-establishment of the reef more and more porpoises come to visit the reef searching for food especially during the night - contrary to the other nearby reef where the porpoises are detected in the daytime.

"As far as we can see, the re-established stony reef at Læsø Trindel provides the basis for a natural food source for the porpoises. This food source may consist of juvenile cods which, we known from another study, move into the reef at sunset and return to deeper waters at sunrise during the summer months" explains Lonnie Mikkelsen.

Her colleague, marine biologist Karsten Dahl, who has followed the development at the reef since 2008, has just finished sampling and observing the animal and plant life development at the reef for this year. The outlook for the reef is positive.

"For the first time, we saw seals at the reef, they are also an indicator of the positive development of the re-established reef" says Karsten Dahl, whose report on the 2012 monitoring of the reef will be available in the beginning of 2013.

Contact: Marine biologist Lonnie Mikkelsen, mobile phone +45 8715 8716, lomi@dmu.dk
DCE - Danish Centre for Environment and Energy
Department of Bioscience, Aarhus University.

Marine biologist Karsten Dahl, tel. +45 8715 8566, kda@dmu.dk,
DCE - Danish Centre for Environment and Energy
Department of Bioscience, Aarhus University.

Re-established stony reef in Kattegat, Denmark attracts harbour porpoises (Phocoena phocoena). Lonnie Mikkelsen. 2012. Aarhus University. 38 pp.

More information about the Blue Reef project at Læsø:
Ecology of Læsø Trindel – a reef impacted by extraction of boulders