Restore habitat for marine and freshwater mammals

How is the evidence assessed?
  • Effectiveness
  • Certainty
  • Harms

Study locations

Key messages

  • One study evaluated the effects of restoring habitat for marine mammals. The study was in the Kattegat sea (Denmark).



  • Abundance (1 study): One before-and-after study in the Kattegat sea found that harbour porpoise activity increased at a stony reef after it was restored with boulders and fishing was prohibited.


About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A before-and-after study in 2006–2012 of a stony reef in the Kattegat sea, Denmark (Mikkelsen et al. 2013) found that restoring the reef, along with prohibiting fishing, resulted in harbour porpoise Phocoena phocoena echolocation clicks being recorded more often and for longer periods than before restoration and protection. The average number of minutes with porpoise recordings and the average duration of porpoise encounters were higher at the reef in each of four years after the reef was restored and fishing prohibited (13–15 minutes/day; 4–5 minutes) than during two years before (6–10 minutes/day; 3 minutes). Porpoise activity at an intact reef 10 km away decreased over the same period (‘before’: 11–15 minutes/day; ‘after’: 3–7 minutes/day). In June–September 2008, a total of 100,000 t of norite boulders were dumped over 19 days to restore a 45,000 m2 cavernous stony reef. Fishing was prohibited around the restored reef in 2009–2012. Porpoise activity was recorded with acoustic data loggers (two at the restored reef; two at an intact reef) for 33–75 days in June–August in each of two years before restoration and protection (2006 and 2007) and each of four years after (2009–2012).

    Study and other actions tested
Please cite as:

Berthinussen, A., Smith, R.K. and Sutherland, W.J. (2021) Marine and Freshwater Mammal Conservation: Global Evidence for the Effects of Interventions. Conservation Evidence Series Synopses. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

List of journals searched by synopsis

All the journals searched for all synopses

Marine and Freshwater Mammal Conservation

This Action forms part of the Action Synopsis:

Marine and Freshwater Mammal Conservation
Marine and Freshwater Mammal Conservation

Marine and Freshwater Mammal Conservation - Published 2021

Marine and Freshwater Mammal Synopsis

What Works 2021 cover

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What Works in Conservation provides expert assessments of the effectiveness of actions, based on summarised evidence, in synopses. Subjects covered so far include amphibians, birds, mammals, forests, peatland and control of freshwater invasive species. More are in progress.

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