NIOZ Reports (2011)


– This is a conservation plan for the Harbour Porpoise Phocoena phocoena based on the current
seasonal occurrence and abundance of porpoises within waters under Dutch jurisdiction.
– The conservation status of the Harbour Porpoise in The Netherlands has recently been evaluated as
‘Inadequate’, and the population as ‘Vulnerable’.
– From 1900 to the early 1950s, Harbour Porpoises were abundant and widespread in coastal waters
throughout the southern North Sea, including Dutch waters. The animal declined and was considered
locally extinct during the 1960s-1980s.
– Harbour Porpoises have increased markedly in numbers in the southern North Sea in recent decades.
Given changes in distribution and abundance, the conservation status of porpoises in Dutch waters is
likely to require an update in the near-future.
– Harbour Porpoises are most abundant in relatively shallow sea areas and often forage near or at the
sea bottom. Their prey spectrum includes pelagic and demersal prey species: different species of
fish, cephalopods, crustaceans and polychaetes.
– The global abundance of the Harbour Porpoise is at least about 700,000 individuals. Within the North
Sea at large, in 2005, total abundance was estimated at 230,000 individuals. A marked change in
distribution was found, with considerably larger numbers of porpoises in the southern half of the
North Sea in the 2005 than during an earlier census in 1994.
– Aerial surveys covering 50% of the Dutch sector of the North Sea produced 37,000 Harbour
Porpoises in Feb-April 2009 and 56,000 in Mar 2010.
– The current Harbour Porpoise conservation plan is a generic plan rather than area-orientated: recent
research in Dutch waters failed to identify areas or regions of particular ecological significance for
Harbour Porpoises for any significant length of time.
– Incidental capture in fishing gear (bycatch) is considered to be the most significant threat to Harbour
Porpoise populations worldwide. In The Netherlands, some 150-250 animals washing ashore per
annum are at least bycatch-suspect.
– The main type of fishing gear responsible for drowning is currently unknown, but set-nets (passive
gear) are the main suspects. Bycatches occurred year-round and throughout the study area.
– The catch composition during which most porpoise strandings occurred varied and no set-net
fisheries should be excluded a priori from an observer scheme. An onboard observer scheme should
be established with priority in the winter fisheries, Dec-Mar, in the northern coastal zone (IJmuiden-
– While there is concrete evidence for avoidance behaviour of loud (explosive) underwater sounds
(such as pile driving for windfarm construction, seismic exploration, underwater explosions, and
naval sonar operation), there is no factual evidence for lethal damage. Adequate studies of hearing
damage and death as a result of underwater sound are lacking.
– The distributional shift of Harbour Porpoises from more northerly parts of the North Sea into the
Southern Bight may have been caused by a reduction in available prey in the north. Studies of the
ecology of Harbour Porpoises in the southern North Sea are required to shed more light on prey
availability and resources (stocks).
– Siting, vessel strikes, the operational phase of windfarms, offshore mining, marine litter, chemical
pollution, (chronic) marine oil pollution, natural predators, infectious disease, and parasites are all
issues of concern that may in part require additional study, none of which required local (or regional,
i.e. on a southern North Sea scale) mitigation measures, but rather on a higher governance level.
– None of the demonstrated threats can be quantified satisfactory, given the slender factual data
currently at hand. It is obvious that further research is required, before effective mitigation
measures can be proposed and the precautionary approach (UNESCO 2005) could be the safest way
– The Harbour Porpoise is legally protected in The Netherlands following international, European and
national legislation, although the patchiness of current policy does not benefit an adequate
protection of the Harbour Porpoise.
– Implementing the research and mitigation measures, as advised in this species conservation plan,
serves to fulfil the requirements of The Netherlands under the relevant international legal treaties.

Current research needs have been prioritised on a scale from 1 (highest) to 5 (lowest). Following discussions
in Chapter 9, the following research needs are listed with the highest (1) priority
– Assessments of Harbour Porpoise population through state of the art aerial surveys, including
analysis of seasonality and spatial patterns
– Innovative studies of the (foraging) ecology and habitat requirements of Harbour Porpoises in the Southern
North Sea
– Prioritise an observer scheme on all fleets with passive gear to assess bycatch rates according to
internationally accepted protocols
– Continue to assess bycatch rates in the most important fisheries (regarding bycatch) and evaluate
the effectiveness of mitigation measures
– A national scientific research steering group would be a suitable instrument to deal with
aspects such as research needs, research quality and the evaluation of the quality and conclusions of
study reports. Such a steering group should be sufficiently authoritative, but also sufficiently
“distant” from the ongoing research. We propose that such a committee should meet and advice
annually, and be composed of at least two foreign marine mammals experts, one Dutch Harbour
Porpoise expert, and (vitally) one statistician.

Recommended policy and mitigation measures are categorized into measures that should be applied at
present and measures that depend of further knowledge from the suggested scientific research. Regarding
bycatch in fishing gear:
– Investigate alternative gear other than set-nets and/or investigate modification of set-nets; controlled use
of pingers when bycatch is identified
– Facilitate bycatch landing
– Restrictions in recreational fisheries, control illegal fisheries
– Amend EC 812/2004
– Monitor and control compliance fisheries restrictions
Regarding underwater noise (detonation, seismic, sonar, pile driving)
– Develop a system of standards for loud explosive sounds; alert animals ramping up sounds, use acoustic
– License and guidelines seismic surveys, pile-driving, underwater explosions; establish porpoise observer
schemes before during and after and notification of the strandings network prior to acoustic impacts
– Reduce noise using bubble curtains, solid barriers, other solutions if proven to be effective; avoid
explosives and use an alternative method for windfarm demolition