Analysis of harbour porpoise data and evaluation of North Sea monitoring

This short report describes the method and results of the trend analyses that Statistics Netherlands performed in 2019 on various data sources of harbour porpoise observations in the North Sea in the period 1991/1992-2017/2018. At the end summary conclusions are drawn and some recommendations are given for the use of the data and possible adjustments to the monitoring.
There are various data sources for harbour porpoises occurring in the North Sea:
1. KRM porpoise counts (Marine Strategy Framework Directive) from an airplane by Wageningen Marine Research on behalf of the Ministry of Agriculture, Nature and Food Quality for a population estimate of the harbour porpoise for the entire Dutch North Sea; counts have been carried out irregularly since 2009, often several times per season, but since 2014 only in July;
2. observations during the MWTL seabird counts (Monitoring Water Management Status of the Lands) from an aircraft that are carried out six times a year by Bureau Waardenburg / Delta Project Management on behalf of RWS (Directorate-General for Public Works and Water Management); porpoise counts are a kind of "by-catch";
3. observations along the coast by volunteers during the counting of migratory sea birds (;
4. stranding data; these should be regarded as an additional source of information, because these are largely dead animals. (including Naturalis / This short report does not elaborate on this information.

The most standardized measurements are the KRM harbour porpoise counts and the MWTL seabird monitoring. The difference between the two programs is mainly in the purpose of the programs: the KRM counts are completely aimed at determining the total population size of the harbour porpoise at any time as accurately as possible for the entire Dutch North Sea, while the MWTL monitoring aims at the determination of the trend by a very regular and systematic coverage both in space and time. It should be borne in mind here that the harbour porpoise must be regarded as "bycatch" of the MWTL seabird monitoring program.

The analyzes show that the MWTL aircraft counts yield statistically reliable trends for harbour porpoises, both for the entire Dutch North Sea and for sub-areas. Trends corrected for weather factors based on sea migration counts appear to provide the same picture as the trend from the MWTL counts and support the reliability of the MWTL trend. Due to the counting method with accurate positioning of the porpoises and the exclusive focus of the observers on harbour porpoises, the KRM counts are currently the most suitable for determining the international population size, partly due to the additional observations to correct for animals that reside under water.