The harbour porpoise Phocoena phocoena in the Belgian part of the North Sea: trends in abundance and distribution

Jan Haelters; Francis Kerckhof; Thierry G. Jacques; Steven Degraer

doi:10.26496/bjz.2011.145

Authors

Jan Haelters Royal Belgian Institute of Natural Sciences (RBINS), Management Unit of the North Sea Mathematical Models (MUMM), 3e en 23ste Linieregimentsplein, B-8400 Ostend, and Gulledelle 100, B-1200 Brussels, Belgium
Francis Kerckhof Royal Belgian Institute of Natural Sciences (RBINS), Management Unit of the North Sea Mathematical Models (MUMM), 3e en 23ste Linieregimentsplein, B-8400 Ostend, and Gulledelle 100, B-1200 Brussels, Belgium
Thierry G. Jacques Royal Belgian Institute of Natural Sciences (RBINS), Management Unit of the North Sea Mathematical Models (MUMM), 3e en 23ste Linieregimentsplein, B-8400 Ostend, and Gulledelle 100, B-1200 Brussels, Belgium
Steven Degraer Royal Belgian Institute of Natural Sciences (RBINS), Management Unit of the North Sea Mathematical Models (MUMM), 3e en 23ste Linieregimentsplein, B-8400 Ostend, and Gulledelle 100, B-1200 Brussels, Belgium

DOI:

https://doi.org/10.26496/bjz.2011.145

Keywords:

harbour porpoise, Phocoena phocoena, Belgian part of the North Sea, abundance, seasonal distribution

Abstract

The harbour porpoise Phocoena phocoena is currently common in the Belgian part of the North Sea (BPNS), after decades of virtual absence. This article describes the results of aerial surveys to assess its distribution and density in Belgian waters, in concert with the first results of tests with passive acoustic monitoring devices (Porpoise Detectors - PoDs), and a basic analysis of strandings. The strandings data over four decades clearly demonstrate an increase of harbour porpoises in the BPNS, with only few stranded animals between 1970 and 1997 (0 to 6 per year), increasing numbers between 1998 and 2004 (8 to 40 per year), and even higher numbers between 2005 and 2009 (62 to 94 per year). The combined results of aerial surveys, strandings monitoring and tests with passive acoustic monitoring reveal a seasonal pattern, with harbour porpoises being abundant from February to April, and more scarce from May to January. Average densities in 2008 and 2009, as estimated by aerial monitoring covering most of the BPNS (with the exclusion of the nearshore 5 km strip), ranged from 0.05 animals per km², or in total less than 200 animals in an area equivalent to the BPNS in August 2009, to 1.01 animals per km², or in total almost 3,700 animals in an area equivalent to the BPNS during April 2008. In the first quarter of the year porpoises occur throughout the BPNS, including territorial waters (12 mile zone), whereas they are restricted to more offshore and northerly waters later in the year. Erratic invasions in the BPNS however blur general seasonal spatio-temporal patterns, which complicates our understanding of spatial distribution and migration.

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