In 2010-2011, aerial surveys were conducted under the umbrella of the Shortlist Masterplan Wind programme. The aim of these aerial surveys was to assess the seasonal abundance and distribution of harbour porpoises Phocoena phocoena on the Dutch Continental Shelf (DCS), and how their distribution varies in space and by season. Such information is vital if we are to understand, quantify and eventually minimize the effect of human activities, i.c. offshore wind farms, on harbour porpoises.
Three complete aerial surveys of the DCS were conducted along predetermined track lines, in summer (July 2010), late autumn (October/November 2010) and early spring (March 2011). The surveys covered 16013 km on effort (in search modus for marine mammals, Figure 3). In total 1085 sightings (1236 animals) of harbour porpoises were recorded (Table 2), 5 sightings of white-beaked dolphins Lagenorhynchus albirostris (8 animals) and 64 sightings (66 animals) of grey Halichoerus grypus and harbour seals Phoca vitulina (Figure 7). Mother-calf pairs of porpoises were mostly sighted in July, around and west of the wind farm survey area W1 (Figure 6), suggesting that porpoises reproduce in Dutch waters. The data was analysed with standard distance sampling methodology. The resulting density estimates of harbour porpoises for the DCS were 0.44 animals/km² in July, 0.51 animals/km² in October/November and 1.44 animals/km² in March. This means total numbers for the entire DCS (Table 4) of ca. 26000 animals in July (95% Confidence Interval (C.I.): 14000-54000), ca. 30000 in October/November (C.I.: 16000-59000) and ca. 86000 in March (C.I.: 49000-165000). These numbers represent a substantial part of the population where the Dutch porpoises belong to, the so-called management unit South-western North Sea and the Eastern Channel. Based on the SCANS II data from 2005 the estimated number of porpoises in this management unit is less than ca. 180000 animals. Using these figures, the Dutch national waters in March thus comprise at least 48% of the population present in the central and southern North Sea. In July this proportion drops to at least 14%.
Maps of the spatial distribution of harbour porpoises on the DCS have initially been constructed by correcting the data for observation effort. Subsequently, for each of the three surveys, a model was constructed to correct the data for the additional effect of environmental factors (such as location, cloud cover, time of day and sea state) on the sighting rate. This model has been used to predict the distribution of porpoises over the DCS. The results of this prediction (Figs. 13, 15 & 17) give the best estimate of the ‘true’ distribution of porpoises (at the moment the surveys took place) that can be distilled from the data collected in this study. However, the quality of these model predictions depends strongly on the covariates included in the model. This model presented here is a first step towards understanding which covariates influence the sighting rate. In due time our understanding of the behaviour and distribution of harbour porpoises will probably improve, and as a consequence better distribution estimates may be obtained in the future. The modelled distribution of harbour porpoises was not uniform within the DCS and shows strong intra-annual variability (compare figures 13, 15 & 17). In March 2011, high densities were found in the whole DCS, except for Zeeland and in close proximity of the mainland coast. These higher density areas thus include the wind farm survey areas W1 and to a lesser extent W2 (Figure 17). In July, high densities were found near the Brown Ridge, Botney Cut- Dogger Bank and Borkumer Reef (Figure 13). In October, distribution seems more spatially homogeneous. Mother-calf pairs were mostly sighted in July, around and west of the wind farm survey area W1 (Figure 15). Repeated surveys are deemed necessary to ascertain if the established patterns are consistent.