Abstract

Danish coastal waters are a hotspot for recreational boating and harbour porpoises (Phocoena phocoena), but as there is little data on the boats’ speeds and distributions, it is difficult to determine where porpoises are likely to be affected by boat noise. To gain knowledge on the potential impacts of small boats we 1) used coastal observations to assess how boat speed is related to boat size, type, distance to marinas, and wave height, and 2) used satellite images to assess the spatial distribution of small boats in Kattegat, the Sound, the Belt Sea and the Western Baltic areas (see map in Sveegaard et al., 2015). This made it possible to determine how the probability of observing boats was related to distance to the coast and number of people living within a radius of 5 km from a given area. We found that boat speeds varied among marinas and that speed was positively correlated with distance to the marina. The relationship between boat speed and size depended on boat type, with motorboats being the fastest. The satellite image analysis showed that motorboats were the dominant type in coastal waters, constituting 61.4% of the observed boats. The probability of observing boats on a satellite image covering 0.25 km2 dropped from 0.13 by the shore to 0.04 at a distance of 5 km from shore. The probability of observing boats was >10% in 6.6% of the study area, and the probability of observing boats and porpoises in the same area was >10% in 3.0% of the study area. Coastal waters are important habitats for many aquatic species, e.g., fish, invertebrates and porpoises, that use sound for communication or navigation. Our study uses a combination of observation and satellite data to identity areas where boats overlap with harbour porpoise presence and thus where the animals may be affected by boat noise. The study also illustrates how combining these two methods can yield important information on the potential impacts of boats on wildlife.