Using standardised counting methods for seabirds to monitor marine mammals in the Dutch North Sea from fixed platforms
In the period 2003-2011 offshore platforms in the North Sea have been successfully used for ornithological monitoring programs following standardised observation protocols. As most seabird observers have a strong interest in other large marine animals, marine mammal observations were included in these systematic observation protocols for recording seabirds. Some of these programs ran for several years and the collated sightings have the potential to contribute to the knowledge on at-sea distribution of marine mammals around these platforms. However, detection of marine mammals from offshore platform has several limitations. This study showed that detection significantly increased with increasing altitude of the viewing platform and decreasing sea state. Also, in instances where observers aim to record both seabirds as well as marine mammals the latter have the potential to be missed during busy periods. Nevertheless, a total of 167 platform-based sightings of three species of marine mammal were collected during standardised counts on 132 observation days between 2003 and 2011. The ‘seabird’ observation protocols used have limitations for monitoring marine mammals but were useful to elucidate trends. In this study it was demonstrated that densities recorded from platforms were up to three orders of magnitude lower than during dedicated aerial marine mammal surveys but were comparable to ship-based surveys and aerial surveys that recorded both birds and marine mammals. Provided that limitations are taken into consideration, fixed platforms can provide suitable observation bases for recording the presence, relative abundance and seasonal changes of marine mammals in offshore environments. The potential application of survey data collected from platforms is discussed and recommendations for future work with the observation protocols used are given.