The Number and Distribution of Marine Mammals in the Fall of Warness, Orkney July 2005 - July 2006


Title: The Number and Distribution of Marine Mammals in the Fall of Warness, Orkney July 2005 - July 2006
Publication Date:
August 01, 2006
Pages: 45

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Duck, C.; Black, A.; Lonergan, M.; Mackey, B. (2006). The Number and Distribution of Marine Mammals in the Fall of Warness, Orkney July 2005 - July 2006. Report by SMRU Consulting. pp 45.
  • Land-based surveys of the Fall of Warness were carried out from 11th July 2005 to 14th July 2006, encompassing a total of 219 days, and 964 hours of observation. Two observers recorded the information. The change of observers took place on 22 nd October 2005.
  • Since sighting probabilities of marine mammals would be strongly influenced by sea state, wind speed was used as a proxy to remove from the analyses any observation periods when sighting probabilities would be compromised. This reduced the total observation period used in the analyses to 731 hours.
  • Grey seals, harbour seals, harbour porpoises, minke whales, white-beaked dolphins and basking sharks were all seen during the study period. All species of marine mammal (and basking sharks) observed in the study area are protected under international legislation.
  • Generalised Additive Models (GAMs) and Chi-squared (X2) tests were used to estimate the patterns underlying the seal and harbour porpoise observation data. There was insufficient information on the other species to support anything beyond descriptive statistics.
  • Grey seals were the most frequently seen species, with a significant peak in occurrence during September and October (their breeding season). Sightings were concentrated close to the shore area, especially to Muckle Green Holm. The probability of sighting grey seals was lower during windy conditions but did not appear to be influenced by state of tide or time of day.
  • Harbour seals were rarely seen in August, and not at all in November and December. The former may be due to seals remaining ashore during their annual moult (August) and the latter to a change in observer during the study (from 22nd October). A strong distance and location effect was observed in sightings, probably as a function of a reduced ability to identify species with distance. The small numbers of sightings was probably responsible for the lack of any other significant factors in the data analysis.
  • Harbour porpoises were seen in the study area between July and November, mainly in small groups. Observations were concentrated in both the centre of the study area and close to the observation point. The small number of sightings of this species meant that it was not possible to determine any other significant factors in the data analysis.
  • Occasional sightings of minke whale, white-beaked dolphin and basking shark are consistent with existing knowledge of these species behaviour and distribution in Orkney waters.
  • The complexity of the patterns observed using GAMs suggests that straightforward comparisons of average results between times and location is unlikely to be productive. It would be more efficient to repeat these analyses using the additional data collected under changed conditions (e.g. after the installation of a tidal flow device). This would examine the significance of the effect of the change in conditions but it would also be essential to include this as an integral part of the testing regime for any device because of the importance of including the state of the device (e.g. operational versus non-operational) as a covariate in any analysis.
  • The potential effects of tidal stream devices on large-bodied animals in the water column are a subject of considerable concern that requires a careful approach to management and mitigation. The methodological approaches developed in this study to help assess these effects are at the leading edge of the field. We suggest that they have broad application to most circumstances in which tidal power generation is being considered.
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