Determining the Water Column Usage by Seals in the Brims Lease Site

Report

Title: Determining the Water Column Usage by Seals in the Brims Lease Site
Publication Date:
November 29, 2017
Document Number: 0822
Pages: 30
Sponsoring Organization:
Technology Type:

Document Access

Website: External Link
Attachment: Access File
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Citation

Evers, C.; Blight, C.; Thompson, D.; Onoufriou, J.; Hastie, G. (2017). Determining the Water Column Usage by Seals in the Brims Lease Site. Report by Sea Mammal Research Unit (SMRU). pp 30.
Abstract: 

The report describes how adult harbour seals and grey seal pups use the water column within the Brims lease site.

  1. Telemetry data from 12 adult harbour seals and seven grey seal pups diving within the Brims lease site were analysed to extract descriptors of dive behaviour.
  2. Dive data were summarised to provide estimates of the proportion of time seals spent at different depths relative to the sea surface and relative to the seafloor. In addition to estimates of the proportion of time at depth, the number of times seals transited through different depth bins relative to the sea surface and sea floor was also determined.
  3. Water depths at dive locations were estimated from high resolution bathymetry data corrected for tide height at that time and place.
  4. Harbour seals spent approximately 31 % of their time at intermediate depths between 10 m to 25 m from the surface, with a secondary but less pronounced peak between 75 m and 80 m from the surface. When expressed as a distance from the seafloor, harbour seals spent the highest proportion of time within 5 m of the seafloor (mean = 0.16; 95 % CIs: 0.04-0.46) with a secondary peak between 65 m and 70 m from the seafloor (mean = 0.12; 95 % CIs: 0.03-0.26) indicating a significant amount of mid water swimming by harbour seals at this site. Harbour seals spent on average 12.8 % of their time in the band between 5 m and 25 m above the sea floor.
  5. For grey seal pups, the proportion of time spent in depth bands decreases monotonically with depth and the amount of time spent close to the sea floor was lower than in harbour seals. Grey seal pups spent on average 6.5 % of their time in the band between 5 m and 25 m above the sea floor.
  6. Harbour seals in the Brims site entered or transited through the zone between 5 m and 25 m from the sea floor on approximately 27 % of dives. Grey seal pups entered the depth bands closest to the seafloor less frequently than did the harbour seals.
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