490
C. Johnston and A. Davison
south west). In these narrows very strong tidal currents occur, with currents at spring
tides up to 7 knots (kn) in Kyle Rhea, and 3 kn in Kyle Akin (Admiralty Chart 2540).
Freshwater inputs to the system can be high, and due to the sheltered nature of the
lochs, incoming freshwater is often not mixed within the water column. This may
result in up to around 6m depth of fresh water lying on top of salt water within Loch
Long and upper Loch Duich at times of high freshwater inputs.
The biological interest of the two SACs relates to their differing
environmental conditions. Papa Stour cSAC was selected for its reefs and caves, with
biological communities characteristic of northern wave exposed conditions. Lochs
Duich, Alsh and Long pSAC was selected also for its reefs, but particularly those
characteristic of very sheltered conditions and subject to considerable freshwater
influence.
Surveys
Due to time and financial constraints, and the large areas of seabed needing to be
surveyed, Scottish Natural Heritage (SNH) required a broad scale survey strategy to be
adopted, and preferably one which could incorporate previous data. Remote sensing
was therefore required, however such methods using visual or other spectra are not
suitable to map seabed types in deep and turbid waters (Davies, 1997). Acoustic
remote sensing with analysis of data within a Geographic Information System (GIS)
appeared to be the best available option. The methodology selected was developed by
the BioMar team at the University of Newcastle-upon-Tyne, England (now BMap
team), and involved acoustic remote sensing, ground validated by remote video, grab
sampling and diver recording, with analysis of data and display of results within a GIS
(Davies et al., 1997). This combination of methods was also capable of incorporating
previous survey data where accurate site positions were available. Such biological data
were available from previous surveys by various methods for seven sites in Papa Stour,
and sixty eight sites in Lochs Duich, Alsh and Long.
The acoustic and ground validation surveys were carried out by Entec
personnel and subcontractors from 10th to 19th August 1996 for Papa Stour cSAC, and
from 15th to 22nd September 1996 for Lochs Duich, Alsh and Long pSAC.
An acoustic ground discrimination system (AGDS), combined with a Global
Positioning System (GPS) for position fixing, linked to a personal computer with
navigation and data collection software recorded data on acoustic properties of the
seabed at 5 second intervals. Whilst the boat traveled along a set path at a speed (over
ground) of approximately 4 kn., continuous sets of measurements (or tracks) of the
physical nature of the seabed were recorded and displayed on the computer using
navigation software.
For Papa Stour, the seabed over most of the survey area was at less than 30 m
bcd, with a maximum depth at the boundary of the cSAC of approximately 60 m bcd.
A RoxAnn Groundmaster™ AGDS was used, which analyses the primary and multiple
return echoes from a high frequency (200 kHz) echo sounder to quantify the depth,
roughness (echo 1 or E1) and hardness (echo 2 or E2) of the seabed (Chivers et al.
C. Johnston and A. Davison
south west). In these narrows very strong tidal currents occur, with currents at spring
tides up to 7 knots (kn) in Kyle Rhea, and 3 kn in Kyle Akin (Admiralty Chart 2540).
Freshwater inputs to the system can be high, and due to the sheltered nature of the
lochs, incoming freshwater is often not mixed within the water column. This may
result in up to around 6m depth of fresh water lying on top of salt water within Loch
Long and upper Loch Duich at times of high freshwater inputs.
The biological interest of the two SACs relates to their differing
environmental conditions. Papa Stour cSAC was selected for its reefs and caves, with
biological communities characteristic of northern wave exposed conditions. Lochs
Duich, Alsh and Long pSAC was selected also for its reefs, but particularly those
characteristic of very sheltered conditions and subject to considerable freshwater
influence.
Surveys
Due to time and financial constraints, and the large areas of seabed needing to be
surveyed, Scottish Natural Heritage (SNH) required a broad scale survey strategy to be
adopted, and preferably one which could incorporate previous data. Remote sensing
was therefore required, however such methods using visual or other spectra are not
suitable to map seabed types in deep and turbid waters (Davies, 1997). Acoustic
remote sensing with analysis of data within a Geographic Information System (GIS)
appeared to be the best available option. The methodology selected was developed by
the BioMar team at the University of Newcastle-upon-Tyne, England (now BMap
team), and involved acoustic remote sensing, ground validated by remote video, grab
sampling and diver recording, with analysis of data and display of results within a GIS
(Davies et al., 1997). This combination of methods was also capable of incorporating
previous survey data where accurate site positions were available. Such biological data
were available from previous surveys by various methods for seven sites in Papa Stour,
and sixty eight sites in Lochs Duich, Alsh and Long.
The acoustic and ground validation surveys were carried out by Entec
personnel and subcontractors from 10th to 19th August 1996 for Papa Stour cSAC, and
from 15th to 22nd September 1996 for Lochs Duich, Alsh and Long pSAC.
An acoustic ground discrimination system (AGDS), combined with a Global
Positioning System (GPS) for position fixing, linked to a personal computer with
navigation and data collection software recorded data on acoustic properties of the
seabed at 5 second intervals. Whilst the boat traveled along a set path at a speed (over
ground) of approximately 4 kn., continuous sets of measurements (or tracks) of the
physical nature of the seabed were recorded and displayed on the computer using
navigation software.
For Papa Stour, the seabed over most of the survey area was at less than 30 m
bcd, with a maximum depth at the boundary of the cSAC of approximately 60 m bcd.
A RoxAnn Groundmaster™ AGDS was used, which analyses the primary and multiple
return echoes from a high frequency (200 kHz) echo sounder to quantify the depth,
roughness (echo 1 or E1) and hardness (echo 2 or E2) of the seabed (Chivers et al.