Provisional Monitoring Scheme Relevant to SACs
Interpretation of change
Having outlined the capabilities and limitations of the various
monitoring techniques, it is possible to suggest a provisional scheme by which the extent
and composition of brittlestar aggregations within an SAC could be assessed. It is assumed
that although the main responsibility for this work will rest with a local SAC site
officer, manpower and technical support will be available from the relevant national
conservation agency (EN, SNH, CCW, DO/NI) from the JNCC, and ideally also from co-workers
in academic institutions or public sector marine laboratories.
Establishment of the existence of brittlestar beds within an SAC or any
other defined coastal area is likely to be fairly easy. Initial information can come from
historical records, local fishermen, diving surveys, remote sampling or camera
observations. This stage has been achieved for the candidate SACs covered here. The basic
composition of the community is also easily established, as the major bed-forming
brittlestars are distinctive and unlikely to be confused with each other. Other large
epifauna such as crab and starfish species can be readily identified first-hand in the
field or on videotape. If the beds are to form part of an overall SAC biotope monitoring
programme, the most important parameters to measure will be the spatial location and
extent of these features and the population density of brittlestars within them. The
occurrence of potential agents of change (eg. predators such as Luidia ciliaris)
and the scale of human activities in or near the SAC (eg. sewage outfalls, salmon farms)
will also form part of the programme.
Measurement of the spatial extent of beds over large areas could be
achieved using RoxAnnTM, towed video or ROV. In practice, many beds will be
located on grounds where the use of towed video is difficult or impossible, and RoxAnnTM
may prove to be the most effective method. For ground-truthing of RoxAnnTM
findings, ROV observations are probably advisable as a standard method, since many beds
are in waters beyond effective diving range. Even where depth allows, diving is probably
unnecessary for routine monitoring unless is it desired to collect brittlestars or to
sample the fauna underlying the beds. Brittlestar densities can be estimated from ROV
recordings, unless the beds are extremely dense (ie. with multiple layers of animals), in
which case grab sampling could be carried out at selected points (where substratum type
allows) across the extent of the aggregation.
RoxAnnTM can clearly distinguish dense Ophiothrix
fragilis aggregations (Magorrian et al., 1995), but is less likely to detect
relatively low-density beds such as those commonly formed by Ophiocomina nigra.
Where these are present, visual surveys using methods appropriate to the depth and
substratum type (towed video, ROV or diving) will be the best techniques to use.
Quantitative measurement of the extent and density of a brittlestar bed
on a yearly or twice-yearly basis, over a period of several years, would be a relatively
simple undertaking, but is one that has seldom been carried out. Data of this kind would
reveal whether any changes are taking place that can be related to coastal modifications,
predator abundance, nearby inputs of organic matter, or other pollutant sources. On Ophiothrix
beds, monitoring of recruitment would also be valuable and could be achieved quite
easily by counting juveniles on the arms of adult brittlestars collected during the peak
settlement period (September/October).
The basic elements of a monitoring programme for a brittlestar bed
should be (with appropriate techniques indicated):
Spatial location and extent of beds (RoxAnnTM, ROV)
There are no quantitative records of long-term changes in the size and
location of individual brittlestar beds (as opposed to broader-scale observations of their
presence in a particular geographic area). Previous mapping exercises have not been
sufficiently detailed to determine whether beds show subtle changes in location over time.
RoxAnnTM mapping will be the most appropriate method to use for dense Ophiothrix
beds with a strong acoustic signature. Lower-density aggregations such as
those typical of Ophiocomina or Ophiura will require visual observation, for
which an ROV is probably the tool most useful in the likely range of environmental
Brittlestar population density (Grab-sampling, diving, ROV)
ROV observations will often allow the measurement of brittlestar
densities, but the non-random selection of counting areas may introduce some bias in the
results. Grab-sampling at randomly-selected points (or, better still, at regularly-spaced
points along one or more transects) within the mapped extent of a brittlestar bed will
eliminate this source of bias, and allow estimation of the degree of patchiness within the
bed. If the substratum is unsuitable for the use of a grab, diver counts (or
suction-sampling, in very dense aggregations) in quadrats at points along a transect could
be made if water depth allows. If water depth is too great for diving and the substratum
too hard for grab-sampling, densities will have to be estimated from ROV recordings, with
care taken to avoid bias in the areas of sea bottom used for counting.
The various bed-forming brittlestar species are easily recognized and
the species composition of a mixed bed can therefore be determined from visual
observations or grab samples.
The same methods and qualifications will apply if it is intended to
record the densities of large epifauna such as crabs and starfish within the brittlestar
bed. In this case, measurements of densities in areas outside the brittlestar bed could be
made for comparative purposes.
Brittlestar population structure and recruitment (Grab- or diver
For Ophiothrix fragilis, rates of recruitment could be quite
easily monitored by counting the numbers of newly-settled juveniles found on the arms of a
sample of adult brittlestars collected using a grab or by divers (depending on the
substratum). The best time to carry out such a survey would be in September/October, the
period of peak recruitment indicated in most studies.
Regular monitoring of population structure in any species would involve
measurement and analysis of body size (represented by disc diameter) in samples of animals
collected at intervals throughout the year (eg. Ball et al., 1995). This would probably be
too time-consuming to be included within a routine SAC monitoring programme, and is more
within the capabilities of a marine research laboratory or other academic institution.
Interpretation of change
Care will be needed when monitoring spatial extent and population
density of beds. Due to the patchy distribution of brittlestars within many beds, and the
fact that patch sizes and locations will change over time, there may be a danger of
confusing natural changes with those caused by human activities. It would be advisable to
monitor for a number of years to establish a baseline, prior to any attempt to interpret
The limited information available suggests that the abundance of
predators (particularly Luidia ciliaris) will be the most important
naturally-occurring agent of change to be included within a monitoring programme. Starfish
abundance can be easily estimated by diving or ROV surveys. With respect to monitoring
human-induced effects, the activities with the greatest potential to cause change in bed
location and extent are probably organic pollution (eg. from sewage disposal or
aquaculture) and coastal alteration processes such as dredging or breakwater construction.
The extent of these activities in and around a marine SAC will presumably be routinely
monitored at each site, and their effects can be assessed against simultaneous records of
the location and extent of brittlestar beds in the area.