Monitoring techniques

Monitoring objectives

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 RoxAnn^TM, towed video or ROV. In practice, many beds will be located on grounds where the use of towed video is difficult or impossible, and RoxAnn^TM may prove to be the most effective method. For ground-truthing of RoxAnn^TM 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.

RoxAnn^TM 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):

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. RoxAnn^TM 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 circumstances.

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.

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.

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 changes.

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.

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