Broadscale surveys

Quadrat surveys

There are two basic monitoring formats appropriate to CFT biotopes.

Broad-scale surveys use abundance scales and check lists to record the presence and abundance of conspicuous species. This is the In situ surveillance of sublittoral (epibiota) biotopes using abundance scales and checklists at exact sites (ACE surveys), described in detail in Connor & Hiscock (1996) and Hiscock (1998b). The principle is that divers survey specified habitats in a defined location, and record the abundance of listed species. Abundance is recorded as density or percentage cover as specified for each species, subjectively averaged over the area of the survey. These values are then converted to points on a six-point abundance scale (see Hiscock, 1998a, Appendix 6, for details of scales appropriate to different types of organism).

The advantage of this method is that it enables rarer species as well as common ones to be assessed, and it provides a quantitative measure which can be compared with earlier surveys, or with other areas. It can also be carried out quite quickly and requires little specialist equipment beyond that needed for diving.

However, it has disadvantages. The results cannot be subjected to statistical analysis. Workers must have the required knowledge to identify species and assess abundance. There is considerable worker variability, though this can be limited by training, precise protocols, and careful definition of area of survey and included habitats. There is no permanent visual record for validation.

Despite the ‘noise’ in this method, with experienced operators it should be reasonably reliable. Differences of more than one abundance grade should be considered significant - however this does represent a very substantial change in abundance. It involves a change in density of two orders of magnitude, or a fourfold change in percentage cover.

Video recording with subsequent laboratory analysis is not a satisfactory substitute for in situ recording by divers. Species are likely to be missed, and variability will be increased. However both still and video photography can supplement the ACE procedures, and provide a permanent visual record.

This is the recording of the presence and abundance of species in small quadrats, typically 50 cm by 50 cm or similar in size. Recording can be done in situ, but photography is used in almost all programmes, and its use will be assumed here.

The initial decision is to choose between random and fixed quadrats. The advantage of random quadrats is that they can be subjected to extensive statistical analysis, including the calculation of confidence limits and the rigorous comparison of consecutive sampling exercises. Nevertheless, the environmental complexity of most CFT environments, and the very patchy and diverse distribution of biotopes, would pose serious problems. Given the level of effort which would be practicable, it is likely that the discrimination of a random sampling strategy would be coarse and unlikely to reveal subtle biological change (Lundälv, 1985). Lundälv argues the case for the use of fixed quadrats, which remove the problem of environmental heterogeneity, and permit the detection of relatively small changes. There are problems of the ‘representativeness’ of the selected quadrats, and that changes within them may be an artefact of biological processes such as switching between stable states, rather than of ‘real’ community change. There are constraints on the statistical analysis of time series based on fixed quadrats. However, the overall advantage of the fixed quadrat option is convincing, and previous and current programmes on subtidal rock use this procedure. It has the added value that information on the recruitment, growth rate, longevity and survival of individual species is obtained.

The basic procedure is to select and mark a range of quadrats, and then to photograph each quadrat on each sampling occasion. To ensure that the photos can be accurately compared the quadrats are marked by pegs inserted into the rock, and the camera located using a frame which fits over the pegs. Detailed procedural guidelines are given by Hitchcock (1998b) for straightforward photography. There are advantages in the use of stereophotography (Lundälv, 1971; Torlegård & Lundälv, 1974), which provides added information content: computer analysis facilitates the interpretation of such photographs. The underwater component of this form of monitoring can be carried out quickly once the initial establishment of the quadrats has been completed. However, the laboratory processing of the photographs is time consuming. For some purposes the use of image analysis systems may increase efficiency, as may the introduction of digital cameras.

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