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.