Environmental Engineering Reference
In-Depth Information
Box 2.2
Plot sampling in action: underwater visual census for fish.
Underwater visual census (UVC) is a standard technique used to measure the
abundance of fish in clear, shallow water, e.g. over coral reefs (English
et al
. 1997;
Figure 2.2). Ashworth and Ormond (2005) used this method to assess the efficacy
of a marine protected area (MPA) in the Egyptian Red Sea. Transects of 100 m
were swum by divers, counting and identifying all fish within 5 m either side, giv-
ing sampling plots of 1000 m
2
. This is somewhat larger than the plots generally
used in other UVC studies, a deliberate strategy to reduce the variability in counts
between plots, and so increase the precision of estimates. However, this strategy
carries the risk of missing significant numbers of fish, so underestimating density,
and is therefore justified only where there is exceptional visibility. Because of the
diversity of species covered, it was not possible to complete each transect in one
pass. Transects were therefore swum several times, focussing each time on one or a
few related fish families. Transects were placed inside and outside the MPA, and at
three different depths. Twelve transects were placed in each depth/location combin-
ation, and the mean fish density (per m
2
) for each combination is therefore the
total number of fish seen divided by 12,000. At the shallowest depth, seven fish
families had significantly lower abundance outside the MPA, but this difference
was not seen in deeper water, reflecting lower fishing pressure at this depth.
Fig. 2.2
Divers in northern Mozambique counting fish in strips of fixed width
in order to estimate abundance. Training in distance estimation is required in
order to be able to judge whether fish seen are within the strip. The method
only works in very clear water. Photos: © Cabo Delgado Biodiversity and
Tourism Project.
