156 M
. Lindegarth et al. J. Exp. Mar. Biol. Ecol. 245 2000 155 –169
1. Introduction
Physical disturbance due to mobile fishing gear is a serious and growing threat to diversity and production in marine benthic habitats e.g. De Groot, 1984; Jones, 1992;
Lindeboom, 1995; Kaiser, 1998; Lindeboom and De Groot, 1998. Gullmarsfjorden on the West Coast of Sweden was closed to trawling for shrimp, Pandalus borealis Krøyer
in 1990. The ban was introduced to protect the fjord as an important reference area for scientific study and to protect areas deemed valuable for recruitment of commercial fish
stocks. To guide future management, regional government authorities decided in 1995 that a large-scale experiment on potential effects of trawling on benthic fauna should be
done in the area.
This experiment is one example of many recent studies on potential effects of mobile fishing gear on benthic diversity and production e.g. Bergman and Hup, 1992;
Brylinsky et al., 1994; Thrush et al., 1995; Currie and Parry, 1996; Kaiser and Spencer, 1996; Tuck et al., 1998. Most of these experiments involve monitoring biological
response variables before and after trawling. Abundances of organisms may, however, change naturally. Therefore tests of hypotheses about effects of mobile fishing gear also
require contrasting the magnitude of temporal change in fished areas to change in undisturbed, control areas Green, 1979; Bernstein and Zalinski, 1983; Stewart-Oaten et
al., 1986; Underwood, 1992. Several recent experiments on effects of trawling and dredging have involved pair-wise comparisons of one ‘control’- and one ‘treatment’-site
e.g. Thrush et al., 1995; Currie and Parry, 1996; Tuck et al., 1998. These investigators recognise that such experimental designs lack independent replicates of the treatments
and therefore changes in response variables can not logically be attributed to the experimental treatment Hurlbert, 1984; Underwood, 1991, 1992. Nevertheless, differ-
ences in temporal change found between sites have been interpreted from these experiments as if they were caused by the experimental treatment. Such conclusions are
not justified unless explicit comparisons show that repeated experimentation demonstrate similar types of effects.
The validity of experimental designs using one control and one potentially impacted area Before-After-Control-Impact ‘ BACI’, Green, 1979 and Before-After-Control-
Impact-Paired-Series ‘BACIPS’, Bernstein and Zalinski, 1983; Stewart-Oaten et al., 1986 rests on the assumption that what happens in the control area would also have
happened in the potentially impacted area, were it not for the experimental treatment Stewart-Oaten, 1996. Unless more than one control area is monitored, it is however
not possible to evaluate the plausibility of this assumption and how potential violations affect quantitative and qualitative conclusions about effects of experimental treatments
Underwood, 1992, 1994. It is therefore very important to investigate the extent of variability in temporal change among potential control areas and what practical
consequences such variability may have for the interpretation of experiments on effects of mobile fishing gear on benthic soft-sediment fauna.
This experiment was designed to obtain quantitative samples of benthic fauna in multiple experimental and control sites before and after the start of shrimp-trawling in
Gullmarsfjorden. The experiment allowed unconfounded tests of specific hypotheses about effects of trawling abundances of benthic animals. The types and sizes of impacts
M . Lindegarth et al. J. Exp. Mar. Biol. Ecol. 245 2000 155 –169
157
observed in this experiment are reported elsewhere Lindegarth et al., in press, Hansson et al., in review. In this paper, we explore problems associated with interpretation of
results that are obtained from experiments using unreplicated treatment and control sites. This is done by comparing conclusions from analyses of the whole experiment i.e.
including all sites and from analyses of pairs of sites. Analyses of total abundance, number of species and abundances of the most abundant species of polychaetes,
echinoderms and bivalves are used as examples.
2. Materials and methods
