Environmental Engineering Reference
In-Depth Information
are not present. However, dewatering is of course entirely unselective, and restock-
ing with desired species is necessary following a suitable period of conditioning
to allow the recovery of invertebrate and plant communities. Partial dewatering
is also eff ective to enhance the e
ciency of physical removal by netting or other
methods because capture e ciency increases as the stocking density is increased
by dewatering.
A more drastic but equally simplistic measure is the use of explosives to elimin-
ate nuisance fi sh in ponds and has been tried with limited success. Apart from the
obvious risk of collateral habitat damage, the success of explosives depends on the
ability to concentrate the explosive charge throughout the water body. h is tech-
nique is more eff ective in small, shallow water bodies and relatively ineff ective in
deeper water.
More common methods for physical removal are passive capture techniques
such as netting and angling, or active methods such as electrofi shing (Fig. 13.2).
Netting techniques and gear may be targeted to particular species and may some-
times be eff ective in removing nuisance fi sh without harming non-target species
because of the selectivity of the techniques with respect to net type, mesh size, and
the behaviour of target and non-target species. Some net types, and electrofi sh-
ing, also off er the opportunity to release non-target species relatively unharmed
whereas gill nets, trammel nets, and seines either cause signifi cant damage or kill
fi sh. Gill and trammel nets (Fig. 13.3) are also highly size selective (Hamley 1975),
based on net mesh size, so that diff erent meshes may be required to target diff er-
ent life stages of the species of concern. Trap nets and electrofi shing are generally
less size-selective depending on net design and electrofi shing current parameters.
Formicki
et al
. (2004) observed that the attachment of magnets to the entrance of
fyke nets enhanced capture e
ciency by 50% or more. It is well established that
many fi sh are able to detect weak magnetic fi elds and this represents possibilities
for controlling the movement of fi sh and improving the e ciency of fi sh removal
eff orts although this is completely untested.
Electrofi shing is a very common method for surveying fi sh communities but can
cause fi sh mortalities at all life stages. Although electrofi shing equipment design
may be altered to increase eff ectiveness against very small fi shes (Copp 1989) no
technique is likely to be completely eff ective in removing fi sh larvae and removal
eff ort may be best directed at a time prior to spawning when larval density is low
and young-of-the-year juveniles are large enough to be targeted. Dwyer
et al
.(1993)
found that trout eggs were very sensitive to electroshocking at a critical early stage in
their development. h ey caution that electrofi shing over recently deposited redds
can result in signifi cant mortality. h is therefore serves as a possible technique to
destroy the redds of invasive salmonids and possibly other species also.
The following examples show that complete removal or the minimization of
ecosystem damage may be achieved by efforts at physical removal in very large
water bodies.
