Environmental Engineering Reference
In-Depth Information
Wrapping has also been used as a method to treat vessel hulls infected with foul-
ing pest species. Vessels infected with
D
.
vexillum
were successfully treated
in situ
using a plastic encapsulation technique and the addition of chemicals (Coutts and
Forrest 2007). Similarly, the black-striped mussel
Mytilopsis sallei
and Asian green
mussel
Perna viridis
detected on the hulls of fi shing vessels in Darwin Harbour were
successfully eradicated by wrapping the vessel hulls in PVC sheaths and adding
chlorine. At the time of writing, the largest reported vessel to be treated using the
wrapping techniques was a 113m frigate in New Zealand (Denny 2007), although
this attempt was unsuccessful due to strong currents in the area and di
culties in
maintaining the integrity of the plastic wrap around certain vessel structures (e.g.
propeller blades).
On natural substrates, plastic sheeting has again been used to smother the alga
C
.
taxifolia
(Zuljevic and Antolic 1999b; Meinesz
et al
. 2001; Creese
et al
. 2004)
and
D
.
vexillum
(Coutts and Forrest 2007; Pannell and Coutts 2007). Jute mat-
ting, which is cheaper and more environmentally friendly than plastic, has also suc-
cessfully been used for smothering
C
.
taxifolia
over small areas of seabed (Glasby
et al
. 2005); however, deployment proved more di
cult over larger areas due to its
positive buoyancy and the amount of weight required to anchor it in place. In the
French Mediterranean, mats soaked in copper sulphate have been placed over beds
of
C
.
taxifolia,
with the chemicals leaching from the mats resulting in increased
mortality (Uchimura
et al
. 2000).
Dredge spoil and sediment have also been used in attempts to smother benthic
pest species. Studies in the USA found that covering the starfi sh
Asterias forbesi
with
a layer of mud or sand resulted in death as the individuals were unable to escape
(Loosanoff 1961). In contrast however, the alga
Sargassum muticum
was found to
be far more resistant to burial and decayed more slowly than similar macroalgal
species, suggestive of the fact that burial initiated a self-protective response from
the plant (Morrell and Farnham 1982). It must be noted that control programmes
involving physical burial of invasive taxa with sediments must be carefully designed
because of the potential to cause signifi cant environmental damage and alter the
habitat in a way that facilitates other introduced species (McEnnulty
et al
. 2001).
Crucial to the success of encapsulation and smothering techniques is the repeated
monitoring of aff ected structures and the complete treatment and removal of every
individual. Other factors important to success include: the size and topographic
complexity of the infected area; the hydrodynamics of the location; and maintain-
ing the smothering for a prolonged period (McEnnulty
et al
. 2001; Creese
et al
.
2004; Coutts and Forrest 2007). For algal and/or colonial species, it is also import-
ant to limit the amount of fragments generated during deployment as these may
settle and establish nearby (Creese
et al
. 2004). Wrappings on artifi cial structures
such as wharf piles and pontoons are typically able to be left in place for extended
periods of time (
>
12 months), providing further protection from re-infection
(i.e. during reproductive periods). Additionally, should the outside of wrappings
become re-infected, their removal provide a secondary treatment option. h ere
are several environmental and public safety issues that must be considered during
