Environmental Engineering Reference
In-Depth Information
6. Impact of
T. philippinarum
cultivation on marine ecosystem
The minimization of negative effects, both direct and indirect, of fishing and aquaculture
activities is perceived to be an important component of management plans in heavily
exploited ecosystems (Benaka, 1999). Demersal resources, such as Manila clam, need to be
harvested by use of mobile gears (mainly trawl nets and dredges), producing disturbances
which may exceed any other natural and anthropogenic disturbance on the lagoon or the
marine ecosystem (Watling & Norse, 1998). Bottom gear may cause widespread physical
disturbance in sediments affecting benthic communities, removing both target and non-target
species, and altering habitats. Many studies have been carried out to assess the impact of
fishing gear used for harvesting molluscs, particularly dredges, on bycatch species and benthic
communities (e.g. Pranovi et al., 2001; Hauton et al., 2003; Gaspar et al., 2009; Leitão et al.,
2009). The few studies available on the effects of commercial fishing on target species focus
mainly on the qualitative/quantitative assessment of the discarded clams and on the
selectivity of the fishing gear (Gaspar et al., 2003; Morello et al., 2005; Kraan et al., 2007). In
addition the effects of dredging on habitats include changes in the physical structure and
chemistry of the environment, sediment suspension and its redistribution (Gaspar & Chicharo,
2007). The alteration of sediment features may influence the colonization and presence of
benthic species, including target ones, leading to long-term changes in community structure
(Pranovi & Giovanardi, 1994; Pranovi et al., 1998). In this way, there has been increasing recent
interest in evaluating the collateral effects of commercial cultivation of bivalves on the marine
environment (Sorokin et al., 1999; Bartoli et al., 2001; Jie et al., 2001; Dame et al., 2002),
especially on macrobenthic communities (Kaiser et al., 1996; Drake & Arias, 1997; Spencer et
al., 1997; Gaspar et al., 2002; 2003). The main effect on non-target species is a reduction in
species richness and abundance (Commito, 1987; Dittman, 1990; Guenther, 1996; Ragnarsson &
Raffaelli, 1999; Beadman et al., 2004; Pranovi et al., 2004), although in some cases the opposite
effect has been recorded (Mantovani et al., 2006). This latter may be caused by the use of
plastic nets to protect juvenile clams from predation by shorebirds and crabs (Spencer et al.,
1992), which increased sedimentation rates and consequently the density of some species of
infaunal deposit-feeding worms (Spencer et al., 1997).
In the Venice Lagoon the harvesting by “rusca” produces a V-shaped furrow (about 60 cm
wide and 7 cm deep) and a plume of resuspended sediment with a significant increase (up
to two orders of magnitude greater than undisturbed areas) of suspended particulate matter
and increased C
tot
, C
org
, N
tot
and sulphide concentrations in the water column. During
experiments “rusca” hauls significantly reduced macrofauna density, whereas no significant
effect on meiofauna was detected (Pranovi et al., 2004). The resuspension caused by “rusca”
fishing activity could be an important factor in determining food quality and quantity
available to filter feeders as described by De Jonge & Van Beusekom (1992), for other
resuspension sources. All this could explain the “
Tapes
paradox
”, which is the apparent
benefit of
Tapes
populations to exploitation. As reported by Pranovi et al. (2003), to sustain
the huge clam biomass an external energetic input is required, because the concentration of
the suspended food in the Venice Lagoon is occasionally below the threshold demanded by
Manila clams (Pranovi et al., 2004).
One of the main effects of the resuspension activity is an increase in water turbidity (Black &
Parry, 1994), which could profoundly affect primary production (both in the water column
and on the bottom). In the Venice Lagoon a significant increase in water turbidity has been
Search WWH ::
Custom Search