3 Aquaculture and Coastal Space Management
101
3.5.5 Wild Fish as “Natural Bio-filters” Beneath Sea-cage
Fish Farms
Food originating from fish farms is available to wild fish in two forms: as large
food pellets lost through the cage and as a “soup” of particulate organic matter
(POM) of broken pellets and faeces from caged fish. Through consumption of the
food available around farms, high abundances of wild fish may greatly influence
the dynamics of nutrient flows. Two caging experiments that excluded fish from
beneath farms showed that wild fish consumed a large proportion of the total sedimenting nutrients (Vita et al. 2004: 80%, Felsing et al. 2005: 40–60%). The extent
to which waste food pellets and POM derived from a farm are consumed will
depend largely on the biomass of wild fish around cages and the species composition of the assemblages (Dempster et al. 2005).
Wild fish may assimilate nutrients lost through the cages and disperse particles
and nutrients that originate at farms. Mugil cephalus kept in experimental enclosures
on the bottom reduced the impact of sea-cages by mixing, oxygenating and resuspending sediments and enhancing effluent dispersal (Katz et al. 2002). The
abundant large Chondrichthyid rays beneath farms at the Canary Islands may play
a similar role (Dempster et al. 2005). To harness the ability of wild fish to act as
assimilators of wild feed and reduce the benthic impact of fish farms, Dempster
et al. (2005) suggested that the large aggregations of planktivorous and demersal
fish around farms could be protected from fishing.
3.5.6 Managing the Interactions of Aquaculture Sites
and Fishing
Management of fisheries concerning species that interact with aquaculture sites will
greatly affect the outcomes for wild fish populations. Allowing targeting of overexploited fish stocks around fish farms in areas where regulation exists through
gear or fishing time restrictions alone may add to overfishing. Such overfishing will
not appear in estimates of catch per unit effort, since the attractive nature of farms
to wild fish ensures that more fish will arrive to replace those caught. In this case,
fish farms will acts as “ecological traps”, as they have the ecological cues that fish
recognise in preferred habitats, yet their association with the farms diminishes their
rate of survival (Battin 2004). The Mediterranean Sea may be a good example of
where this could occur, as many of the species associated with farms are currently
assessed as fully exploited or over-exploited.
Where potential overfishing presents a problem, the best approach may be to
protect aggregations around farms. Sea-cage fish farms are incompatible with
MPAs designed to protect biodiversity, as assemblages shift away from those naturally observed. However, they may be compatible with the aims of MPAs designed
to enhance fisheries (Dempster et al. 2006) as they concentrate large numbers of a
101
3.5.5 Wild Fish as “Natural Bio-filters” Beneath Sea-cage
Fish Farms
Food originating from fish farms is available to wild fish in two forms: as large
food pellets lost through the cage and as a “soup” of particulate organic matter
(POM) of broken pellets and faeces from caged fish. Through consumption of the
food available around farms, high abundances of wild fish may greatly influence
the dynamics of nutrient flows. Two caging experiments that excluded fish from
beneath farms showed that wild fish consumed a large proportion of the total sedimenting nutrients (Vita et al. 2004: 80%, Felsing et al. 2005: 40–60%). The extent
to which waste food pellets and POM derived from a farm are consumed will
depend largely on the biomass of wild fish around cages and the species composition of the assemblages (Dempster et al. 2005).
Wild fish may assimilate nutrients lost through the cages and disperse particles
and nutrients that originate at farms. Mugil cephalus kept in experimental enclosures
on the bottom reduced the impact of sea-cages by mixing, oxygenating and resuspending sediments and enhancing effluent dispersal (Katz et al. 2002). The
abundant large Chondrichthyid rays beneath farms at the Canary Islands may play
a similar role (Dempster et al. 2005). To harness the ability of wild fish to act as
assimilators of wild feed and reduce the benthic impact of fish farms, Dempster
et al. (2005) suggested that the large aggregations of planktivorous and demersal
fish around farms could be protected from fishing.
3.5.6 Managing the Interactions of Aquaculture Sites
and Fishing
Management of fisheries concerning species that interact with aquaculture sites will
greatly affect the outcomes for wild fish populations. Allowing targeting of overexploited fish stocks around fish farms in areas where regulation exists through
gear or fishing time restrictions alone may add to overfishing. Such overfishing will
not appear in estimates of catch per unit effort, since the attractive nature of farms
to wild fish ensures that more fish will arrive to replace those caught. In this case,
fish farms will acts as “ecological traps”, as they have the ecological cues that fish
recognise in preferred habitats, yet their association with the farms diminishes their
rate of survival (Battin 2004). The Mediterranean Sea may be a good example of
where this could occur, as many of the species associated with farms are currently
assessed as fully exploited or over-exploited.
Where potential overfishing presents a problem, the best approach may be to
protect aggregations around farms. Sea-cage fish farms are incompatible with
MPAs designed to protect biodiversity, as assemblages shift away from those naturally observed. However, they may be compatible with the aims of MPAs designed
to enhance fisheries (Dempster et al. 2006) as they concentrate large numbers of a