96
T. Dempster and P. Sanchez-Jerez
specific area to fishing lies in its relative contribution to the overall catch of a particular fishery and finer detailed information on the scale of hundreds of metres can be
obtained from the fishers themselves. Areas where catch rates are high, where catches
are economically or socially important, or where particularly important habitats for
juveniles of important fisheries species exist (e.g., seagrass meadows, macroalgal forests) should be deemed unsuitable as sites for aquaculture to avoid conflict between
the fisheries and aquaculture sectors.
During farm placement, consideration must also be given to the proximity of the
site to areas that may be of particularly high importance to wild fish stocks, such as
known points of natural aggregation for feeding, spawning or migratory pathways
of anadromous fish. For salmonid aquaculture, two substantial environmental effects
are of concern: 1) escape of cultured fish and their subsequent mixing with wild
stocks (see review by Weir and Grant 2005); and 2) that the large numbers of cultured fish held in coastal areas may increase parasite loads of their wild counterparts
(Bjorn et al. 2001; Morton et al. 2004; Krkošek et al. 2005). Presently, much is
known about the causes and environmental effects of escapes for salmonids (Naylor
et al. 2005), while comparatively little is known for other species such as sea bream,
sea bass and Atlantic cod (but see Moe et al. 2005). Inter-breeding and competitive
interactions of escapees with wild salmon within rivers may have detrimental effects
on wild populations. Likewise, high parasite loads on seaward-migrating salmon
smolts have been implicated as a potential cause of high mortality at sea and reduced
return of adults to rivers (Bjorn et al. 2001).
Assessment of the risk that escapees and other effects pose to wild populations
when placing farms has been suggested (Naylor et al. 2005; WWF 2005). Declaration
of the “national salmon fiords” throughout Norway in 2003 and the consequent
restriction on placing new fish farms in these areas is an example of considering
important wild fish stocks when locating farms (Sivertsen 2006). In response to
concerns regarding escapees and parasite loads of seaward-migrating smolts, particular rivers flowing into coastal fiords in Norway were regarded as of such high
importance to wild salmon populations that sea-cage salmonid farms were restricted
or removed from these fiords.
An emerging issue regarding escapes is that certain fish species are being raised to
sizes within sea-cages at which, if they become sexually mature, they are capable of
spawning. This requires the concept of escape from mariculture to be redefined to
include the escape of reproductive gametes into the environment. Jørstad and van der
Meeren (2006) allowed 1000 gene tagged cod to spawn within a small fiord
system in Norway. Upon sampling larvae in the waters surrounding the farm, 25%
were traced back to caged parents. This indicates that if spawning occurs within commercial cod farms where numbers of animals are far greater, the contribution of
“escaped” larvae to cod recruitment within fiords may be substantial. Spawning of sea
bream within sea-cages has also been observed in Greece (Dimitriou et al. 2007).
If breeding programmes shift the genetic diversity of aquacultured fish away from
wild stocks, the extent of spawning within sea cages and whether larvae subsequently survive and recruit into natural populations in significant numbers will
likely greatly affect siting of farms.
T. Dempster and P. Sanchez-Jerez
specific area to fishing lies in its relative contribution to the overall catch of a particular fishery and finer detailed information on the scale of hundreds of metres can be
obtained from the fishers themselves. Areas where catch rates are high, where catches
are economically or socially important, or where particularly important habitats for
juveniles of important fisheries species exist (e.g., seagrass meadows, macroalgal forests) should be deemed unsuitable as sites for aquaculture to avoid conflict between
the fisheries and aquaculture sectors.
During farm placement, consideration must also be given to the proximity of the
site to areas that may be of particularly high importance to wild fish stocks, such as
known points of natural aggregation for feeding, spawning or migratory pathways
of anadromous fish. For salmonid aquaculture, two substantial environmental effects
are of concern: 1) escape of cultured fish and their subsequent mixing with wild
stocks (see review by Weir and Grant 2005); and 2) that the large numbers of cultured fish held in coastal areas may increase parasite loads of their wild counterparts
(Bjorn et al. 2001; Morton et al. 2004; Krkošek et al. 2005). Presently, much is
known about the causes and environmental effects of escapes for salmonids (Naylor
et al. 2005), while comparatively little is known for other species such as sea bream,
sea bass and Atlantic cod (but see Moe et al. 2005). Inter-breeding and competitive
interactions of escapees with wild salmon within rivers may have detrimental effects
on wild populations. Likewise, high parasite loads on seaward-migrating salmon
smolts have been implicated as a potential cause of high mortality at sea and reduced
return of adults to rivers (Bjorn et al. 2001).
Assessment of the risk that escapees and other effects pose to wild populations
when placing farms has been suggested (Naylor et al. 2005; WWF 2005). Declaration
of the “national salmon fiords” throughout Norway in 2003 and the consequent
restriction on placing new fish farms in these areas is an example of considering
important wild fish stocks when locating farms (Sivertsen 2006). In response to
concerns regarding escapees and parasite loads of seaward-migrating smolts, particular rivers flowing into coastal fiords in Norway were regarded as of such high
importance to wild salmon populations that sea-cage salmonid farms were restricted
or removed from these fiords.
An emerging issue regarding escapes is that certain fish species are being raised to
sizes within sea-cages at which, if they become sexually mature, they are capable of
spawning. This requires the concept of escape from mariculture to be redefined to
include the escape of reproductive gametes into the environment. Jørstad and van der
Meeren (2006) allowed 1000 gene tagged cod to spawn within a small fiord
system in Norway. Upon sampling larvae in the waters surrounding the farm, 25%
were traced back to caged parents. This indicates that if spawning occurs within commercial cod farms where numbers of animals are far greater, the contribution of
“escaped” larvae to cod recruitment within fiords may be substantial. Spawning of sea
bream within sea-cages has also been observed in Greece (Dimitriou et al. 2007).
If breeding programmes shift the genetic diversity of aquacultured fish away from
wild stocks, the extent of spawning within sea cages and whether larvae subsequently survive and recruit into natural populations in significant numbers will
likely greatly affect siting of farms.