144
T.F. Cross et al.
treatments or at excessively high water temperatures, there is considerable industry
objection to the process.
Sex determination mechanisms are poorly known in other finfish species used in
aquaculture (but see Mank et al. 2006) and need further study before triploidy
should be attempted with all-female strains. Where used, the incorporation of
triploid induction into existing and future breeding programmes is also
recommended, so as to avoid lowered genetic variability in triploids. Early attempts
to induce triploidy in bivalve shellfish using chemicals like cytochalasin-B were
only partially successful and rarely produced 100% triploids. These methods are
not therefore reliable as a means of inducing sterility. However, using a proprietary,
patented, process, a USA company can now produce tetraploid brood stock. Males
are identified from select brood stock lines and sacrificed for their sperm. The
sperm from these tetraploid males is naturally diploid (2N), instead of haploid (1N)
as it would be from typical diploid males. The diploid sperm is then added to
haploid eggs from the customer’s normal diploid brood stock. No chemical or
pressure induction treatment is used. The resulting zygotes are genetically triploid,
with two sets of chromosomes contributed by the sperm and one set contributed by
the egg (Benoit et al. 2000).
4.8 Risk Analysis
Important questions must be raised with respect to species in which wild populations
are most at risk from incursions of reared conspecifics. Table 4.2 addresses some
of these questions. At present the numbers of marine and anadromous fish and
invertebrate species cultured in Europe, other than for salmon, flat oyster and
mussel is much lower than for wild populations. However, this is completely
opposite to the situation in salmon in the eastern North Atlantic, where cultured fish
outnumber wild conspecifics by more than two orders of magnitude.
The marine species that are currently farmed in Europe (such as cod, halibut,
turbot, sea bass, sea bream, lobster, scallop, mussels, native and Pacific oysters,
abalone and Manila clams) differ from Atlantic salmon in many important respects.
As noted above, census population sizes of native wild marine fish and invertebrates
are several orders of magnitude greater than salmonids and genetic population
(stock) structure appears to be much less well defined (Table 4.2). In addition, as
mentioned above, the extent of local adaptation for the majority of marine species
has not been established. Thus, from a genetic viewpoint the risk to wild populations
might be considered to be far less (but see Bekkevold et al. 2006). However, as
farmed production of some or all of these species is predicted to increase greatly,
with extensive ocean ranching being promoted in several quarters (Leber et al.
2004), and wild populations are steadily decreasing due to overfishing (Worm et al.
2006), the situation may change in the future. Furthermore, diseases, which are
likely to be much more of a problem under high density rearing conditions and yet
are usually controllable by medication in captivity, might become a severe problem
T.F. Cross et al.
treatments or at excessively high water temperatures, there is considerable industry
objection to the process.
Sex determination mechanisms are poorly known in other finfish species used in
aquaculture (but see Mank et al. 2006) and need further study before triploidy
should be attempted with all-female strains. Where used, the incorporation of
triploid induction into existing and future breeding programmes is also
recommended, so as to avoid lowered genetic variability in triploids. Early attempts
to induce triploidy in bivalve shellfish using chemicals like cytochalasin-B were
only partially successful and rarely produced 100% triploids. These methods are
not therefore reliable as a means of inducing sterility. However, using a proprietary,
patented, process, a USA company can now produce tetraploid brood stock. Males
are identified from select brood stock lines and sacrificed for their sperm. The
sperm from these tetraploid males is naturally diploid (2N), instead of haploid (1N)
as it would be from typical diploid males. The diploid sperm is then added to
haploid eggs from the customer’s normal diploid brood stock. No chemical or
pressure induction treatment is used. The resulting zygotes are genetically triploid,
with two sets of chromosomes contributed by the sperm and one set contributed by
the egg (Benoit et al. 2000).
4.8 Risk Analysis
Important questions must be raised with respect to species in which wild populations
are most at risk from incursions of reared conspecifics. Table 4.2 addresses some
of these questions. At present the numbers of marine and anadromous fish and
invertebrate species cultured in Europe, other than for salmon, flat oyster and
mussel is much lower than for wild populations. However, this is completely
opposite to the situation in salmon in the eastern North Atlantic, where cultured fish
outnumber wild conspecifics by more than two orders of magnitude.
The marine species that are currently farmed in Europe (such as cod, halibut,
turbot, sea bass, sea bream, lobster, scallop, mussels, native and Pacific oysters,
abalone and Manila clams) differ from Atlantic salmon in many important respects.
As noted above, census population sizes of native wild marine fish and invertebrates
are several orders of magnitude greater than salmonids and genetic population
(stock) structure appears to be much less well defined (Table 4.2). In addition, as
mentioned above, the extent of local adaptation for the majority of marine species
has not been established. Thus, from a genetic viewpoint the risk to wild populations
might be considered to be far less (but see Bekkevold et al. 2006). However, as
farmed production of some or all of these species is predicted to increase greatly,
with extensive ocean ranching being promoted in several quarters (Leber et al.
2004), and wild populations are steadily decreasing due to overfishing (Worm et al.
2006), the situation may change in the future. Furthermore, diseases, which are
likely to be much more of a problem under high density rearing conditions and yet
are usually controllable by medication in captivity, might become a severe problem