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T.F. Cross et al.
larvae was 40% lower than the natives, when grown under identical conditions in
the hatchery. It would appear that during the hatchery period of the reared parents
there had been artificial (though-inadvertent) selection pressure, which resulted in
reduced fitness of the reared progeny.
In Hokkaido, Japan, scallop seed (Patinopecten yessoensis) are settled on midwater collectors, half grown in suspended culture and then released locally onto the
seabed for final ongrowing. Stock enhancement has been carried out in this region
for over 30 years and now yields about 300,000 t per year (Uki 2006). Because the
pelagic larvae are retained by local gyres and the juveniles usually remain within
0.5 km of the release site, the area could be considered as an extensive marine farm
since reseeding areas are rotated annually and predators are removed by dredging.
As a result starfish (Asterias amurensis and Asterias pectinifera) and the sea urchin
Glyptocidaris crenularis have been almost eliminated from the ongrowing areas.
In many areas there is almost a monoculture of scallop on the seabed.
The situation in Europe is very different. Despite over 30 years of research and
technology transfer from Japan, scallop culture is still in its infancy with total
production of less than 1,000 t. A small proportion of this is obtained from seabed
ranching. The most successful projects are in France (150–200 t per annum),
Ireland (50–100 t per annum) and Norway (50–100 t per annum) (Shumway 2006).
In each case, the industry has been careful to use local broodstock for their hatchery
programme but, as has been demonstrated with lobster restocking, the hatchery part
of the process will inevitably induce some genetic selection with possible loss of
fitness in the F 1 and subsequent generations.
As mentioned above, a very different rearing strategy is generally employed for
enhancement exercises, than when producing animals for farming. While it is
generally assumed that such animals will have greater survival in the wild than farm
strains, this may in fact be more detrimental to wild populations, as will be
discussed below. Apart from genetic considerations, the success of stocking
exercises is crucially dependent on the strategy employed, viz. the number and life
stage of the animals used for stocking, the location/s and timing of where the
introductions take place, and whether the exercise is undertaken once or repeated
on a regular (annual) basis. As noted in Cross et al. (2007), there has been very little
detailed follow-up monitoring and it is generally presumed that the aim of stocking
is a larger self-sustaining “wild” population. However, this may not be possible
because of limits to environmental carrying capacity or because of environmental
constraints, such as the presence of dams on salmon rivers, which inundate natural
spawning areas (Cross et al. 2007). In these cases, the stocking will have to be
repeated on a regular basis and the exercise becomes, in effect, a type of ranching.
Unfortunately, not all returning adults from such an exercise will be caught in
terminal fisheries, so the potential for large numbers of ranched individuals to be
introduced into the wild is high and this situation will usually be maintained by
continued regular introductions, even if the reproductive fitness of reared animals
or their hybrid progeny is somewhat lower than “pure” wild individuals. While with
commercial ranching, the aim is to recover or capture all animals of marketable
size, this is probably never achieved so “ranched wanderers” or strays must be
considered as a potential threat to wild populations.
T.F. Cross et al.
larvae was 40% lower than the natives, when grown under identical conditions in
the hatchery. It would appear that during the hatchery period of the reared parents
there had been artificial (though-inadvertent) selection pressure, which resulted in
reduced fitness of the reared progeny.
In Hokkaido, Japan, scallop seed (Patinopecten yessoensis) are settled on midwater collectors, half grown in suspended culture and then released locally onto the
seabed for final ongrowing. Stock enhancement has been carried out in this region
for over 30 years and now yields about 300,000 t per year (Uki 2006). Because the
pelagic larvae are retained by local gyres and the juveniles usually remain within
0.5 km of the release site, the area could be considered as an extensive marine farm
since reseeding areas are rotated annually and predators are removed by dredging.
As a result starfish (Asterias amurensis and Asterias pectinifera) and the sea urchin
Glyptocidaris crenularis have been almost eliminated from the ongrowing areas.
In many areas there is almost a monoculture of scallop on the seabed.
The situation in Europe is very different. Despite over 30 years of research and
technology transfer from Japan, scallop culture is still in its infancy with total
production of less than 1,000 t. A small proportion of this is obtained from seabed
ranching. The most successful projects are in France (150–200 t per annum),
Ireland (50–100 t per annum) and Norway (50–100 t per annum) (Shumway 2006).
In each case, the industry has been careful to use local broodstock for their hatchery
programme but, as has been demonstrated with lobster restocking, the hatchery part
of the process will inevitably induce some genetic selection with possible loss of
fitness in the F 1 and subsequent generations.
As mentioned above, a very different rearing strategy is generally employed for
enhancement exercises, than when producing animals for farming. While it is
generally assumed that such animals will have greater survival in the wild than farm
strains, this may in fact be more detrimental to wild populations, as will be
discussed below. Apart from genetic considerations, the success of stocking
exercises is crucially dependent on the strategy employed, viz. the number and life
stage of the animals used for stocking, the location/s and timing of where the
introductions take place, and whether the exercise is undertaken once or repeated
on a regular (annual) basis. As noted in Cross et al. (2007), there has been very little
detailed follow-up monitoring and it is generally presumed that the aim of stocking
is a larger self-sustaining “wild” population. However, this may not be possible
because of limits to environmental carrying capacity or because of environmental
constraints, such as the presence of dams on salmon rivers, which inundate natural
spawning areas (Cross et al. 2007). In these cases, the stocking will have to be
repeated on a regular basis and the exercise becomes, in effect, a type of ranching.
Unfortunately, not all returning adults from such an exercise will be caught in
terminal fisheries, so the potential for large numbers of ranched individuals to be
introduced into the wild is high and this situation will usually be maintained by
continued regular introductions, even if the reproductive fitness of reared animals
or their hybrid progeny is somewhat lower than “pure” wild individuals. While with
commercial ranching, the aim is to recover or capture all animals of marketable
size, this is probably never achieved so “ranched wanderers” or strays must be
considered as a potential threat to wild populations.