4 Interactions of Wild and Reared Fish and Invertebrates
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are broadcast spawners and, providing the correct environmental cues of temperature and light are present, they will reproduce whether in cages or on the seabed.
Increased production of cultured strains greatly increases the potential for
huge escapes or deliberate introductions into the wild. Added to this, is the trend
in marine finfish culture to farm large piscivorous fish (e.g., cod, tunas), to use
bigger cages (both for performance and cost reasons) and to site these cages
further offshore (so as to rapidly dilute waste and unused food). The accidental
break up of one of these cages will result in the release of a very large number of
reared fish.
A major, yet rarely addressed, question is: “How detrimental is the present level
of culture to natural populations, where ecological, epidemiological or genetic
interactions occur in the wild, and additionally, will these problems increase as
production expands?”
The generalised genetic issues for marine and anadromous animals are addressed
in the current chapter. The majority of previous work on interactions between wild
populations and conspecific reared strains has been on Atlantic salmon, Salmo
salar, an anadromous species where reared production, chiefly from sea cage
farming, now approaches 1.5 million tonnes. The findings of these investigations
may be summarised as follows:
●
Cultured salmon are introduced inadvertently (as in farm escapes) or deliberately
(as in stocking/ranching) to the wild, where they can interact genetically with
natural populations (of the same or closely related species), either directly (by
interbreeding) or indirectly (by ecological competition or disease introduction).
●
Cultured strains of salmon usually have lower Darwinian fitness in the wild,
compared with natural native populations.
●
With direct effects, hybrid progeny of interbreeding may have reduced fitness
(resulting in reduced survival and overall productivity), whereas indirect effects
may drastically reduce the size of natural populations, exponentially increasing
genetic drift, and possibly leading to inbreeding depression and to loss of local
adaptation, where the latter occurs.
●
Although hybridisation of Atlantic salmon with the close congener Salmo trutta
occurs at low levels in wild populations, the incidence of hybridisation can also
increase greatly following reared fish intrusions, leading to inter-specific hybrid
progeny of very low reproductive fitness.
●
Indirect genetic effects, having an ecological basis, result from the fact that
reared salmon are usually better competitors in the short term (faster growing
and being more aggressive) than their wild relatives, but survive substantially
less well, leading to an overall loss of production per unit area of suitable
habitat.
●
There are several examples of indirect effects involving diseases:
1. Furunculosis, the bacterial disease caused by Aeromonas salmonicida, was accidentally introduced to Norway with farmed smolts and spread to wild salmon,
which were naive and highly susceptible to the disease, resulting in high
mortalities.
119
are broadcast spawners and, providing the correct environmental cues of temperature and light are present, they will reproduce whether in cages or on the seabed.
Increased production of cultured strains greatly increases the potential for
huge escapes or deliberate introductions into the wild. Added to this, is the trend
in marine finfish culture to farm large piscivorous fish (e.g., cod, tunas), to use
bigger cages (both for performance and cost reasons) and to site these cages
further offshore (so as to rapidly dilute waste and unused food). The accidental
break up of one of these cages will result in the release of a very large number of
reared fish.
A major, yet rarely addressed, question is: “How detrimental is the present level
of culture to natural populations, where ecological, epidemiological or genetic
interactions occur in the wild, and additionally, will these problems increase as
production expands?”
The generalised genetic issues for marine and anadromous animals are addressed
in the current chapter. The majority of previous work on interactions between wild
populations and conspecific reared strains has been on Atlantic salmon, Salmo
salar, an anadromous species where reared production, chiefly from sea cage
farming, now approaches 1.5 million tonnes. The findings of these investigations
may be summarised as follows:
●
Cultured salmon are introduced inadvertently (as in farm escapes) or deliberately
(as in stocking/ranching) to the wild, where they can interact genetically with
natural populations (of the same or closely related species), either directly (by
interbreeding) or indirectly (by ecological competition or disease introduction).
●
Cultured strains of salmon usually have lower Darwinian fitness in the wild,
compared with natural native populations.
●
With direct effects, hybrid progeny of interbreeding may have reduced fitness
(resulting in reduced survival and overall productivity), whereas indirect effects
may drastically reduce the size of natural populations, exponentially increasing
genetic drift, and possibly leading to inbreeding depression and to loss of local
adaptation, where the latter occurs.
●
Although hybridisation of Atlantic salmon with the close congener Salmo trutta
occurs at low levels in wild populations, the incidence of hybridisation can also
increase greatly following reared fish intrusions, leading to inter-specific hybrid
progeny of very low reproductive fitness.
●
Indirect genetic effects, having an ecological basis, result from the fact that
reared salmon are usually better competitors in the short term (faster growing
and being more aggressive) than their wild relatives, but survive substantially
less well, leading to an overall loss of production per unit area of suitable
habitat.
●
There are several examples of indirect effects involving diseases:
1. Furunculosis, the bacterial disease caused by Aeromonas salmonicida, was accidentally introduced to Norway with farmed smolts and spread to wild salmon,
which were naive and highly susceptible to the disease, resulting in high
mortalities.