142
T.F. Cross et al.
Chesapeake Bay (Brown et al. 2005b). Results of both trials indicated that performance was related to level of resistance, salinity of water and virulence of Perkinsus
marinus.
Box 4.3 Burrishoole experiments
A two generation experiment was undertaken in the Srahrevagh river, a natural spawning tributary of the Burrishoole system, in western Ireland to
measure the relative lifetime reproductive success of the progeny of wild
salmon, escaped farmed salmon and their hybrids in the wild (McGinnity
et al. 1997, 2003). The experiment was conducted as a “common garden”
experiment in a natural river. Wild and farm salmon, and first and second
generation hybrids and backcrosses between them, were planted as eyed
eggs, thus removing the influence that hatchery rearing might have on performance if the fish were introduced at a later life stage. The study was also
designed to eliminate behavioural differences between spawning adults and
to examine the effect of solely genetic differences on survival and performance. Offspring of farm and “hybrids” (i.e., all F 1 , F 2 and BC1 groups)
showed reduced survival compared with wild salmon. The relative estimated
lifetime success ranged from 2% (farm) to 89% (BC1 wild) of that of wild
salmon, the various hybrids having intermediate levels of fitness, indicating
additive genetic variation for survival (see Fig 4.1). There was also clear
evidence of out-breeding depression in the F 2 hybrids. The progeny of farm
salmon grew faster as juveniles and displaced wild parr, which as a group
were significantly smaller. The offspring of farmed salmon showed a
reduced incidence of male parr maturity compared with native fish. The latter also showed a greater tendency to migrate as autumn pre-smolts. Growth
of hybrids were generally either intermediate or not significantly different
from the wild fish. Wild salmon primarily returned to fresh water after one
sea winter (1 SW), but farm and “hybrids” produced proportionally more 2
SW salmon. However, due to an overall reduced survival, this would result
in reduced recruitment despite increased 2SW fecundity. The experiment
showed that the interaction of farm with wild salmon results in lowered fitness, with repeated escapes causing cumulative fitness depression and
potentially an extinction vortex in vulnerable populations.
An additional experiment has since been carried out in the Srahrevagh
river using the “common garden” approach. This study was a comparison of
the relative lifetime success and performance characteristics of communally
reared offspring of wild native Burrishoole, ranched native and non-native
salmon from the Owenmore River; a river that is in the same geographic area
as the Burrishoole (McGinnity et al. 2004).
T.F. Cross et al.
Chesapeake Bay (Brown et al. 2005b). Results of both trials indicated that performance was related to level of resistance, salinity of water and virulence of Perkinsus
marinus.
Box 4.3 Burrishoole experiments
A two generation experiment was undertaken in the Srahrevagh river, a natural spawning tributary of the Burrishoole system, in western Ireland to
measure the relative lifetime reproductive success of the progeny of wild
salmon, escaped farmed salmon and their hybrids in the wild (McGinnity
et al. 1997, 2003). The experiment was conducted as a “common garden”
experiment in a natural river. Wild and farm salmon, and first and second
generation hybrids and backcrosses between them, were planted as eyed
eggs, thus removing the influence that hatchery rearing might have on performance if the fish were introduced at a later life stage. The study was also
designed to eliminate behavioural differences between spawning adults and
to examine the effect of solely genetic differences on survival and performance. Offspring of farm and “hybrids” (i.e., all F 1 , F 2 and BC1 groups)
showed reduced survival compared with wild salmon. The relative estimated
lifetime success ranged from 2% (farm) to 89% (BC1 wild) of that of wild
salmon, the various hybrids having intermediate levels of fitness, indicating
additive genetic variation for survival (see Fig 4.1). There was also clear
evidence of out-breeding depression in the F 2 hybrids. The progeny of farm
salmon grew faster as juveniles and displaced wild parr, which as a group
were significantly smaller. The offspring of farmed salmon showed a
reduced incidence of male parr maturity compared with native fish. The latter also showed a greater tendency to migrate as autumn pre-smolts. Growth
of hybrids were generally either intermediate or not significantly different
from the wild fish. Wild salmon primarily returned to fresh water after one
sea winter (1 SW), but farm and “hybrids” produced proportionally more 2
SW salmon. However, due to an overall reduced survival, this would result
in reduced recruitment despite increased 2SW fecundity. The experiment
showed that the interaction of farm with wild salmon results in lowered fitness, with repeated escapes causing cumulative fitness depression and
potentially an extinction vortex in vulnerable populations.
An additional experiment has since been carried out in the Srahrevagh
river using the “common garden” approach. This study was a comparison of
the relative lifetime success and performance characteristics of communally
reared offspring of wild native Burrishoole, ranched native and non-native
salmon from the Owenmore River; a river that is in the same geographic area
as the Burrishoole (McGinnity et al. 2004).