| Foreword |
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R
G J Shelton
Officer-in Charge,
Freshwater Fisheries Laboratory, Faskally |
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| Fisheries
for salmon and trout are among Scotland's most valuable
rural resources. They are based upon fish populations
that spend all or part of their lives in fresh water.
Salmon and sea trout use fresh waters for spawning and
juvenile development but make most of their growth in the
sea. Brown trout complete the whole of their life cycle
in fresh water. For migratory and non-migratory
salmonids, habitat constraints in fresh water set upper
limits on their numerical abundance. Whether or not such
limits are attained depends in the first instance on an
adequate number of spawners and secondarily on the
subsequent fate of juvenile fish in the river system. |
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| Predation by
fish-eating birds is widely perceived as one of the
factors which reduces the numbers of fish subsequently
available for exploitation. Reduction of avoidable losses
to predation is therefore seen as a high priority by
fishery managers. Sawbill ducks (goosanders and
mergansers) and cormorants are the main bird species of
concern to fishery authorities in Scotland. All three
species are protected under the terms of the Wildlife and
Countryside Act (1981, as amended 1995) but may be shot
under licences that The Scottish Office can grant for the
purpose of preventing serious damage to fisheries
(including fish farms). Under the present arrangements,
licences are granted only where alternative non-lethal
methods are unsuccessful or impractical. The first
premise upon which the licences are issued is that the
sectors of the fish populations predated by the birds
have limited capacity to compensate for losses through
density-dependent changes in the growth and survival of
the fish remaining. The second premise is that
goosanders, mergansers and cormorants have a value of
their own and that licensed shooting should be organised
in such a way that fishery resources are adequately
protected but with due regard for the conservation status
of the birds. |
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| For species
with such a high unit value as Atlantic salmon, any
uncompensated loss may be regarded as
"serious". Data from monitored rivers suggest
that the capacity for compensation diminishes as juvenile
life as a parr in the river proceeds and is entirely lost
by the smolt stage when the young fish enter the sea. |
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| Large parr
are believed to be at their most vulnerable when they
undergo migratory movements in the autumn and early
winter and smolts during their seaward migration in the
spring. Licensed shooting has largely been confined to
these times and in no instance has shooting been
permitted in late spring and early summer to avoid undue
impact upon the breeding success of the birds. |
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| In this
document we report the results of some recent parallel
investigations into the lives of both predators and prey
and useful insights have been obtained into the ways in
which they interact. The results also form the basis for
the critical recommendation that, in future, an iterative
combination of modelling and experimentation is likely to
provide the best predictions both of the impact of bird
predation and of the effectiveness of protective
measures. |
