- This report details
studies (1990-96) of predation on salmonid fish
by three species of fish-eating birds in
Scotland: goosanders, red-breasted mergansers and
great cormorants. Studies have focussed on
assessments of (i) sawbill duck abundance at the
catchment scale on the Rivers Dee and North Esk,
(ii) bird diet throughout Scotland based on
stomach contents analysis and (iii) salmon
recolonisation of artificially depleted areas of
stream and river.
- On both the Dee and
North Esk seasonal variation in the abundance of
mergansers and goosanders was predictable, as was
the number of goosanders in winter. Numbers in
spring declined over the study and numbers in
summer varied from year to year dependent on the
survival of small ducklings.
- Sawbills on these
rivers lived at densities averaging 0.15 birds/ha
and rarely exceeding 0.33 bird/ha. Goosander
density was highest on lower sections of both
rivers in midwinter, and on the mainstem of the
Dee in summers of high duckling production.
Red-breasted merganser density was highest on the
North Esk mainstem in May.
- Throughout Scotland,
the diet of goosanders, mergansers and cormorants
was diverse and varied between seasons and sites.
All three bird species appeared to be generalist
predators. The prey taken commonly and most
widely were brown trout, salmon parr, eel and
minnow. Larger bird species took larger fish and
fewer salmon than did smaller ones, and all bird
species had a less diverse diet in the north than
in the south.
- Using latitudinal
trends in the proportion of salmon in the diet
during winter and spring, red-breasted merganser
diet averaged about 30% salmon (by mass) on
southern rivers and 56% in the north. The
equivalent figures for goosander diet were 9% in
the south and 41% in the north, and for
cormorant, <1% in the south and 18% in the
north. Trout were a greater dietary component
than were salmon in all 3 bird species, but its
proportion showed no discernible latitudinal
trend.
- It is calculated that
sawbills consumed 3 to 8 salmon per bird per day
on southern rivers and 5 to 61 on northern ones.
The numbers of larger salmon (>89mm) consumed
by sawbills were fewer; 0 to 3 on southern
rivers, 1 to 19 in the north. Similar
calculations for cormorants suggested they
consumed <1 to 3 salmon per bird per day on
southern rivers and 1 to 8 in the north.
Corresponding figures for larger salmon
(>89mm) were <1 to 3 per cormorant per day
on southern rivers and 1 to 4 in the north.
- Some of the trout
taken on both standing waters and rivers were of
a size (>199mm in length) that could be kept
by anglers. Mergansers took no such trout,
goosanders took few (0 to <1 per bird per day)
and cormorants most (<1 to 3).
- Further calculations,
incorporating both bird diet and abundance data,
suggested that in April, goosanders consumed 2 to
3 salmon per hectare of river per day, on both
the Dee and North Esk. Higher numbers were taken
by goosanders in winter on the lower Dee (4), by
ducklings on both rivers (4 to 9) and by
mergansers at the mouth of the North Esk in April
(12).
- The numbers of salmon
consumed by ducks were compared with the best
available data on the densities of these fish in
the Dee and North Esk. This suggested sawbills on
these rivers consumed 0.02 to 0.30% of the salmon
standing stock per day on narrow (<30m) river
sections and 0.40 to 2.4% on wider sections when
and where birds were at their highest density.
- These calculations
suggest for instance that on the Dee in summer,
birds would remove all salmon within two months,
which clearly does not happen. Either our
estimates of consumption are too high and/or the
estimates of salmon stock are too low or the
salmon population is somehow able to compensate
for this apparently high level of predation.
- A review of recent
studies suggested that there may be little scope
for populations of salmonid fishes to compensate
for losses of parr and smolts.
- During summer there
was some movement of salmon parr into areas of
stream in which populations were experimentally
depleted. By comparison with densities in
undepleted areas nearby, estimates of maximum
levels of recolonisation averaged 21.1% (range 7
to 54%).
- A review of case
studies attempting to control populations of
sawbill ducks together with observations on the
study rivers, suggested that shooting can reduce
the numbers of birds locally but its
effectiveness is compromised by turnover of
birds.
- Under some
circumstances, fish-eating birds can consume
large numbers of trout and salmon. In order to
continue to improve understanding of the
importance of this predation in terms of impact
on both populations of these fish and on their
commercial fisheries, we recommend an iterative
procedure of modelling and experimental testing
of quantitative predictions.
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