Title: Impact of naturally-spawning captive bred Atlantic salmon on wild populations:
depressed recruitment and increased risk of climate mediated extinction.
Authors
Philip McGinnity1, 2, Eleanor Jennings3, 4, Elvira deEyto2, Norman Allott3, Patrick
Samuelsson5, Gerard Rogan2 Ken Whelan2 and Tom Cross1.
1. Department of Zoology, Ecology and Plant Science, UNIVERSITY
COLLEGE CORK, Ireland.
2. Aquaculture and Catchment Management Services, Marine Institute, Newport,
Co. Mayo, Ireland.
3. Centre for the Environment, School of Natural Science, Trinity College,
Dublin 2, Ireland.
4. Department of Applied Sciences, Dundalk Institute of Technology, Dundalk,
Ireland.
5. Rossby Centre, Swedish Meteorological and Hydrological Institute, 60176
Norrköping, Sweden.
Corresponding author
Philip McGinnity
Address:
Department of Zoology, Ecology & Plant Science, Distillery Fields, North
Mall, University College Cork, Ireland.
Phone: 353 98 42317
Email: p.mcginnity@ucc.ie
Fax: 353 98 42340
ELECTRONIC SUPPLEMENTARY MATERIAL.
Materials & Methods
Study Site
The Burrishoole river system is situated in NW Ireland (5359N, 0937W). Lough
Feeagh, the largest lake in the catchment, has a surface area of 3.9 km2, a mean depth of
14.5 m and drains an area of 83 km2. The water in the lake is soft and distinctly coloured.
Catchment soil is composed primarily of blanket peat and exposed rock and the principal
land uses are forestry and sheep grazing. Burrishoole is situated close to the Atlantic
coast and has a temperate oceanic climate. Average annual rainfall at the meteorological
station on the eastern shore of Lough Furnace, is 1560 mm year-1 while mean summer
and winter water temperatures are 15oC and 6 oC respectively. The lake stratifies
thermally between June and September although surface water temperatures are seldom
greater than 20oC in summer. The surface water temperature in winter is typically greater
than 4oC and prolonged periods of ice cover are unknown. The lake and its catchment are
an important habitat for salmon (Salmo salar), trout (Salmo trutta) and eel (Anguilla
anguilla). Adult Atlantic salmon in the Burrishoole river system typically return to
freshwater between June and September and spawn the following December. At this
latitude (53o 59N) juveniles generally spend three winters in freshwater, including the
winter when eggs are in the gravel, before migrating to sea in May or June as smolts
(Metcalfe & Thorpe 1990).
Estimating fecundity and egg to smolt survival
There are some unpublished data to suggest that there has been a progressive reduction in
size of both adult wild and ranched fish returning to spawn in Burrishoole over the period
of our study and that the fecundity values applied above may not hold for the latter part
of the time series, such that egg to smolt survival rates in the most recent years may
actually be higher than those calculated. We have not adjusted the fecundity values to
account for this potential change as there are no contemporaneous fecundity estimates
available. The consequence of not doing so is we suggest, to reduce the sensitivity of the
model to determine the population response, specifically the performance of the progeny
of wild fish, to environmental variability, as the natural spawning cohorts from 1990 to
2007 are dominated by wild fish. It might therefore be expected that a reduction in the
size of spawning fish in the latter part of the time series underestimates the performance
of wild fish and not that of the ranched fish. The survival values for the period when
wild fish predominate would therefore be higher than those reported. It is also possible
that hatchery females carry proportionally more eggs per unit weight than wild females;
the ranched eggs being smaller (McGinnity et al. 1997). More eggs would mean that we
have underestimated the negative effect of ranched fish on egg to smolt survival values,
and that our model outputs are quite conservative in this regard.
We have also considered that winter temperature and a reduction in adult size may be
correlated and that as a result our interpretation of the model response might be erroneous
because of a co-variation effect. However, as neither January water temperature nor the
proportion of ranched fish show any significant linear trend over this period, this
unmeasured factor does not co-vary with the terms contributing to the interactive effect.
Jan and Aug were included as they produced the best fit. All individual months and
seasons were assessed in the stepwise regression process but only those that contributed
significantly were included in the final model.
The alternative models used were:
Low contribution of ranched eggs:
Egg to smolt % survival = (-0.050  Jan SWT year 0) + (-0.092* Jan SWT year 1) +
(0.131* Spring SWT year 2) + (0.010 * 90 ppt Jan year 1) + (-0.011*90 ppt Aug year 1)
+ 0.265.
High contribution of ranched eggs:
Egg to smolt survival = (-0.151  Jan SWT year 0) + (-0.092* Jan SWT year 1) + (0.131*
Spring SWT year 2) + (0.010 * 90 ppt Jan year 1) + (-0.011*90 ppt Aug year 1) + 0.606.
Other factors considered in development of the Multiple regression model
In addition to the model parameters reported the impact of year, numbers of sea trout
eggs in the catchment, air temperature and the North Atlantic Oscillation (NAO) was also
assessed. There was no significant relationship with the number of sea trout eggs. The
winter NAO index is known to be highly correlated with surface temperatures in Lough
Feeagh (Jennings et al. 2000; Blenckner et al. 2007). The relationship between air
temperature and survival was weaker than corresponding water temperatures. The NAO
index in year 0, the winter when eggs are incubating, explained a similar percentage of
the variability to water temperature in that year. However, the NAO in the second winter,
when salmon are 0+ juveniles (parr), was not significant. This is not surprising, as the
final model indicated that egg to smolt survival is influenced negatively by high
temperatures and positively by high rainfall in that winter. High NAO index years are
associated with both milder and wetter conditions on the west coast of Ireland (Jennings
et al. 2000; Blenckner et al. 2007). Density effects can also be important in population
regulation (Ricker 1958) but were found not to be a contributing factor in this case.
Inputs to the DYRESM model (Imberger & Patterson 1981; Imerito 2007)
Model inputs include daily solar radiation, cloud cover, wind speed, vapour pressure, air
temperature, and precipitation (Imerito 2007). Data are also required inflow volume and
inflow water temperature. Steam flow data for the main inflows were only available for
the period 2002 to present. Simulated streamflow data were generated using the
hydrology sub-routine of the Generalized Watershed Loading Functions (GWLF) model
(Schneiderman et al. 2002). These data were used in the validation runs of DYRESM and
for all future climate runs. The Nash-Sutcliffe efficiency coefficient for measured daily
stream flow v simulated daily stream flow was r2 = 0.78. Stream water temperature data
were available for the period 2002-present only. A five day average air temperature was
used as a surrogate for stream water temperature. Temperature at 0.5 m depth was output
from the model to represent mean daily lake SWT. The Nash-Sutcliffe efficiency
coefficient for measured daily lake SWT v daily SWT simulated using DYRESM for a
validation period of 1983 to 1996 was r2 = 0.97.
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