A Comparison of the Growth Histories of Pre- and Post-Decline Brood
Years of Sockeye Salmon (Oncorhynchus nerka) from Rivers Inlet Based
on Otolith Analysis, and in Relation to Environmental Conditions
Claire Li Loong, Hakai Network for Coastal People, Ecosystems and Management, Simon Fraser University
Abstract
History of Rivers Inlet Sockeye Salmon
The widths of growth zones
recorded on otoliths of sockeye
salmon (Oncorhynchus nerka)
from Rivers Inlet were measured
to assess the impacts of
environmental conditions on
growth at different life history
stages. Otoliths from the predecline (1984-1987) and postdecline brood years (1997-2005)
were compared. Post-decline
juveniles were significantly larger
than pre-decline individuals
during the migration to sea, but
grew less in the first marine year.
Final size was not significantly
different between time periods.
Otolith growth was positively
correlated with salinity during
September and October.
Decreased salinity under a 30 ppt
threshold appears to have
contributed to recent
unfavourable oceanic conditions.
•  Sockeye salmon of Rivers Inlet have been
commercially fished since 1882.
•  Rivers Inlet has been one of the largest
producers of sockeye salmon along with the
Fraser and Skeena River.
•  Returns in the 50’s and 60’s exceeded a
million on several occassions.
•  There was a substantial decline in returns of
sockeye salmon in the 1990’s. Fewer than
3,600 fish returned to spawn in 1999.
•  The fishery was closed indefinitely in 1996.
Methods
Discussion
•  I used otoliths (ear bones) to examine annual
growth in the fish. Incremental distance between
annuli (rings) was measured. Each width
represents the annual growth of the fish.
•  Salinity and temperature was obtained from
McInnes Island lighthouse which is along the
migratory route of the salmon.
•  The effect of the environmental conditions on
growth at different life stages was studied.
•  Growth and environmental conditions were
compared in pre- and post-decline years.
•  The relationship between chlorophyll production
and salinity was compared.
Results
•  Post-decline smolts are larger than pre-decline
smolts (Figure 3) but attain the same final adult
size as pre-decline fish (Figure 4).
Possible causes of decline
•  Although juvenile post-decline fish are larger
when they leave Owikeno lake, they grow
less in the ocean than pre-decline fish.
•  Final adult size is the same between preand post-decline fish.
•  Size is a good predictor of survival in the
ocean.
•  Adverse marine conditions have likely
contributed to the decline of Rivers Inlet
sockeye salmon.
•  The salinity conditions that the juvenile
sockeye salmon experience during fall while
they migrate past McInnes Island are a good
predictor of how much they will grow that
year.
•  If the juveniles experience unfavourable
ocean conditions during their first year at
sea, they may not attain the large size and
energy reserves required to survive their
first winter at sea.
•  Salinity values over 30 ppt at McInnes Island
during September and October had a strong
positive effect on fish growth in their first year at
sea (Figure 5).
•  Freshwater habitat degradation from logging
around Owikeno Lake.
•  Increased predator range due to warming
climate.
•  Changes in ocean conditions.
•  More chlorophyll was present when salinity was
above 30 ppt at McInnes Island (Figure 6).
•  There was a significant seasonal effect of
salinity. Fall (September and October) was a
good predictor of otolith growth in the first marine
year. Summer influenced otolith growth the least
(Figure 7).
0.7
Reason for study
550
Hypural (mm)
1st FW
0.6
•  Despite elimination of fishing pressure, the
population has still not returned to historic
abundances.
0.5
0.4
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Conclusions
600
500
450
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350
300
250
(a) Pre
(b) Post
Time period
(a) Pre
Figure 3. Juveniles are smaller in
pre-decline years than in postdecline.
(b) Post
Time period
Figure 4. There is no significant
difference in final size between
adults in pre- and post-decline
years.
1800
1700
Chlorophyll
(mg/m2/day)
1600
Questions
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1000
900
800
Poster Design & Printing by Genigraphics® - 800.790.4001
0.5
0.45
0.4
0.35
0.3
0.25
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(f) NovDec
(e) SepOct
(d) JulAug
(c) MayJune
0.1
(b) MarApr
0.15
(a) JanFeb
Tula Foundation
NSERC
Fisheries and Oceans Canada
Wuikinuxv First Nation
Bachan Family
Bart Proctor and Dave Rolston
Laurie Ainsworth
Dr. Ron Tanasichuk
Dr. Isabelle Côté
Dr. Richard Routledge
Figure 5. Average salinity over 30
ppt at McInnes Island was correlated
with otolith growth in the first marine
year.
Correlation
Acknowledgements
•  What are the causes of the decline?
•  What changed from the pre-decline to postdecline era?
•  Why has the population not recovered?
Time window
Figure 1. Otolith showing
annual growth zones.
Figure 2. Study site.
Figure 7. Fall salinity at McInnes Island
affected otolith growth the most strongly,
with the weakest correlation occurring in
summer.
Above
Below
Salinity =30
Figure 6. Significantly more
chlorophyll was present at salinities
above 30 ppt.
•  The sockeye salmon of Rivers Inlet have
encountered unfavourable changing ocean
conditions.
•  Sea surface salinity has decreased to
unprecedented levels in recent years.
•  Growth in the first marine year is lower than
that of the historical population.
•  Decreased growth rates in the first marine
year may lead to decreased survival and
may negatively impact the ability of the
population to rebuild.