Diel Feeding Patterns in Juvenile Chinook Salmon

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Diel Feeding Patterns in Juvenile Chinook Salmon
-Alisa Bieber, Wetland Ecosystems Team,
University of Washington
How does stomach fullness vary over the course of
the day?
What is the gastric evacuation rate and daily
ration?
Background A small (337 ha of historic marshes)
estuary along the Oregon Coast, the Salmon River
estuary has been the site of intensive research,
particularly into restoration science. Sections
of the estuary have been restored to tidal
inundation by removing dikes in 1978, 1987, and
1996. In 2003 and 2004, the Salmon River estuary
was part of a coast-wide study evaluating Chinook
salmon and their habitat in several estuaries
along the Oregon coast, gathering data on
abundance, diet, prey resources, genetics,
otoliths, RNA/DNA ratios, and, in the Salmon
River, growth through a mark/recapture study.
The diet data will be used as part of a
bioenergetic model to estimate relative growth of
the Chinook in different estuaries, and compare
the models results to the RNA/DNA ratios and the
mark/recapture study. Diets, however, only offer
a snapshot of what the fish has been eating over
the last few hours, while the bioenergetic model
requires the total caloric value of prey consumed
over the entire day. This leads to potentially
problematic assumptions in the model regarding
both diet composition and the quantity of food
consumed. In order to estimate the diet with
more precision, this study was designed to
evaluate some of the factors influencing diet and
better estimate a daily ration.
Gastric evacuation rate StSo-rt Where St is
the stomach content weight at time t, So is the
initial stomach content weight, r is gastric
evacuation, and t is time elapsed (Doble and
Eggers 1978). Assuming no feeding occurred after
full dark, r-0.476 for the 2400-300 time, and
r-0.247 for the 2300-600 time, and r-0.346
for the 300-600 time. Higher absolute values
indicate faster digestion. On average, it would
take 3.5 hours to digest half of a full (2400 hr)
stomach.   Daily Ration (D, percentage of
bodyweight eaten/day) D100 F/W and F24
Sr Where F is the total amount of food eaten in
a day, and W is the fish weight. Where S is mean
level of food present in the stomach in a 24 hr
period (Doble and Eggers 1978). 5.23ltDlt10.08
g/g/d using high and low r values

• Highlights
• Marked decrease in fullness after midnight,
  particularly in foregut
• Feeding resumes around daybreak
• Highest feeding rates in the evening and the
  following noon
• No reduction in feeding during daytime observed
• This data is consistent with the visual, often
  diurnal feeding behavior of salmonids. The night
  of the study was rainy, so no moonlight was
  available for feeding, and daytime (on day 2) was
  not very bright. Crepuscular feeding was
  observed during only the evening twilight, not in
  the morning, perhaps because the rain affected
  feeding intensity.

• Methods
• 9 AM May 28 2004- Noon May 29, Salmon River
  estuary, reference marsh
• 5 fish captured every 3 hours by pole or beach
  seine
• Fish were weighed, measured and
• Preserved in 10 formalin
• In the lab,
• Total stomach fullness estimated (1-6)
• Fore- and hindgut fullness estimated (1-6)
• Total contents weighed
• Contents identified, weighed individually

Comparing the Results
How does diet composition vary over the course of
the day?
This studys calculations for the daily ration of
Chinook match closely with Sagar and Glova (1988)
and are slightly lower than Healey (1980). The
gastric evacuation rate found in this study
brackets the value offered by Doble and Eggers
(1978) for similar-sized sockeye.

• Highlights
• In the evening, aquatic and benthic prey
  predominant
• Daytime feeding on day 1 (not raining) included
  mostly aquatic and benthic prey and plant matter
• Daytime feeding on day 2 (raining) included
  primarily neuston
• Tides do not apparently have a large effect on
  diet composition
• Higher-calorie food was consumed at 2400
  (polychaetes) 900 and 1200 on day 2 (insects)
• Lower-calorie food (corophium) was consumed in
  the evening

• Green patternsneustonic (surface) prey
• Blue patternsaquatic or benthic prey
• Orange patterns either
• Yellowrest or unidentified

• Conclusions
• Feeding was heaviest in the evening and dusk
• As visual feeders, Chinook do not eat when it is
  fully dark
• Gastric evacuation rate estimated in this study
  matched that given by other authors for sockeye
  salmon of similar size.
• Daily ration for the fish was estimated at
  between 5.23 and 10.08 g/g/d in this study,
  similar to or lower than other studies have
  reported.
• Light cycles are a more important determinant in
  both feeding intensity and diet composition than
  tidal cycles
• Although repetition of this study would lend more
  weight to the results, these results will
  increase the accuracy of bioenergetic modeling
  for this system.

References
Pole seining in an estuary
Salmon River estuary
Doble, BD and DM Eggers. 1978. Diel feeding
chronology, rate of gastric evacuation, daily
ration and prey selectivity in Lake Washington
juvenile sockeye salmon (Oncorhynchus nerka).
Trans. Am. Fish. Soc., Vol 107, 36-44. Sagar PM,
and GJ Glova. 1988. Diel Feeding periodicity,
daily ration and prey selection of a riverine
population of juvenile chinook salmon
Oncorhynchus tshawytscha (Walbaum). J. Fish
Biol. 33, 643-653. Healey, M.S. 1982. Juvenile
pacific salmon in estuaries The life support
system. Pages 315-341 in V.S. Kennedy (ed.),
Estuarine comparisons. Academic Press, NY.
Acknowledgements Many thanks go to Margot
Hessing-Lewis, Trevan Cornwell, Lisa Krenz, Dave
Hering and Sam Perry for field assistance,
despite bad weather for camping on a marsh.
Thanks to Si Simenstad and the WET lab. Finally,
I am grateful for the funding provided by the
Oregon Sea Grant program and the UWs School of
Aquatic and Fishery Sciences.