Chilko bull trout fed hardest on smolts and peak spawn

STEP 1 · Pulse Windows

Arrival period had the highest empty-stomach share

The 2021 captures ran from 22 April to 10 May during the sockeye smolt outmigration, from 23 July to 5 August, from 9 September to 22 September as spawners arrived and spawning started, and from 11 October to 22 October at peak spawning.

The authors describe 10 to 70 million sockeye smolts funnelled into the Chilko River each spring. About 300,000 sockeye spawn there in an average fall. Of that spawning, 85% to 90% sits in the first 2.5 miles (about 4 km) below the outlet. Those figures describe the system and were not counted from the 299 stomachs.

In spring, juvenile sockeye were in 85% of river stomachs and averaged 75% of stomach-content mass on the paper's per-stomach average. Lake fish were at 49% of stomachs and 41% on that same average. The authors assumed most of those spring juveniles were smolts leaving the lake.

Early-fall eggs were in 8% to 13% of stomachs and averaged 5% to 8% of stomach-content mass. At peak spawn eggs were in 65% of lake stomachs and 76% of river stomachs. Those eggs averaged 24% of stomach-content mass in the lake and 51% in the river.

Late-fall carcass flesh was in 21% to 25% of stomachs and averaged 12% to 22% of stomach-content mass. The lab scored flesh as a carcass only when fins, organs or red skin remained. Our read is that carcass flesh records consumption and can include scavenging.

Empty stomachs were 9.4% in late fall, 16.6% in spring, 24.0% in summer and 32.8% in early fall. The fitted model gave a lower probability that a stomach held food in early fall than in spring. It did not separate summer or late fall from spring on that fitted probability. A difference the study could not detect stays an open question here.

Among 239 fish with food, stomach contents reached 15.2% of body mass. Spring meals averaged 3.0% of body mass and late-fall meals averaged 2.7%. Every meal above 5.0% of body mass fell in those two periods. Summer averaged 0.3% and early fall averaged 0.7%. These means pool every capture location in the season.

In spring a larger body carried a relatively larger meal. The estimate rose 1.85-fold for each extra 1.3 pounds (590 g) of body mass.

STEP 2 · Modeled Line

Summer fish stayed under the daily maximum

The authors defined binge-feeding as a stomach whose wet contents beat a daily maximum from the Wisconsin bioenergetics model for bull trout. The inputs were fish mass and the average hourly water temperature for the 24 hours before capture. That maximum is a model calculation for these fish.

In spring, 71% of bull trout with food in the stomach were over the line. Among those spring fish the mean was 6.55 times the maximum and smolts averaged 77% of stomach mass. No fish in the summer sample was over the line. One early-fall fish reached 1.20 times the maximum on a single large fish. In late fall, 47% of fish with food were over the line and their mean was 2.85 times the maximum. Among those late-fall fish, eggs averaged 59% of stomach mass and carcass flesh averaged 11%.

Juvenile sockeye were still in 29% of summer stomachs. Across summer and both fall periods their average share of stomach-content mass was 4% to 20%.

Spring and late fall had similar meal sizes as a share of body mass. The binge ratio was still higher in spring. The authors suggest late-fall water up to 2.6 times warmer than spring water raised the modeled maximum. A similar meal would then cover less of that maximum. They also say faster digestion and eggs that burst during lavage could have understated late-fall mass. Unpublished numbers cited in the paper put Chilko sockeye eggs at about twice the energy density of smolts. Our read is to keep the smolt run and the peak spawn as paired windows. The binge ratio alone is a weak way to rank one pulse above the other.

STEP 3 · North End

Three seasons ate more toward the outlet

North in these captures points toward the lake outlet and the Chilko River. Relative consumption was higher farther north in spring, summer and late fall. Early fall was the period when that location pattern weakened.

In spring, relative consumption rose 1.71-fold per standard-deviation step north. The step was about 8.0 miles (12.9 km) of north-south capture position inside the model. The late-fall location effect on relative consumption was similar to the spring effect. The summer location effect was stronger than spring and its fold estimate had an interval too wide to use as a size. The early-fall location effect was significantly reduced from spring.

For the binge ratio, the spring step north was estimated at 1.21-fold and the interval included no change. Late fall was estimated at 2.78-fold per step and that location effect was significantly stronger than spring. Large meals marked the smolt run and peak spawn, along with a high share of fish that already had food over the modeled line. A northward tilt in meal size also showed up in summer.

STEP 4 · Organ Sample

Place and season are mixed together

Organ dry mass came from a permit-limited subset of 14 fish from 5 to 10 May, 15 fish from 24 July to 5 August and 14 fish from 13 to 21 October. The fish were not spread evenly through the study area. In fall, 12 came from the river and two from the northern lake. The summer text states a sample of 15 and also states one river fish plus 15 lake fish. Season and capture place are tangled in every spring or fall comparison with summer.

The stomach, pyloric caeca, intestine, liver and spleen were significantly heavier for their length in spring and fall than in summer. Spring stomachs were 70% heavier for their length than summer stomachs. Fall livers were 307% heavier for their length than summer livers. Our read is that these gaps belong to this uneven subset. They mix a seasonal contrast with a river and lake contrast.

STEP 5 · Shore Log

Prey and place are what you can check

Bull trout rules vary by water. Check the current agency page before you fish. Our rule is a log on a sockeye lake you already fish legally, or a watch from shore with no hook in the water. Record the date and whether you are at the outlet, the upper river or well down the lake. Record whether smolts are moving, whether spawners have just arrived or whether spawning is at its peak. Make the check during the smolt run, in midsummer, when spawners first arrive and again at peak spawn.

If smolts or spawners bunch near your outlet in those windows, your water shares the prey setup the Chilko fish fed on. Where you saw bull trout is your own observation. The paper measured stomach contents, not how many fish sat at the outlet. If the fish use the same places in every window, treat this paper as a weak guide for where you spend time on that water. The log can establish which prey were visible and where you saw bull trout on those visits. It cannot establish stomach mass, the modeled multiplier or a population-wide rate. In their discussion the authors say it remains unclear why some Chilko bull trout stay away from the outlet during the smolt run and spawning. A bull trout seen well down the lake in a pulse week does not throw that pattern out on its own. The measurements cover Chilko Lake and the Chilko River in 2021.