Quick summary: Bull trout movements match the life history of sockeye salmon: consumers can exploit seasonally distinct pulses (Transactions of the American Fisheries Society, 2017)
In this paper, we studied the movements of bull trout at a lake-river transition in British Columbia. Bull trout are a largely freshwater fish, and is a species of conservation concern throughout its range in both the United States and Canada. At our study site (Chilko Lake), bull trout feed heavily on juvenile sockeye salmon that leave the lake and swim downstream (towards the ocean) each spring (more on that in later posts). We were curious if bull trout would return to this region year after year to feed on these juvenile salmon.
We implanted acoustic tags into bull trout: these tags send out a short acoustic code (at a frequency we can't hear) at set intervals. We placed receivers (listening stations) at the lake outlet, and ~1-km downstream in the river, to listen for the tagged bull trout. The tags lasted for well over a year, so that we can determine if bull trout returned to the area the following year.
Nearly half (40%) of the tagged bull trout did return to the outlet the following spring during the sockeye smolt downstream migration. In addition, bull trout use of the river was increased in the fall, when adult sockeye salmon spawn. A fish closely related to bull trout (Dolly Varden) has been shown to feed on spawned eggs in other systems, so it is possible bull trout in our system are doing something similar. Use of the outlet was relatively low during other times of the year (so the fish appear to move into the area specifically to exploit sockeye salmon). Bull trout are also a coldwater species (and don't do well in warm waters), and we found these fish avoided the river when waters were warmer than 16 C.
In short, this simple paper showed that bull trout use the outlet during 1) spring juvenile sockeye salmon migrations downstream, and 2) when sockeye salmon spawn in the fall. Some bull trout, but not all, do return to the outlet in successive years, indicating feeding on juvenile sockeye salmon could be very important for bull trout.
Why it's important to science: This paper demonstrates that a species (here, bull trout) can move around their landscape to exploit not just a single pulse of energy, but two that occur in different seasons (salmon smolts in the spring, and spawning salmon in the fall).
Why it's important to conservation and management: This paper demonstrates that bull trout behavior is inherently tied to sockeye salmon life history, underscoring the potential importance of salmon to bull trout.
As always, if you have any thoughts or questions, let me know! Please note I may update this summary as I write more posts for other publications.
Full citation:
NB Furey, and SG Hinch. 2017. Bull trout movements match the life history of sockeye salmon: consumers can exploit seasonally-distinct pulses. Transactions of the American Fisheries Society 146: 450-461.