I finally got out in the field today, a welcome break from office work.  I went on a mini backpack electrofishing trip to collect fish for part of my research on a local stream. 

Because the area I work in contains an important Bonneville cutthroat trout metapopulation, the presence of brown trout and their role in the system is a major concern.  We’re trying to figure out whether or not brown trout are piscivorous (eat fish), and more specifically, if they’re eating cutthroat trout. 

While brown trout are known to be piscivorous in many other systems, our data thus far has not shown that they eat other trout.  Basically, they seem to eat what is most available.  Since the water is so incredibly productive here, most of the food items we find in fish stomachs are aquatic insects such as caddisflies, mayflies and stoneflies.  However, there seems to be a fairly strong shift in diet composition that is directly tied to seasonality.  In late summer, aquatic and terrestrial insects are the main diet staple.  In late fall and early winter, when these insects are less prevalent in the drift, they feed on fewer insects and more sculpins, which are a small minnow-like fish species (Cottidae) that live near the stream bottoms. 

We hypothesize that if brown trout are feeding on cutthroat trout, they’re most likely to do it when cutthroats are most available, in areas where the two species co-exist.  This could very well happen in mid-summer, when juvenile cutthroat trout are first emerging from their redds in the gravel.  Many of these fish emerge and drift downstream, if they aren’t strong enough to swim to more suitable areas.  This is when I suspect that the browns may be feeding on cutthroat trout, which is why I’ll conduct most of my sampling the second half of July. 

Sampling includes collecting fish via electrofishing, measuring and weighing them, collecting scales for aging, and pumping stomachs to extract gut contents.  We pump stomachs by placing a small hose, which is attached to a pump, into the fish’s stomach and pumping water into the stomach to flush out all of its contents.  Some fish will be sacrificed (killed and opened up) to validate the stomach pumping technique.  

Out of the 60 fish collected today, I found no fish in stomach contents.  Most of the items collected were aquatic and terrestrial insects such as caddisfly larvae, mayfly larvae, stonefly larvae, cicadas, wasps, and ants.