Thoughts from Shepherd Research Center – Stewarding for Fish Productivity
Here at the Shepherd Research Center, we have a lake with very high fish productivity. It’s called Fish Fry Lake, and it lives up to its name. For instance, last Sunday five of us fished for two hours and caught 141 fish, mostly one- and two-year old perch. However, at about 7.5 inches, even the one-year-old fish were large enough to fillet. In the mix were also twenty one three-, four- and five-year-old perch, with the largest of these being females and weighing nearly two pounds, bulging with eggs. We even caught three Yellowstone Cutthroat trout. Amazingly, just four years ago this water was so nutrient-saturated that its dissolved oxygen levels were only about one third of what Cutthroat require to survive. Today Cutthroat are doing well, despite being located thirty miles east of Billings, out on the plains, where water can get very warm in the summer.
Two Minutes per Fish
Over a four month period last year we tracked our catch rate, as well as the pond’s overall fish productivity. We found that the average catch rate was one fish every two minutes. Now this spring, after harvesting nearly three thousand fish from the 6.5 acre pond over the course of the year, there are still an estimated 5,400 harvestable fish present in the lake. Their average weight is just under one-quarter pound. And if we harvest 110 pounds of these fish every month, we can keep up with the incoming non-point source phosphorus.
What we have learned is that the lake can sustain a 110 pound/week harvest level. But it’s not that simple. For example, even with enough phosphorus to sustain such a harvest level, we run out of nitrogen. So to use up all the phosphorus, we have to gauge the correct volume of nitrogen and add it to reach the correct ratio of phosphorus to nitrogen to carbon. This is not a simple, bake-a-cake recipe. It means we have to measure. All of a sudden this stewardship thing is harder work.
Nitrogen and Circulation
We are between a rock and a hard place, because if the phosphorus is not harvested in the form of fish, it will very likely become free floating algae, underwater plants, or cyano-bacteria. Any one of these other optional life forms would result in our Cutthroat trout going away.
The lake is a wild fish fishery; we do not feed these fish. Instead, we provide substrate upon which biofilm-generating bacteria grow. They are more aggressive consumers of nutrients than any of the other optional life forms. However, they are limited: if there is not enough surface area, especially inert, non organic surface area, one of the other life forms will dominate. On the other hand, if the biofilm generators win out, fish growth and productivity can be very high. Extremely high. By adding circulation to this equation, we can set the stage for the biofilm generators to win, because they do about 4.5 times better with circulation than without it. So life got simple again.
Looks like all we have to do is add nitrogen so the correct nutrient ratio is in place. Then circulate. The circulation brings aeration with it, and that’s good, since three of the four nutrient conversion cycles are aerobic. But we’re not through with our stewardship challenge quite yet, because even though those nutrients become fish in this process, someone’s got to catch them buggers! Even at a fish every two minutes, 110 pounds translates into 440 quarter pound fish, caught over 880 minutes of fishing time, or fourteen hours of fishing per month. And today, even with a drum scaler, that’s probably at least another fourteen hours of fish cleaning time. Since a lot of the folks that fish here are new fisherpeople, you can imagine who gets stuck with fish cleaning duty.
By the way, here is an excellent article on the nitrogen cycle written by an expert for aquarium owners.
Swapping Computer Screens for Fishing Poles
What’s amazing to me is that so many young people are no longer in touch with fishing. So many kids have never been fishing, and even fewer have caught fish. Still fewer know how to clean fish. Still fewer know how to fillet fish. But rest assured, lots of kids have experienced “all of the above” here at Fish Fry Lake. Maybe a little fish slime on their computer key boards isn’t a bad thing!