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Ronan's report


Wednesday 3rd March, 2010

Last time I wrote about Trophic Cascades, and the trickle down effect (sometimes catastrophic) within an ecosystem that can happen if its top predators are removed.

I picked up an interesting book the other day at Powell’s, the local book nerds Mecca, all about the topic. It’s called Where the Wild Things Were: Life, Death, and Ecological Wreckage in a Land of Vanishing Predators by William Stolzenburg. Pretty good so far. And a bit frightening. Check it out.

While I was reading on the bus yesterday I got to thinking about another predator vs. prey relationship. This one is less complex in theory than the Trophic Cascade, but is surprisingly hard to prove in the wild. The idea is that the number of predators is directly related to the number of prey available, and in search of an equilibrium between numbers, a cyclical relationship develops when either predator or prey numbers is pushed out of balance by outside factors. A pretty basic example is something like mice (prey) and owls (predator). When there is a surplus of mice, the theory goes, the population of owls will be well fed and their reproductive success will improve to the point where their populations grow. Often the predator population will grow to the point that the result is an overall drop in prey (mouse) population. But fewer mice means less food for the owls, and some will end up starving, and the population will drop again in search of equilibrium. Of course, things are rarely this simple, and there are other factors that influence predator and prey populations in the wild, but the theory makes sense and my basic summary will have to do for today’s FP.

What I was thinking about in regards to this theory was the predator vs. prey population relationship between fly anglers (the most common predator on Sexyloops), and the fish that we catch.

In fact, I managed to find a reference that links the actual theory of population dynamics that I’ve just described to an Italian mathematician who may have been inspired to develop the theory based on his observations of fishermen.

“The Italian mathematician [Vito] Volterra is said to have developed his ideas about predation from watching the rise and fall of Adriatic fishing fleets. When fishing was good, the number of fishermen increased, drawn by the success of others. After a time, the fish declined, perhaps due to over-harvest, and then the number of fishermen also declined. After some time, the cycle repeated.“

Clearly it isn’t this simple. Just as weather or other factors might influence the predator prey relationship between owls and mice, there are other factors influencing the population of anglers other than simply fish populations. Commercial fishing is one thing, and yes, we are seeing fewer and fewer commercial fishermen every year as our salmon stocks, tuna stocks, cod stock, etc crash across the globe. But in today’s era of C&R flyfishing and vastly reduced fish populations there are other factors at work that determine the overall population of anglers vs. the overall population and availability of fish. Otherwise, it seems that flyfishers would be rapidly declining in numbers if not going extinct outright. And, despite my lack of real data, I just don’t think that is happening. But maybe it is, and maybe what we perceive as crowding of our favorite rivers is really just relocation of flyangling predators from other rivers where the fish populations have indeed crashed or disappeared, while the overall population of us is decreasing as individuals “starve” or simply get disgruntled by what it takes nowadays to find consistent fishing and quit the game.

I feel like I’m getting very close to opening up another giant can of worms here as well - one that I don’t want to mess with on this FP. That is, trying to find an answer to the question “Why do we flyfish?”. Based on the above example and the predator vs. prey relationship, it’s certainly not only “to catch fish to eat” or even “to catch and release fish”.

If anyone is interested, feel free to open that can up over on the Board!

Be Well & Fish On,
Matt


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