First, regarding the current state of rivers in Hokkaido. Unlike Honshu, Hokkaido's inland waters generally lack fishing cooperatives, and fishing rights are not established. Consequently, the vast majority of these waters do not require permits like fishing licenses. Without cooperatives, there are no stocking programs. In some areas, volunteers do stock rivers; I myself have experience stocking Yamame trout as part of an elementary school class activity. However, these are truly only a small fraction of Hokkaido's numerous rivers. In other words, cases of artificially increasing stream fish populations are not particularly common these days. We generally consider them to be naturally reproducing.

Second premise: Regarding anglers in Hokkaido. Specifically for freshwater trout fishing, catch-and-release is likely the overwhelming majority practice. It seems human involvement is minimal in the decline of fish numbers. While it wasn't uncommon in the past for people to take home large numbers of one-year-old and two-year-old fish, this is rare now.

Building on these premises, the concept I focused on this time is “natural carrying capacity.” This term comprehensively describes the extent to which a region's natural environment can sustain the healthy growth of a given species and quantity of flora and fauna. To illustrate with a human and societal analogy: what happens when too many people gather in a certain area? Various problems arise, such as difficulty securing supplies for survival or failing to achieve an ideal living environment. Many people can probably relate to this concern, as a politician who ran for Tokyo governor from a city mayor position also expressed similar fears. Think of natural carrying capacity in a similar way.

It's something you might not notice casually observing, but nature is remarkably well-balanced. Trees grow, creating habitats for animals and insects. Birds, animals, and insects aid in their propagation by consuming fruits and pollen, thereby ensuring their own survival. Trees surrounding mountains and rivers stabilize river environments by retaining water through their roots. Overgrown trees decay and fall naturally, allowing sunlight to reach younger, emerging trees. Animal carcasses become nutrients for other animals, insects, and the soil.

The red fox I encountered walked effortlessly up the steep slope. By eating plants and defecating, it plays a role in dispersing seeds to other locations.

Nature is sustained through the constant cycle of life and decay as various creatures thrive and perish. However, when the balance somewhere is disrupted and the normal cycle cannot be maintained, the state where the entire area continues to change can be called a state exceeding the carrying capacity of nature.

Regarding the second river mentioned earlier, rivers teeming with small fish aren't particularly rare when you're fishing. Viewing this situation from an angler's perspective, many probably can't help but feel disappointed. It's the sad nature of anglers to inevitably seek out larger fish.

“But you catch a lot, so isn't that good enough?”

True. True, but it's not the same. When you catch lots of small ones, you somehow even start to feel apologetic. Even though you're supposed to be fishing. Anglers are peculiar creatures.

“Then why not just stop fishing?”

That's not it. Try fishing yourself, and you'll understand this feeling. If you've never fished, I urge you to give it a try.

Having many small fish means the environment isn't conducive for fish to grow large. So, focusing on the fish living in the river, what do they need to grow big? Food and shelter. When the number of fish exceeds the natural carrying capacity of the area, there isn't enough food to support the excess fish, nor enough safe places for them to live. This means fish get caught by predators or reach the end of their lifespan before they can grow large.

This is how rivers end up with lots of small fish and no large ones.

Furthermore, when small fish proliferate, the aquatic insects they prey upon also decline. Small fish are often voracious eaters. As one species increases, the species it preys upon decrease. Within this dynamic, the balance gradually shifts, and the imbalance grows larger. Can't we faintly glimpse a future where nature quietly deteriorates in this manner?

Amidst such phenomena, what can we do? One answer might be “thinning.” Reducing the number of fish that have become too numerous. By removing fish in moderation, food and comfortable habitats could be distributed more evenly, potentially allowing the remaining fish to grow larger. While not specific to trout, there are apparently examples where thinning different species of freshwater fish has helped balance generational turnover. However, this is truly an effective measure only for very limited periods or situations. Intervening in nature is incredibly difficult. Precisely because everything is interconnected, it's nearly impossible to fully grasp how far the impact of human intervention might ultimately spread. Reading Frank Sawyer's book “Trout Fishing in England” (translated by Mamoru Kuramoto), written by someone who worked as a “river keeper” in England, the birthplace of fly fishing, really brings home this difficulty.

When viewed as a fishery resource, trout reveal significant distortions in nature. Yet attempting to “manage” this with human intervention presents enormous difficulties. What we should be doing now is not trying to manage nature, but rather offering just a little help for nature to regain its former state.