How to catch the "Master of the River"?
This is the 30th post in a series that translates scientific papers into fishing information.
This time, I would like to consider the theme of "How to catch the 'Master of the River'?" as a topic.
When we talk about the Master of the River, it's familiar from the retro game "Kawa no Nushi Tsuri" or the giant char episode of "Tsurikichi Sanpei".
It might sound a bit dry to put it this way, but what we really want to know is, "Where do large trout, the kind where there are only one or two in the entire river, tend to exist, and in what kind of rivers is their appearance rate high?" is what we want to know.
There is a study that simultaneously investigated the habitat patterns of more than 10 species of fish, including trout, in two rivers in England, the birthplace of fly fishing.
The results of this study report that there is a clear difference in habitat patterns between large trout and other fish.
It goes without saying, but in the same section of a river, many fish live together with other fish, and in the two rivers that were the subject of this survey, about 3 to 6 species were caught in the same location.
However, the only exception was trout.
This is because, trout live alone in more than 40% of the entire river area, and it was found that the maximum body length of the trout living there is inversely correlated with the population density (biomass) of other fish (see the illustration below).
In other words, this shows that trout do not live with other fish, but rather occupy a certain place (for example, a deep pool), and this tendency becomes stronger as they grow larger.
Behind these characteristics are "competitive exclusion" and "predator-prey relationships."
Competitive exclusion refers to so-called "territorial disputes," which is the act of driving others away to secure convenient locations for themselves, such as feeding grounds or resting places.
"Predator-prey relationship" is exactly as the words suggest, but in this case, the trout is the predator and other fish are the prey, so naturally, the prey cannot help but flee from the predator.
If that's the case, you already know why larger trout live alone in certain places, right?
As they grow larger, they require more food, so they will exclude other fish (including their own kind) and expand their feeding grounds themselves. Also, it might be better for their resting place to be larger as well.
So, in what kind of rivers are large, master-like trout likely to live?
The results of this study suggest that large trout are more likely to live in rivers where the scale of "pools," such as deep pools and drop-offs, is large and numerous.
On the other hand, in places with fast currents like mountain streams, the "exclusion of other species" shown by large trout is not observed, so it seems that the habitat of the pool is indeed important.
When you think about it, it might seem like common knowledge to anglers, but I think this is the actual scientific background behind it.
It is known that in places like New Zealand, large trout (such as brown trout) can often be caught even in small streams.
These rivers, called "creeks," do not have a large volume of water and are truly small enough for a person to walk across, so it was very mysterious why large trout could be caught there so easily.
When I asked a local guide about this before, they said, "Large trout are found in creeks where the current is slow and heavy (though the density of the fish population itself also has an effect)," so perhaps the entire creek is like a "pool" as we imagine it.
Also, it is often said in recent years that fish populations are decreasing due to river improvement projects that advance bank protection work and straighten rivers.
Considering the characteristics of their habitat, these large trout might be the first to be directly affected.
This was another thought-provoking topic.
Well then, see you next time.