Mature Fish Are Found In Deeper Water Because of Humans (arstechnica.com)

An anonymous reader quotes a report from Ars Technica: When studying populations of a flounder-like North Sea fish called plaice in the early 1900's, a man named Heincke noticed that older, larger fish are found deeper in the water than younger, smaller fish. The same phenomenon was subsequently found for other North Atlantic species like cod, haddock, pollock, and some species of flatfish; it was thus dubbed Heincke's Law and treated as an established fact. Biologists assumed it was ontogenic in nature, meaning that it must be connected to how the fish age and mature.

All the species in which older, bigger fish are found in deeper water have something else in common: we eat them. Could it be, some Canadian scientists wondered, that all the big fish are found in deeper water because we fished them out of shallower water? Apparently (and somewhat astonishingly) this possibility had never been evaluated. And the scientists found that not only could this be the case -- it in fact was. "[T]he researchers added a simulation in which the depth and mass of fish were tied to the rate of mortality by fishing," the report adds. "When set to mimic the actual fishing rate over the two decades spanning the dataset, the model outcomes were consistent with both the new and old fish data. When fishing mortality rates were increased in the model, larger fish moved progressively deeper. And when fishing rates were set to zero in the model, there was no age-related deepening seen at all." The study has been published in the PNAS journal.

Comments

Even in the fishtank

[Oswald McWeany]

Even in a controlled environment like a fish tank, in lots of species, the younger fish hang out at the top of the water more than older fish. (now the reverse can be true in some species if the fry hide at the bottom instead- but most fry go to shallow or higher waters). No one is fishing fish out of my fishtanks.

In the ocean (or a pond, or a fishtank) small microscopic lifeforms are found in higher densities at the surface- because that is where the sunlight is. Whatever eats those lifeforms needs to be nearby... whatever eats those needs to be near them.

The larger you get, the more varied food you can eat- you don't necessarily have to stick with micro-fauna and micro-flora at the surface.

It may be that we eat the fish at the surface- and that's why no big ones there... or it could be because large fish simply don't NEED to be near the surface like small ones do.

Re:Evolutionary pressure

[Oswald McWeany]

"When set to mimic the actual fishing rate over the two decades spanning the dataset, the model outcomes were consistent with both the new and old fish data. When fishing mortality rates were increased in the model, larger fish moved progressively deeper.

I think this falls under confirming the obvious. Our fishing creates an evolutionary pressure. Fish that survive our hunting will tend to be the ones that prefer places where our nets don't reach as often. As long as we don't hunt them to the point where the population collapses it's perfectly obvious that we would see them evolve in response to our fishing tactics over time.

If this were primarily evolutionary driven, we would find more smaller ones in deep water too as the surviving big fish pass on the behavioral genes to go deep to their young.

I think this could be behavioral. Small fish need smaller foods than big fish. Smaller foods are nearer the surface because that's where the primary conversion of sunlight to growth is with micro-flora and the micro-fauna that feed on them.

This might be more complex than the knee-jerk "it must be humans" response. Of course, it could turn out to be that we just eat the big fish in the upper-levels, but it's not OBVIOUS until we test it and find out that this is true.