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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.
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.
Very interesting. The abstract of the paper is somewhat more clear than the summary on Slash Dot. We need to teach the public about how these simple simulations relate to reality and help avoid the magical thinking that 'the simulation finds this effect' so it must be the key to the story when the effect was basically put into the simulation. The key piece of rigorous thinking that justifies putting this paper in PNAS is that using a simple model with empirically measured fishing rates for various sizes of Cod, they reproduce 70% of the observed deepening of large fish, suggesting that selective fishing is the largest contributor to the observations that led to Heincke's Law.
Evolution takes place over thousands of generations.
No it does not require thousands of generations. Have you ever seen a purebred dog? Humans applied selective breeding and can develop a completely new breed of dog with just a few generations. Evolution CAN happen slowly but it does not have to. It can happen quite quickly given the proper evolutionary pressures.
Heck... it's hardly migration. "Deepening" differences are measured in ranges from 60 to 120 meters deep.
Evolutionary pressure don't not care about what you perceive to be a small difference in distance. All that matters is whether that difference in depth creates an advantage in reproduction. If the difference in depth causes a difference in reproductive rates within a population then voila, you have an evolutionary pressure.
I.e. Young and inexperienced fish don't know how to hide from the nets OR the easy picking food (bottom dwelling crabs and crustaceans) they're munching on isn't available that deep.
The ones that prefer the locations where they do not get hunted (the reasons why don't matter) are the ones that will be selected to breed again. Small fish that don't prefer the deep get removed from the gene pool before they reproduce and so they never become big fish. Do this enough times and you will have selected for fish that prefer deeper waters. That my friend is an evolutionary pressure at work and it happens all the time.