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June 21, 2018 01:00 pm

Mature Fish Are Found In Deeper Water Because of Humans

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.

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