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.

Explaining the law

Starting in the 1990s, a number of hypotheses were posited to explain Heincke’s Law. One is that larger, older fish gravitate down to cooler waters where the diminished metabolic demands can increase their lifespans. Another suggested that all fish prefer to be in shallower water, but when the population gets too big, the seniors get shunted out of prime territory by the youngsters and have to live in deeper waters. A third holds that juveniles hide in shallower waters from the threatening adults down in the depths.

A new hypothesis, posited in 2005, contends that the older fish are going deeper to find favorable temperatures as ocean water warms. It has even been suggested that the depth at which we find these fish can act as an indicator of how much the seas are warming.

The new research starts by reevaluating a dataset of cod stock in the Eastern Scotian Shelf, in the Northwest Atlantic just south of Nova Scotia, from 1970-1989. In 1993, Canada put a moratorium on cod fishing because the stock was so depleted, ending an industry that had sustained humans in the region for the past 500 years. Cod stocks have started to rebound, but the moratorium is still (mostly) in effect.

Simulated fishing

This new analysis recapitulated previous findings from 1992: older, larger cod were found in deeper, cooler waters. But the researchers added a simulation in which the depth and mass of fish were tied to the rate of mortality by fishing. 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.

Between 2006 and 2010, fishing rates were in fact zero—so larger cod were not being fished out of shallower water. During this time, codfish of all ages and sizes were found living together in shallower water.

This process has implications beyond fisheries management. If hypotheses that related the maturity of fish to the water temperature were correct, then fisheries data would give us a way to estimate water temperatures, helping us track the oceans' warming. But it looks like we probably shouldn’t use the depth at which commercially fished species are swimming to tell us anything about the temperature.

PNAS, 2018. DOI: 10.1073/pnas.1802096115 (About DOIs).