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Has fishing given cod a ‘depth sentence’?

07 Jun 2018 Liz Kalaugher
Photo of Atlantic cod and other fish species

Back in the early 1900s British scientist Walter Garstang and Germany’s Friedrich Heincke discovered that plaice in the North Sea swam deeper as they grew bigger. This trend was found in many other species of fish and became known as Heincke’s law.

Now a team from Canada has discovered that the “law” may not be due to fish biology but instead down to fishing. The Canadians’ interest began through recent reports that marine fish are heading deeper and moving north because of climate change. These studies, however, did not consider the bigger-deeper trend, which puzzled Kenneth Frank of Bedford Institute of Oceanography, Canada.

Initially, Frank and his colleagues reasoned that reduced fishing pressure in many areas of the ocean may have permitted fish to live longer and grow bigger, so that they would move deeper.

“But we also realized that most commercial fishing operations prefer to capture bigger fish – as well there are often minimum size regulations – and to fish in the most productive areas, such as on banks or plateaus that are relatively shallow,” says Frank. “All fishermen know about the importance of banks and the high catches they can yield relative to other ocean areas.”

The researchers decided to look at the role that size-selective fishing, i.e. targeting larger fish, could play in the distribution of commercial species. They used data on the depth distribution by age of Atlantic cod on Canada’s Scotian Shelf, to the southwest of Nova Scotia. There have been scientific surveys here every year since 1970.

“Virtually all the fish species that exhibit the bigger-deeper trend are targeted by commercial fishing,” says Frank. “As a result of their commercial importance, such species tend to have long-term, readily available data describing their distribution and abundance.”

The data from the Scotia Shelf indicated that cod swam deeper by about 100 m over the course of their lives, according to Frank. But the team found that the commercial fishing itself could have altered the distribution of the fish by size and age.

“We discovered that a simple population model that simulated the behavior of the fishery – in terms of selectively capturing older fish in shallower water – reproduced the depth distribution of cod that was observed during the scientific surveys,” says Frank. “The agreement between the model results and the field observations were stunning.”

Intensive fishing took place on the Scotian shelf until cod stocks collapsed in the early 1990s; the Canadian government imposed a moratorium in 1992.

“When there was no fishing, the bigger-deeper trend was not evident,” says Frank of the Scotian Shelf data. “Rather, adult cod of all age groups occurred at similar, rather shallow depths located on the productive banks. There was a suggestion of deepening among the younger cod – two and three year olds – amounting to about 10 m but beyond that age, up to age 12, no change in depth was evident.”

This indication that when there’s no fishing, Heincke’s law does not apply to older fish has several implications.

“There is a vast literature attempting to explain both the ecological and evolutionary significance of the bigger-deeper trend that we, as a scientific community, will want to revisit,” says Frank. “Further, the many recent studies that have interpreted deepening as a response to climate change need to be tempered by the fact that these studies did not consider age or sizes of any of the species. Moreover, the effects of fisheries exploitation were not considered.”

Frank believes that all future studies of changes in the distribution of marine and freshwater fishes must factor in the effects of selective fishing while addressing the potential role of climate change and habitat preferences. “It is not sufficient to look just at one factor in isolation from the others,” he says.

Now the team plans to look at the depth distribution patterns of haddock, pollock and silver hake. “These other species have different life styles and different types of fisheries exploiting them so we will modify our population models,” says Frank.

The researchers are also interested in the repercussions of removing fish from depths they would normally occupy. “When subjected to high fishing pressure it is possible that older/larger fish living in deeper water may be experiencing food shortages or possible metabolic stresses which may influence their reproductive capacity and ultimately their survival,” says Frank. “One might consider this a ‘depth sentence’ as a by-product of size-selective fishing.”

Frank and colleagues reported their findings in PNAS.

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