Female lobsters living in marine reserves have been found to grow faster than those in unprotected areas in a study by Norwegian scientists.

Marine reserves are areas of ocean where a government has placed limits on human activity and removing or destroying natural resources is prohibited. Tonje Knutsen Sørdalen, a researcher at the University of Agder (UiA) studied lobsters in three protected areas and found that marine reserves especially help female lobsters to grow faster

Lobsters have exoskeletons and they must shed their old shells in order to grow. They take on lots of water, which forces the shell open, then they walk out of their old shell, ready dressed in their new one.

In the past it was thought that with competition over food in protected areas, lobsters would grow less. The study shows the opposite.

“The most likely explanation is that fast-growing lobsters are also the ones that are the hungriest and therefore seek out lobster traps to look for bait," says Sørdalen. "In fishing areas, the fast-growing lobsters disappear from the population, and such areas are left with the lobsters that grow slowly and are hesitant to enter traps.”

For male lobsters, the results point in the same direction, but the differences between areas were significantly smaller than for females. Sørdalen explains that this difference in growth means the results can only be applied to female lobsters.

“We came to the same conclusions in all the three areas, which means we can rule out local variations,” Sørdalen says. “We’ve previously shown that male lobsters have larger claws relative to body size in marine protected areas.

"We therefore have a theory that the difference in growth would hold for male lobsters had we looked at weight instead of length."

Lobster data

The researchers compared 14 years of lobster data from three different areas along the Skagerrak coast of south-east Norway - all have been lobster reserves since 2008 and lobster fishing is prohibited. The ecosystem of the wider region is strained and negatively affected by shipping and intensive fishing.

A total of 2,303 lobsters were caught, tagged and later recaptured. In the reserves, it was found 77% of female lobsters of 25cm length will change their shells next year, compared to 66% in areas affected by fishing. Significantly, lobsters in reserves grow 9% on average with each shell change.

The reserves have proven very effective in increasing local lobster populations and Sørdalen thinks the protection of fast-growing lobsters is key to understanding why.

“We did not find this difference for lobsters below the minimum size of 25cm; these are clear signs that selection is taking place due to fishing,” Sørdalen says. “Bold and hungry lobsters are fished out of the population.”

The researchers say that marine protected areas (MPAs), along with regulations around maximum lobster landing size, make an unexpectedly good combination. One theory is that lobster reserves can supply surrounding areas with more lobsters because some are forced out when the reserve gets crowded.

In 2017, a maximum size of 32cm for landing lobster in the Skagerrak was introduced. Sørdalen thinks that this regulation in combination with reserves will help increase the populations that live outside the reserves.

“They will be protected. If they grow big inside the reserve, they can move out without fearing the lobster traps. Fast-growing lobsters develop faster and reproduce earlier and are therefore also great for breeding,” Sørdalen says.