Lobsters left alone by fisherman are growing in size and number, a study has found.
And the subsequent rise in population has led to vicious territorial battles between the creatures, who are tearing lumps out of each other's shells, causing infection and disease.
The research by Swansea University and the Shellfish Association of Great Britain (SAGB) was conducted at Lundy island, off the North Devon coast.
It was published as 31 new marine conservation zones (MCZ) across the UK were announced by the Government.
The results have brought to light some of the negative effects of allowing populations to grow unchecked by commercial fishing.
The work compared European lobsters in the protected "no-take" part of Lundy's MCZ, and the "refuge" zone, where pot fishing is allowed.
Animals grew bigger when not fished and numbers soared, but this led to "increased levels of physical injury and shell disease", the study found.
This was said to be due to "higher levels of aggressive behaviour between competing lobsters", whose damaged carapaces became infected.
Scientists suggested the cost and benefits of such protected areas should be critically assessed and that health monitoring be included in management strategies to "improve design and efficacy".
SAGB director David Jarrad said: "We wholeheartedly support and are actively involved in improving marine conservation and fisheries management.
"This timely study highlights the importance for independent scientific evidence to inform the use of marine protected areas.
"We really do need to assess the role of no-take zones to ensure they are the appropriate tool for the job."