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They consist of an electron-abundant benzene ring fused to an electron-deficient pyridine ring; these electronically distinct rings can be modified independently by adjusting reaction conditions.

However, a panel of 12 experts said in January no definitive cause had been found, but they ruled out pyridine and concluded it was possible a disease or parasite new to UK waters was responsible.

However, the panel of 12 experts said in January no definitive cause had been found, but they ruled out pyridine and concluded it was possible a disease or parasite new to UK waters was responsible.

Measures should include "thorough sampling and testing for chemical pollutants such as pyridine," it said.

Research last year by academics, backed by the fishing industry, suggested the industrial pollutant pyridine, possibly from dredging at the mouth of the River Tees, as a potential cause.