In a recently released paper the findings of an investigation of the potential trends in the age and length at maturation of cod (Gadus morhua), haddock (Melanogrammus aeglefinus) and whiting (Merlangius merlangus) from the Firth of Clyde have been published.
It will come as no surprise that they found over fishing causes a reduction in the of age of maturity which in turn causes smaller mature fish.
The Firth of Clyde is a large estuary in the west coast of Scotland that once supported substantial demersal fisheries for a range of species such as cod, haddock, saithe, whiting and hake (Ross et al., 2009). A ban on trawling for vessels over 15 m was repealed in 1962. Consequently, the landings of demersal fish quickly rose, peaking in 1973 before falling into decline (Hislop, 1986), reaching near-zero levels by 2005 when the fleet stopped targeting demersal fish. Fishing effort shifted away from demersal fish during the 1990s as more vessels began converting to exploit the abundant Nephrops stock, and since 2005 the Clyde demersal fish catch has been mostly bycatch from the Nephrops fishery.
It is well known that trawl fishing always truncates length structures, lowering the abundance of large fish, but if it has also been causing increasingly early maturation then the fishing process has induced a response which may have further reduced the probability of individuals growing to a large size.
A reversal of these trends in maturation may promote increases in the abundance of large fish, which is needed if the Clyde demersal fishery is to be restored.
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