If you need proof that UK fish stocks have been in decline you need look no further than the British sea fish angling records.
These records give you little indication of the size of fish prior to large-scale commercial exploitation (archaeological records show that cod in medieval times could be several meters long), but for an insight into the current state of our marine environment it's enlightening.
If you take an average of the years in which the record fish were caught the heyday for sea angling in UK waters was roughly the mid-1980's, it's been pretty much downhill ever since.
The largest bass was caught in 1988, cod 1992, haddock 1978, halibut 1979, herring 1973, ling 1989, mackerel 1984, monkfish 1984, skate 1986 and plaice 1989 (Source: British Record Fish Committee). It would be surprising if any of these records were ever broken again. The oldest record still standing is from 1933; Atlantic bluefin tuna - they left UK waters when the North Sea herring fishery collapsed.
Whilst this can in no way be described as a scientific measurement of the health of UK seas, it's a sobering reminder of what we've lost beneath our surrounding seas.