Harvey estimated the mean annual biomass of demersal fish in the English Channel at 1.0-1.25 g dry wt^m-2 (Table 7.1). With a calorific value of 5 kcal^g-1 dry wt the energy content of this biomass is 6 kcal •m-2 approximately. Harvey put the annual yield at 30-50 per cent of stock (say 40 per cent) so annual production can be calculated as 6 X 0.4 = 2.4 kcal^m-2^yr-1. If the GCE is 0.1, this level of production requires an energy intake of 24 kcal^m-2^yr-1.
Demersal fish feed partly on benthic and partly on pelagic prey. We have calculated above that from the annual production of 17 kcal • m-2 •yr-1 of pelagic predators 1.5 kcal. m-2. yr-1 is removed by pelagic fisheries, and we have assumed that 3.4 kcal^m-2^yr-1 is lost by natural death, i.e. not eaten by other predators. This leaves approximately 12 kcal^m-2^yr-1 available as food for demersal fish. If this is all consumed by demersal fish, there is a balance of 24 - 12 = 12 kcal^m-2^yr-1 to be made up by devouring benthic fauna. As a fraction of the annual production of benthic herbivores this is 12 X 100/37.5, or about 30 per cent. Comparing this with Thorson's estimate quoted earlier (see page 241) that demersal fish take only 1-2 per cent of available fish food, with the rest consumed by invertebrate predators, it is clear that some assumptions are considerably astray. More information is necessary.
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