Conclusion

In the preceding computations we may note that from a total energy fixation of approximately 1500 kcal-m-2-yr-1, the energy content of the annual crop taken by fisheries in the English Channel has been only about 1.7 kcal -m-2-yr-1, i.e. only a little over 0.1 per cent. This figure is a reasonably conservative estimate for sustainable yields of shelf fisheries. From the North Sea and Icelandic shelf a slightly higher proportion of GPP is harvested. But for fisheries over the deep ocean it seems unlikely that even 0.1 per cent of GPP could be gathered as fish because of greater difficulties of capture, even allowing for greater production at lower latitudes.

Despite many uncertainties there have been several attempts to compute a global quantity for total GPP over the whole ocean surface. A figure of the order of 1017kcal-yr-1 seems reasonable. Taking the optimistic view that world fisheries extended over the deep ocean could take 0.1 per cent of this, we

Table 7.2 The energy balance sheet

Energy input

Value

Energy output

Value

(kcal-m_2-yr_1)

(kcal-rnm-yr-1)

GPP by phytoplankton

1500

Respiratory losses at each

trophic level, i.e. heat loss:

Additions of organic

Phytoplankton

300

materials from shore and

Pelagic herbivores

672

land, plus some

20 approx.

Pelagic predators

136

chemosynthesis,

Bacteria

213

utilization of DOM

Benthic herbivores

150

by saprophytes

Demersal fish

21

and NPP by

Benthic predators

24

benthic plants.

Loss to permanent sediment and

inorganic oxidations

2

Balance to fisheries and benthic

predators approx.

2

Total approx.

1520

Total approx.

1520

arrive at a maximum sustainable annual yield for world fisheries of about 1 X 1014 kcal-yr-1. Taking a calorific value of 5kcalg-1 dry wt for fish, and a wet weight of six times dry weight, this converts to about 12 X 1013 g wet wt or 12 X 107 tonnes per annum of fresh fish. For an economic return on the effort of catching fish, it is more realistic to suppose that substantially less than this could be captured. Considering that world sea fisheries are already taking over 6 X 107 tonnes of sea fish per annum, it is clear that our calculations certainly do not support ideas that the future food needs of the rapidly rising world population can be greatly alleviated by a large extension of ocean fisheries. During recent years, the world catch of fish has not increased appreciably despite greater efforts at capture (see Figure 9.25).

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