Tertiary pelagic production

Compared with herbivores, predators generally use a larger proportion of their food for respiration. A figure of 10 per cent (GCE = 0.1) for conversion of food to new tissue by carnivores is generally quoted. This permits us to divide the energy transfers at this trophic level as follows:

Energy inflow by predation = 170 kcal-m-2-yr-1

10% egested unassimilated = 17kcal-m-2-yr-1 to detritus

80% utilized in respiration = 136 kcal-m-2-yr-1 lost from the system

10% to tertiary production = 17kcal-m-2-yr-1

Taking 5 kcal g-1 dry wt as a mean calorific value for first-rank carnivores, tertiary production may be expressed as 17/5 = 3.4 g dry wt-m-2-yr-1 or (if 1g dry wt = 0.44 gC) approximately 1.5 gC-m-2-yr-1.

The mean annual biomass of pelagic fish in the English Channel has been estimated at 1.8 g dry wt-m-2 (Table 7.1). If we assume that this is 90 per cent of the weight of first-rank carnivores, the total biomass of pelagic predators is 2.0 g dry wt-m-2. We note that annual production at this level is only 3.4/2 = 1.7 times the mean weight of standing stock. Compared to plankton, fish are long lived but slow growing.

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