Suspension, surface deposit and sediment feeding animals of the sea bottom are preyed upon by a variety of carnivores, including many of the food fish highly prized by man. Studies of biomass and composition of benthic communities therefore give some indication of the extent to which areas can support stocks of fish. The production of animal material on the sea floor can be regarded as 'useful' or 'wasteful' according to whether it contributes to the formation of commercially valuable species of fish or other creatures of no value to man. For example, starfish and brittle-stars are not used as human food and are relatively unimportant as fish food; but they are carnivores and compete with fish for the same sort of prey, especially molluscs. From a human viewpoint, the growth of these predatory echinoderms can be regarded as a wasteful type of production.
The invertebrate predators of the sea bottom are far more numerous, comprise a far greater weight of living material, and have a much higher rate of food consumption than bottom-feeding fish. Thorson (1960) reviewed what is known of feeding habits and food requirements of predatory benthos in the north-east Atlantic area, and concluded that:
it seems reasonable to assume that most bottom-dwelling fishes in temperate waters will consume an average portion of food per day corresponding to from 5 to 6% of their own living weight during the summer half-year, and will reduce this rate significantly when the temperature decreases. During the coldest months of the year they may almost completely cease to feed . . . Summarizing the data for invertebrate predators, we find that at very young stages they are extremely voracious, taking an amount of food corresponding to about 25% of their own living weight per day. Those species which remain active as adults and continue to increase in size do require somewhat less food, although their average consumption is about 15% of their living weight per day. Finally, those predators which nearly or totally stop growing when mature and are sluggish, will in their adult phase, reduce their food claims to such an extent as to be cheap or fairly cheap to run.
On an average, growing invertebrate predators seem to consume four times as much food per day and unit weight as bottom-dwelling fishes, which seems reasonable when we realize that the life cycle, or at least the time from birth to maturity, is much shorter in the invertebrate predators than in most of the fishes. A species of invertebrate predator will often produce three or more generations (i.e. build up three or more units of meat) while a flounder is producing only one generation (i.e. build up one unit).
On the basis of our information that invertebrate predators consume food about four times as fast as the flounders, we must recognize the amazing fact that only 1-2% of the 'fish food' on the sea bottom is actually eaten by fish; the rest is taken by invertebrates. If the standing crop of 'fish food' is increased for some reason, the invertebrate predators with their shorter life cycles and quicker growth will furthermore be ready to take the advantage of this circumstance long before the fishes are able to do so.
Hardy (1959) in his classic work The Open Sea discusses Thorson's ideas in the context of 'weeding' the sea-bed of some of these predators, thereby leaving more food for 'useful' fish species (see Section 9.6.2).
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