Photosynthesis and fish production in the sea.

Numerous attempts have been made to estimate the production in the sea of fish and other organisms of existing or potential food value to man (1-4). These exercises, for the most part, are based on estimates of primary (photosynthetic) organic production rates in the ocean (5) and various assumed trophic-dynamic relationships between the photosynthetic producers and the organisms of interest to man. Included in the latter are the number of steps or links in the food chains and the efficiency of conversion of organic matter from each trophic level or link in the food chain to the next. Different estimates result from different choices in the number of trophic levels and in the efficiencies, as illustrated in Table 1 (2). Implicit in the above approach is the concept of the ocean as a single ecosystem in which the same food chains involving the same number of links and efficiencies apply throughout. However, the rate of primary production is known to be highly variable, differing by at least two full orders of magnitude from the richest to the most impoverished regions. This in itself would be expected to result in a highly irregular pattern of food production. In addition, the ecological conditions which determine the trophic dynamics of marine food chains also vary widely and in direct relationship to the absolute level of primary organic production. As is shown below, the two sets of variables primary production and the associated food chain dynamics may act additively to produce differences in fish production which are far more pronounced and dramatic than the observed variability of the individual causative factors.