The results in Table I can be used to make an estimate of the efficiency of transfer from one trophic level to another. The primary production is expressed in tons C.106/yr produced and so is the secondary production. The efficiency of energy transfer is given by the ratio of secondary production to primary production and the percentages are given in columns O1 and O2; in column O1, the trophic level efficiency with a "lengthened" generation time of secondary production is given, and in column O2 that with an uncorrected generation time is shown. The estimates in column O2 are higher than those in column O1. They range from 1.44 per cent to 23.67 per cent (or 1.92 per cent to 31.51 per cent), and the average is 9.75 per cent (O1) and 13.09 per cent (O2). There is no evidence of a trend with temperature or with latitude. It should be pointed out that the higher value of trophic level efficiency is derived in the simpler way by merely taking the number of generations and not the lengthened generations. The efficiency (13.09 per cent) is a little higher than the 10 per cent usually accepted. The method with the lengthened generation time yields an efficiency of 9.8 per cent. The efficiencies of conversion in the Pacific upwelling areas, excluding the eastern tropical sector, New Guinea and the Marquesas areas, are higher (13.89 per cent and 18.53 per cent) than in other areas. Because of the fair uniformity of method over extensive areas, it is possible that the Pacific estimates are the best ones. Hence a figure of more than 15 per cent in the upwelling areas is a distinct possibility.
The production at the third trophic level may be estimated by taking 1 per cent of the primary production or 10 per cent of the secondary production by both of the estimates used. The estimates are given in Table I. Where possible an average has been taken of 1 per cent of primary production and 10 per cent of the secondary production, with the two methods used. This average has been expressed as tons C.104/yr and as tons wet weight/yr. The two estimates proceed from independent estimates of primary and secondary production respectively, but both make an assumption of 10 per cent efficiency between secondary and tertiary production. They are not therefore entirely independent, but the degree of correlation between them suggests that the basic methods are sufficient for our present purposes. The weights from the Marquesas divergence and the eastern tropical Pacific are excluded from the sum because the oceanic production cycle has a third trophic level composed of very small fish, for example, the myctophids of the Deep Scattering Layer. It is assumed that these fish should not be exploited at present because they are widely dispersed and costly to catch. The sun of third level production available in coastal upwellings (as opposed to equatorial upwellings) is about 125-150 M tone wet weight per year. If the true transfer coefficient of energy is higher, for example, 15 per cent, the available production in the third trophic level should be increased to 190-300 M tons wet weight/yr.
The question which now arises is how much of the third trophic level is composed of fish. In temperate waters the competitors with fish at the third trophic level are carnivorous copepods, arrow worms, jellyfish, ctenophores, squids and other pelagic molluscs. So the weight of secondary production in Table I includes part of the third trophic level as invertebrates. Very little is known of the abundance of squids and since they are in economic and fishery terms comparable with fish they will be treated as fish. To account for part of the invertebrate third trophic level, 9 per cent of the secondary production is taken, and the tertiary production would then amount to about 120 M tons or ca 210 M tons at a 15 per cent conversion rate. All estimates given here are averaged across the upwelling area and through the whole season.
If the tertiary production is largely composed of fish and squid, then the annual production is the annual increment to populations in recruitment and in growth. It is assumed that the maximum biomass of a fish stock is reached during adult life, because from one generation to the next the weight of gonads in the stock must increase by three or six times to create the filial stock. Fish in subtropical areas do not appear to live very long. Peruvian anchoveta live for 2 to 3 years and Californian and South African sardines live for 3 to 6 years (Davies 1958, Marr 1960). Hake, which feed on euphausiids, may live somewhat longer. A rough effective life span in the fishery of about three years would be reasonable. Hence the stocks available in the upwelling areas might range from 340 M tons upwards and the yield, to all consumers of the tertiary production, of which man can probably take no more than half, equivalent to one year's recruitment, ranges from 113 M tons upward.
This result is above most present prognostications for world fisheries, save that of Chapman (1967). It includes all fish in the pelagic zones, - sardines, anchovies, scads, hakes and part of the tuna-like fishes. Although the proportions are known off California, the same proportions may not apply elsewhere. For example, in the Peru Current, egg samples taken throughout the year show that the anchoveta out-numbers all other species at all seasons by a factor of ten. In some distributions, the eggs of other species are excluded in space by those of the anchoveta. Off California, the eggs of sardine and anchovy at the coast and jack mackerel and hake are distributed in succeeding bands offshore, hake and anchovy abundant and sardine less so (Ahlstrom, 1966; Alverson, 1967, California, Department of Fish and Game, 1956). The two best known areas, Peru and California, are so profoundly different that no generalization can be made about the division of the annual third level of production into groups of fish.
Most production of harvestable animals in the upwelling areas is tertiary, but that in the divergences of the equatorial region is in the fourth level, the larger tuna-like fishes feeding on a level of small fish and macrozooplankton. In the eastern tropical Pacific and in the Marquesas divergence the production at the fourth level, may be estimated as 95 and 30 tons C.104/yr, or 1.6 and 0.5 tons wet weight/yr.
The tuna stocks are oceanic and the larvae are distributed all over the ocean. Hence the stocks need not depend upon the divergences of the coastal upwellings or the equatorial system to sustain themselves and it would be difficult to disentangle the contribution from the upwelling areas. However, the low oceanic catches of tuna are explained. A very rough estimate of the total production of tuna-like fishes in the world ocean would not be greater than perhaps 3-5 M tons in the equatorial region.
Columns S1 and S2 of Table I give the wet weights of tertiary stocks in M tons/year. It will be seen that the major areas are those in the Peru and Benguela Currents (about 20-30 M tons). The medium areas are California, Chile and southwest Arabia (5-10 M tons). The rest are small ones (1-5 M tons). Gulland (1968) has suggested that the total annual deaths (equal to total annual production) of the anchoveta stock off Peru is 18-20 M tons. As there is now only one year-class in the stock (and most of our carbon and zooplankton observations were taken during the period of exploitation), the correspondence between Gulland's estimate and this one is quite good.
Taking all upwelling areas, the production of fish and squid may be as much as 120-130 M tons. If we suppose that one third to one half can be taken by fishing, then a potential catch of 40-60 M tons is available. As some of this catch (ca 15 M tons) is already being taken, perhaps we may guess that a further 25-50 M tons of fish and squid can be taken from the upwelling areas.
