222

Whether this very large catch will continue, only time will tell; but, with this success in Lake Kioga, it is hardly surprising that no vigorous steps were taken to stop the spread of the species to Lake Victoria where it is now gaining a foothold. It is worth noting, also, that the original wishes of those who wanted sport fishing are also being met; angling for Nile Perch in the Nile above the Murchison Falls has now become an important tourist attraction.

It may well be that the catches from Lake Kioga will diminish in the future, perhaps drastically, and the Nile Perch accounting for half the total catch certainly suggests an unstable population structure. It would need a brave biologist, however, to predict at what level the catch will stabilize!

Although Lake Victoria, to which Fryer refers specifically, is very much larger than Lake Kioga, in terms of fish catch per unit area it has a very much smaller production. Its fishery for Tilapia, which is limited to the shallow bays and a narrow strip of inshore waters, is a classic example of over-fishing brought about mainly by the use of small-meshed nets as Fryer (1972) points out. But the introduced Nile Perch needs large-meshed nets--up to 8 or 9 inches (19.3 or 22-9 cm) compared with the 5 inches (12.7 cm) which was recommended by Graham (1929), as well as by Garrod (1963), as the minimum to ensure a prosperous Tilapia fishery. It might be that, as the Nile Perch increase in Lake Victoria, the fishermen will take to large-meshed nets as they have already done in Lake Kioga. This could give the Tilapia a chance to recover, even though some of them, together with other species of less commercial value, will be gobbled up by the Nile Perch.

The moral of this story is that a complex ecological system, such as the food-web which links primary, secondary, and tertiary, biological production in an African lake, raises issues which make it no simple matter to predict the effect of a change such as the introduction of a new predator. While I agree with Fryer's comment that in our present state of knowledge of such systems mathematical models are of no great help to prediction, I would add that the extremely simple model which he puts up when stating that 'the addition of predators reduces the efficiency of production by 80 per cent', may be highly misleading when it comes to the practical returns from a fishery. However, I wholly associate myself with Fryer in his pleas for better systems of control and conservation of the extremely important resources of these lakes, and, in particular, for inter-state cooperation, as every lake needs a common policy and nearly all the big ones are shared by at least two countries.

Biological Conservation

References

FRYER, Geoffrey (1972). Conservation of the Great Lakes of East Africa: a lesson and a warning. Biological Conservation, 4(4), pp. 256-62, iUustr., and pp. 301-2.

GARROD, D. J. (1963). An estimate of the mortality rates in a population of Tilapia esculenta Graham (Pisces, Cichlidae) in Lake Victoria, East Africa. J. Fish. Res. Bd Can., 20, pp. 195-227.

GRAHAM, Michael (1929). The Victoria Nyanza and its fisheries: a report on the fishing survey of Lake Victoria, 1927-28. Crown Agents for the Colonies, London: 225 pp., illustr.

UGANDA (1971). Fisheries Department Annual Report for 1970. Entebbe, 22 pp. (typescript).

WORTHINGTON, E. Barton (1929). Report on the fishing survey of Lakes Albert and Kioga. Crown Agents for the Colonies, London: 136 pp., illustr.

E. BARTON WORTHINGTON, Scientific Director, International Biological Programme, 7 Marylebone Road, London N W 1, England.

HOPEFUL EMPIRICISM v e r s u s SCIENTIFIC METHOD: THE NILE PERCH CONTROVERSY REVIVED

Dr E. B. Worthington's note, while not specifically supporting the introduction of the Nile Perch (Lates niloticus) into Lakes Victoria and Kioga, nevertheless suggests tacit approval.* Such a stance I regard as regrettable. Whatever the present status of the Lake Kioga fishery, of which more anon, the long-term prospects for fisheries involving piscivores are much less attractive than those based on herbivores. Relevant facts, which need not be repeated here, were put forward several years ago (Fryer, 1960), and neither a temporarily flourishing fishery nor the satisfaction of a few anglers causes me to change the views then expressed. Theoretical reasoning, simple observation, and practical results, all show that better long-term returns are obtained by harvesting a herbivore than a carnivore, and as an approximation the statement that: 'The addition of p r e d a t o r s . . , reduces this

* Dr Worthington comments: 'My intervention was in- tended, not to give 'tacit approval' to the introduction of Nile Perch to Lake Kioga or Lake Victoria, but to indicate that the argument against it, which was invoked by Michael Graham and myself many years ago and is now used again by Geoffrey Fryer, is too facile. There is no possibility now of getting rid of the introduced fishes from these lakes, so let us make the best of things. If their influence is followed carefully through the years to come, the introductions might throw light on some interesting unresolved questions concerning aquatic ecology, including the reason why they have proved to be such valuable commercial additions to the Kioga fish fauna up to this time.'--Ed.

Short Communications

efficiency [of production] by 80 per cent' is fully justified. I agree that those dealing with fisheries can- not always concern themselves with the niceties of conversion rates, and must cope with the situations that present themselves; but this does not invalidate this simple fact, nor does it justify reducing the efficiency of an existing ecosystem by the introduction of a predator.

In arguing for the beneficial effects of such an introduction, proponents of the idea make much of the concept of cropping small species through the agency of large predators. Such exploitation is grossly inefficient. The real challenge presented by these small species--which are now assuming great importance as a source of human food in Lake Victoria--is how to crop them directly without seriously damaging the complex ecosystems of which they are part. Such direct cropping can yield more protein per unit area than can the harvesting of their predators, which produce only one unit of flesh for each five units consumed.

As to Lake Kioga, the situation is highly complex, involving as it does not only the introduced Lates but also another introduced species, Tilapia nilotica, which, while trophically a herbivore, obtains some plant material as detritus. It so happens that I have been attempting to follow the progress of the Lake Kioga fishery and have figures obtained by the Uganda Fisheries Department for catches of both Tilapia and Lates for each of the past 13 years. In February 1972 I was provided by that Department with figures for 1970. These are completely different from--and much lower than--those quoted by Dr Worthington! As in previous years, the figures supplied are of the reported numbers of fishes caught and their weights in pounds--which to some extent enables their authenticity to be checked. The 1970 figures are:

Tilapia--7,101,890 fishes weighing 11,464,350 lb,* Lates --1,454,606 fishes weighing 20,039,758 lb,*

--or, in round figures, Tilapia 5,120 tons and Lates 8,950 tons. According to the Uganda Report, the Tilapia catch was more than three times and the Lates catch almost four times as high as indicated here. The discrepancy cannot be accounted for by confusion of measured catches and estimated yields, as the 1970 figures quoted here are in line with previous years (and indeed are approximately 40 per cent higher than the totals claimed for each species in 1969). Regrettably, statistics relating to the yields of certain African lakes are unreliable. The vital point to make, however, is that the 9,000 or so tons of Lates harvested represents

* 1 lb = 0.454 kg.

223

a potential loss of up to 36,000 tons of other species. As to Lake Victoria, the possibility of fishermen

voluntarily shifting to large-meshed nets to catch Lates must remain a pious hope. Only a shift exclu- sively to such nets would fulfil the hope expressed, and this is almost impossible to visualize.

Relevant here is a recent proposal to introduce Lates niloticus to Australasia (Midgley, 1968, 1969). This, based on observations whose significance is obscure, and on extremely crude experiments whose relevance to any ecological situation is difficult to envisage, rightly caused alarm in that region. As a result, the case for introduction was effectively con- demned by Williams (1971), whose plea was supported by quoted extracts from letters received from those best qualified to judge. There is, however, a frightening appendix to Midgley's report which indicates that, when the pelagic Tanganyikan clupeid Stolothrissa, now apparently introduced successfully into Lake Kariba, has become fully established, Tanganyikan species of Lates may also be introduced. Such a proposal is biologically suicidal, and it is sincerely to be hoped that those who realize that one rears cows for direct consumption and not to feed to lions in order that the latter may be eaten, will forbid this for all time. In the long term, human welfare may be far more dependent on this decision than on many a seemingly more important but ephemeral political edict.

It is pleasing to end in wholehearted agreement with Dr Worthington regarding the need for cooperation in planning the controlled exploitation of these valuable resources. Too often in the past, important decisions have been left to those unqualified to appreciate the consequences of their actions. In the future they should be based on sound principles of ecology and conservation.

References

FRYER, G. (1960). Concerning the proposed introduction of Nile Perch into Lake Victoria. E. Afr. Agric. J., 25, pp. 267-70.

MIDCLEY, S. H. (1968). A study of Nile Perch in Africa (and consideration as to its suitability for Australian tropical inland waters). Winston Churchill Mem. Trust Fellowship Rep. No. 3, 20 pp.

MIDCLEY, S. H. (1969). A study of Nile Perch in Africa. Aust. Soc. Limnol. Bull., No. 1, p. 5.

WmLIAMS, W. D. (1971). On the proposed introduction of Lares niloticus (L) to Australia. Aust. Soc. Limnol. Bull., No. 3, pp. 33-5.

GEOFFREY FRYER, Principal Scientific Officer, Freshwater Biological Association, The Ferry House, Ambleside, Westmorland, England.