8. EFFECT OF THE PROPOSED MANAGEMENT MEASURES ON THE SHRIMP FISHERY IN THE IVORY COAST

The biology and fishery of P. duorarum is well known in the Ivory Coast. The simulation techniques (Garcia, 1978) could be used to study interactions between the lagoon artisanal fishery and the industrial sea fishery and also the effect of closed seasons at sea only and of combined sea and lagoon closures on the yield in weight and value, biomass and, therefore, catch rates and also on potential fecundity. The range of possible measures is very wide and the final selection will depend on their comparison with government options in terms of development. The more reasonable solutions, only taking into account the knowledge acquired on this stock, have been tested.

The combined use of global and structural models has shown that one could hope, by reducing the lagoon fishery, to obtain an optimum catch of 1 260 tons, which would be caught within 2 800 fishing days by eleven 250 HP shrimp trawlers working 250 days a year. For the present 350 HP vessels having higher fishing power (about 25 percent higher), the optimum potential catch (F_0.1) would be reached with seven or eight units.

At that level of exploitation in the lagoon observed in 1973 (55 percent), the optimum catch at sea drops to 570 tons for an identical level of optimum effort. The cpue would therefore be 200 kg/d (250 HP trawler) or 250 kg/d (350 HP trawler). The increase of fishing pressure in the lagoon results in a significant decline in catches at sea but also in an increase of catches in lagoon. It is therefore important to examine the development of total catches (sea + lagoon) in terms of the various combined exploitation schemes at sea and in the lagoon. Since the overall value of catches depends upon the sizes of shrimps caught, this development should be examined in terms of weight and market value. In the present situation, presented in Figure 13 by the year 1973 rate, an overall reduction of sea or lagoon effort would bring about but little change in terms of total tonnage (3 to 8 percent), but the improvement of the total value of catches would be very appreciable (12 to 43 percent, depending on the reduction considered in the lagoon).

Fig. 13 - Evolution of theoretical annual catches in weight and value for various levels of exploitation (combined sea/lagoon) (after Garcia, 1978)

After examining the simulated consequences of a variation in fishing effort at sea or in lagoon or simultaneously it is interesting to examine the theoretical consequences of seasonal interruptions: closed seasons at sea, in lagoon or both. These measures tend to modify the distribution of fishing mortality in terms of age, by protecting juveniles when their growth is still rapid and they have not yet reached sexual maturity. This protection can be given in the lagoon at the time of migration to sea immediately after this. Migration and recruitment at sea take place mainly from January to April, a period during which recruitment accounts for 53 percent of its total. The most effective closure would, therefore, correspond to this period. The management hypotheses tested are the following:

- Closures at sea: in January, January-February, February-March, March-April, February-March-April. The basic effort has been chosen equal to 300 d/month, a level corresponding to the maximum equilibrium yield;

- Closed seasons in lagoon: only the most favourable case, i.e. the one corresponding to February-March at the time of peak migration, has been envisaged. A longer period is theoretically possible but does not seem compatible with the present social and economical situation of artisanal fishermen. The level of exploitation considered is that of 1973, viz. 55 percent.

The results concerning combined catches (sea + lagoon) have been calculated in the same way and are given in Tables 7 and 8. In the case of closed seasons at sea only (Table 7), total catches (sea + lagoon) diminish very slightly in weight (-1 to -4 percent), but remain stable in value; in the case of closure at sea and a simultaneously closing of the lagoon (in February-March), the diminution in weight would be 3 to 7 percent but the gain in value would be 2 percent (Table 8).

This means that if, for social and economic reasons, exploitation in lagoons in the Ivory Coast were to be maintained at its present level, not much perceptible improvement of the overall catches (sea + lagoon) can be expected in weight or in value (whatever the management scheme adopted). On the other hand, it would be possible to make a 25 percent fishing effort saving at sea without causing any decline in the annual total catch, whether in weight or in value. If lagoon exploitation could be reduced, really interesting results could be obtained only in terms of value of the catches.

Whatever the scheme envisaged, the biomass and theoretical fecundity of the stock would be improved appreciably, which would reduce the risks of collapse.

Table 6

EFFECTS ON MARINE CATCH OF CLOSED SEASONS AT SEA ONLY OR COMBINED SEA AND LAGOON

Date of sea closing	SEA CLOSING ONLY (variable date)						CLOSING AT SEA (variable date) AND IN LAGOONS (February/March)
	Weight (t)	Gain %	Market value	Gain %	Theoretical Fecundity	Gain %	Weight (t)	Gain %	Value	Gain %	Fec.	Gain %
January	648	- 2	1 213	- 1	47	+ 09	748	+ 13	1 404	+ 14	54	+ 25
January/February	631	-5	1 200	- 2	50	+ 16	731	+ 10	1 395	+ 14	58	+ 35
February/March	637	-4	1 214	- 1	49	+ 14	738	+ 11	1 417	+ 15	57	+ 32
March/April	638	- 4	1 221	- 0	49	+ 14	735	+ 11	1 422	+ 16	57	+ 32
Feb./March/April	620	- 6	1 206	- 2	53	+ 23	716	+ 8	1-411	+ 15	62	+ 44
No closing	663 1/		1 228 /1		43 1/		764	+ 15	1 422	+ 16	50	+ 16

1/ Reference values in the absence of closed season.

Table 7

EFFECTS OF CLOSED SEASONS AT SEA ONLY ON VARIOUS OCCASIONS ON CATCHES (SEA + LAGOON)

	Catches (tons)				Market value (106 F. Francs)
Period of closure	Sea	Lagoon	Total	Gain %	Sea	Lagoon	Total	Gain %
January	648	517	1 165	- 1	1 213	569	1 782	- 1
January/February	631	517	1 148	- 3	1 200	569	1 769	- 2
February/March	637	517	1 154	- 2	1 214	569	1 783	- 1
March/April	638	517	1 155	- 2	1 221	569	1 790	0
Feb./March/April	620	517	1 137	- 4	1 206	569	1 775	- 1
No closed season	663	517	1 180 1/	0	1 228	569	1 797 1/	0

1/ Basic values in the absence of closed season.

Table 8

EFFECTS ON CATCHES (SEA + LAGOON) OF COMBINED CLOSED SEASONS AT SEA ON VARIOUS OCCASIONS AND IN THE LAGOON IN FEBRUARY/MARCH

	Catches (tons)				Market value (106 F. Francs)
Closing period	Sea	Lagoon	Total	Gain %	Sea	Lagoon	Total	Gain %
January	748	375	1 123	- 5	1 404	412	1 816	+ 1
January/February	731	375	1 106	- 6	1 395	412	1 807	+ 1
February/March	738	375	1 113	- 6	1 417	412	1 829	+ 2
March/April	735	375	1 110	- 6	1 422	412	1 834	+ 2
Feb./March/April	716	375	1 091	- 8	1 411	412	1 823	+ 1
No closing	764	375	1 139	- 3	1 422	412	1 834	+ 2

Values used for the calculation of gains are from Table 7