ANNEXE X

BIOSTATISTICAL DATA ON THE GHANAIAN DEMERSAL FISHERY, 1979–1985

by

KORANTENG, K.A. and S.N.K. QUAATEY

Fishery Research Unit, Tema, Ghana

1. INTRODUCTION

Demersal fishery resources in Ghanaian waters are exploited by three of the four main sectors of the marine fishing industry; namely canoes (or artisanal), inshore (or semi-industrial) and distant water (or industrial) vessels.

Since the decline of the sardinella fishery in Ghana in 1972/73, greater emphasis has been placed on the exploitation of the demersal fishes. Except for the fishery for tuna, the fourth sector, the flow of investment into the fishing industry has been principally, into boats for bottom trawling (Kwei, 1973). Fishing effort on demersal resources has increased considerably in the last two decades. Between 1966 and 1970, for example, the fishing effort doubled (Bernacsek, 1986). This situation has been worsened by the re-deployment of State Fishing Corporation's distant water vessels to fish on the Ghanaian shelf; and also by the operation of additional fleets owned by companies like AFKO, GHAKO, etc.

In the artisanal sector, demersal or bottom and near-bottom dwelling fishes are caught mainly by set nets, long line and beach seines. The fishing craft is a dug-out canoe which measures up to 20 meters long (depending on the gear used). About 52% of these canoes are motorised (Odoi-Akersie, 1986); outboard motors of up to 40 HP are used.

Most semi-industrial vessels are trawler/purse seiners which exploit demersal fishes throughout the year. During the sardinella season (July – September) however, most of these vessels use their purse seine gear. These inshore vessels are locally built boats which are of wooden hull, measure between 9 and 37 m in length and are powered by inboard engines of between 100 and 400 HP. There are two categories of these vessels, namely 9–12 m and 12,3–37 m. There are also some imported steel vessels. These are generally bigger than those built locally (although they rarely exceed 30 m) and have engines of between 350 and 650 HP.

Distant water (or industrial) vessels are all over 35 meters overall length and have main engines of more than 800 HP. Many of these vessels, especially those belonging to the State Fishing Corporation (S.F.C.), were built for fishing outside Ghanaian waters. Until the advent of the 200 mile EEZ, these vessels were fishing mainly off Angola and Mauritania.

In this paper, catch and effort of these fishing fleets with respect to demersal fish are presented. For the purpose of this exercise, demersal fish includes semi-pelagic fishes like the big-eye grunt or burrito (Brachydeuterus auritus) and the trigger fish (Balistes capriscus). Because of the high rate of their occurrence in bottom trawls, species like Selene dorsalis and Chloroscombrus chrysurus have also been included here.

The fishery for the trigger fish (mainly Balistes capriscus) has assumed greater importance in Ghana since 1973; the records of its catches have, therefore, been give special prominence in most of the tables.

In table 1, a list of the most important demersal and semi-pelagic fishes that have been considered in this work, is given.

For statistical purposes, the Fishery Research Unit has adopted the following as measures of fishing effort.

Canoes: - number of operations (or trips). An operation lasts for about a day or a few hours depending on the type of fishery and also the season. Duration of absence from port, which is in hours, has also been given in some of the tables. There is a tendency, currently, for some canoes to stay at sea for about three days; such canoes carry ice.

Inshore vessels: - number of operations (trip). Similarily, the duration of an operation varies between one and seven days. This also depends on the size of vessel and the season. In general though, a trip is about five days long. The imported, steel vessels, having better crew accommodation, stay at sea for up to about two weeks.

Distant water vessels: - duration of absence from port (days). Carrier vessels are not used in this operation.

2. STOCK ASSESSMENT

Since 1957, a number of resource surveys have been conducted in Ghanaian waters. A list of these surveys are given in Bernacsek (1986). The most recent intensive survey conducted on the Ghanaian shelf was between 1981 and 1982 (Koranteng, 1984) by the Fishery Research Unit, Tema.

Estimates of stock size from the 1981/82 survey are given in Koranteng (1984) and are summarised here as follows:

Total demersal and semi pelagic: 159,700 ± 38,800 tons

(average density of 85.9 kg/ha)

Total demersal component of Balistes capriscus: 79,200 ± 36,500 tons

(average density = 42.6 kg/ha)

Total demersal excluding Balistes: 80,500 ± 18,400 tons

(average density = 43,3 tons/ha)

These put the potential yield of demersal fish on the shelf of Ghana (up to 75 m deep) at 41,500 ± 10,000 tonnes. This estimate excludes Balistes capriscus, the corresponding figure for which is 21,000 ± 2000 tonnes. These estimates are slightly different from those given in Koranteng (1984) as incidental catches of pelagic fishes have been left out here.

Results of three demersal fish surveys in Ghanaian waters are given in Table 2.

3. CATCH AND EFFORT

The tables give the relevant catch and effort data in respect of the Ghanaian demersal fishery. In addition to presenting recent data, some historical data have been updated. In all the tables, “total catch” refers to catch of all demersal (and semi-palagic) species. This, however, excludes trigger fish which are mainly Balistes capriscus (= B. carolinensis).

Table 3 gives a record of the number of canoes and motor fishing vessels of all categories that operated in Ghana between 1980 and 1986. Of these, the inshore and industrial trawlers, the set net, line and to some extent the beach seine canoes, are the main harvesters of demersal fishes.

In the artisanal sector (Tables 4.1 – 4.7) data on the pelagic encircling gear “Poli” are also given separately. This component includes catches of “Ali” (an encircling gill net) and “Watsa” which is also a surrounding net. Catches of these gears are considered together because they are operated with the same type of canoes. Although these gears are essentially for pelagic operation, they contribute about 23% of total demersal fish caught by the artisanal fleet. They, however, account for nearly 55% of the total canoe effort. This situation has direct effect on the CPUE of demersal fishes in the Ghanaian artisanal fishery.

Also given separately in tables 4.8 – 4.15 are data on the beach seine fishery. This fishery has been described as probably the most destructive marine fishery in Ghana. This is because it thrives on the catch of juvenile fishes, both pelagic and demersal, which are yet to recruit to the main artisanal fishery. A number of measures have been suggested in the past, to arrest the situation. These include the institution of closed seasons for beach seine operations and mesh size regulations. The Fishery Research Unit, Tema, is currently looking at the first and certainly the most difficult of these options; and are trying to identify the most appropriate period for closure.

Because the catch of beach seines is mainly juvenile fish, sorting into species can be extremely difficult, and quite often this is avoided. On the average though, demersal fishes constitute about 36.4 % of the total catch of beach seines. In the last 13 years this percentage has varied between about 19.5 and 53.3 % and it includes catches of Brachydeuterus auritus, Selene dorsalis and Chloroscombrus chrysurus.

The data on the inshore fleet (Tables 5.1–5.7) come only from their trawling operations hence any catches of demersal fish made by inshore purse seiners have been ignored. The vessels operate as purse seiners mainly during the sardinella season, during which catches are made up nearly entirely of pelagic fishes.

Landings of the industrial fleet are from catches made only in Ghanaian waters (being part of CECAF statistical area 34.3.4). These are presented in tables 6.1 – 6.7.

The annual catches of demersal fish in Ghanaian waters are summarized in table 7. This includes trigger fish, the landings of which are given in table 8.6. Annual landings of some major fish species are given in tables 8.1–8.11. From the industrial fishery, fine species identification is not normally done. This has made it rather difficult, in fact impossible, to get precise catch values for some of the species. For example, most of the time, sparids are grouped into “Sikasika” (local name) or sea breams. The former is made up of Sparus caeruleostictus, Dentex canariensis and sometimes Dentex qibbosus; and the latter embraces Pagellus bellottii Dentex angolensis and Dentex congoensis. The landings of Pagellus bellottii between 1972 and 1976 (and up to 1978 in some cases) are most probably the total landings for all sparids except where stated otherwise. The required distinction between species for the industrial fleet is hardly made, even, to date.

In tables 9–11, the total landings of each fleet, comprising of both demersal and pelagic fishes, are given for the years 1972–1985.

These landings are further broken down into the contribution from each sector (gear) of each fleet (Table 12).

In table 10, the classification between 27–39 and 40– 120 footers was discontinued in 1984.

If one compares the annual landings of demersal fishes as given in table 7 with the biomass and potential yield estimates as reported in Koranteng (1984), it is clear that the current level of exploitation of demersal fishes is too close to the upper limit of acceptable levels.

It has been demonstrated in a recent work on the sea bream, Pagellus bellottii (Koranteng and Pitcher, 1987) that with proper management methods, the effort on the resource could in fact be increased and still obtain higher yield per recruit. This, needless to say, calls for, inter alia, stringent mesh size regulations, especially its enforcement, as this will reduce the risk of increasing the fishing mortality rate. The authors advise that for bottom trawling, the semi-industrial vessels must use codends of 70 mm minimum stretched mesh while 75 mm is recommended for the highly efficient industrial vessels. It is worthly of note that although current mesh regulations require that a 60 mm mesh be used for bottom trawling on the Ghanaian shelf, there is the tendency in the industry, to use nets of meshes smaller than 60 mm. Since Pagellus bellottii is a relativey small fish in this multispecies fishery the findings imply that a number of species, especially those that ultimately grow bigger than Pagellus bellottii are subjected to heavy exploitation probably before they have the opportunity to reproduce for the first time. Catches can thus be expected to be more variable or less predictable.

Balistes capriscus, on the other hand, is grossly under exploited. Except in 1979 when the total landing was about 62 % of the potential yield of its demersal component (Koranteng, 1984) average landings are around 20 % of the potential yield. Until recently, though, the trigger fish was not a target species and because of its low commercial values it is believed that a sizeable amount of its catch is discarded at sea. It appears, therefore, that the actual amount of Balistes caught, especially by the industrial fleet is not precisely known.

4. FUTURE WORK

In the last two or three years it has been observed that the modal sizes of a number of prime fishes, especially the sparids, have become fairly small. The Fishery Research Unit, consequently, mounted a programme to monitor fish landings with particular emphasis on fish sizes. Samples of fish were procured from commercial fish landings and the lengths of fish measured. It is sad to note here that due to financial difficulties, the programme was discontinued.

It is hoped that this programme will be re-activated in the shortest possible time.

The Fishery Research Unit also has plans to explore new fishing grounds beyond the 75 m depth contour which presently marks the upper limit of the Unit's trawling surveys. It is believed that there is a great potential for the development of a fishery for some prime demersal fishes beyond 75 m. Mensah (1981 and 1984) states clearly, the Fishery Research Unit's plans to carry out more research, for example, on the deap-sea red crab, Geryon quinquedens, the deep-water sparid community and the stock of Paracubiceps ledanoisi (Arioma bondi or butter fish) off Axim in waters deeper than 60 m. Troadec and Garcia (1980) also mention the great potential of fishing for deep-water fishes off Ghana.

Koranteng and Ofori-Adu (in preparation) have examined the difference in quantities of demersal fishes caught during and outside the coastal upwelling period in Ghanaian waters. The present investigation centres around the Saltpond and Winneba areas, but it has planned that the area of investigation be expanded to cover the entire shelf of Ghana.

Another extensive reappraisal of the demersal stocks has been planned to begin during the first half of 1987.

5. ACKNOWLEDGEMENTS

We are grateful to Mr. M.A. Mensah for carefully reading through the script and making very useful suggestions.

6. REFERENCES

Bernacsek G.M. (1986): Profile of the marine resources of Ghana. CECAF/TECH/86/71. 105p.

Koranteng K.A. (1984): A trawling survey off Ghana. CECAF/TECH/84/63. 72p.

Koranteng, K.A. & Pitcher, T.J. (1987): Population parameters, biannual cohorts, and assessment in the Pagellus bellottii (Sparidae) fishery off Ghana. J.Cons.Int.Explor.Mer, 43:10 p.

Kwei, E.A. (1973): Demersal fisheries. pp. 50–59. In Proc. of symposium on the Ghana Fishing Industry. Fishery Research Unit, Tema.

Kwei, E.A. (1973): Demersal fisheries. pp. 50–59. In Proc. of symposium on the Ghana Fishing Industry. Fisery Reserrah Unit, Tema.

Mensah, M.A. (1981): Report for the Triennium 1977–79 on the Fishery Research Unit, Tema. 82p.

Mensah, M.A. (1984): Report for the Triennium 1980–82 on the Fishery Research Unit, Tema. 45p.

Odoi-Akersie, W. (1986): A preliminary analysis of canoe frame survey - 1986. Fisheries Department, Accra.

Rijavec, L. (1980): A survey of the demensal fish resources of Ghana. CECAF/TECH/80/25. 28p.

Troadec, J.-P. and S. Garcia. (1980): The fish resource of the Eastern Central Atlantic. part 1. The resources of the Gulf of Guinea from Angola to Mauritania. FAO Fish. Tech. Pap. (186.1) : 166p.

* : Provisional

*** : From 1977 canoa census

Source : Fisheries Department, Accra

TABLE 4.1 : 1979

TABLE 4.2 : 1980

TABLE 4 : (continued)

TABLE 4.3 : 1981

TABLE 4.4 : 1982

	ALL GEARS				POLI
	Total	Balistes	Effort		Total	Balistes	Effort
MONTHS	(mt)	(mt)	Trips	Hours	(mt)	(mt)	Trips	Hours
Jan.	2542.9	749.8	46621	336570	641.2	726.4	25438	169836
Feb.	2868.7	318.5	58887	420495	450.4	306	23982	147967
March	1990.8	25.7	50584	363412	246.6	22.7	25887	156430
April	2557.4	2.7	68667	613901	594.3		27542	206886
May	3007.3	30.9	57053	875278	560.4		23781	175882
June	3349.1	190.5	56292	527694	247.6	187.4	22247	197543
July	3564.5	26.7	93927	450282	491.0	26.8	59616	159872
Aug.	3511.5		40735	354956	453.1		22058	186314
Sept.	4170.1	421.6	56208	477940	94.3	421.6	30172	222112
Oct.	3590.5	2.7	27005	2195136	639.8		38237	1798508
Nov.	3042.8	297.3	50285	405908	821.9	296.6	23269	209925
Dec.	5350.2	299.3	88926	801423	1082.6	299.3	40592	404603
TOTAL	39545.8	2365.7	745190	7822995	6323.2	2286.8	362821	4035378

TABLE 4 : (continued)

TABLE 4.5 : 1983

TABLE 4.6 : 1984

TABLE 4 : (continued)

TABLE 4.7 : 1985

TABLE 4.9–4.15 : (continued)

TABLE 4.15 : 1985

TABLE 5.7 : 1985

TABLE 6.7 : 1985

Note : Landings in mt

TABLE 8.11 : Dentex congolensis

Note : Landings in mt