INTRODUCTION

Maputo Bay is located immediately adjacent to the capital city of Mozambique. It has a surface area of 381 km2 and an average depth of 10 metres. Estimates of the landings from the Bay during 1984 include 1,914 tonnes of fish by-catch and 722 tonnes (whole weight) of shallow water shrimps from trawling, and about 3,000 tonnes of fish from (drift) gillnetting. There is a small additional catch from other methods.

The main contributors to the catch are the artisanal fishermen. Their boats are generally small, of about 3 to 7 metres in length, and powered by outboard engines, sails or oars. Some fishermen operate from small rafts, while others fish from shore without craft. The landing sites are the Costa do Sol, Muntanhana, Macaneta, Inhaca Island, Machangulo, Catembe, and Matola (see Figure 1).

An estimate of the landings by this group for 1984, is 1,100 tonnes of fish by-catch and 350 tonnes of shrimps from 152 shrimp boats, and about 2,600 tonnes of fish from 303 gillnet boats. The catches by the fishermen using handlines, estimated as about 500 tonnes, are not included in the above values.

The other susbstantial contribution to the catch comes from the larger privately owned boats. These are all operated from Maputo Harbour, are of 7 to 15 metres in length, and powered by inboard diesel engines.

An estimate of the catch from Maputo Bay for this group during 1984, is 666 tonnes of fish by-catch and 316 tonnes of shrimps from 14 shrimp trawlers, and 100 tonnes of fish from 6 gillnet boats.

The remainder of the catch comes from the larger government owned boats operated by SULPESCA. These are very similar in size and construction to the privately owned boats, and also based from Maputo Harbour.

An estimate of the catch from Maputo Bay for this group in 1984, is 148 tonnes of fish by-catch and 56 tonnes of shrimps from 6 shrimp trawlers, and 307 tonnes of fish from 15 gillnet boats. Since 1984 four of the boats have been engaged in pair trawling for fish. In that year the catch amounted to 33 tonnes. Some SULPESCA boats are also used in handlining for fish outside Maputo Bay.

The source of the above catch information is M. I. Sousa, Actividade de Pesca de Emalhe e de Arrosta na Baia de Maputo em 1984., Boletim de Divulgacao., No. 10.

CATCH AND EFFORT DATA COLLECTION

Servico de Pesca da Cidade de Maputo:

Amongst its many functions the Servico administers the collection of catch and effort statistics for Maputo Bay. This is a log-book type system, in which the fishermen in-charge of licensed boats and gear, are required to provide the Servico each month with returns, describing the weights of catch landed on each fishing day. Each catch is usually categorised by type; for example as fish, shrimp, octopus, crab etc. Following a date when all (or most) of the returns are thought to be in-hand, the Servico undertakes a tabulation of the data.

As a check on the accuracy of these data, the Instituto de Investigacao Pesqueira undertook a comparison with data they had collected, from weighing the catches of the private fishermen landing at Maputo Harbour, and the artisanal fishermen landing at Catembe. This weighing of all the landings during one day each week is ongoing, and was commenced during 1984.

The comparison excercise revealed that the extent of agreement was good for Catembe, but was very poor in respect to the landings by the private fishermen at Maputo Harbour. It is believed that the result for Catembe was unlikely to be representative for the other artisanal landing sites.

The fishermen here are required to submit the catches from each fishing trip for examination by the local port officer. It is presumed they would be reluctant to provide false information in the monthly returns submitted to the Servico, as this could readily be checked with the information recorded by the port officer.

At the other landing sites there is either no local port officer, or the requirement to show the catches is much less rigorously enforced.

Administracao Maritima de Maputo:

The Administracao operates posts adjacent to the landing sites of Maputo Harbour, Costa do Sol, Inhaca, and Catembe. Amongst other duties, the port officers stationed at these posts are required to issue certificates, to indicate the weight of catch by type in each landing.

These certificates are required by the fishermen as proof of catching the fish, and in turn allows them to sell their catches through the official marketing system.

The information recorded on the certificates also includes the name of the fishermen in-charge, the name of the boat, and the date. The catch weights are given as determined 'by eye'.

The booklets containing copies of the certificates are periodically forwarded to the head-office, where they are stored. The data recorded on the certificates are not used for statistical purposes.

The issuance of certificates was seen to be done effectively at Catembe. At this site they are issued in respect to all the landings. This is assisted by the location of the post, which is on a jetty, adjacent to the landing site. The fishermen position their boats against the jetty on returning from each fishing trip, so the catches can be seen by the port officer.

Similarly, at Maputo Harbour there are certificates issued in respect to each landing (except for the SULPESCA boats for which the catches are taken directly to the company cold storage facilities). The landing site is enclosed within a security fence, with the post located adjacent to the entrance gate.

The port officer, however, does not usually examine- the landings himself, but records on the certificates the quantity of catch as indicated by the skippers.

At the Costa do Sol, the other landing site visited during the mission, the port officer issues about ten to twenty certificates on each fishing day. This is less than twenty percent of the actual landings.

The problem here is that the landings occur over some four kilometres of beachfront, and as such it is usually inconvenient for the fishermen to present their catches for inspection.

The port officer does, however, attend the local fish market each day, and is available to issue certificates prior to the auctioning of the fish. Many of the certificates are issued in this way, although it seems that most of the landings are being sold outside the official marketing system.

There are no posts adjacent to the landing sites of Matola, Muntanhana, Macaneta, and Machangulo. The one post on Inhaca Island is insufficient, to service all the fishermen operating around the island.

An additional function of the Administracao is the issue of fishing licences. These are of twelve months duration, and in respect to fishing boats and certain gears. Every year a brigade of the Administracao undertakes an inspection of all the boats and fishing nets at each of the landing sites in Maputo Bay. This takes approximately one month.

The possibility of transfering the administration of licensing to the Servico de Pesca da Cidade de Maputo is presently under consideration.

Instituto de Investigacao Pesqueira:

Since the beginning of 1984, the Instituto has been collecting data on the catches landed at Maputo Harbour, Catembe and the Costa do Sol. These landing sites are visited on one day each week, at which time the weights of catch are recorded for each landing on that day. The exception is the Costa do Sol where the landing site is too dispersed to enable access to all the landings. Here the weights are recorded for some of the landings.

In respect to Maputo Harbour, data are collected in respect to the privately owned boats used in trawling for shrimp and gillnetting for fish. Data are not collected for the other categories of privately owned boats, such as those used in the handlining of fish outside Maputo Bay.

The weights are estimated as the product of the number of boxes landed and the conversion factor of 23 kg/box. Some additional data, obtained from the port officer, are the numbers and identity of the boats landing on all fishing days.

The categories of catch for which weights are recorded are the three commercial grades of fish, and two grades of shrimps. The fish and shrimps are sorted by the fishermen according to size.

The catch weights for the SULPESCA boats engaged in shrimp trawling and gillnetting is obtained directly from the company records.

At Catembe and the Costa do Sol, data are collected only for the artisanal fishermen engaged in trawling for shrimps and gillnetting for fish. No catch information is collected in respect to the many fishermen engaged in lining for fish. The weights of the catches recorded are as determined by weighings.

The additional data used in respect to Catembe, but not for the Costa do Sol, were the monthly returns, which gave the number of boats landing on all fishing days. At the Costa do Sol it was not possible from any source to determine the numbers of boats being landed, even on the days when the catches were being sampled.

When estimating the monthly catch weights at the Costa do Sol, it was assumed that the mean numbers of fishing days per month per boat were the same as for Muntanhana. The latter were obtained from the monthly returns to the Servico de Pesca da Cidade de Maputo.

An additional joint activity, involving the Instituto and the Servico de Pesca da Cidade de Maputo, is the census of fishermen, boats and fishing gear deployed in Maputo Bay. This was undertaken for the first time in mid 1986, and is expected to be repeated at yearly intervals. The results from the census are not yet available.

The information that were collected are the birthplace, nationality, I.D. card number, and residential address, for each fisherman, type of boat (motorised, sail, rowing, wood, fibreglass), and licence number, engine horsepower, make, fuel type (gasoline, diesel), and engine number, type of fishing gear (trawl, gillnet, seines, handlines, etc), number of crew, and identity of the boat owner.

SULPESCA:

Within Maputo Bay, SULPESCA has cold storage facilities at the landing sites of Maputo Harbour, Costa do Sol, Macaneta, Inhaca, and Machangulo. Its own boats are all unloaded at Maputo Harbour.

Complete data of the catches landed from these boats are available from the company records. The categories of catch being recorded are the three commercial grades of fish (small, medium, and large), and two commercial grades of shrimps (small and large).

At the other landing sites where it has facilities, SULPESCA seeks to purchase the catches of the artisanal fishermen. It is successful in purchasing most of the landings at Machangulo, where there is no local fish market and no possibility of transporting the catches by road to alternative buyers in Maputo.

Similarly, most of the landings in the vicinity of its facilities at Inhaca, are purchased by SULPESCA. It purchases relatively little of the catches at the other landing sites.

CATCH AND EFFORT DATA FOR 1984

The most complete compilation of catch and effort data for Maputo Bay are for 1984 (in M.I.Sousa, Actividade de Pesca de Emalhe e de Arrosto na Baia de Maputo em 1984., Boletim de Divulgacao., No. 10: 42 p.). Similar data were collected during 1985, but have not yet been analysed.

These data specifically relate to the use of trawlers for catching shrimp and fish by-catch, and to the gillnetting of fish. No reliable data are available concerning the use of other fishing methods.

Shrimp Fishery:

During 1984 there were twenty of the larger sized trawlers operated in Maputo Bay, six owned by SULPESCA, and the rest privately owned. Two of the latter were engaged for only four to five months of the year. The combined landings were 372.5 tonnes of shrimp (whole weight), and 812.9 tonnes of fish, from a total of 3,499 fishing days.

The mean catch rates were highest for the privately owned boats, at 126 kg/boat day of shrimps and 267 kg/boat day of fish, compared with 56.5 kg/boat day of shrimps and 146.5 kg/boat day of fish for the SULPESCA boats.

Fishing trips were generally from morning to the afternoon or evening. The better daily catches from the privately owned boats were in part the consequence of being used for more hours in each day. The trawls in use had horizontal openings of seven to nine metres and vertical openings of one to two metres. The cod-end mesh size was one and a half inches (stretched).

Boats were operated during all months of the year. The catch rates of shrimps were highest in May and June, and highest for fish in August through January (see Table 1).

Much less reliable data are available for the shrimp and fish by-catch from the artisanal fishermen. The combined catch during 1984 for all the landing sites (except Machangulo) was estimated as 1,100 tonnes of fish and 350 tonnes (whole weight) of shrimps.

This is based on catch data available for the landing sites of Costa do Sol, Muntanhana, and Catembe (see Table 2). The catches for the other sites were estimated, from the number of boats operated at each site, multiplied by the mean annual catch per boat for the Costa do Sol, Muntanhana, and Catembe combined.

The numbers of boats at each site used in these calculations were;

The number of boats alongside Catembe include those operated from Machangulo and Catembe combined.

The mean daily catch rates of fish and shrimp (whole weight) respectively, determined for those landing sites for which there were data, are 43.3 kg/boat day and 10.0 kg/boat day for the Costa do Sol, 35.1 kg/boat day and 11.9 kg/boat day for Muntanhana, and 17.5 kg/boat day and 13.0 kg/boat day for Catembe.

Gillnet Fishery:

During 1984 there were 21 of the larger gillnet boats operated in Maputo Bay, fifteen by SULPESCA, with the rest being privately owned. Three of the SULPESCA boats and four of the private boats were operated for only part of the year.

The combined landings were 406.8 tonnes, from a fishing effort of 2,049 fishing days. The mean catch rates were slightly higher for the private boats, at 213 kg/boat day, compared with 194 kg/boat day for the SULPESCA boats. The months of April, May, June and July comprised the season having the highest catch rates (see Table 3).

Fishing trips were generally from the early morning to the afternoon or evening. Usually three nets were used, each consisting of two panels of 100 metres, and having mesh sizes of two inches (stretched). The depths of the nets being used were 100 meshes.

The same gear were used from the 303 artisanal fishing boats. The combined catch from these boats during 1984, for the landing sites other than Machangulo, was estimated at about 2,600 tonnes.

This is based on the catch data available for the Costa do Sol and Catembe (see Table 4). The catches for the other sites were estimated, from the product of the number of boats operated at each site, and the mean annual catch rate for the Costa do Sol and Catembe combined.

The numbers of boats at each site used in the calculations were:

The boats operated at Machangulo are included in the figure given for the Costa do Sol.

The mean daily catch rates determined for the Costa do Sol and Catembe were 35.4 kg/boat day and 63.0 kg/boat day respectively. The most productive months were July through December at the Costa do Sol, and March through June at Catembe.

* Greatest values
** Sharks and rays not included

NI   No data
*    Greatest values
**   Estimated to account for missing and incomplete data

* Greatest values

NI  No data
*   Greatest values
**  Estimated to account for missing data

INTERPRETATION OF THE DATA

Shrimp:

In utilising the available data to determine whether the fishery is already being fully exploited, it would first be necessary to seperately estimate the rates of recruitment, growth, and the mortalities from each of natural causes and capture. Unfortunately this was not possible.

It might have been possible if the catch weights in each month were able to be converted to catch numbers; and if seperate data were available for each of the principal species of shrimp found in the catches. The main species are Penaeus indicus and Metapenaeus monoceros. Ideally the catch data should have been further stratified by sex, and with length compositions available for each species/sex combination for each month.

Another serious impediment is the inadequacy of the data on the catches by the artisanal fishermen, who take about half the total catch of shrimp from Maputo Bay. It is likely that they operate in shallower waters, and hence tend to catch the shrimp before they are available for capture on the grounds fished from the SULPESCA and private boats (see Table 2).

Notwithstanding, it is possible to make the following qualitative interpretation from the data for the SULPESCA and private boats. This is assisted in part by the information contained in papers published by L. Brinca and L. Palha de Sousa, dealing with the growth and mortality of Metapenaeus monoceros occuring in Maputo Bay (see Revista de Investigacao Pesqueira No. 11 ;1984).

During the period from January through April/May, the mean catch weights per fishing day increased progressively (see Table 1 and Figure 2). This can be interpreted as the consequence of the migration of juveniles onto the fishing grounds from inshore nursery areas, and the growth in weight of the individual shrimps; with the combination of these effects being substantially greater than the effect of shrimps dying from natural causes and capture.

In the months from May/June through August, there was a progressive decline in the mean catch weights per fishing day. During this period the recruitment of young shrimps seems to have been negligible. The effect of numbers dying (and possibly also dispersing across or migrating away from the fishing grounds), is much greater than the effect from the growth in weight of the surviving shrimps.

In the period August through October, the mean catch weights per fishing day increased slightly. This has been interpreted as in part due to a second migration of post juveniles onto the fishing grounds. This, along with the growth of the individual shrimps, is greater than the decline in numbers due to natural mortalities and capture.

Following October, the mean catch weights per fishing day again decline, possibly through to the following January. In these months the recruitment of young shrimps is presumed to have been negligible. The effect of numbers dying was again greater than the effect from increases in the individual weight of the shrimps.

In the above interpretation, the possibility of the older shrimps migrating away from the fishing grounds has not been included. Whether this occurs or not is open to speculation. If it does, it would be most likely towards the end of the year.

Gillnet Catch:

The obvious difficulty with interpreting these data relates to the fact that the gear is highly selective. Only the fish within a certain range of intermediate sizes are able to be caught in the meshes of the nets. As such, any increase or decline in the catches per unit effort, will not correctly reflect changes in the abundance of the fish population as a whole. Rather it will be an indication of the abundance of fish having lengths within a certain size range.

The consequence of this is that the changes in the monthly catches per unit effort cannot be used to estimate the mortality rates, either from fishing or natural causes. Similarly, even if the lengths of the fish in the catches had been measured, these data would have provided little or no information on either the mortality rates or growth rates.

The monthly catches per unit effort are useful, however, in identifying the periods of the year when the fish of catchable sizes commence to be recruited into the catches. According to the data for 1984, this seems to occur during March, April and May (see Table 3 and Table 4, and Figure 3). It is not clear whether the high catch per unit effort in January is the reflection of a seperate and earlier recruitment.

Such data, also provides an indication of the relative magnitude of the recruitment. The mean catch per unit effort during March, April and May 1984, was 207 kg/boat day. If the mean for the same months in the next year were around 400 kg/boat day, for example, this might be presumed to indicate that the recruitment had doubled.

Such an approach might have been useful in identifying whether there is a relationship between the fishing effort in particular years and the recruitment in the next or subsequent years. In the context of fishery management, administrations would presumably seek to ensure that the fishing efforts do not reach a level that might be reflected by reduced recruitment.

Similarly, if catch weights and fishing efforts had been available for a number of years, it might have been possible to determine a relationship between the mean annual effort and the resultant annual catch. Again, mamagement would presumably seek to avoid increases in the fishing effort, not associated with at least some increase in catch.

The above pre-supposes that there are seperate catch and effort data for each of the important species in the landings. In Maputo Bay, by far the most important species in the gillnet catches is the kelee shad (Hilsa kellee). Unfortunately, in respect to most of the data, the catches of this species were not seperated from that of the other species.

Another serious impediment is the inadequacy of the data for the artisanal fishermen. This group contributed more than 85 percent of the total catch from gillnets during 1984. If more comprehensive and seperate data for each of the landing sites were available, it would have been possible to determine the distribution of the fish within the Say in both time and space, and hence to possibly understanding their patterns of movement.

Fish By-catch:

In respect to the fish by-catch from the SULPESCA and privately owned shrimp trawlers, the period when the catches per unit effort were highest is from about August to January (see Table 1 and Figure 4). It is tempting to conclude that these are the months when the abundance of these fish is highest.

It may be a reflection, however, of a shift of fishing effort to grounds which are more productive of fish, during the off-season for catching prawns. This might have been clarified, if catch and effort data had been available for each of the seperate fishing grounds within the Bay.

Whatever the situation, it would still not have been possible to utilise the monthly catch and effort data alone, to identify whether the fishes comprising the by-catch were fully exploited or otherwise. This task would require the availability of at least a series of annual catches and efforts over a number of years (preferably by species), and/or the length (or age) composition of the priority species in the landings.

If the catch weights and fishing efforts had been available for a number of years, it might have been possible to determine a relationship between the mean annual effort and the resultant annual catch. Over-fishing of the resource might then have been identified by increases in effort associated with no increase in catch.

The availability of lengths (or ages) for the priority species would again have been very useful in enabling estimates of the mortality rates, from fishing and natural causes, and of the growth rates; and hence providing a better basis for assessing whether the resource was over-fished or otherwise.