Clams as a Resource in Maputo Bay - Mozambique - TMBL

Clams as a Resource in Maputo Bay - Mozambique
Supervisor: Lars-Ove Loo
Examiner: Kerstin Johannesson
Maria Perpétua Janeiro Scarlet
Master Thesis in Marine Ecology, 20 points
Department of Marine Ecology, Göteborg University
Contribution nr. 294
Date: 27 05 2005

Abstract
Clams represent an important infaunal resource, which are of considerable dietary and
economic importance, providing incomes for coastal communities. However, despite
their value to local communities, studies of clams in Mozambique are scarce. This
study aims to investigate the occurrence of clam species and quantify the population
aspects of the species throughout Maputo Bay and Espírito Santo Estuary. Human
exploitation activity was analyzed to evaluate its impact on the clam populations.
Three areas of the western coast of Maputo Bay were selected to achieve and relate
some generalities. Sediment and clams were sampled randomly. Catch per unit effort
was the model used to estimate abundance. Five species were recorded of which two
were dominant. Both dominant species belonged to the Veneridae family: Eumarcia
paupercula (Holten 1802) and Meretrix meretrix (Linneaus 1758). The last species is
an isolated population in Maputo Bay. At Maritimo (the outer part of Maputo Bay) E.
paupercula was the dominating species and reached the maximum density of 10 to 14
individuals m -2 between September and October 2002. At Luís Cabral (the inner part
of Maputo Bay) the dominating species was M. meretrix and the highest density
measured was (9.61 ± 1.89) in November. Mean length and biomass differed
significantly between species and locations. Analysis of size-class frequency of M.
meretrix and E. paupercula suggest that recruitment is diffuse. The numbers of
collectors recorded were different for the three sites and 50 were the maximum
number registered. From these results it is estimated that 12% of standing stock is
removed monthly from an intertidal area of 19 km 2 .

Introduction
Clams are widely distributed in intertidal areas throughout Maputo Bay and Espírito
Santo Estuary in Mozambique where they inhabit sand, mud flats and sea grass beds.
They also represent an important infaunal resource, which are of considerable dietary
(Hernroth 2002) and economic importance providing incomes for coastal
communities (Balidy 2003, Bento and Guissamulo 1998, Pereira 1998, de Boer and
Longamane 1996). They constitute one of the main seafood products sold in and
around Maputo City markets and, if properly harvested, they could provide a steady
source of protein for a much larger population.
Exploitation of clams in Mozambique is a traditional activity involving mainly
women and children. Harvesting is performed with relatively small investment. The
oldest evidence of shellfish collection around Maputo Bay dates from 8000 BC,
according to Barradas (1967). De Boer et al. 2000 analyzed shell middens and his
results correspond with testimony from other middens in southern Africa Siegfried et
al. (1994) and Griffiths and Branch (1997 cited by de Boer et al. 2000). Intertidal
areas are important issues in eastern Africa with the majority of the people living
along the coast, mainly depending on the exploitation of natural resources
(Coughanowr et al. 1995).
Distribution patterns of infaunal beach assemblages typically result from a
combination of biotic (food availability and intraspecific interactions) and abiotic
(beach slope, sand particle size, salinity and chemical cues (McLachlan and Hanekom
1979; McLachlan et al. 1990) factors. Human activity may represent an additional
regulatory agent acting either directly, by removal of individuals, or indirectly, by
removal or disturbance of potential competitors. Studies of intertidal invertebrate
fauna suggest that clams are a limited resource and their exploitation leads to loss of
this resource (Lercari and Defeo 1999, Defeo and Alava 1995). Clams are especially
vulnerable as many of the growing human population in and around Maputo City are
dependent on them for nutrition and income. It has been estimated that Maputo City
had 966 837 inhabitantsin 1997 rising to 1 018 938 inhabitants in 2004 (INE, 2004).
Mozambique has no functioning system for controlling shellfish resource exploitation
(Ribeiro 2002).
Despite their value to the local communities, clam studies in Mozambique are scarce.
Basic biological information of these invertebrates is restricted to some taxonomic
studies. Rosario (1970) identified four species of clams consumed in Maputo such as
Gafrarium divaricatum, Venus (Meretrix) meretrix, Semele radiata and Arca
natalensis. According to Besteiros (1997) five species Donax faba, Anadontia
edentula, Macoma retrosa, Dosinia hepatica and Pillucina pisidium were found at
Costa do Sol beach, western part of Maputo Bay. The limited scientific information
on clams in Maputo Bay make it difficult to assess the relative importance of this
resource and to set guidelines for its sustainable exploitation.
The aim of the present study was to investigate the occurrence of clam species,
quantify population density, population size structure and biomass for each species in
western shore of Maputo Bay and the Espírito Santo Estuary. Human exploitation
activity was measured to evaluate the impact on clam populations. The data should
serve as a baseline for this resource in the western shores of Maputo Bay.

Materials and Methods
Study Area
Maputo Bay and Espirito Santo Estuary are near the southern border of the tropical
climate zone. There are two seasons: the warm/rainy season which occurs from
October to March and the cold/dry season occurs between April and September.
Yearly precipitation is 780 mm with maximum in December/January. The daily mean
air temperature varies from ca. 20ºC in winter to 25ºC during summer (Hoguane
1996). Tides in Maputo Bay are semi-diurnal with a spring tidal range of about 3 m
(Hoguane 1996).
Maputo Bay is a semi-enclosed shallow bay located in southern most part of
Mozambique at latitude 25°55’S to 26°10’S and longitude 32°40’E to 32°55’E (Saide
2000).The western shore of Maputo Bay is a long sandy beach. It is flat to concave
and no set or fluctuating wave break zone exists. The sand flat zone is a complex area
of channels at low tide. Occurrence of small dunes shows the closest point to land
where the waves break. A lot of shells can be found on the beach and birds are
commonly seen foraging at intertidal areas in low tide (see list of birds in Appendix
A).
Clams were sampled over a period of 18 months, during low tide of spring tide, from
January 2002 to June 2003 at three localities along the western shore of Maputo Bay
and the Espirito Santo Estuary: 1) Bairro dos Pescadores, 2) Maritimo Beach and 3)
Bairro Luis Cabral respectively (Fig.1). The three localities border Maputo City and
present varying estuarine conditions and anthropogenic influences.
1) Bairro dos Pescadores is located 12 km north of Maputo City at latitude
25°54.730’S to 25°54.147’S and longitude 32°39.009’E to 32°38.712’E. This is an
intertidal area characterised by patches of sea grass beds of Nanozostera capensis,
interspersed with macro algae Enteromorpha sp. and Ulva sp.. This locality has the
most local fishing activity.
2) Marítimo is part of the northeast shore of Maputo City proper and accepts the
incoming sewage from two separate city discharges. Maritimo is located 6 km from
Maputo city at latitude 25°57.497’S to 25°56.985’S and longitude 32°36.441’E to
32°38.010’E. Within its intertidal zone, three 70 m man-made stone barriers, 120 m
apart, and perpendicular to the coastline, have been built to prevent erosion. These
are the only hard substrates in the area. The route Avenida Marginal borders the
beach. The islands of Inhaca and Xefina (Grande and Pequena) shield both Maritimo
and Bairro dos Pescadores from the Indian Ocean proper. In some periods of the year
Enteromorpha sp. and Ulva sp. are seen at Marítimo and B. Pescadores.
3) Bairro Luis Cabral is located on the west side of the south shore of Maputo City
within the Espírito Santo Estuary and the city harbour. It is located 6 km from the
town centre, at latitude 25°56.763’S to 25°57.121’S and longitude 32° 32.713’E to
32° 32.133’E. Espirito Santo estuary accepts the discharge of Matola, Maputo,
Umbeluzi and Tembe Rivers (Saide 2000). The estuary and the channel of entrance of
the Port of Maputo are subject from time to time to dredging.

Fig. 1. Location of the study areas, at western shore of Maputo Bay and Estuary Espírito
Santo. (Orange = intertidal area; Yellow =sampling area)
Investigation methods
From the pilot study, clams were sorted into preliminary types and representative
photos were sent to Richard Neil Killburn at the Natal Museum Pietermaritzburg SA
for identification (Fig. 2). All clams were subsequently classified to species according
to Richmond (1997).
Fig. 2. Eumarcia paupercula (Holten, 1802) left, and Meretrix meretrix (Linneaus, 1758)
right.
Water surface temperature (°C) and salinity (‰) were monitored using a digital
thermometer and a hand-held refractometer (Atago s-10) for each locality monthly,
just prior to monthly sampling. Sediment temperature was recorded in situ from the
exposed intertidal flat, 2 m inshore from the water line, from June 02 to June 03.

Moist sediment was collected randomly from the intertidal zone, using a PVC core
tube with 5 cm diameter to a depth of 20 cm. Samples were taken in January 02 and
June 02 (representing respectively warm/rainy and cold/dry season) and taken to the
Soil Analysis Laboratory, at Faculty of Agronomy at Eduardo Mondlane University.
Here pH, percent carbon, organic matter content and phosphorous content were
measured. For purposes of soil classification, percent content of lime, sand and argyle
were recorded. Identification of birds observed was done according to Parker (1999).
A random sampling design was adopted for this study (haphazard sampling, Quinn &
Keough 2002). A quadrat frame (1 m 2 ) was placed randomly over a surface area for
each replicate. Between 27 to 42 replicate samples were taken at B. dos Pescadores,
27 - 39 at Maritimo and 24 - 40 at Luís Cabral on each sampling occasion. The
replicates were sampled at low tide of spring tide from an intertidal area, ranging from
low to high water mark. Clams were collected from each quadrat using a hand grab
(covered with a mesh of 2 mm) and clams from each quadrat were kept in separate
labelled bags and taken to the Ecological Laboratory at Department of Biological
Science for measurements. Some samples were frozen for shell length measurement,
usually within a month from the date of sampling.
Density
To estimate number of individuals per m -2 belonging to each species the clams were
separated and recorded for each locality during the study period. To compare
variation in densities between species and localities, two-way ANOVA was used.
Shell lengths
For all clams shell length (expressed as anterior-posterior length) was measured to the
nearest 0.1 mm, using a Digital Vernier Calliper (DIGIMATIC) with a resolution of
0.01 mm, and the population frequency size distribution was presenting using a class
length of 5 mm. Mean shell length differences between species and location were
analyzed using a two-way ANOVA.
Weight determination
From each quadrate and each location, individuals were taken for separate weightmeasurements.
The shell and the wet flesh were separated by a brief immersion in hot
water (30 – 40 °C). Shells and wet flesh were weighed separately. The water content
in the flesh was removed by putting in an aluminium cake-tin foil with small holes in
bottom. This cake-tin with the flesh was place on a paper towel. All weights were
made to 0.01 g on a top loading balance. Linear regression between total fresh weight
(shell and wet flesh) (y) and shell length (x) were calculated:
Log y = b log x + log a
Table. 1. Regression analyses for log length-log weight with a, b and r-values. These values
were used in the biomass calculation.
Species/Location a b W = a " x
!
b r 2 n
Eumarcia/B. Pescadores 0.00293 2.217 5.52 0.710 809
Eumarcia/Marítimo 0.04688 1.495 7.57 0.794 195
Meretrix/Marítimo
Meretrix/Luis Cabral
0.00081 2.777
0.00538 2.192
10.30
9.31
0.964
0.910
16
279

Two-way ANOVA was used to analyze the differences in mean biomass between
species and location.
Human exploitation
To evaluate the human exploitation on the resource, distribution of clam collectors at
each site was counted and registered during the daily sampling between June 2002 to
June 2003. To quantify catches, 5 collectors were randomly selected each day and the
catches were weighed (after the clams were separated from the other species
collected). Abundance of the clams in field (mean biomass (g) m -2 sampled) against
CPUE (mean biomass (g) colleted by the collectors) in a correlation analysis were
performed in StatView 5.01 to detect what type of relationship could be expected
between CPUE and abundance (Hilborn and Walters 1992).
Impact on clam populations
The total area of the three locations studied was estimated by calculating the area
from a map using image analyzing program Carnoy 2.0. The calculated area was
multiplied by the mean fresh biomass m -2 to achieve the total standing stock. This
estimated value was compared to total biomass caught by the collectors and then the
impact on clam population could be evaluated.
Results
Mean monthly variations of temperature (°C) and precipitation (mm) in Maputo City
between January 2002 and June 2003, (National Institute of Meteorology , Fig. 3),
followed the temperature and precipitation predicted for the seasons (dry/cold; April
to September and warm/rainy; October to March) with an exception in June for
precipitation.
Fig. 3. Mean air temperature and mean precipitation from January 2002 to June 2003 in
Maputo.
Monthly mean surface water salinity at the three locations varied between 32 and 45
‰ (Fig. 4). This variation could be due to high exposure of the intertidal areas. High
salinity was recorded between September and March at three locations, while B.
Pescadores showed lower salinity than the other two locations. Input of fresh water
from Incomati River (Fig. 1) could have influenced the salinity value recorded at B.
Pescadores.

Fig. 4. Monthly means of surface water salinity (‰), and water temperature (°C) and
sediment temperature (°C) with maximum and minimum ranges, from the three locations of
the study (B. Pescadores, Marítimo and Luis Cabral).
Monthly mean surface water temperatures at the three locations varied from 20.5 to
34.2 °C between winter and summer. The variation of sediment temperature showed
the same tendencies as water temperature (Fig. 4). The distribution of carbon (% C),
nitrogen (% N) and phosphorus (% P) followed the organic matter content (% O. M.)
in two season recorded; rain and warm season and dry and cold season (See Appendix
B, Tab. 1 and 2 respectively). The sediment was classified as sandy and comprised
97% sand and 3% lime.
Five species of bivalve were recorded two of which were dominating, both belonging
to the Veneridae family, Eumarcia paupercula (Holten, 1802) and Meretrix meretrix
(Linneaus, 1758). E. paupercula (H.) is variable in shape, often tapering and
flattening to posterior, with fine concentric lines on the shell. The colour varies from
cream to pale brown often with darker, jagged markings. M. meretrix (L.) is a nearly
equilateral triangular-shaped clam with highly coloured shells. The colour varies from
pale to dark brown with purple bands or dashes and presents a lustrous periostracum.
The common vernacular and regional Portuguese name of clams is “amêijoa”, and
“timbatsana” is the local name for all species,

Density
Fig. 5. Mean density (individuals m -2 ±SE) of intertidal clams at Bairro dos Pescadores,
Marítimo and Luís Cabral.
Mean density (Fig. 5), showed a remarkable fluctuation during the study at B.
Pescadores. One-way ANOVA showed significant differences (p< 0.0001) between
species density at all locations (Appendix C).
At Bairro dos Pescadores, E. paupercula was present in high density in February 2002
(12 ± 2.53 individuals m -2 ) but density declined dramatically in the last two months of
the study. Two peaks of 5 individuals per square meter of E. paupercula were seen in
May and September 2002. M. meretrix was present only in March and September
2002 and present at a low density in all months of the study 2003.
At Maritimo, E. paupercula reached a maximum abundance of 10 to 14 individuals
m -2 between September to October 2002. M. meretrix occurred in low density in all
months except in April and May where it was absent.
At Luís Cabral E. paupercula was recorded in December and June 2002 with a
population of 5 individuals. M. meretrix reached two peaks, one in January and

February 2002 and one in October and December and compared to B. Pescadores and
Luís Cabral M. meretrix reached the highest density (9.61 ± 1.89) in November.
Mean length
Fig. 6. Mean shell length (mm) of clams (mean ± SE) at Bairro dos Pescadores, Marítimo and
Luis Cabral between January 2002 and June 2003.
No trend or cohort of mean length was clear along the time of study. The distribution
of mean size was conservative in general for both species (Fig. 6).
The shell length measurements were recorded from 3019 E. paupercula distributed
as: 51.47% for Maritimo, 48.43% for B. Pescadores and 0.1% for L. Cabral. Only
three individuals were registered at L. Cabral. Mean length (and range) recorded was
25 (66.0 - 7.6) mm. Mean length differed significantly for E. paupercula (ANOVA)
between the three locations, with a significance level of 5% (Fisher’s Protected LSD)
(p

(and range) recorded was 26 (67.8 ; 7.2) mm. Mean length of M. meretrix differed
significantly between B. Pescadores and Marítimo, but between L. Cabral and
Marítimo locations, with a significance level of 5% (Fisher’s Protected LSD)

Length-frequency distribution
Fig. 7. Shell length frequency of E. paupercula (H.) and M. meretrix (L.) at B. dos
Pescadores, Marítimo and Luís Cabral

Juveniles (individuals ≤ 35 mm) of both E. paupercula and M. meretrix were the most
abundant size classes at all sites (Fig. 7). The much larger size class from 40 to 45
mm for E. paupercula and M. meretrix were found at B. dos Pescadores and Marítimo
with a contribution of 5% and 6% respectively.
At B. Pescadores the population of E. paupercula was dominated (30 %) by two shell
length size classes, which varied from 25 to 35 mm. At Marítimo the length showed a
different distribution, 34% within 20 to 25 mm while 18 % and 16% were in much
lower size classes. At Luís Cabral the frequency was dominated in 24 % by two shell
length classes within 15 to 20 and 20 to 25 mm respectively.
Biomass
Fig. 8. Mean biomass (g m -2 ) of Eumarcia paupercula and Meretrix meretrix at three
locations B. Pescadores, Marítimo and Luís Cabral between January 2002 and June 2003.
Biomass peaked between January and February/March and between September and
December (Fig. 8). The fluctuation of biomass follows the spatial and temporal
pattern of density (Fig. 5).

The mean biomass of E. paupercula (Fig. 8) was significantly different (p< 0.0001)
(ANOVA) between Luís Cabral (Matola) and B. Pescadores (Appendix E, Fig.1 (A)
and Tab 1 (A)).
The mean biomass for M. meretrix showed in Fig. 8 differed significantly between the
locations (ANOVA) with a significance level of 5% (Fishery’s Protected LSD)
(Appendix E, Fig.1 (B) and Tab 1 (B))
Impact of collectors on clam populations
Fig. 9. Daily number of collectors at B. Pescadores, Marítimo and Luís Cabral between June
2002 and June 2003.
September and November were the months with the greatest number of collectors
with peak numbers occurring in October (Marítimo), December (B. Pescadores) and
February (L. Cabral, Fig 9). The number of collectors at B. Pescadores was high
compared to the other two areas.
Fig. 10. Mean biomass collected (kg) by five collectors at B. Pescadores, Marítimo and Luís
Cabral between June 2002 and June 2003.

In general the mean values of biomass collected followed the trend of number of
collectors.
Fig. 11. Total biomass collected per day at Bairro dos Pescadores, Maritimo and Bairro Luis
Cabral.
The total biomass collected per day during low tide was low in cold dry season and a
cohort of increased biomass occurred between October to February during warm and
rainy season (Fig. 11).

Fig. 12. Total biomass collected by the collectors one day, plotted against biomass per square
meter (g) and regression equation.

Fig. 13. Show regression equation of mean biomass collected by the collectors (CPUE)
plotted against biomass per square meter (g m -2 )
Total biomass collected per day, plotted against biomass per square meter (g m -2 ) are
shown in Fig. 12 and mean biomass collected by the collectors (CPUE) plotted
against biomass per square meter (g) Fig. 13. The curves show proportionality
between total biomass and mean biomass collected by collectors but large variances
were observed.
The mean biomass (g m -2 ), at B. Pescadores, Maritimo and Luis Cabral, was 23 g and
the total collecting area was 19 km 2 . This gives a total standing stock of 437 tonnes.
The mean catch per day for all three locations was 86 kg and the mean number of

collecting days per months was 16 . This yields 1.37 tonnes per month and 49 tonnes
per year. The biomass harvested per year in three areas was 49 tones, equivalent to
12% of the total standing stock of 437 tonnes.
Discussion
The two main species of clams harvested by local communities were Eumarcia
paupercula (Holten, 1802) and Meretrix meretrix (Linneaus, 1758). Chatterji et al
(2002) found that the Meretrix species is an estuarine clam, although no comparable
data are available for E. paupercula. According to Kilburn (2001) Meretrix meretrix
it is not known from the East African Coast, except for a population in Maputo Bay.
This population may have been artificially established early last century (Kilburn
pers. comm.). In 2004 some clams of Meretrix were observed in the estuary near
Beira (personal observations). Meretrix meretrix is found in most parts of China’s
coast and is used in aquaculture in East Asia.
Differences in the number of species observed in the present study and in the studies
of Rosario (1970) and Murta (1995) may be attributed to the sampling method used
by both. Rosario (1970) and Murta (1995) sampled the principal markets in and
around Maputo City. These samples represent clams from large areas along the coast
of Maputo Bay. In contrast, samples in the present study were restricted to three
intertidal areas along the coastline of Maputo Bay and Espírito Santo Estuary.
According to Schoemam and Richardson (2002) it is often difficult to identify
processes that affect distribution and abundance of organisms in physically stressed
environments. Consequently it is difficult to attribute any cause to the differences
observed in this study. Reasons could be intra and/or interspecific competition
between the two species for space and food, or decrease of organic matter due the
high correlation found between high exposure and low amount of organic content
(Möller 1986, and see Tab. 1 and 2 of Appendix B). However, according to Guo et al.
(1999) interspecific competition is not relevant to M. meretrix because beds of M.
meretrix aquaculture are used in Jiangsu Province, China, for growing other clams.
For Eumarcia paupercula no relevant information was found. In general, most
animals in the intertidal area are eliminated by anthropogenic factors (Balidy 2003),
natural mortality, physical stress in the sediment and bird predation (Boer and
Longamanae, 1996). My assumption that physical stress caused by the digging
equipment used by catchers affects the distribution of clams, is in accordance with
Balidy (2003) and Boer et al. (2002) who declared that in general, most animals in
intertidal area are eliminated by anthropogenic factors. Whimbrels, egrets and greater
flamingos were observed as the most important birds with regard to food consumption
in the intertidal areas (Appendix A, bird list) and according to de Boer and
Longamanae (1996), birds such as little egrets, grey herons and grey headed gulls,
were estimated to feed 50% on the benthic fauna while sacred ibises and green herons
fed entirely on benthos.
The higher densities was found in this study for E. paupercula at Maritimo during
September to October and for M. meretrix at Luís Cabral between September and
November 2002. This results could be compared with the high densites of bivalves
reported for Inhaca Island by de Boer et al. (2002) during October. Ribeiro (2002)

eported the same pattern for another bivalve species Perna perna at Xai-Xai
province just north of Maputo.
In this study E. paupercula reached a maximum shell length (anterior- posterior) of
45 mm which is similar to the maximum previously reported for this species (i.e. 40
mm, Richmand 1997). Meretrix meretrix in this study reached up to 67.8 mm in
comparison to the maximum reported length of 70 mm (Richmond 1997). Like the
mussel, P. perna at Xai-Xai province in south of Mozambique, E. paupercula and M.
meretrix don’t appear to have a standard pattern of recruitment and growth. This in
concordance with Ribeiro (2002) who indicated that the heavy fishing effort (all day
of low spring tide), over the resource was the principal cause of the diffused
recruitment observed. This assertion is supported by low frequencies of adult clams
i.e. more than 45 mm.
Size categories of clams have been defined as recruits (which are in the intertidal
areas for two months or less and are ≤ 15 mm, Schoeman, 1997), adults who are
sexually mature at ≥ 45 mm (McLachlan and Hanckorn 1979) and the remainder of
the population defined as juveniles” (McLachlan and Hanckorn 1979). These
categories were used by McLachlan and Hanckorn (1979), Donn (1987) and Defeo
(1996). De Boer et al. (2002) and Ribeiro (2002) found from other species of bivalves
two peaks of recruits, in January and October with frequency classes < 5 mm and <
10 mm representing 90% of the entire population. However, no recruits were found in
this study, so the question is, “where are they settling?” Some explanations can,
however, be suggested for the diffuse settling recorded. The hypothesis constructed
by Donn (1987) (hereafter referred to as Donn’s hypothesis) states that D. serra
larvae settle in the surf zone adjacent to major river mouths (in response to some
unknown cue) and their subsequent exposure to prevailing near shore currents during
semi lunar migrations, transport individuals consistently eastwards, away from river
mouth. This statement compares favourably with what Murta (1995) observed. The
clams in Maputo Bay grow in the mouth of Incomati river (Montanhana, Xefinas
Island and Macaneta areas) located in the north part of the bay and then are spread
along the east coast. In this case the Donn hypothesis could be applied to explain the
sporadic settlement patterns of recruits observed in the present study. According to
Parry et al. (1999) sampling methods could influence the recruitment analysis and
different methods of sampling over a greater area could be required. For example,
sampling from high, mid and low water in low tide of spring tide could be done to
study density, and length class distribution along the gradient could help to analyse
the pattern of settlement of recruits. However, it is difficult to assess the recruitment if
the rate of growth is rather slow (Kausky 1982; Richardson et al. 1990; Richardson et
al. (1995).). The continuous impact of human and other predators on the population,
observed in this study, together with continuous recruitments can result a large
overlap in the size distribution. This means that the different year classes could not be
separated.
According to Levinton (2001) reproduction of many invertebrates occurs within the
estuary and larvae and juveniles then move out to the coastal waters, later moving
back to the estuary where they spend their adult lives. Thus, sampling larvae in the
Incomati estuary may be crucially important to understanding the breeding site,
pattern of larvae migration, settlement and growing areas of the two species.
According to Guo et al. (1999) Meretrix clams, live close to the surface like surf
clams and are capable of moving and relocating. This behaviour could explain the low
numbers of “recruits” observed. Alternatively, low recruit numbers could be due to

the stress or/and disturbance caused by higher predation pressure by human, birds and
crabs. Witlatch et al. (1997) who stated that predatory infaunal invertebrates typically
act to reduce the recruitment supports this and according to Guo et al. (1999) before
reaching 10 mm (length reached by the end of first year and considered in commercial
size), predation is a major problem in Meretrix aquaculture.
The mean biomass per unit area was related to the density and not to the length of
clams (most clams counted were in the juvenile stage between 15-35 mm; Fig. 7 and
Fig. 8). Growth of suspension feeding bivalves is related to food availability (Jensen
1993 and Nakaoka 1992 cited by Laudien et al. 2003), which can be limited at
exposed sandy beaches (Brazeiro and Defeo 2000). Laudien et al. (2003) and
McLachlan et al. (1979) compared the biomass of bivalves inhabiting tropical and
subtropical regions with the biomass of upwelling and temperate zones. They showed
lowest overall growth compared to the upwelling regions which show highest grow
performance. Therefore, the lowest values of biomass found in this study may have
been influenced by food availability and Rainer (1982) suggested excessive
harvesting of bivalves by humans as the main reason for low biomass and thus low
production by suspension feeders. The growth constant b for M. meretrix in this study
was 2.19 (Tab. 1) at Luís Cabral and 2.78 (Tab. 1) in Marítimo for the regression line.
This results approximate to the values found by Park (2002), for Meretrix lusoria
from coastal waters of Korea which showed an isometric relationship with b value
estimate to 2.93. In same study Park (2002) found for Ruditapes philipinarum and
Cyclina sinensis (both of Veneridae family) from coastal waters of Korea, an
isometric relationship with estimates of b values at 3.04 (± 0.05 SD) and 3.06 (± 0.13
SD) respectively and it has been reported that coefficients of b varied between 2.286
and 3.295 and for Anadara cornea was found to be within this range and the range
could be in concordance with this study. No results was found to compare with E.
paupercula.
The number of human collectors recorded in the present study was high compared
with Inhaca Island (maximum number recorded 50; de Boer and Longamane, 1996)
and low compared with collectors of mussel Perna perna at Xai-Xai Province, where
maximum number of collectors were 270 in March with each catcher collecting 5 to
15 kg each (Ribeiro, 2002). The peaks of collectors registered in this study were
between October and December for all three sites (Fig. 9). This could be explained by
the movement of collectors between the sites and by the density of clams on the sites.
According to de Boer et al. (2002), people from Inhaca regarded October as a better
season for bivalve exploitation and the increased number of collectors supported this.
In the present study an increasing number of collectors was observed around October
(September to February), which could be in accordance to de Boer et al. (2002). In
my opinion, traditional and local ecological knowledge about the distribution of clams
helps the collectors to select and decide the best location, best time for collecting and
to manage natural resources available.
Catch per unit effort (CPUE) methods (Hilborn and Walters 1992) provide three
possible relationships (Hilborn and Walters 1992) to explain the patterns observed.
The relationship between CPUE and abundance (Fig. 13) show that the correlation
between catches and unit of effort was proportional. At Luís Cabral small variance
was observed in the curve comparing with B. Pescadores and Marítimo where the
variance was large (Fig. 12 and 13). According to Hilborn and Walters (1992) the
proportionality with small variances, means that handling times are small and the
search is random. High variances between CPUE and abundance could be explained

y the way that humans collect the bivalve, however my observations lead me to the
conclusion that they know exactly where to find the clams in the field. The total
impact of human exploitation is probably underestimated because the number of
collectors and mean biomass collected, was obtained during one day of sampling per
month (moment sampling) and many factors could influence the mean biomass
collected. Women play a more important role in collecting invertebrates than men
(who dominating mozambicanan fishing) and invertebrates form an important part of
the diet. Similar observations have been made elsewhere e.g. Samoa (Wass, 1980;
Mattews 1995 cited by UNESCAP 2003) where age, sex and time of collecting could
be important factors. Although for Inhaca, according to de Boer et al. (2002), a total
harvest of 0.12 AFDW m –2 y -1 is low compared to the available biomass of 6.4 g
AFDW m –2 y -1 corresponding to a daily removal of < 0.01% or total harvest of < 5%
of the biomass, the results of the present study show an estimated removal from an
intertidal area of 19 km 2 of 12% of the standing stock monthly. It is difficult to
compare the results of these two studies because de Boer et al. (2002) did not express
the area used by the collectors and he had evaluated the biomass of all invertebrates
caught. Nonetheless, the estimated human exploitation per month is assumed to be
less than the maximum sustainable level. To estimate the real impact it could be
necessary to estimate the ratio production/biomass and seasonality of recruitment
settling. Defeo (1996) agrees that exploitation of invertebrates living in muddy or
sandy substrates can be sustainable.
Conclusions
Two main species are harvested in eastern coast of Maputo Bay and Espírito Santo
Estuary; E. paupercula and M. meretrix. Differences in distribution, density, length
and biomass have been seen between areas and species and also within species.
Exploitation by humans and birds is suggested to be the main cause of low values of
biomass, length frequency, and diffuse recruitment observed in this study. The level
of exploitation by humans was estimated to be 12% of the standing stock removed at
an intertidal area estimated of 19 km 2 .
Some studies that are needed to be clarified in this species ecology and dynamics are:
- hydrodynamic patterns between the estuaries, bay and coastal areas to
understand the larvae migration and settlement.
- recruitment, settlement and mortality-rate (including impact on predation from
species other than humans) to estimate the “real” production.
The Meretrix meretrix species in this study is a potential resource for the Mozambican
people and economy. These results constitute a base of information for future studies.

Acknowledgements
I would like to thank to SIDA/SAREC-UEM project for the financial support from
which this thesis could not have been possible. I’m particularly grateful to Lars-Ove
Loo my supervisor who has always been supportive and taken time to hear me and
discuss my work. To Per Nilsson for his advice with a hoard of ideas with expertise
about my work when I needed for which, I’m grateful. Thanks to Jon. Havenhand,
Lillemor Svärdh and Kerstin Johannesson for the comments to improve my work.
Apart from direct scientific input certain people have given me the harmony needed
and I’m bound to forget some in this acknowledgment. To Tjärno Marine Biological
Laboratory and staff “ tack så mycket” for cordiality, hospitality and provision of
facilities during my visits. Thanks to Lars Hernroth, coordinator of the Biological
Project.. To Mrs. Albano and Rosário from the Biological Department –UEM thanks
to be with me during the sampling time. To Ana “my little daughter” thanks to be
patient with me.
For those without names here, you know who you are, thanks!

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Appendix A
List of species of birds identified in the study area (Specie (vernacular name) Robert’s
number*) :
Phalacrocorax carbo (White breasted Cormorant) 55,
Egreta garzettae (Little Egret) 67,
Ardea Cinerea (Grey Heron) 62,
Phoenicopterus ruber (Greater flamingo) 96,
Phoeniconaias minor (Lesser Flamingo) 97,
Dendrocygna viduata (Whifaced Duck) 99,
Numenius phaeopus (Wimbrel) 290,
Charadrius tricollaris (Threebaanded Plover) 249,.
Charadrius marginatus (Whitefronted Plover 246,
Haematopus moquini (African Back Oystercatcher) 244,
Xenus cinereus (Terek Sandpiper) 263, Tringa ochropus (Green Sandpiper) 265,
Calidris ferruginea (Curlew Sandpiper) 272,
Actilis hypoleucos (Common Sandpiper) 264,
Larus cirrocephalus (Greyheaded Gull) 315,
Ardea melanocephala (Blackheaded Heron) 63,
Butorides striatus (Greenbacked Heron)74,
Pelecanus onocrotalus (White Pelican) 49,
Threskionis aethiopicus (Sacred Ibis) 91,
Himantopus himantopus (Black Winged Stilt) 295,
Charadrius Pecuarius (Kittlitz's Plover) 248, Charadrius hiaticula Ringed Plover
245,
Arenaria interpres (Turnstone) 262,
* Robert’s number it’s a pattern used in eastern Africa to standardised the taxonomic
classification

Appendix B
Tab. 1 and 2.Values of ph, % C, % O.M, % N, %P and % of sand, lime and argyl for
sediment classification .registered from rain and warm season (Tab. 1) band from dry and
cold season (Tab.2).
Tab.1.
Determination
Sites
1. B. Pescadores 2 – Marítimo – L. Cabral
PH 8.44 8.35 8.06
Carbon (% C) 0.05 0.0 0.10
Organic Matter (% O.M.) 0.9 0.0 0.18
Nitrogen (% N) 0.028 0.017 0.028
Phosphorus (%P) 0.03 0.81 0.31
Sand (%) 97 97 97
Lime (%) 3 3 3
Argyll (%) 0 0 0
Classification Sandy Sandy Sandy
Tab.2.
Determination
Sites
1. B. Pescadores 2 – Marítimo – L. Cabral
PH 6.89 9.11 6.79
Carbon (% C) 0.59 0.05 0.26
Organic Matter (% O.M.) 0.96 0.09 0.45
Nitrogen (% N) 0.05 0.004 0.02
Phosphorus (%P) 0.98 0.71 0.58
Sand (%) 78.95 93.11 91.54
Lime (%) 2.70 4.01 2.71
Argyll (%) 18.36 2.88 5.75
Classification Sandy Sandy Sandy

Appendix C
Tab. 1. Statistical analysis (1-way ANOVA) of mean density for clams E. paupercula
collected from different locations.
Tab.2. Statistical analysis (1-way ANOVA) of mean density for clams M. meretrix collected
from different locations.

Appendix D
Tab. 1a. Statistical analysis (2-way ANOVA) of mean length for clams E. paupercula
collected from different locations and date.
Type III Sums of Squares
Source df Sum of Squares Mean Square F-Value P-Value
Date 17 20786.388 1222.729 49.311 .0001
Location 2 4129.541 2064.770 83.270 .0001
Date * Location 16 22716.166 1419.760 57.257 .0001
Residual 3515 87158.336 24.796
Dependent: Length(mm)
Tab. 1b. Fisher’s Protected LSD statistical analysis of mean length for clams E.paupercula
collected from different location and date.
Fisher's Protected LSD
Effect: Location
Dependent: Length(mm)
Significance level: .05
Vs. Diff. Crit. diff. P-Value
Matola Marítimo 1.548 5.642 .5907
Bairro dos Pescadores 9.094 5.641 .0016 S
Marítimo Bairro dos Pescadores 7.546 .330 .0001 S
S = Significantly different at this level.
Tab.2a. Statistical analysis (2-way ANOVA) of mean length for clams M. meretrix collected
from different locations and date.
Type III Sums of Squares
Source df Sum of Squares Mean Square F-Value P-Value
Date 17 15305.843 900.344 16.907 .0001
Location 2 16177.146 8088.573 151.89 .0001
Date * Location 22 13090.530 595.024 11.174 .0001
Residual 2254 120031.902 53.253
Dependent: Length(mm)
Tab. 2b. Fisher’s Protected LSD Post hocs statistical analysis of mean length for clams
M.meretrix collected from different location and date.
Fisher's Protected LSD
Effect: Location
Dependent: Length(mm)
Significance level: .05
Vs. Diff. Crit. diff. P-Value
Bairro dos Pescadores Matola 1.437 1.460 .0537
Marítimo 8.671 1.545 .0001 S
Matola Marítimo 7.234 .709 .0001 S
S = Significantly different at this level.

Appendix E
Statistical analysis (ANOVA) with posterior Fishery’s Protected LSD of biomass of E.
paupercula (1) and M. meretrix (2) at B. Pescadores, Marítimo and Luís Cabral.
A) B)
30
25
20
15
10
5
0
- 5
Interaction Bar Chart
Effect: Place
Dependent: Biomass
With 95% Confidence error bars.
Cell Means of Biomass
Bairro dos Pescadores Maritimo Matola
Place
Bairro dos Pescadores Maritimo Matola
Fig. 1. Mean total of biomass(g) with error bars of E. paupercula (A) and M. meretrix (B) at
B. Pescadores, Marítimo and Luís Cabral.
Tab. 1. Statistical analysis (ANOVA) with posterior Fishery’s Protected LSD of biomass of
E. paupercula (A) and M. meretrix (B) at B. Pescadores, Marítimo and Luís Cabral.
A) B)
Fisher's Protected LSD
Effect: Place
Dependent: Biomass
Significance level: .05
Vs. Diff. Crit. diff. P-Value
Matola Maritimo 14.333 8.813 .0020 S
Bairro dos Pescadores 21.306 8.813 .0001 S
Maritimo Bairro dos Pescadores 6.972 8.813 .1184
S = Significantly different at this level.
25
22.5
20
17.5
15
12.5
10
7.5
5
2.5
0
-2.5
Interaction Bar Chart
Effect: Place
Dependent: Biomass
With 95% Confidence error bars.
Cell Means of Biomass
Fisher's Protected LSD
Effect: Place
Dependent: Biomass
Significance level: .05
Place
Vs. Diff. Crit. diff. P-Value
Bairro dos Pescadores Maritimo 11.722 4.873 .0001 S
Matola 18.200 4.873 .0001 S
Maritimo Matola 6.478 4.873 .0102 S
S = Significantly different at this level.