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3.1. Plankton
Several studies confirm the close correlation between
the seasonal oscillations of hydrological conditions of
the habitat, abundance variability of planktons, and
fish abundance on the Sierra Leone shelf and the West
African waters.
The most recent study on plankton on the Sierra Leone
shelf is found in Lamin (2011) and much emphasis is
placed on the taxonomic diversity in relation to physic-
chemical processes prevalent in 2008–10. The study
recorded a taxonomic diversity of 49–61 zooplankton
species from 20–24 genera/families, Calanoid copepods
(30%) predominated throughout, followed by decapods.
For over a period of about 50 years now the mean depth
of the thermocline has not changed, and accordingly the
bathymetric distribution of the major species assemblages
has not changed either. Lamin (2011) further noted
that surface temperature increased from 28.43 °C in
May 2008 to 30.20 °C in May 2010, but this should be
viewed as synoptic and inconclusive to be attributable to
climate change.
It could therefore be concluded that the plankton
biodiversity has not shown any significant change over the
past 2–3 decades, spatial distribution has remained fairly
unchanged and oscillating with the seasons, whilst the
relative predominance among plankton species in terms
of biomass has changed with time as ecosystem trophic
indices change.
3.2. Macro algae
There are three major categories of algae with about
thirty species in Sierra Leonean waters belonging to
the following groups:
Chlorophyta
,
Phaeophyta
and
Rhodophyta
. In 2011, the seaweed
Sargassum vulgare
and
recently (2014),
Sargassum natans
and
Sargassum fluitans
invaded the country’s coastal waters in unusually large
amounts for the first time littering the entire coastline
beaches. Since then this phenomenon seems to be a usual
occurrence appearing around June and disappearing
around October. During this period, activities such as
fishing, navigation and tourism are being affected. The
decaying plants produce an offensive odor on the beaches.
This occurrence is now a national concern triggering
studies initiated by the Environment Protection Agency
Sierra Leone.
3. Marine Biodiversity
3.3. Coastal fisheries
3.3.1. Fish species of the Sierra Leone coastal
estuarine environment
The Sierra Leone River Estuary has been studied in Sierra
Leone with respect to fisheries resources (Watts 1957,
Longhurst 1965, Sentengo and Ansa-Emmin 1986),
individual studies on single species or assemblages have
also been undertaken by various workers including
Pseudotolithus senegalensis
and
P. typus
,
Drepane africana
(Beresford-Cole 1982),
Pseudotolithus elongalensis
,
P.
brachygnathus
,
P. typus
,
Pterocion peli
,
Pomadasys jubelini
,
Drepane africana
,
Chaetolipterus goreensis
,
Psettodes
selchen
,
Galeoides decadatylus
and
Pentanemus quinquarus
(Fofana 2000).
As a matter of convention shrimps and crabs of estuaries
and deltas are also included here. Only 3 (
Penaeus notialis,
P. atlantica and P. kerathurus
) out of 6 species of shrimps
are found in the coastal waters of Sierra Leone. There may
be post larval stages and juveniles found in plankton. The
species of crabs of commercial importance found in the
Sierra Leone River estuary are the entire genus Callinectes
and are
Callinectes pallidus, C. amnicola and C. maginatus
.
3.3.2. Ichthyofauna
A large number of species of fish have been recorded for
the Sierra Leone River estuary (as high as 80 species). The
fishes of Sierra Leone estuary belong to two categories
(Longhurst 1969, Fager and Longhurst 1968, Longhurst
and Pauly 1987): Pelagic fish species and estuarine and
creek species.
3.3.3. Pelagic fish community
This is a rather diverse group and has been the subject of
investigations for several years (Longhurst 1963, Williams
1968, Williams 1969, Villegas and Garcia 1983, Nieland
1980, Nieland 1982, Sentengo and Ansa-Emmin 1986,
Anyangwa 1988, Coutin 1989). The dominant members
of this group are the Clupeidae (
Ethmalosa fimbriata
,
Sardinella maderensis
,
Ilisha africana
). Others include:
Carangidae (Caranx) and
Chloroscombrus chrysurus
.
Some members of the Carangidae may make periodic
incursions into the estuary at high tide:
Decepterus
rhonchus
and
Trachurus tracea
. Tetraodontidae
(
Lagocephalus cephalus
,
Liza falcipinis
), Sphyraenidae
(
Sphyraena barracuda
),
Pristis pristis
, Dasyatidae
(
Dasyatis margarita
).
The inshore demersal stocks include mainly the Sciaenid
fauna. Members of the Sciaenid assemblage live above the
thermocline on shallow muddy bottoms. Although some