International Journal of Marine Science 2016, Vol.6, No.27, 1-22
4
places. Sexes of fish were determined through manual sexing (Nielson and Johnson, 1985). Total number of
individual species as well as total weight were then quantified and extrapolated to the catch using the raising
factor (R), which is obtained by dividing the total catch over the sampled catch.
2.3.2 Physico-chemical parameters
Seawater samples were collected within the perimeter where fishing occurred from which fish samples were taken,
within the hours of 0700 and 0900 GMT, from a depth of between 2 and 3 centimetres beneath the water surface
from both the artisanal and semi-industrial fisheries. Measurement of physical parameters such as temperature,
salinity, pH and pressure were undertaken in situ offshore, aboard a motorised vessel with a HANNA 9828
multi-parameter probe. Three replicates of these readings were taken and three replicates of water samples also
collected, using water sample bottles. The samples were kept on ice for transportation to the laboratory where they
were preserved in a refrigerator at a temperature of 4
o
C and analyzed within 48 hours of preservation.
Analysis of physico-chemical parameters was done using standard protocols according to APHA (1998). Nutrients
(nitrates, phosphates, silicates and sulphates) and alkalinity analyses were carried out using Spectrophotometer
DR2800 and alkalinity test kit respectfully. Iron, calcium, magnesium and carbon dioxide determination analyses
were done using a photometer. Seawater partial pressure of Carbon dioxide (pCO
2
), Dissolved Inorganic Carbon
(DIC), Carbonate ion concentration [CO
23-
], mineral saturation states for calcite (Ωcalcite) and aragonite
(Ωaragonite), and the Revelle factor (β) were computed from pH, Total alkalinity, temperature, salinity and
pressure data using the programme CO
2calc
(Robbins et al., 2010). Partial pressure of carbon dioxide, dissolved
inorganic carbon, total alkalinity and the Revelle factor constituted seawater carbonate chemistry parameters,
while pH, carbonate ion concentration, aragonite saturation state and calcite saturation state constituted ocean
acidification parameters (Bates et al., 2012). Samples were analyzed for nutrients using standard protocols
according to APHA (1998).
2.3.3 Ichthyoplankton biomass and composition
An ichthyoplankton net of 200 microns was towed in a step-oblique pattern within the perimeter of the beach
seine haul for a standard total time of 18 minutes. This was within 10 m to 50 m depth. The net was lowered
vertically, towed horizontally for 3 minutes, further lowered and towed horizontally again for 3 minutes until the
standard 18 minutes, at a boat speed of 3 knots. Sample collects in the sampler of the net and this was backwashed
into a sample bottle and fixed with 40% formalin for preservation.
In the laboratory, formalin was drained off the sample with a sieve of 500 microns and made up to 200 ml in a
round bottom flask. Using a 10 ml measuring flask, 10 ml portions of the sample were taken and smaller portions
of it poured in a petri dish and viewed under the microscope with a magnification of 4x to identify fish eggs and
larvae. Identification of larvae was done to the lowest taxonomic level possible using FAO species identification
keys (Fischer et al., 1981). Calculations were then carried out to estimate the total biomass of fish eggs and larvae
as follows (Perry et al., 1993):
Total number of ichthyoplankton in 10ml of sample = a
Total number in 200 ml (N) = (200 x a) / 10 ml (this represents the total number of larvae in volume of seawater
filtered by the net).
Volume of seawater filtered by the net (V) = RF m
3
Where R = number of revolutions and F = flow meter constant given as 0.11960
Therefore the number of larvae per m
3
of seawater = V/N
