International Journal of Marine Science 2016, Vol.6, No.27, 1-22
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crumenophthalmus
,
Chloroscombrus chrysurus
,
Auxis thazard
,
Ephippion guttifer
and
Sphyraena sphyraena
.
Sphyraena sphyraena
was positively related with total alkalinity in the lean season of the artisanal fishery (Figure
9), indicating that as total alkalinity increased, abundance of this species increased and vice versa. Total alkalinity
is a seawater carbonate chemistry parameter and as such a determinant of ocean acidification, thus, ocean
acidification could be said to result in decreasing abundance of
Sphyraena sphyraena
, a very important
commercial species in the artisanal fishery. With time, this could likely lead to a decreased diversity for this
fishery.
In the peak season of the semi-industrial fishery,
Sardinella aurita
positively related to pH and carbonate ion
concentration (Figure 12). The decreasing trend in pH and carbonate ion concentration observed in the
semi-industrial fishery (Figure 5) thus relates to a decrease in abundance of the species
Sardinella aurita
, another
very important commercial species for both fisheries. Odulate et al. (2014) also observed the abundances of
Sardinella aurita
to be influenced by pH levels in marine coastal waters of the Gulf of Guinea, Southwest Nigeria.
Selar crumenophthalmus
,
Chloroscombrus chrysurus
and
Ephippion guttifer
were also identified to relate
positively with pH in Figures 13, 9, and 8 respectively. These findings underscore the evidence that ocean
acidification as a result of decreased pH levels would impact fish species, as reported by authors such as Fabry et
al. (2008); Doney et al. (2009) and Munday et al. (2009).
The relative abundance of shellfish was higher in the peak season as compared to the lean season in the artisanal
fishery. Change in abundance could probably be influenced by increased pH and carbonate ion concentration as
shown from the canonical correspondence analysis. Sepia officinalis was found to relate positively with pH from
the CCA biplot (Figure 10), implying a decrease in abundance of the species as pH decreased. Thus ocean
acidification as a result of decreased pH would impact the abundance of S. officinalis and thus the diversity of the
fishery. A decrease in abundance would result in a loss of the species over time and the fisheries that depend on
them, thus reducing diversity and vice versa. Gutowska and Melzner (2009) also found linear relationships
between mass of S. officinalis and pH, confirming that decreased concentrations of pH could result in the
observed decreased abundance of the species. Also, since S. officinalis is a mollusc which makes use of calcium
carbonate in the formation of its internal shell, decreasing carbonate ion concentration in its environment as
evidenced by the results could also account for decreased abundance. In the semi-industrial fishery, abundance of
S. officinalis, a high value export species from Ghana, was again influenced by carbonate ion concentration.
There was no clear evidence that ocean acidification was affecting the relative abundances and diversities of the
shellfish species
Callinectes sapidus
and
Penaeus notialis
identified in the artisanal fishery, since these species did
not relate with any of the ocean acidification indicators or the seawater carbonate chemistry parameters. Since
these are calcifying organisms, the findings are in contrast to reports from Vezina and Hoegh-Guldberg (2008);
Doney et al. (2009); Munday et al. (2009) and Barnard and Grekin (2010) which indicate that ocean acidification
would impact negatively on calcifying organisms. However, Ries et al. (2009) has shown that this could occur, as
their work on laboratory simulations revealed that marine calcifiers such as crustaceans may exhibit varied
responses and further observed that
C. sapidus
did not exhibit any significant effects to ocean acidification.
Although results for calcite and aragonite saturation states for the artisanal fishery show a decreasing trend with
respect to the seasonal means (4.43 ~ 2.60 for aragonite saturation; 6.67 ~ 3.99 for calcite saturation; Table 1),
none of the saturation states is yet below the value of 1, the value at which under saturation takes place according
to Royal Society (2005). This could perhaps explain why the crustacean species were neither influenced by any of
the seawater carbonate chemistry parameters nor ocean acidification parameters from the CCA. Thus ocean
acidification could be a contributing factor to the observed abundances of molluscs but inconclusive for observed
abundances of crustaceans in the artisanal fishery.
Ichthyoplankton has been identified to be more susceptible to ocean acidification and other environmental factors
than adult fish (Sumaila et al., 2011). In this study, Ichthyoplankton was found to be generally higher in the peak
season than the lean season (Figure 14). Sardinella larvae observed confirm the abundance of
Sardinella sp.
in
