Factores de influencia en el clima

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% WFD = Estimated degraded watershed area X 100 %

Estimated watershed area ……. 5 3.2.3.5 Frequency of dams’ gate opening

Assessment of frequency of opening of the dams’ gate (Dam gate-valve opening- DGO) and its effects on fisheries was used to determine opening of the gate of the dam as potential threat to Asejire Lake fisheries. Formal document on the frequency of opening of the gate valve of the dam was sought from Water Corporation of Oyo State which manages the dam. This was followed by bi-monthly data capture of observations on frequency and type of opening of the dam’s gate.

Gate opening was taken as Partial Dam Gate Opening (PDGO) when the dam’s gates were opened but not entirely, while Complete Dam Gate Opening was identified as a situation where the gates were entirely opened. Frequency of each type was documented bi-monthly. Interval (number of days) in between a complete opening of the dam’s gate and any other gate opening was also documented.

Pictures were obtained to show the effects of the gate opening on shore activities and the lake’s fishery. Monitoring of opening of the dam’s gate was concurrently carried out with sampling for water quality.

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Major and minor tributaries along each of the strata (OYS and OSS) were also selected for sampling. Four sites were selected on a minor tributary. The sites were located at point of entry and the last accessible point on the leeward and windward sides of each minor tributary. Six sites were selected on each of the major tributary of each of the stratum. The sites were located at point of entry, mid-point and the last accessible point on the leeward and windward sides of each of the stratum. Compensation for relatively larger size in major tributary was the basis for selecting mid-point sites for major tributaries. This made the total sampling sites to be 38. Information on the sampled sites and their identity is presented in Appendix 2. Map of the sampled sites is presented in Plate 1.

3.2.4.2 Sampling procedure

Water quality parameters were sampled bimonthly in wet and dry seasons for 24 months from the 38 selected sites of the catchment. Samples were collected as described in Omoike (2004) with modifications. Samples were obtained on board dug-out canoes between January and December, 2010-2011. Samplings were carried out in January, March, May, July, September and November. Samples obtained during October-November, December-January, and February-March sampling periods were taken as dry season samples while those of April-May, June-July and Aug-September were taken for wet (rainy) season samples. Samplings were completed within one week during each sampling period. Water samples for analysis of physico-chemical parameters were taken at about 30cm depth from each of the 38 sites between 7.00a.m. to 9.00 a.m. during each sampling period.

3.2.4.3 Determination of values of water quality parameters of Asejire Lake

The spatio-temporal values of each of the studied water quality parameters were determined. Values were determined for strata across wet and dry seasons. All the parameters were measured per site at every sampling period. Examination of water quality followed Omoike (2004). Temperature was determined on-site and measured using mercury-in-glass thermometer. Water samples for determination of Dissolved Oxygen Content (DO), Total Alkalinity (TA) and Total Hardness (TH) were obtained in water sampling bottles and were immediately taken to the Central Chemical Laboratory of the Water Corporation of Oyo State upon landing. Determination of values of the parameters followed Boyd (1982).

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Dissolved oxygen was determined by the Winkler’s method. Water samples were fixed with 2ml of Winkler’s solutions A and B shaken properly then 2ml H2SO4 was added and vigorously shaken. The treated samples were then titrated with 0.025N Sodium thiosulphate using fresh starch solution as indicator until the colour changed from yellow to colourless. Titrations were repeated and the average value recorded. Volume of the titrant is assumed to be equal to the amount of the iodine liberated and this is equivalent to the original dissolved oxygen content of the sample (Boyd, 1982).

Total Alkalinity and Total Hardness were also determined by titrating sampled water against standard indicators. For determination of Total Alkalinity, sampled water were titrated with 0.02N H2SO4 after 2 drops each of Sodium thiosulphate mixed with methyl orange had been added. Total hardness was determined as the portion of Calcium and Magnesium ion in the samples. 2ml of ammonia buffer solution and a pinch of erichrome Black T was added to 50ml of samples and titrated with EDTA solution until the colour changed to blue. Values of each physico-chemical parameter obtained throughout the sampling period was used to compute mean values per site and strata across seasons.

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Plate 1: Map showing the 38 sampled sites at Asejire Lake

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3.2.4.4 Assessment of variability in water quality parameters of Asejire Lake

Variability of the obtained mean value of the water quality parameters of the lake was studied. The assessed indices of variability were Heterogeneity of Spatial Values (HSV), Limiting Spatial Value (LSV) and Extremely High Spatial Values (EHSV). The presence of significantly heterogeneous Coefficient of Variability (CV) in any of the studied parameters was taken to indicate HSV. This was determined from the obtained mean values of the parameters across the strata and seasons. The minimum and maximum columns of the descriptive statistics of each of the parameters were assessed for sites that reflected LSV and EHSV. The presence of spatial values below and above the minimum and maximum ranges of recommended values of water quality parameters for healthy fish production presented in Omitoyin (2011) were taken as benchmark for determining LSV and EHSV.

Values below and above the recommended range were taken to indicate LSV and EHSV, respectively. The percentage number of sites that ever showed deviation (LSV and EHSV) during sampling was also obtained across seasons. This was calculated using the formula:

% Deviant sites = Number of sites which reflected deviation X 100%

Total number of sampled sites (38) …………. 6

3.2.4.5 Determination of factors responsible for variability in WQP

Determination of number of the principal factors that were contributory to variability of water quality parameters utilized statistical methods. The expected number of principal factors (components) responsible for variability was electronically generated along with the matrix of the studied parameters on the extracted component. The matrixes were assessed for deviations from their normal relationships presented in Boyd (1982).

Significant variations in seasonal data as well as catchment fragments (strata) data were hypothesized as major possible sources of variability and were therefore analyzed for significant differences. Relationship of parameters on the seasons and strata were compared with that extracted by statistical factor analysis to get their alliance.

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3.3 Assessment of morphometric, meristic and phenotypic variability of Clarias