Study to detect impacts of pollution on fishery biology of tilapias in lake Manzalah, Egypt

الملخص EN

The effect of pollution on fishery biology of tilapias was investigated on specimens caught seasonally by trammel nets from two differently polluted localities in Lake Manzalah during 2000.

The less polluted one lies southwest (region A) and the other highly polluted lies in southeast (region B) of the Lake.

Significant differences in fish abundance by weight (F = 7.153, p < 0.05) and by number (F = 11.299, p < 0.05) were observed between the two regions.

Abundance by species revealed that Oreochromis aureus, Sarotherodon galilaeus and Tilapia zillii were significantly differed in number and S.

galilaeus in weight among tilapias in areas of study.

Size composition revealed regional differences in the case of O.

aureus (F = 23.155, p < 0.01) and S.

galilaeus (F = 36.793, p < 0.01).

Individuals of age group 1 dominated the catch of all species in areas of study except for O.

aureus whose individuals of age group 2 dominated its catch in region B.

The overall ratio of males to females was 1:1.26 for Oreochromis niloticus, 1:1.11 for O.

aureus, 1:1.03 for S.

galilaeus and 1:0.72 for T.

zillii.

Chi-square test (X2) revealed that the percentage of females is significantly higher than males in region B for all tilapia species except T.

zillii comparing with the sex ratio for region A fishes.

The mean values of condition factor of O.

niloticus and S.

galilaeus were higher in region A than region B.

In addition, the natural mortality coefficient for O.

niloticus inhabiting in region B was comparatively higher than that for region A.