| dc.description.abstract | This study explores the trophic status of pelagic fishes in Mtwapa creek and Shimoni channel on the Kenyan coast. Both qualitative and quantitative spectra of the diets of these fish were investigated in relation to the biotic and abiotic conditions in the study areas. In- depth trophic analyses were undertaken on Sardinella gibbosa and Atherinomorous lacunosus. Mtwapa (3° 55’S, 39° 45’ E) is estuarine, surrounded by extensive mangrove swamps and mudbanks with freshwater input from a seasonal river. The creek is relatively eutrophic due to raw sewage disposed from the Shimo la Tewa GK prison. Shimoni channel on the other hand (4° 37’S, 39° 41’ E) is more open oceanic with no freshwater input, and has few patches of mangrove vegetation along the rocky shoreline.
Samples were taken at 3 sites from each study area between January and August 2001. Fish were collected using a beach seine net of 20m long and 20mm stretched mesh. All fish were immediately preserved in 10% formaldehyde in seawater solution. Diets of the 8 most abundant fish from Mtwapa creek: Sardinella gibbosa, Pellona ditchella, Spratelloides delicatilus, Atherinomorous lacunosus, Selar crumenophthalmus, Gerres oyena, Secutor insidiator and Leiognathus equulus were studied. Since S. gibbosa and A. lacunosus were found to be common and abundant in both study areas, they were given an in-depth treatment to investigate the spatial and temporal differences in their diet.
This study established a clear spatial separation of Mtwapa creek from Shimoni channel in terms of the abiotic and biotic data. Highly significant differences (t-test, P<0.05) were observed between the two study areas in temperature, salinity, transparency, conductivity, chlorophyll a and in zooplankton abundance and diversity. There were however, no differences in pH and dissolved oxygen (DO) values between the two areas (t-test, P>0.05). It is suggested that the observed differences in the abiotic environment and the habitat heterogeneity, i.e. presence or absence of mangrove swamps and the nature of the substratum, may have caused the observed habitat difference between the two areas. This observation was further reflected by the differences in the diets of S. gibbosa and A. lacunosus collected from the two study areas. There was a significant difference (t-test, P<0.05) in monthly abundance of zooplankton between Mtwapa creek and Shimoni channel. Mtwapa creek had lower mean densities than Shimoni channel. Diversity of zooplankton species was however, higher in Mtwapa creek (H’ = 0.92 ± 0.48 SE) than in Shimoni channel (H’ = 0.89 ± 0.02 SE).
Temporal pattern was observed in dissolved oxygen, transparency, temperature and salinity between the two monsoon seasons. Higher values were recorded during the Northeast monsoon (January - March) as compared to the lower values of the Southeast monsoon (April - August). This pattern was dictated by the reversing monsoon winds and the rainfall regime.
Total fish catch from the experimental fishing was higher in Shimoni channel (11 270 specimens) than in Mtwapa creek (7 179 specimens). While diversity of fish species and evenness of species distribution in the study areas were higher in Mtwapa creek (H’ = 0.97, J = 0.63) than in Shimoni channel (H’ = 0.42, J = 0.62). The size range for most of the fish caught from the two areas was much lower compared to their adult sizes at maturity which suggests that the majority were juveniles.
Copepods were the principal food item in the diets of studied fish species with the exception of Selar crumenophthalmus, which fed mainly on fish scales. Polychaetes were an important diet for Gerres oyena and Leiognathus equula. Two trophic categories, i.e. carnivores and omnivores, were identified among the studied fishes. Spratelloid.es delicatilus was basically a carnivore having fed only on zooplankton and zoobenthos with the lowest diet (H’ = 0.40). The rest were omnivores. S. gibbosa, P. ditchella, L. equula, S. insidiator, G. oyena and S. crumenophthalmus fed on phytoplankton, zooplankton, zoobenthos and detritus with a relatively higher diet diversity (H’ = 0.68 - H’ = 0.96). A. lacunosus did not take detritus in their diet and had a lower diet diversity compared to the other omnivores (H’ = 0.47).
The examined species highly overlapped each other in the feeding niche. The Morisita feeding niche overlap values ranged between 0.84 and 1.00. Most of them were generalised opportunistic feeders except for S. dilicatilus, which exhibited a specialised feeding strategy.
The in-depth analyses on spatial and temporal variation in diets of S. gibbosa and A. lacunosus showed that the two species took a greater number of prey categories in Mtwapa creek than in Shimoni channel (%2 - test, P<0.05). Copepods were important prey items for both species in Mtwapa creek for both the NE and SE monsoons. However, S. gibbosa from Shimoni fed mostly on copepods during the NE monsoon but took more nematodes and brachyuran megalopas during the SE monsoon. A. lacunosus from Shimoni channel fed mostly on lemellibranch larvae during the NE monsoon and on nematodes during the SE monsoon. No pattern was discernible in their feeding strategies. On the other hand, S. gibbosa from Mtwapa creek highly selected sergestids in their diets, and nematodes in Shimoni channel. A. lacunosus selectively took brachyurans in Mtwapa creek, and nematodes in Shimoni channel. The size of copepods eaten by S. gibbosa and A. lacunosus differed significantly (ANOVA, P<0.05; Tukey HSD, P<0.05) with S. gibbosa eating larger and wider copepods than A. lacunosus. However, there was no significant difference in the size of nematodes eaten by the two species (ANOVA, P>0.05; Tukey HSD, P>0.05). No significant ontogenetic changes were observed in prey sizes eaten by either species (P> 0.05).
The spatial variation in feeding intensities of S. gibbosa and A. lacunosus shows that, the two species fed with higher intensities in Mtwapa creek than in Shimoni channel. A. lacunosus fed with a higher intensity at dawn and dusk, and lower activity during the day. The number of prey categories was also higher at 0600 hours and 1800 hours. At these hours, copepods constituted the highest percentage of prey items in the stomachs of these fishes. Overall, this study has shown that the small pelagic fishes in the studied inshore waters had a flexible diet, which relied
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greatly on the prevailing biotic and abiotic conditions in their habitats. | |
