Influence of salt works’ hyper-saline waste-brine on distribution of mangrove crabs (Decapoda) within the Gongoni-Kurawa intertidal area, Kenya

Abstract

Evaporation is one of the oldest methods employed in sea salt production, a process that involves pumping seas water into a series of ponds where solar evaporation concentrates it into brine, and precipitates the salt; which is then harvested manually. Lack of baseline information on the effect of the discharged brine at the Gongoni-Kurawa region caused the need to determine its impact on the marine ecosystem. The objective of this study was to identify the effect of brine on mangrove crab species diversity and distribution. The impact of the discharged hyper-saline waste-brine waters on the mangrove crabs along the intertidal habitats bordering two of the biggest salt works; Krystalline and Kurawa salt industries was investigated in this study. Sampling was done during both spring and neap season in the year 2015 between the month of February to June using 1m2 quadrats, and crabs collected from the quadrats used to estimate the densities. A total of 34 mangrove crab species were recorded, with abundances significantly higher (p>0.05) within the inlets’ habitats as compared to the outlets (discharge-point) habitats. Six species were most dominant and occurred in all of the transect samples and they were Machrophthalmus grandidieri, Uca chlorophthalmus, Terebralia palustris, Machrophthalmus latreillei, Uca tetragonon and Amaea acuminata. Higher species diversity and evenness were recorded in inlet habitats at Kurawa compared to the outlets. The inlet habitats reported higher Maximum Shannon-Wiener diversity, whereas outlets recorded lower diversity, with Marereni recording considerably lower Hmax, at 0.95. Species distribution showed a significant reduction of the genera Uca and M. grandidieri (p<0.05) at Marereni outlet habitats, but an increase in U. vocans at the inlet habitats. Similarly, there was a higher abundance of genera Uca, M. ovalina, M. grandidieri, M. latreillei, Amaea acuminata and Cerithidea decollata (p<0.05) during the spring tide period while the abundances of U. vocans dropped during the same period. This asymmetric distribution between inlets and outlets was explained by significant variations in salinity as well as site specific salinity gradients at the two study sites; Marereni and Kurawa in north coast, Kenya. Suggestions for improving salt production and quality while minimizing adverse environmental effects were recommended.