The influence of European sea bass farming on the level of heterotrophic bacteria in marine water assessed by SimPlate® substrate test

Intense fish farming could affect the microbial quality of marine water in aquacultures and surrounding areas. Our aim in this study was to investigate the influence of several sea bass farms along the Croatian Adriatic coast on the abundance of heterotrophic bacteria (HPC) in marine water as indicator of organic enrichment. For that purpose, substrate SimPlate® test was introduced for HPC enumeration in marine water. Optimal conditions for HPC monitoring were established based on comparison of the results obtained at two incubation temperatures (22C and 35C), as well as comparison of the results obtained by substrate test and by traditional spread plate method. Combination of SimPlate® test and incubation temperature of 22°C was best suited for monitoring purposes, since it sustains growth of both naturally present and potentially pathogenic marine bacteria, and enables growth of marine bacteria which cannot grow on solid media. Preliminary assessment of marine water in selected aquacultures (June 2007 - October 2008) indicated that farming activities still did not reach critical impact on HPC abundance (0.23×103-1.04×105 MPN/mL). However, observed increasing trend of HPCs over the studied period was an indication that marine water in the vicinity of aquacultures should be occasionally monitored for organic pollution.