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Aquatic organisms are often subject to intermittent exposure to pollutants in real ecosystems. This study aimed to compare mercury accumulation and the physiological responses of mussels, Mytilus edulis during continuous and intermittent exposure to the metal. Mussels were treated in a semi-static, triplicated design to either a control (no added Hg) or 50 µg l(-1) Hg as HgCl2 in continuous (daily) or intermittent (2 day exposure, 2 days in clean seawater alternately) exposure for 14 days. A time-dependent increase in Hg accumulation was observed in the continuous exposure, while the intermittent treatment showed step-wise changes in Hg concentrations with the exposure profile, especially in the gills. At the end of the experiment, tissue Hg concentrations were significantly increased in the continuous compared to the intermittent exposure for digestive gland (4 fold), gonad and remaining soft tissue (>2 fold), but not for the gill and adductor muscle. There was no observed oxidative damage at the end of the experiment as measured by the thiobarbituric acid reactive substances (TBARS) concentrations in tissues from all treatments. However, total glutathione was significantly decreased in the gill and digestive gland of both the continuous and intermittent exposure by the end of the experiment. The neutral red retention ability of the haemocytes was not affected, but total haemocyte counts were significantly decreased (<2 fold) in the intermittent compared to the continuous exposure. Histopathological examinations showed less pathology in the gill, but more inflammation in the digestive gland of mussels for the intermittent compared to the continuous exposure. Overall, the results showed that Hg accumulation from intermittent exposure was less than that of the continuous exposure regime, but the sub-lethal responses are sometimes more severe than expected in the former.

The study of the socioeconomic characteristics of any group is a requirement to a successful implementation of effective government assisted programme. The study examined the socioeconomic characteristics, production processes and production constraints of small-scale catfish fishing enterprises in Obio/Akpor Local Government Area in Rivers State, Nigeria. Simple randomized sampling technique was used to select 60 small-scale catfish farming enterprises. Structure questionnaires were used to collect primary data from the respondents. The collected information were analyzed using simple descriptive statistics. The results showed that majority of the farmers were females (58%), married (74%), within the age group (31-40 years), Christians (92%), Igbo tribe (22%), and had university degree (58%) with household size (6-10). Majority have fish farming as primary occupation (32%), 1-5 years of experience, non-members of association (83%) and chose fish farming for profit maximization (50%). Based on this study, different aquaculture production systems were identified at the study area such as concrete tanks (34%), plastics tanks (20%), concrete+plastic (20%), earthen ponds (12%), earthen+concrete (8%), and earthen+plastics (6%). Respondents purchased land (66%), kept record (52%), used imported feed (52%), flow-through techniques (88%), monoculture as culture system (66%), stocked Clarias gariepinus (88%), borehole as water source (96%), procured fingerlings from private farms (84%) and personal savings as source of funding (86%). Some of the constraints like lack of extension service (76%), disease outbreak (52%), lack of capital (44%), lack of electricity (38%), preservation and processing (18%), lack of skilled human labour (10%), and lack of government assisted programmes (0%) were faced. In conclusion, catfish farming promises to improve in the area if there is adequate government assistance.