Tracing the flux of aquaculture-derived organic wastes in the southeast arm of Lake Malawi using carbon and nitrogen stable isotopes (original) (raw)
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Journal of Great Lakes Research, 2011
The initiation of cage aquaculture in the shallow southeast arm of Lake Malawi has raised concerns about its possible impact on the surrounding environment and the highly diverse fish community. To evaluate the impact of the cage operation on the surrounding environment, observations were made over an annual cycle in 2007 at a production capacity of~200 tonnes fish/year. Impacts of the cage wastes in the water column in the vicinity of the cages were minimal despite the substantial discharges from the cages. No significant differences were observed in concentrations of dissolved and particulate nutrients (ammonia, nitrate, phosphate, particulate C,N and P), chlorophyll, chlorophyll fluorescence, dissolved oxygen, total suspended solids, Secchi depths and extinction coefficient of photosynthetically active radiation between the cage site and the control stations upstream or downstream of the fish farm. Although sedimentation rates measured in traps were higher under the cages than at control sites, the sediment flux was a small percentage of total feeds added to the cages. Apparently cage wastes were efficiently dispersed by water currents which averaged 9.3 cm/s below the fish cages. Consumption of the wastes by wild fish species which aggregated around the fish cages and their movement in the vicinity of the cages also contributed to the dispersion of the cage wastes and served to dilute impact of the cages. In combination, these physical and biological processes reduced the immediate impact of the cage farming operation and must be considered in the siting of future cage farms.
Journal of Great Lakes Research, 2011
is a great lake not only because of its large size (30,800 km 2); but also because it hosts between 700 and 1000 fish species, mostly endemic cichlids. The fish are most abundant and more diverse in the littoral zone. Cage aquaculture in Lake Malawi started in 2004 in the nearshore waters of the southeast arm. Approximately 50 cages were operational by 2009 harvesting about 20 tons fresh fish/cage/year. In 2007, data were aggregated from cage feed and production records to produce a mass balance for carbon (C), nitrogen (N) and phosphorus (P) added to cages in the form of feed and juvenile fish for 22 production cycles. Nutrient losses from cages to the surrounding environment accounted for between 81 and 91% for C, 59 and 80% for N and 85 and 92% for P. Production strategies such as stocking density and feeding rates were also analysed to assess their effect on fish harvests and nutrient loads from the fish cages. The study showed that nutrient loads from the cages were significant and can be exacerbated by poor feed quality, overstocking, stocking of premature fish and use of lower than recommended feeding rates to grow the fish to target sizes. The inefficient production strategies necessitated longer production cycles (376 ± 42 days) and more feed usage to achieve market sizes of ≥ 300 g/fish than if generally recommended stocking and feeding rates were followed. If cage aquaculture continues to expand, it will become an important new source of nutrients, not only locally, but on a lake-wide basis.
Uganda Journal of Agricultural Sciences
Growing of fish in cages is currently practiced in Uganda and was first introduced in northern Lake Victoria in 2010. An environment monitoring study was undertaken at Source of the Nile, a private cage fish farm, in Napoleon gulf, northern Lake Victoria. In-situ measurements of key environmental (temperature, dissolved oxygen, pH and conductivity) and biological (algae, zooplankton, macro-benthos) variables were made at three transects: Transect 1-the site with fish cages (WC); transect 2-upstream of the fish cages (USC-control) and Transect 3-downstream of the cages (DSC). Upstream and Downstream sites were located approximately 1.0 km from the fish cages. Environment parameters varied spatially and temporally but were generally within safe ranges for freshwater habitats. Higher concentrations of SRP (0.015-0.112 Mg/L) occurred at USC during February, September and at DSC in November; NO 2-N (0.217-0.042 mg/L) at USC and DSC in February and November; NH 4-N (0.0054-0.065 Mg/L) at WC and DSC in February, May and November. Algal bio-volumes were significantly higher at WC (F (2,780) =4.619; P=0.010). Zooplankton species numbers were consistently lower at WC with a significant difference compared to the control site (P=0.032). Macro-benthos abundance was consistently higher at the site with cages where mollusks and low-oxygen and pollution-tolerant chironomids were the dominant group. Higher algal biomass, concentration of low-oxygen/pollution-tolerant macro-benthos and depressed zooplankton diversity at WC suggested impacts from the fish cages on aquatic biota.
Environmental impacts of cage culture in Lake Victoria: the case of Shirati Bay‑Sota, Tanzania
Abstract The experimental cage culture was conducted at Shirati bay, Lake Victoria from February to August 2013, to investigate the impacts of the small scale cage culture on the environment. Three locations along the cages, at the intermediate and one in the offshore (control) were sampled for water quality parameters, phytoplankton and macro invertebrates. A notable increase in nutrient concentration was observed after the set of cages among the stations. However DO, pH, and water transparency showed no major changes and was within the recommended ranges. Cyanophytes an indicator of inorganic pollution dominated before and after the set of cages, an increase in phytoplankton numerical abundance was observed after stocking of fish in cages. In addition there was an increase in the invertebrate community especially bivalves and gastropods. In conclusion we found no consistent environmental change caused by cage culture, and therefore it can be allowed in Lake Victoria, Tanzania part, with close monitoring of its impacts. Keywords: Cage culture, Nutrient enrichment, Water quality, Waste food
Changes in fish community structure associated with cage aquaculture in Lake Malawi, Africa
Aquaculture, 2015
Local fishermen claim that introduction of a commercial cage aquaculture farm in Lake Malawi resulted in low fish catches as fish take refuge within the farm. Fish specimens were caught in three fishing sites, one at the farm and two 5 km southeast and northwest of the farm in February, April, June, and August 2012 using four experimental multi-mesh gillnets with similar dimensions to catch different fish species and sizes. Data was used to determine changes of fish community composition, abundance, biomass, and fish diversity. Site 2 located at the farm and 3 to the southeast were dominated numerically by many small bodied fishes which often is an indication of disturbance by fishing; therefore, they were classed as disturbed while site 1 in northwest was classed as undisturbed site. A near field impact of the farm on fish community structure was detected as revealed by significantly different fish community structure from site 2 compared with that found at sites 1 and 3, but with similar number of fish species and diversity as the remote sites. Overall community structure and the abundance of the fish community in the vicinity of the cage site were improved primarily through providing protection from fishing while incidence of large number of small bodied fish at site 3 resulted from removal of large fish by fishing pressure. This study suggests that protected areas such as the cage site can be a practical strategy to reduce fishing pressure in Lake Malawi and allow recovery of native fish stocks.
A preliminary assessment of water quality in fish cages on Lake Kariba, Zimbabwe
The water quality in fish cages on Lake Kariba was studied. The Kruskal-Wallis test was used to determine whether the water quality parameters differed significantly among the cage-types (cage-type effect). Secchi disç readings ranged from 2 to 4 metres, while temperature ranged from 23°C to 29.5°C. The observed conductivity was between 98.7pS cm and 102.3yS cm.
Aquaculture, 2011
The potential impact of Nile Tilapia (Oreochromis niloticus) cage culture on water quality and pelagic community composition was investigated in two Ethiopian small water bodies, one located in the highlands (Yemlo) and the other in the Great Rift Valley (Allage). This study was designed to assess the difference between the cages and open water in relation to those water quality changes attributable to intensive inputs of fish waste and left-over fish feed. All physico-chemical water quality parameters including inorganic nutrients varied temporally, coupled with dry and wet periods. The reservoir's trophic state ranged from eutrophic to hypereutrophic, with a strong correlation between chlorophyll-a and total phosphorus. The phytoplankton community was dominated by Cyanobacteria (84% of total phytoplankton abundance), in particular by Anabaenopsis sp. in Allage reservoir, whereas Chlorophyta (70%), with Pediastrum simplex as the dominant taxon, prevailed in Yemlo reservoir. A total of 23 zooplankton taxa were recorded during our sampling; rotifers were the richest group with 14 taxa distributed in 6 genera, followed by cladocerans represented by 6 taxa (5 genera) and copepods by 3 taxa (1 genus). Dissolved inorganic nutrient concentrations and other physical parameters showed no significant differences between the cages and open water. The exceptions were dissolved oxygen and ammonium nitrogen, which were lower and higher in the cages, respectively. For the whole study period of 240 days, the mean net weight and daily growth rate per fish were 183.3 g and 1.1 g d − 1 , respectively.
Heavy metal concentrations and sediment quality of a cage farm on Lake Volta, Ghana
Aquaculture Research, 2020
This study involved the determination of sediment quality and heavy metals in the water column and bottom sediment of a selected cage fish farm in Lake Volta, to assess the potential impact of metals and organic matter pollution on the lake due to cage fish farming. Sediment analysis indicated that the texture of all sampling sites was sandy clay loam with sand dominating with a range of 31.5-81.2%. The Organic matter (TOM) ranged from 4.42-8.89%, while organic carbon (TOC) was from 2.57-5.22%. Total nitrogen (TN) fluctuated between 0.22 and 0.45%; total phosphate ranged between 0.22 and 5.30%. The TOC, TOM, and TN content in the farm sediment were significantly lower than those of the reference sites (ANOVA, p<0.05). Lead, copper, cadmium, and selenium were not detected in the water. The results revealed that heavy metals concentrations in the water column and sediments were low and within tolerable levels, indicating no influence of metals from fish feed was observed on the lake water quality. However, the farm seemed to have moderate impact on sediment quality from organic matter. Water and sediment quality monitoring should be embarked upon periodically to ensure sustainable cage culture in the Lake Volta.