Simulation of optimal harvesting strategies for small-scale mixed-sex tilapia (Oreochromis shiranus Boulenger 1896) ponds using a bio-economic model

Abstract A cohort-based bio-economic biomass growth and economic model, validated with data from experiments conducted in Malawi, was used to identify an optimal harvesting strategy for mixed-sex tilapia ponds. Three harvesting scenarios (baseline, economic optimum time 110 days and economic optimum time) were used. In each harvesting scenario four options were explored: (i) no further harvest, harvest every (ii) 60 days, (iii) 90 days and (iv) 120 days after initial harvest. The lowest simulated yield (487 kg ha À1 year À1 ) was obtained when no partial harvesting was carried out and ¢sh were harvested after 365 days. Maximum yield (4416 kg ha À1 year À1 ) was obtained when partial harvests were carried out every 90 days starting with a ¢rst harvest of ¢sh weighing 60 g or more at day 90. Maximum ¢nancial returns (US$2561 ha À1 year À1 ) were obtained when partial harvests were carried out every 120 days starting with the ¢rst harvest at day 90 and removing all ¢sh ! 60 g. The model simulations indicate that mixed-sex tilapia culture may be pro¢table for tilapia farmers in Africa where markets accept small (60^150 g)-sized ¢sh. The study further shows that a cohort-based population growth model can be reliably incorporated in tilapia production models to simulate ¢sh yields in mixed-sex tilapia production systems. However, incorporation of intergenerational competition e¡ects could improve the model's utility as a decision support tool for managing mixed-sex tilapia production

Molecular MR Imaging of Liver Fibrosis: A Feasibility Study Using Rat and Mouse Models

Background & Aims: Liver biopsy, the current clinical gold standard for fibrosis assessment, is invasive and has sampling errors, and is not optimal for screening, monitoring, or clinical decision-making. Fibrosis is characterized by excessive accumulation of extracellular matrix proteins including type I collagen. We hypothesize that molecular magnetic resonance imaging (MRI) with a probe targeted to type I collagen could provide a direct and non-invasive method of fibrosis assessment. Methods: Liver fibrosis was induced in rats with diethylnitrosamine and in mice with carbon tetrachloride. Animals were imaged prior to and immediately following i.v. administration of either collagen-targeted probe EP-3533 or non-targeted control Gd-DTPA. Magnetic resonance (MR) signal washout characteristics were evaluated from T1 maps and T1-weighted images. Liver tissue was subjected to pathologic scoring of fibrosis and analyzed for gadolinium and hydroxyproline. Results: EP-3533-enhanced MR showed greater signal intensity on delayed imaging (normalized signal enhancement mice: control = 0.39 ± 0.04, fibrotic = 0.55 ± 0.03, p <0.01) and slower signal washout in the fibrotic liver compared to controls (liver t1/2 = 51.3 ± 3.6 vs. 42.0 ± 2.5 min, p <0.05 and 54.5 ± 1.9 vs. 44.1 ± 2.9 min, p <0.01 for fibrotic vs. controls in rat and mouse models, respectively). Gd-DTPA-enhanced MR could not distinguish fibrotic from control animals. EP-3533 gadolinium concentration in the liver showed strong positive correlations with hydroxyproline levels (r = 0.74 (rats), r = 0.77 (mice)) and with Ishak scoring (r = 0.84 (rats), r = 0.79 (mice)). Conclusions: Molecular MRI of liver fibrosis with a collagen-specific probe identifies fibrotic tissue in two rodent models of disease

Changes in the biomass of chambo in the southeast arm of Lake Malawi: A stock assessment of Oreochromis spp.

Lake Malawi has one of the most diverse fish faunas in the world (500-650 species) and is a major source of protein for the people of Malawi. Chambo (Oreochromis spp.) is one of the most important food fishes; its abundance has declined sharply over the last twenty-years. Surveys by the Malawi Department of Fisheries have shown a decrease in chambo density in the southeast arm of the lake and the annual harvest has dropped substantially since 1985. We conducted a dynamic stock assessment of Oreochromis spp. which included all vessel and gear types and covered the entire southeast arm of Lake Malawi. Chambo biomass peaked in 1982 and then declined continuously through the early 2000s. The biomass is highly correlated with the mean lake height two years prior suggesting that recruitment may be linked to increased nutrient input, and spawning and nursery habitat associated with the flooding of low lying areas. The main driver of chambo biomass, however, was fishing pressure which was above the level that would achieve maximum sustainable yield during the entire time series. This study provides a baseline from which to measure changes due to future management actions or climate variations. © 2012 Elsevier B.V

The natural history and fisheries ecology of Lake Chilwa, southern Malawi

Lake Chilwa produces between zero and 24,000 metric tons of fish per year, making it one of the most productive but variable lakes in Africa. The size of the lake varies seasonally and among years, sometimes drying completely. Its surrounding wetland and floodplain provide habitat for a diversity of birds and economically valuable grasses and reeds. When the lake has water, there is considerable activity on its shores and temporary fishing villages spring up. People move in and out of the lake basin in concert with these seasonal and longer term changes. This paper examines the environmental dynamics of Lake Chilwa and its surrounding wetlands, presents an overview of the socio-economic context of the area and discusses threats to this resilient system that might occur as a result of climate change. We conclude that management of Lake Chilwa must place the lake in the wider economic and ecological system in which it is situated. Ultimately, land-use practices within the basin present more of a threat to the resilience of the fishery and people\u27s livelihoods than overfishing or a strict focus on the lake\u27s resources. These perspectives present significant challenges to conventional fisheries governance