PurposeThe purpose of the work presented in this paper is to capture the current state of Six Sigma as well as to document the current practices of Six Sigma through a systematic literature review so as to extend and update the previous work of Brady and Allen.Design/methodology/approachThe approach to this paper is to answer the questions such as “what is Six Sigma?”, “what are the applications of the Six Sigma?”, “what are the main enablers and barriers to its application?” and “what are the emerging trends?” These questions are used to guide the search of papers from various publication databases even if it is expected that existing literature might not be sufficiently developed to translate each question directly into a finding. The literature is then analysed and the major emerging themes are presented.FindingsSeven key findings (topics on which the views of the authors converged) and two issues (topics on which authors had differing views) have been established. These include the interpretation of Six Sigma, tools and techniques, implementation of Six Sigma, benefits, adoption, enablers and links to other disciplines.Originality/valueThe systematic literature review approach used in this paper allows emerging trends and issues in Six Sigma to be highlighted in a structured and thematic manner, enabling the future work to progress as Six Sigma continues to develop and evolve. The findings also open up new opportunities to apply Six Sigma in the fields that are not widely explored before for instance sustainability and product‐service systems.

In this study, we document an increasing trend of catches of alien fish species from the Ganga River system. The changing fishery during 1980 to 2011 and the fish yield rate (kg km −1 ) are positively correlated with an invasion coefficient index (I xi ) of alien species within the river. The reproductive propagule pressure (PPP) of alien fishes (Cyprinus carpio, Oreochromis niloticus, Aristichthys nobilis, Ctenopharyngodon idella, Hypophthalmichthys molitrix and Clarias gariepinus) was determined and found to be 19.45×10 6 for C. carpio, 0.33×10 6 for O. niloticus and 0.82×10 6 for A. nobilis at confluences suggesting that these alien species may be migrating into new habitats. Further, these invaded species exhibited all reproductive stages, indicating their likely establishment within the river. The trophic spectrum of alien fishes spanned all levels and the gut repetitive index (GRI) indicated that that food items in most of the fishes were similar showing early trends of food-web alteration and biotic homogenization. The results of this study suggested a continuous decline in the catch of local fishes by wet weight, particularly Indian major carps (IMC) from 128.91 kg km −1 to 38.58 kg km −1 owing to increased catches of alien species from nil to 384.27 kg km −1 . Invasion of alien species in the Ganga River represented one of many possible causes of the decline in river health and overall loss of native aquatic communities.

Monitoring of heavy metals was conducted in the Yamuna River considering bioaccumulation factor, exposure concentration, and human health implications which showed contamination levels of copper (Cu), lead (Pb), nickel (Ni), and chromium (Cr) and their dispersion patterns along the river. Largest concentration of Pb in river water was 392 μg L(-1); Cu was 392 μg L(-1) at the extreme downstream, Allahabad and Ni was 146 μg L(-1) at midstream, Agra. Largest concentration of Cu was 617 μg kg(-1), Ni 1,621 μg kg(-1) at midstream while Pb was 1,214 μg kg(-1) at Allahabad in surface sediment. The bioconcentration of Cu, Pb, Ni, and Cr was observed where the largest accumulation of Pb was 2.29 μg kg(-1) in Oreochromis niloticus and 1.55 μg kg(-1) in Cyprinus carpio invaded at Allahabad while largest concentration of Ni was 174 μg kg(-1) in O. niloticus and 124 μg kg(-1) in C. carpio in the midstream of the river. The calculated values of hazard index (HI) for Pb was found more than one which indicated human health concern. Carcinogenic risk value for Ni was again high i.e., 17.02 × 10(-4) which was larger than all other metals studied. The results of this study indicated bioconcentration in fish due to their exposures to heavy metals from different routes which had human health risk implications. Thus, regular environmental monitoring of heavy metal contamination in fish is advocated for assessing food safety since health risk may be associated with the consumption of fish contaminated through exposure to a degraded environment.

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Water quality monitoring of Clarias gariepinus culture ponds (n = 27) revealed poor physico-chemical conditions and metal contaminants in fish tissues (n = 324). Human health risk assessment for some heavy metal contamination delineated low risk in general except for Aluminium (Al), Iron (Fe) and Lead (Pb) which accumulated significantly (p < 0.05) high in tissues. Health risks values were 6.3 × 10(-3)-9.6 × 10(-3) for Al; 3 × 10(-3)-9.7 × 10(-3) for Fe and 1.15 × 10(-5)-9.3 × 10(-6) for Pb respectively suggesting that contamination of Pb particularly in ponds fed with chicken waste (CW) was posing high risks.

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Invasion, spread and predominance of alien invasive fishes were studied in 950-km-long stretch of the Yamuna River. The fish yield ranged between 32 t/km/yr to 240 t/km/yr and the catch of alien invasive fishes dominated over local fishes in the fishery, which was over 96% at Etawah. The calculated Invasion Coefficient Index (I xi) of alien invasive fishes was in the range of 0.12 to 0.31 indicating a major loss to the local fish diversity. The calculated Pearson's correlation coefficient was found to have positive correlation of 0.757 and 0.710 for common carp and tilapia respectively with locally captured Indian major carps (IMC). The results of this study highlighted how unintentional spread of alien invasive fishes due to rapid aquaculture diversification and intensification has invaded into the Yamuna River causing severe loss to the local fishery. The invasion of alien fishes has been mapped for the Yamuna River at different locations using GIS tools. The results of this study invite attention on the management and sustainability of the local fishery and ecosystem health of the river.