The clam and the dam: A Bayesian belief network approach to environmental flow assessment in a data scarce region

Afua Owusu,Marloes Mul,Michael Strauch,Pieter Van Der Zaag,Martin Volk,Jill Slinger

doi:10.1016/j.scitotenv.2021.151315

Abstract

The Volta clam, Galatea paradoxa, is a freshwater macrobenthic bivalve which is endemic to the Lower Volta River in Ghana. The range of occurrence of the clam has been influenced by the flow regime in the Lower Volta which is in turn controlled by operation of two dams located upstream. Previous research has documented the changes to the Lower Volta due to the dams and attempts have been made to design environmental flows (e-flows), freshwater flows to sustain ecosystems, to inform the re-operation of the dams. The past attempts were based on the pre-dam, natural flow regime of the Lower Volta. In this study, a designer e-flow approach is explored using the Volta clam as an indicator species. Using knowledge garnered from various sources on the lifecycle, habitat and the local conditions corresponding to historical and current states of the Volta clams, the factors influencing its extent are visualized and quantified in a Bayesian belief network (BBN). Based on this BBN, an e-flow recommendation for the Lower Volta is for low flows, between 50 m3/s and 330 m3/s, for four months during the Volta clam veliger larva and recruitment life stages which occur in November to March. In addition, it is recommended that full breaching of the sandbar which regularly builds up at the Volta Estuary is done annually and that sand winning on the river bed is prohibited. These e-flow and management recommendations will have consequences for other water users and these have to be investigated, for instance by flow experiments and trade-off analysis. The results show that a BBN is potentially suitable for modelling the linkages between flows, management practices and the status of ecological indicators for the development of e-flows for highly modified rivers in data scarce regions.

Highlights

Using knowledge garnered from various sources on the lifecycle, habitat and the local conditions corresponding to historical and current states of the Volta clams, the factors influencing its extent are visualized and quantified in a Bayesian belief network (BBN). Based on this BBN, an e-flow recommendation for the Lower Volta is for low flows, between 50 m3/s and 330 m3/s, for four months during the Volta clam veliger larva and recruitment life stages which occur in November to March
The Volta clam, Galatea paradoxa, is a freshwater macrobenthic bivalve which was previously abundant in West African rivers such as the Lower Volta River in Ghana, Sanaga River in Cameroon and the Nun and Cross Rivers in Nigeria (Etim and Brey, 1994; Obirikorang et al, 2013)
Life cycle of the Volta clam and associated flow conditions The specific lifecycle for G. paradoxa is yet to be fully determined empirically, literature on the general life cycle of clams (Abraham and Dillon, 1986; da Costa, 2012) together with empirical knowledge on the gametogenic cycle of G. paradoxa in the Lower Volta River and Cross River in Nigeria (Kwei, 1965; Etim and Taege, 1993; Etim, 1996; AdjeiBoateng and Wilson, 2013b, 2016) and interviews with local clam fishers provide a good indication of its key life stages, the timing of these stages and its habitat preferences

Summary

Introduction

The Volta clam, Galatea paradoxa, is a freshwater macrobenthic bivalve which was previously abundant in West African rivers such as the Lower Volta River in Ghana, Sanaga River in Cameroon and the Nun and Cross Rivers in Nigeria (Etim and Brey, 1994; Obirikorang et al, 2013). In Ghana, it is an important source of protein and supports an artisanal fishing industry near the estuary (Obirikorang et al, 2009; Adjei-Boateng et al, 2012a). Before construction of two dams in the Lower Volta, the Akosombo Dam in 1965 and the run-of-the river Kpong Dam in 1982, natural G. paradoxa beds covered a river length of approximately 80 km, stretching from Akuse to Sogakope (Lawson, 1972; Tsikata, 2008) (Fig. 1). Due to the flow alteration associated with the operation of the dams over the decades, this range has shifted downstream and narrowed to a fraction of the pre-dam state, stretching approximately 10 km from Agave-Afedume to Big Ada (Fig. 1) (Adjei-Boateng et al, 2012a). In a survey in 1954 at Battor, one of the riparian communities in the Lower Volta, about half of the women in the community were engaged in clam picking while for two-thirds of these women, clam-picking was their main source of livelihood (Tsikata, 2008)

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