Assessing the use of West African Mangrove oyster as a bioindicator of aquatic environment in Densu Estuary economic viability

Invisible fisheries such as oysters though minimal and less harvested in comparison to fishes, are of nutritional, medicinal and economic value. Oysters are a source of nutrients in the form of protein to many people in West Africa including Ghana.

The West African Mangrove Oysters (WAMO) is a source of protein and contains low amount of cholesterol as compared to other animal protein. It’s a source of amino acids and fatty acids vital for human wellbeing.

The shells provide calcium and are used in the preparation of poultry and livestock feeds. In addition, the shells serve as an ingredient in paint preparations, a rough base for footpaths, and cement for building and raw material for pharmaceutical industries.

Ecologically, sessile organisms like oysters are important in the aquatic food chain. Oysters are filter feeders and while they feed on plankton, they help improve on water quality. The settling behavior of oyster spats with time, form reefs which provide structured habitat in estuaries and lagoons for many fish species and crabs.

In Ghana, the West African Mangrove Oyster (WAMO) is widely distributed occurring in mangroves, sediments and compact substrates of coastal water bodies. In the country, oyster populations in estuaries and lagoons are declining. As at 1996, about nine (9) out of the 41 wetland ecosystems lost their oyster populations according to research.

The statistics is likely to increase as the study dates back to two (2) decades ago with this possibly to be exacerbated by recent state of increase in pollution of Ghana’s water resources. Meanwhile, there is a huge potential for most of these wetlands to be used for culture of oysters on commercial basis for economic gains to the country as practiced in some countries like the Gambia, Egypt and Australia (Obodai, 1999).

Furthermore, according to research, 48 % of the estuaries and lagoons in Ghana were found to be suitable for commercial cultivation of oysters suggesting a high potential for their use for aquaculture to augment catches from the wild which the Densu estuary is no exception.

The Densu estuary in Ghana is a habitat for Crassostrea tulipa, also known as West African Mangrove Oyster. The population is currently harvested for commercial use and support the livelihood of riparian coastal communities. There exist several studies on oyster fisheries in Ghana and elsewhere.

Most of these studies were centered on oyster socio-economics, biology and culture potential of oysters. Meanwhile, there is no information on the interactions between the size and abundance of the wild mangrove oyster with the environment for use as an indicator of natural aquatic variability in the Densu estuary.

Water quality factors, invertebrates, algae, foraminefera, birds, macrophytes and fish have been the conventional proxies used in Ghana for assessing environmental health and managing aquatic systems.

The high mobility of birds, fin fish, short life span of algae, frequent changes in water quality requiring a longer period of monitoring, presents a challenge to their sustainable usage. However, oysters are sedentary organisms that are cosmopolitan in nature, have the ability to filter pollutants and store biogeochemical data thereby reflecting the health of estuarine environments better than other known aquatic bio indicators.

In many regions of the world, extensive studies have been carried out on oysters and mussels as ecological indicators of environmental variability.

However, in Ghana scanty scientific studies have been done on the use of oysters. Also, it has been documented that as filter feeders, oysters filter a lot of pollutants and are considered as good indicators of environmental changes in water.

Conversely, due to the possibly different environmental conditions of the Densu estuary, and the rising need for identification of proxies in the current environmental uncertainty, there is a need to undertake this current research to guide stakeholders to make informed decisions on the possible adoption or otherwise of the West African Mangrove Oyster as an early warning signal of changes in estuarine environments for the development, management and sustainable exploitation of oyster fisheries in Ghana.

Challenges
Out of ten estuaries investigated in the coast of Ghana for their suitability for culture of oysters, two, namely the Densu and Lower Volta are located in the Greater Accra Region. The Densu estuary is the most important in terms of Oyster fishery in the region.

It currently supports a thriving commercial oyster fishery under the Development Action Association (DAA) project funded by USAID and remains an important coastal water body in Greater Accra. Information on aspects of the ecology of C. tulipa bordering on food habits of the West African Mangrove Oyster and its interaction with the natural environment in the Densu estuary is lacking.

Also, despite scientific proofs on the detrimental impacts of estuarine acidification on oysters, there is dearth of information on the influences of acidification factors on the Densu oyster population in Ghana.

There are indications that many coastal wetlands along the Gulf of Guinea Large Marine Ecosystem (GLME) of Ghana which the Densu estuary is no exception is under increasing threat of contamination from incessant human activities such as oil drilling, farming activities, improper disposal of waste from household and industrial sources.

Several works have reiterated the deteriorating state of the Densu estuary and its tributary with impacts arising from nutrient and trace metal loads.

Feedback from oyster collectors in Densu indicates oyster size and harvest have currently reduced. Currently most of the brackish systems in Ghana which had thriving oyster populations are recording decreases.

The declining state of oyster stocks in the country and the concurrent contamination of the delta give concern for detailed studies to heighten the need for conservation and urgent management attention.

No known study on the Densu estuary and any other coastal wetland in Ghana have attempted to investigate the interactions between C. tulipa and the aquatic environment for use as a proxy of environmental changes.

Meanwhile, in this period of climate change and variability, estuaries are classified as highly vulnerable due to their close tie to the sea according to World Bank report. The likely impacts will stem from rise in the sea level, changing temperature and estuarine acidification.These impacts are likely to have profound effects on shell bearing organisms like the West African oyster.

According to reports of the regional climate vulnerability studies by the World Bank in 2017, Ghana will be significantly impacted by variability in climate. The impacts would arise from high annual temperatures from heat stress and precipitation with warming which will impact greatly on fisheries resources in sub-Saharan Africa.

Despite these envisaged challenges, the uniqueness of this sub-artisanal fishery lies in the fact that, although fishing is generally recognized as a male-dominated field in Ghana, the harvesting, processing and marketing of oysters is dominated by women.

These roles of women in the oyster industry have contributed significantly to the reduction of poverty and enhanced food security within deprived communities.

Sustainable management of the fishery would ensure a continual nutritional and economic support to this vulnerable group and the society as a whole.

Therefore, for a viable management and development of the oyster industry as well as its culture, there is a need to acquire scientific knowledge on the environment and aspects of the biology of the species. This work would not only be useful for the management of the fishery but would inform frontiers of knowledge on the growth and use of the oyster as an early warning signal of environmental perturbations.

The findings of this research would be useful in policy formulation by being leveraged into the Ghana national fisheries policy in its bid to enhancing and deepening marine stock recovery as planned in the 2020 Ghana budget and economic policy.

It is said that uncontrolled, unsustainable mangrove cutting without replacement depletes the habitats of oysters and thus limit their ability to absorb about 5 - 8 times atmospheric carbon while contributing to global warming.

More so, the discharge of sewage into estuaries and unsustainable farming activities, introduce high levels of nutrients and harmful chemicals like heavy metals which kill organisms, harm humans and degrades the quality of water.

Furthermore, he presence of pollutants reduces oyster production, interrupts with the filtering of pollutants from upland run-off, destabilizes bottom sediments where oysters thrive, increases shoreline erosion and reduces the protection capacity of the estuary from shoreline erosion.

It has also been discovered that heavy silted water is vulnerable to flood occurrences during heavy rains and may destroy property and lives.

According to the Senior Lecturer at the Department of Aquaculture Dr. Sandra Atindana , the main aim of this study is to assess the possible use of the West African Mangrove Oyster as a bio-indicator of aquatic environmental variability to enhance sustainable management of the fishery in the perspective of global environmental change in Ghana.

For sustainable oyster fisheries, Government, the metropolitan assembly and all relevant stakeholders should institute policies on treatment of sewage before discharge into water, sensitize populace on sustainable ways of cutting mangroves and creation of buffer zone along the wetland.

We as researchers at the Department of Fisheries and Aquatic resources management of the University for Development Studies (UDS) have been undertaking research activities and also train most of our students to undertake research that would yield good result .

We are appealing to government and corporate organisations for collaboration to undertake research activities that would help identify the economic viability of the sector.

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