The increase in the global human population has an inevitable impact on the need for food as well as on the need for more-sustainable livelihood options. While greater awareness is rapidly being gained of the impact of fishing and the risks of overfishing as a source of seafood, the demand for more sustainable seafood sourcing is increasing. Concomitantly, interest and focus has greatly shifted towards aquaculture in which global production has increased by more than 500% during the past three decades (FAO 2020). In comparison, capture fisheries only increased by 14% during the same period (1990–2018) globally and are often exceeding their sustainable limits for the species in question (Zhou et al., 2015, Link & Watson, 2019).
The Indonesian archipelago has a combined coastline of over 81,000km, ensuring a large aquaculture potential which in many area’s is already being utilized. Nevertheless, only part of this potential is currently in use for aquaculture with estimates ranging from 22.5–40% of the brackish water area and between 0.01% and 2.32% of the coastal marine areas (KKP 2017, Amelia et al., 2021). Notwithstanding the underused potential, Indonesia is one of the largest exporting countries of cultivated shrimp globally (Hendriksson et al., 2019),. Even so, Indonesia is being outcompeted by other countries like Thailand, China and Vietnam because of lower business efficiency, lower market penetration and quality, in part due to sanitation issues (Widowati et al., 2018). A key problem in lower vitality and competitiveness of the industry is the intensification of the cultivation practices, which leads to poor water quality as result of accumulation of unused feed and faeces from the shrimp. This in turn results in higher vulnerability to epizootic disease outbreaks and more frequent shrimp mortality (Izzati, 2011, Jescovitch et al., 2017).
Two key shrimp species need to be mentioned: the Pacific white shrimp (Litopenaeus vannamei) which accounts for about 70% of the national shrimp production, and the Giant tiger prawn (P. monodon) which accounts for about 19% (Hargiyato and Sumiono, 2013, Anshary et al., 2017, Wiradana et al., 2020). Cultivation of both species takes place in ponds with additional feeding. Post larvae shrimp are placed in the ponds and left to grow out to marketable size. The time needed for this process differs substantially between the two species with approximately eight weeks for L. vannamei, and 16 weeks for P. monodon, respectively. Even though the latter species has a higher market value, its longer maturation time and higher vulnerability to disease makes it the less preferred in culture (Widowati et al., 2021a). To cultivate P. monodon, post larvae (PL) are released in the ponds with a maximum density up to 25ind·m− 2. In this life stage the shrimp are benthic feeders feeding on detritus and are capable of surviving salinity chances down to 2‰ (FAO, 2021).
As ponds used for intensive aquaculture typically deteriorate rapidly and become unusable (Primavera et al. 2014), the current government strategy to expand coastal aquaculture is to abandon ponds once unproductive and expand into vital mangrove areas, thereby destroying the mangroves. It is predicted by Ilman et al. (2016) that brackish-water aquaculture will continue to be the main driver of mangrove destruction in Indonesia for the next two decades. This comes at a high cost to society due to the loss of critical natural capital (Henriksson et al., 2019). Mangrove systems have been shown to represent a much higher total economic value than coastal aquaculture systems for which they are destroyed. For instance, mangrove ecosystem input to coastal fisheries may vary from ~ 3500US$ ha− 1·yr− 1 in Australia (Taylor et al. 2018), 7,002 US$ of shrimp ha− 1·yr− 1 in Gujarat, India (Das 2017), an average of 13,223 US$ ha− 1·yr− 1 in several countries considered by Ronnback (1999), and up to as high as 37,000 US$ ha− 1·yr− 1 in Mexico (Aburto-Oropeza et al., 2008). The economic value of coastal protection by mangroves can range between 1,586 US$ ha− 1·yr− 1 in Kenya (Kairo et al., 2009) and 5,812 US$ ha− 1·yr− 1 in the Philippines (Menéndez et al. 2018). These numbers should be added up to calculate the economic value of these systems. Furthermore, the Net Present Value (NPV) of mangroves has been calculated to be between 4 and 8 times that of intensive shrimp farms in India (Khor 1995) and Sri Lanka (Gunawardena & Rowan, 2005), while Total Economic Value (TEV) for Sulawesi, Indonesia, has been calculated at 36,000 US$ ha− 1·yr− 1 (Mangkai et al., 2012) and for a case in India, at 57,000 US$ ha− 1·yr− 1 (DebRoy, 2012). This not only means that there is a high socio-economic priority to protect the remaining mangroves but also a high premium on restoring the productivity of coastal ponds through remediation so that pond productivity can be restored and the drive to continue destroying mangroves can be reduced. Based on cases from around the world, Van Wesenbeeck et al. (2015) illustrate how mangrove destruction for aquaculture expansion induces erosion of tropical coastlines, and ultimately results in poverty, socio-economic upheaval and forced displacement of coastal communities due to loss of livelihoods (Abidin et al., 2013; Joseph et al., 2013).
The aim of this work was to study the potential of seaweed culture to help remediate and restore shrimp pond productivity in largely disfunctional coastal shrimp ponds and reduce the deterioration of coastal zones. Co-cultivation of plant and animal crops has been proven to enhance productivity thanks to circularity (e.g. Bashir et al., 2020, Fierro-Sañudo et al., 2020, Hargrave et al, 2022). Seaweed is not only a valuable crop in itself (Gentry et al., 2020), but also can absorb excess nutrients to potentially give better water quality, heathier shrimp and higher shrimp production (Kang et al., 2020). In this way it may be utilized to help restore ecosystem health by lowering impacts on the natural system and improving the livelihood of the coastal communities. Seaweeds are efficient at absorbing and using surplus nutrients, resulting in lower farm expenses as well as lower eutrophication for a more integrated and circular production system while also diversifying farm production (Widowati et al., 2019, Widowati et al., 2021a). As productivity of shrimp farming has been declining, more and more farmers have been transitioning to seaweed cultivation which today has become a major activity in Indonesia’s coastal areas (KKP, 2017). This transition has been enhanced by the ambition of the Indonesian government to become the world leading producer of agar-agar and carrageenan products (Saleh and Sebastian, 2020).
Both agar-agar and carrageenan, are hydrocolloids which are extensively used as binding agents in both food and non-food products. uses About 99% of the cultivated seaweed in Indonesia is already destined for this industry and makes it the key market for seaweed producers (Rimmer et al., 2021). The main seaweed species are the red algae Kappaphycus alvarenzii, Euchema denticulatum and Gracilaria spp. Species of latter genus grow in temperate to tropical marine environments and are prized because of their fast growth and food-grade quality agar. Therefore, Gracilaria culture has been pinpointed as a priority for further development (Lüning and Pang 2003, FAO 2014).
Our specific objective in this study was to compare and contrast pond yields for separate and mixed cultures of seaweed and shrimp. By measuring inputs, and yields, product and water quality parameters for, respectively, the monoculture of shrimp, monoculture of seaweed, and combined culture of seaweed and shrimp, we aimed to assess the potential for environmental remediation and evaluate the net effect of combined culture on pond system health and pond economic output.