Restoration potential of Asian oysters on heavily developed coastlines

Abstract

Reef-building oysters historically provided the main structural and ecological component of temperate and subtropical coastal waters globally. While the loss of oyster reefs is documented in most regions globally, assessments of the status of Asian oyster reefs are limited. The feasibility of restoration within the regional biological and societal contexts needs to be assessed before implementation. Here, we quantified the current distribution of natural oyster reefs (Crassostrea spp.) in the shallow coastal waters of Hong Kong, assessed the biological feasibility of reestablishing reefs using natural recruitment, and examined their current and potential water filtration capacity as a key ecosystem service provided by restoration. We found natural low-relief oyster beds in the low intertidal coastal areas at a subset of the locations surveyed. These areas are, however, degraded and have sparse densities of oysters generally <2 years old. Recruitment was high in some areas (>500,000 indiv./m 2) and while survival to maturity varied across sites there was adequate larval supply and survival for restoration. Filtration rates for a 1-year-old recruit (90 mm length, approximately 30 L/hour per individual) at summer temperatures (30 C) meant that even the small remnant populations are able to provide some filtration services (up to 31.7 ML/hour). High natural recruitment means that oyster reef restoration can be achieved with the addition of hard substrate for recruitment, increased protection of restoration sites, and would not only increase the ecological value of reefs regionally but also serve as a model for future restoration in Asia.

Some Like it Hot: The Ecology, Ecosystem Benefits and Restoration Potential of Oyster Reefs in Tropical Waters

“…There is very little information documenting the decline of tropical oyster reefs outside South America. Some local declines have been documented for pearl oyster reefs in Sri Lanka and Qatar (Smyth et al, 2016;Ostroff, 2020), mixed Magallana belcheri and Magallana bilineata reefs in India (Alagarswami and Narasimham, 1973;Nagi et al, 2011), and Magallana bilineata and Magallana hongkongensis reefs in Hong Kong (Lau et al, 2020). Talonostrea salpinx reefs in Kuwait have also declined, with historical photographs showing how extensive and well developed the reefs once were (Al-Kandari et al, 2021).…”

“…We chose the Pearl River Delta, China (which includes Hong Kong and Macao) for our first case study. This region was selected due to the presence of existing, degraded oyster reefs (Beck et al, 2011;Lau et al, 2020) and the world's longest recorded history of oyster cultivation dating back 1000 years (Cheung, 2019). Oyster reef restoration projects in this region are also underway, however monitoring and results have not yet been reported on.…”

“…By filtering large volumes of water and particulates, oysters are capable of assimilating and bioaccumulating nutrient loads in the water (Kellogg et al, 2013) and, in developed areas, pollutants, microplastics and other contaminants can be traced by using tropical oysters as bio-indicators (Patterson et al, 2019;Patra et al, 2021). The tropical species Magallana hongkongensis and M. bilineata have some of the highest known filtration rates in the world and can filter 30 L/hour when they are only 90 mm in size (Lau et al, 2020). Filtration rates of other tropical species is missing from literature, but given that M. hongkongensis can reach adult sizes of 160 mm under current high harvest pressures (Lam and Morton, 2003), though individuals >300 mm have been recorded, and that tropical species experience warm conditions year-round thus avoiding slower feeding rates in cool temperate conditions (Gray and Langdon, 2018;Eymann et al, 2020), it is likely that mature tropical oysters could have higher rates of filtration than temperate species.…”

“…Restoration of tropical marine systems such as coral reefs (Meesters et al, 2015), seagrass (Paling et al, 2009) and mangroves (Hashim et al, 2010;Su et al, 2021) is occurring, but to date there has been no documentation of tropical oyster reef restoration in the scientific literature. Oyster reefs occur throughout tropical inshore coastal and estuarine systems (Naik and Gowda, 2013;Lau et al, 2020) but knowledge of these assemblages, including the extent of occurrence, the identity of the dominant ecosystem engineers, their biology and ecology, and the benefits they provide, remains poorly understood. Oysters exhibit high levels of morphological plasticity and molecular techniques are required for accurate identification (Sekino and Yamashita, 2016).…”

Population Genomics, Transcriptional Response to Heat Shock, and Gut Microbiota of the Hong Kong Oyster Magallana hongkongensis

“…It was previously believed that marine populations were highly interconnected due to high potential for dispersal [17,18]. More recently, studies clarified the local adaptation mechanism of several oysters, with underlying mechanisms including the differences on genotype heterogeneity and nucleotide polymorphism [19][20][21]. Magallana oysters have high fecundity and prolonged pelagic larval durations (two to three weeks) [22], likely providing high gene-flow among populations and reducing the likelihood of local adaptation [23].…”