Study on bait contribution level of shellfish culture area 

in Yalu River Estuary

Abstract: The Yalu River Estuary of Dandong of Liaoning Province is the main breeding area for the edible saltwater shellfish Ruditapes philippinarum. The breeding area is 45 000 hectares, and the annual production reaches 240 000 tons. The production accounts for 20% of the total production in Liaoning Province, and 6% of the national total production, making a great contribution to fishery carbon sink. However, in recent years, the culturing of Ruditapes philippinarum has faced with some problems, including reduced plumpness, prolonged growth cycle, and increased occurrence of diseases and mortality. Preliminary analysis showed that deterioration of the strains and variations in ecological conditions both contributed to the issue, which could be a complex systematic process. In this paper we studied the process by analyzing the bait contribution level to the shellfish in the living space.

There is feeding selectivity on the size of microalgae particles for filterfeeding shellfish. Ruditapes philippinarum prefers large sized microalgae. Attention should thus be paid to the variation in feeding capacity of large sized microalgae in seawater. Since most filterfeeding shellfish trapped microalgae larger than 10 μm, the dominant microalgal species were divided into two groups based on size, large microalgae larger than 10 μm, and small microalgae less than 10 μm, to investigate the feeding capacity of microalgae to shellfish. The size structure of microalgae was mainly analyzed using chlorophyll a fractionation method to roughly distinguish the biomass of different sized microalgae in previous studies. Due to the high interception rate of large pore filter membrane, the contribution of small sized microalgae to the feeding capacity to shellfish was seriously underestimated, which resulted in overestimated contribution of large sized microalgae. This further resulted in overestimated carrying capacity for shellfish in the study area. With the development of highthroughput sequencing technology, the number of sequences obtained through the molecular identification of eukaryotic microalgae can greatly improve the detection rate of small sized microalgae. The highthroughput sequencing technology could thus be used for the statistical analysis of the whole size structure of microalgae, and the characterization of biomass of effective feeding microalgae.

The monthly average feeding supply distribution of the Yalu River Estuary was plotted using the spatial inverse distance weight (IDW) model of GIS science for spatial interpolation. In the model, the feeding supply in unmeasured sea area was estimated through the index of the measured value of feeding supply of evenly distributed stations in measured sea area. The bait contribution index of different stations had weighted impact on the unknown stations, that the closer the distance was, the greater the impact would be.

Samples of filtered seawater were collected once a month from March to December at 16 stations in the Yalu River Estuary in 2020. We used highthroughput sequencing to identify eukaryotic microalgae. The microalgae fractionation was used to analyze the microalgae particle size structure. The IDW model was used in the plotting of feeding supply distribution by spatial interpolation. The results showed that the biomass of large microalgae accounted for 20%, 29%, 63% and 56% in spring, summer, autumn and winter, respectively. Large microalgae dominated in autumn and winter. Small microalgae dominated in spring and summer. The biomass of small microalgae accounted for 80%, 71%, 27% and 44% in spring, summer, autumn and winter, respectively. The overall feeding capacity of microalgae in the Yalu River Estuary was at a medium level, with the highest feeding capacity found in October and the lowest in July. The annual carrying capacity of Ruditapes philippinarum ranged from 41 g · m-2 to 338 g · m-2, with an average of 195 g · m-2, including 248 g · m-2 in spring, 117 g · m-2 in summer, 262 g · m-2 in autumn and 67 g · m-2 in winter. The carrying capacity was mainly contributed by spring and autumn. In the central part, the index was positively affected by chlorophyll a, inorganic nitrogen, nitrate, and total nitrogen, and negatively affected by salinity. At the east part, the index was positively affected by chlorophyll a, inorganic nitrogen to phosphorus ratio, inorganic nitrogen, nitrate and total nitrogen, and negatively affected by salinity. In general, on the spatial scale, the bait contribution index of Yalu River Estuary was mainly positively affected by chlorophyll a, inorganic nitrogen phosphorus ratio, inorganic nitrogen, nitrate and total nitrogen, and negatively affected by salinity. In terms of time series, the feeding index of each station was mostly affected by environmental factors in April and December, which was positively affected by total nitrogen, total phosphorus, COD, inorganic phosphorus, silicate, nitrate, nitrite, inorganic nitrogen, organic nitrogen and chlorophyll a, and negatively affected by pH, dissolved oxygen and inorganic nitrogen to phosphorus ratio.

The estimation of the carrying capacity of Ruditapes philippinarum in the study area was lower than that of previous studies. This was because the parameters of microalgae biomass were optimized in the nutrient dynamic model in this work. The biomass of large sized microalgae preferred by shellfish was selected for the estimation, which made the estimated carrying capacity of shellfish lower, while more in line with the reality. In addition, reduced size of microalgae in the breeding area possibly also contributed to the reduced shellfish carrying capacity. Further studies are needed to have a better understanding of the problems faced by the culturing of Ruditapes philippinarum. It is worth noting that the biomass of Karlodinium veneficum in this study area is high. This species is microdiatom, and it is also the main contributor to feeding power, which may affect the health quality and safety of shellfish.

Keywords: microalgae; Ruditapes philippinarum； bait contribution; size fraction; high throughput sequencing; Yalu River Estuary