Impacts of intensive mariculture on coastal ecosystem and environment in China and suggested sustainable management measures
By Qisheng Tang and Jianguang Fang, Yellow Sea Fisheries Research Institute, Qingdao, 266071

Abstract

Mariculture in China has grown dramatically since the later 1970s. In 2000, the total mariculture production of China was 10.6 million MT. Among them, production of both bivalves and macroalgae, mostly cultivated in coastal zone by the suspension culture method, consisted of 82% and 11% respectively. As we know, the growth and reproduction of bivalves rely mainly on the supply of microalgae and detritus, but the growth and reproduction of macroalgae, microalgae and seaweeds depend mostly on the supply of inorganic nutrients coming from their living water body. Any variation in phytoplankton, the basic food chain element in marine ecosystems, will influence not only the growth and reproduction of bivalves directly, but will also cause changes in the whole marine ecosystem in the coastal zone by restraining the secondary production and then influencing the fisheries resources. Similarly, if the supply of inorganic nutrient is limited in the mariculture region, competition for inorganic nutrients will take place between phytoplankton and macroalgae. In addition to that described above, great attention should be paid to the study of the impacts of mariculture on the coastal zone environment. All aquaculture, however, is based on an underlying requirement for a clean environment for two very practical reasons. First, since the final marketed product is for human consumption, the product must meet high quality standards . Second, as in any husbandry operation, production is highest when environmental stresses are minimized. To some extent, then, environmental degradation due to mariculture is directly related both to human health and to the water quality of coastal zone. Therefore, some projects have been launched by the Chinese government to investigate the impact of mariculture on ecosystems and the environment. Summarizing the projects, some research results are showed as following:

1. effects of intensive macroalgae mariculture on ecosystem and environment
   Macroalgae such as kelp Laminaria japonica, porphyra spp. etc., are important economic species cultivated in the coastal zone of China. In 2000, the total macroalgae mariculture yield of China was more than 1 million MT. Studies results showed that 1MT Kelp in dry weight would absorb about 10-15kg N from seawater. Such quantities of N can produce 100-150 kg POC. If the mariculture sites are N deficient, the intensive macroalgae culture will certainly reduce the primary production. Therefore, intensive macroalgae mariculture on a large scale will influence the stabilization of coastal ecosystem by depleting the nutrients. In addition, the decay of kelp will occur during the summer season if they cannot be harvested in time. The detritus dropped from the decayed front part of kelp can make the seawater so mucous that it will be harmful to the hatcheries, or may be cause bloom of some species of harmful macroalgae in the mariculture sites.
2. Effects of intensive bivalves mariculture on ecosystems
   Study results showed that the mean feeding ration of scallop Chlamys farreri with shell height of 5-6 cm is about 11mg POC/d/ ind. Moreover, bivalves have a strong capability of filtering water at optimal temperatures. For example, scallops Chlamys farreri with shell height of 3-4cm, 4-5cm, 5-6cm can filter about 60 L, 100 L and 120 L water during a period of 24 hours respectively. Experimental results showed that one scallop Chlamys farreri (shell height: 5cm) could decrease the phytoplankton concentration from 8800cell/ml to 200 cell/ml within 1h in 3000ml water volume at temperature 15~200,. If the culture biomass of bivalves exceeds the carrying capacity of mariculture regions, intensive bivalve mariculture will decrease the quantity of phytoplankton rapidly so as to decrease the primary production. Without sufficient phytoplankton in the coastal seawater, the growth and reproduction of zooplankton and other herbivorous marine animals will be affected and then the coastal eco-system will change.
3. Effects of intensive mariculture on benthos ecosystem
   Experimental results show that scallop Chlamys farreri (shell height: 5~6cm) and oyster (shell height: 8~10cm) can produce amounts of faeces up to 50~60g and 120g in dry weight respectively each year. Some part of such faeces will be decomposed into dissolved matter and will be carried by the current away from the culture site, but most will sediment to and accumulate on the sea bed under the suspension culture facilities. This particulate organic matter is so fine that it will change the texture composition of the seabed. When the texture of seabed is changed, the population of organisms living on or in the mariculture seabed will consequently change, and then the benthos ecosystem will change going with the accumulation of the bio-sediment on the seabed year by year. Comparing the present data of benthos biomass of organisms with the historical data, we found that the biomass of seaweed and bivalves in the seabed of intensive mariculture areas has declined dramatically since the 1970's. For example, although the resources of eelgrass Zostera marina were so rich that it could be found almost everywhere along the coast zone from north to south of China before the 1970's, now it is very difficult to find. Although there are many factors causing the decline of eelgrass resources, the accumulation of bio-sediment from the intensive suspending culture may be the one of the most important factors among them.
4. effects of resuspension of bio-sediment on cultivated organisms
   Generally speaking, the longline culture density of bivalves (scallops, oysters) is about 50ind./m2, and most of the intensive mariculture areas in China have been developed for about 20 years. This means that the bio-sediment accumulated on these intensive mariculture areas is about 10 cm in the seabed so far. This bio-sediment matter can not only change the texture of the seabed, but can also be disturbed into water column, especially during the storm season. This resuspended particulate not only can cause heavy mortality by blocking the gills of bivalves, but may sometimes induce the occurrence of harmful microalgae blooms because it can increase nutrients concentration such as N, P, etc., in the intensive mariculture areas very rapidly during a very short period after storms. According to the statistical data, heavy mortality of bivalves cultivated in coastal zone has increased year by year since early 1990's in China. Though it is not known whether such heavy mortalities are caused by the turbidity of seawater, or by disease, it is recognised that the accumulation of bio-sediment on the intensive mariculture seabed is harmful to the ecosystem, the environment, and mariculture.

In order to develop the marine aquaculture resources in a sustainable way, some suggestions or recommendations are made as follows:

1. Establishing the models to predict the potentiality of the new sites for mariculture based on their mariculture carrying capacity and ecological carrying capacity.
2. Paying great attention to study the impact of mariculture on ecosystem and environment, the interaction between mariculture and environment, the relationship between the intensive mariculture in coastal zone and the variation of marine fisheries resources , etc.
   
3. Re-evaluating intensive mariculture sites in coastal zones not only based on the carrying capacity, but also according to standards of human health.
   
4. Establishing a sustainable management system that can determine and control the mariculture species, areas and scale, density, culture models in different sites based on the specific ecological and environmental conditions of different regions.