Environmentally friendly mariculture - challenges of marine fish culture in Mainland China

Environmentally friendly mariculture - challenges of marine fish culture in Mainland China
Mariculture production in Mainland China, inclusive of fishes, crustaceans, molluscs and seaweeds, has rapidly increased over the last two decades. To date, more than 50 marine fish species from over 20 families are cultured, mainly in earthen ponds and inshore (shallow coastal water) floating cages (Hong & Zhang, 20034) and offshore (deep water) submersible cages have recently been developed (Chen Jiaxin, unpublished data).

I shall discuss several of the problems that the marine fish culture industry in Mainland China faces. From both biological and ecological perspectives, these problems cannot be ignored, and greater focus is needed on a more environmentally friendly approach to mariculture. Major issues are the practice of grow-out of wild-caught fish (therefore continued exploitation of wild populations), alien species introductions, loss of genetic diversity and pollution from mariculture operations. Since pollution aspects associated with mariculture are well-known in the region (Yang et al., 2004), I will focus only on the first three issues.

Of more than 50 marine fish species cultured in Mainland China, only about 10 have achieved hatchery-based mariculture (i.e. both broodstock and cultured fish are from captive breeding) (Hong & Zhang, 2003); for the majority, wild-caught juveniles are the main source of grow-out culturing. The activity of ‘growing-out’ wild-caught fish in captivity is not relevant for addressing overfishing because it does not necessarily reduce fishing pressure on wild stocks of these species (Sadovy, 2001). For instance, for the Hong Kong grouper (Epinephelus akaara), listed as ‘endangered’ on the IUCN Red List in 2003 and the most expensive grouper in Hong Kong’s live food fish market today, captive breeding has never been successful on a large scale since first attempted in the 1960s. The mariculture production of this species is still mainly from ‘growing-out’ wild-caught juveniles (Wang Hansheng, personal communication). Shortage of governmental financial support for biological and ecological studies, and with lack of management, its fishing condition and biological status have little chance to improve. Information on its spawning behaviour in the wild, spatial and temporal movement patterns and habitat use throughout its life cycle, population size, landing volumes and catch per unit effort are needed for carrying out management initiatives. Such information, in turn, can also provide guidelines to improve captive breeding techniques (Liu & Sadovy, 2006).

Second, there is no sufficient and effective management of alien fish species introductions in Mainland China, in general. Shandong Province, as one example, has introduced about 30 fish species (both marine and freshwater) for mariculture purposes over the last decade (Liu et al., 2003). However, most of the species fail to establish themselves in culture operations in the region, mainly because of insufficient evaluation of the species before introductions, and because there is no effective management to maintain good economic characteristics following introduction. For instance, the turbot has to be repeatedly introduced from Europe due to the rapid loss of its ‘good’ characteristics (e.g. flesh quality, high growth rate) after one or two generations of captive breeding, which result in ‘bad’ characteristics (e.g. early maturation and albinism) (Liu et al., 2003). For relatively successful species, current management is also not enough. For instance, the red drum (Sciaenops ocellatus), introduced from the United State in the mid-1990s, has had more than 10 million juveniles produced annually (Hong & Zhang, 2003), and the estimated mariculture production was about 43,500 tonnes (about 7.5% of the total marine fish culture production) in 2003 (Zhou & Wang, 2004). Considering its adaptability, the possibility of the species establishing itself in the wild with unknown consequences cannot be ignored (Sadovy, 2000). In another case, an introduced fish species, the redtail catfish (Phractocephalus hemiolioterus), was caught from the Pearl River Estuary. Using closed systems and sterile individuals for culturing introduced fish species should be considered.

Third, apparent losses of characteristics considered to be of economic value (e.g. flesh quality, high growth rate and disease assistance) and genetic diversity in maricultured fish species, are not uncommon. For instance, cultured individuals of the large yellow croaker, after several generations in captivity, mature at younger age and smaller size, and have slower growth rates and lower genetic diversity than wild-caught individuals (Liu & Sadovy, unpublished manuscript). It is also noted that the genetic diversity of its wild stocks is particularly low. There are at least three possible explanations. First, this species was heavily exploited at its spawning and over-wintering aggregations between the 1950s and 1970s with almost no fishery management. Its fishery collapsed in the 1980s. Since then its spawning aggregations have never reappeared. Second, captive breeding started, following over-exploitation, by using only small number of broodstock from the wild and then a small number of broodstock was taken from captive-bred individuals. This may reduce the genetic diversity of the species after several generations in captivity. Third, large-scale and long-term restocking programmes in the last decade in the East China Sea, the major source area for this species, by restocking captive-bred juveniles with low genetic diversity, may further contaminate the gene pools of its wild stocks. The case of the large yellow croaker sends a clear message of the importance of maintaining genetic variation, conserving biodiversity, and of timely, sufficient and effective management.

To date, offshore submersible cage culture is considered to be one of the environmentally friendly mariculture modes in Mainland China, and has been promoted by fishery departments through financial support to investment companies. The economic benefits reported relate to high survival and grow rates, and, presumably, reduced pollution risks for inshore waters. Because of the high cash input necessary for the submersible cage culture industry, most traditional, inshore, small-scale, floating cage culture farmers cannot become involved. Therefore, inshore cage culture operations, have little prospect of becoming reduced or abandoned, at least in short term, and the associated environmental problems in inshore waters will not be improved or become solved. Developing offshore cage culture should be planned together with reducing inshore culture scales to achieve the ideal plan for environmentally friendly mariculture.

taken from: http://www.hku.hk/ecology/porcupine/por34/34-misc1.htm