Original articleModelling the effect of food depletion on scallop growth in Sungo Bay (China)
Introduction
The development of shellfish aquaculture raises questions regarding its sustainability defined via the carrying capacity concept, i.e. the maximum production achievable in a given ecosystem given the biological constraints and characteristics of the aquaculture activity. Assessment of the maximum yield is relevant if one considers that little was known until recently on regarding the capacity of ecosystems to support aquaculture activity apart from some empirical knowledge or successful/unsuccessful trials to adapt different species in coastal areas. Since shellfish production is an important component of the fisheries resources of coastal communities, carrying capacity assessment has become a major focus of scientific studies facilitating coastal zone management.
These topics are of particular importance in China where traditional aquaculture has been ongoing for centuries, but has been undergoing especially rapid growth in the past 10 years (Guo et al., 1999). Coastal zone management in China has become a major concern because of the following reasons: (i) the impact of human activities on environmental and water quality, and (ii) the need for optimization of aquaculture strategy (Fang et al., 1996). Within this context, a project was funded by the European Union to build tools capable of characterizing the carrying capacity and impact of shellfish and kelp aquaculture in two Chinese bays situated in Shandong province. Cooperative studies were conducted on the feeding responses and growth of cultivated species, variation in key environmental parameters in the field, and modelling hydrodynamics, filter-feeder growth and ecosystem dynamics.
When addressing carrying capacity assessment, an important first step is to describe and quantify the relationship between filter-feeders and the environment, considering ecophysiological processes such as food filtration, ingestion, assimilation and metabolic losses (Dame, 1993). Physiological processes are driven by temperature, food concentration (particulate organic matter (POM), phytoplankton) flow and total suspended matter concentration which act on the ability of the individual to ingest or to reject a fraction of the available food as pseudofaeces. Modelling these processes allows prediction of the relationship between individual scope for growth (SFG) and environmental factors. Ecophysiology models have been published recently for Mytilus edulis Scholten and Smaal, 1998, Grant and Bacher, 1998 Crassostrea virginica (Powell et al., 1992), Crassostrea gigas Raillard et al., 1993, Barillé et al., 1997, Ren and Ross, 2001, pearl oyster Pinctada margaritifera (Pouvreau et al., 2000), Tapes philippinarum (Solidoro et al., 2000), which can be used to identify food limitation. A second step is to define the geographical scale of any food limitation. Carrying capacity may be defined at the ecosystem scale when major parts of the bay are occupied by aquaculture Raillard and Menesguen, 1994, Dowd, 1997, Bacher et al., 1998, Ferreira et al., 1998). Tidal currents and the geographical position of filter-feeders may also result in a low percentage of food used by those filter-feeders (Grant, 1996), so that investigations at local scales are relevant when rearing density and/or low currents are suspected to influence growth through food depletion Incze et al., 1981, Pilditch et al., 2001, Pouvreau et al., 2000. An understanding of food limitation in cultured populations assists managers in defining the suitability of sites for aquaculture (Nath et al., 2000).
This paper is one of a series dealing with carrying capacity assessment in Sungo Bay at different spatial scales. Sungo Bay is a small bay with a mean depth of 10 m, total area of 140 km2, opening to the ocean and occupied by several types of aquaculture, e.g. kelp (Laminaria laminaria), oysters (Crassostrea gigas) and scallops in lantern nets (Chlamys farreri) (Fig. 1). It is one of the most intensively cultured bays in China. The current velocity is driven by the tide and is usually <20 cm s–1. Due to low nutrient inputs from rivers, primary production originates from the import of organic matter and nutrients from the sea, including recycling of nitrogen within the bay. The total production and standing stocks have changed over the past 20 years, including a shift from kelp to shellfish production. However, overexploitation is apparent from reduced shellfish growth and the increased incidence of disease. Scallops are the dominant cultivated filter-feeders, and production is estimated as ~50 000 tonnes (total weight) per year. Selection of sites favourable for scallop growth, and determination of suitable rearing densities have become important issues. We focus here on the local scale where rearing density, food concentration and hydrodynamics interact. The first objective was to assess individual scallop growth by combining a hydrodynamic model to predict current velocity and food delivery (Grant and Bacher, 2001) with an ecophysiological model to predict responsive adjustments in scallop feeding and growth (Fig. 2) (Hawkins et al., 2002), taking into account any food depletion. The combined “depletion model” was used to simulate individual growth at several sites where food concentration had been measured, determining the sensitivity of annual growth to scallop density. The second objective was to integrate the model within a Geographical Information System (GIS) to assist in making decisions about the appropriate densities suitable for aquaculture at different sites throughout Sungo Bay.
Section snippets
Depletion model
The depletion model is coupling food transport, food consumption by the scallop population and scallop growth at the scale of a cultivated area—e.g. within a domain of a given length (typically 1000 m). Since we restricted the computations to local scales, we considered the main direction of the current at a given site. The model is based on a one-dimensional (1D) equation comparable to Pilditch et al. (2001) and Wildish and Kristmanson (1997) when vertical mixing prevents a vertical gradient
Field survey
The temperature of Sungo Bay oscillated between almost 0 °C in February 2000 and 26 °C in August 1999 with an average of ~14 °C (Fig. 4). The temperature was generally uniform among the seven sites, but large differences appeared in May 1999 and April 2000. For these dates, temperatures at sites 2 and 5 were lower compared to other sites, probably due to the more rapid warming of water masses in shallower waters. The mean chlorophyll a concentration was equal to 1.3 μg l–1, with a maximum of
Discussion
We have developed a depletion model coupling a detailed dynamic model of C. farreri feeding and growth and a 1D transport equation. The model was first applied to assess the effect of spatial variability in environmental parameters (e.g. TPM, POM, temperature, chlorophyll a) on growth. In the second step, effects of scallop density on growth through food depletion were simulated for a density of 50 ind m–3. In all the simulations, food concentrations enabled a substantial weight increase to
Acknowledgements
This work was supported by the INCO-DC project “Carrying capacity and impact of aquaculture on the environment in Chinese bays” contract number ERBIC18CT980291, EU.
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