Elsevier

Aquatic Living Resources

Volume 16, Issue 1, 1 January 2003, Pages 10-24
Aquatic Living Resources

Original article
Modelling the effect of food depletion on scallop growth in Sungo Bay (China)

https://doi.org/10.1016/S0990-7440(03)00003-2Get rights and content

Abstract

Sungo Bay (China) has a mean depth of 10 m, a total area of 140 km2 and is occupied by several types of aquaculture, whilst opening to the ocean. The production of scallops (Chlamys farreri) cultured on long lines is estimated to exceed 50 000 tonnes (total weight) per year. Selection of sites for scallop growth and determination of suitable rearing densities have become important issues. In this study, we focused on the local scale (e.g. 1000 m) where rearing density, food concentration and hydrodynamics interact. We have developed a depletion model coupling a detailed model of C. farreri feeding and growth and a one-dimensional horizontal transport equation. The model was applied to assess the effect of some environmental parameters (e.g. food availability, temperature, hydrodynamism) and spatial variability on growth, and to assess the effect of density according to a wide range of hydrodynamical and environmental conditions. In the simulations, food concentrations always enabled a substantial weight increase with a final weight above 1.5 g dry weight. Compared to a reference situation without depletion, a density of 50 ind m–3 decreased growth between 0% and 100%, depending on current velocity when maximum current velocity was below 20 cm s–1. The mean ratio between food available inside and outside the cultivated area (depletion factor) varied with the percentage of variation in scallop growth that was due to density. Our model suggests that scallop growth was correlated with maximum current velocity for a given density and current velocity below 20 cm s–1. The model was integrated within a Geographical Information System (GIS) to assist in making decisions related to appropriate scallop densities suitable for aquaculture at different locations throughout the bay. Concepts (depletion), methods (coupling hydrodynamics and growth models), and the underlying framework (GIS) are all generic, and can be applied to different sites and ecosystems where local interactions must be taken into account.

Résumé

Modélisation de l’effet de la diminution de nourriture sur la croissance du pétoncle dans la baie de Sungo (Chine). La baie de Sungo (Chine) est une baie largement ouverte sur l’océan, qui occupe une surface de 140 km2 pour une profondeur moyenne de 10 m et dont une grande partie est consacrée à l’aquaculture. La production annuelle de pétoncles (Chlamys farreri) dépasse ainsi les 50 000 tonnes (poids total). La sélection de sites et la définition de densité d’élevage favorables à la croissance sont devenues un enjeu important. Nous avons développé un modèle mathématique prenant en compte les interactions entre densité d’élevage, concentration de nourriture et hydrodynamisme à un niveau local, défini par une distance typique de 1000 m, afin de prédire la diminution de nourriture liée à la consommation par les pétoncles (appelée par la suite « déplétion ») et son effet sur la croissance. Ce modèle s’appuie d’une part sur des équations détaillant la nutrition et la croissance du pétoncle et d’autre part sur un modèle de transport horizontal unidimensionnel. Il permet d’évaluer l’effet des conditions environnementales (nourriture, température, hydrodynamisme) et de leur variabilité spatiale sur la croissance et de tester l’influence de la densité d’élevage pour ces différentes conditions. Nous avons comparé une situation sans déplétion (où la croissance est maximale) à une situation avec une densité d’élevage de 50 ind par m3. Le modèle indique des diminutions de croissance entre 0 et 100 % en fonction de la vitesse du courant tant que la vitesse maximale reste en dessous de 20 cm s–1. Cette variation de croissance annuelle peut être mise en relation avec le rapport entre la concentration moyenne de nourriture à l’intérieur et à l’extérieur du domaine cultivé, qui est un indice de déplétion reflétant principalement l’effet de la vitesse du courant. Le modèle a été intégré à un Système d’Informations Géographiques (SIG) ce qui permet de simuler et cartographier rapidement et automatiquement la croissance annuelle et de fournir ainsi des recommandations sur la densité d’élevage appropriée. Le concept de déplétion, le couplage d’un modèle de croissance et d’un modèle hydrodynamique et l’utilisation d’un SIG sont transposables à d’autres systèmes comparables dans lesquels les interactions locales doivent être considérées.

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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