Summer Mortality of Crassostrea gigas in relation to environmental rearing conditions

Journal of Shellfisheries Research, Jan, 2005 by Patrick Soletchnik, Christophe Lambert, Katherine Costil

ABSTRACT The purpose of this study is to investigate summer mortality of the cupped oyster, Crassostrea gigas, in relation to culture practices in the traditional oyster production region of Marennes-Oleron (France). Four oyster rearing conditions, varying culture location ("on-" or "off-bottom"), and site depth (65% to 80% daily immersion termed "deep" or 45% to 65%, termed "shallow") were studied to compare biologic performance and maturation status of oysters, in relation to sediment and water column parameters. The most severe mortality occurred in June to July in "on-bottom" reared oysters (25%), as compared with 10% mortality in "off-bottom" cultured oysters. Oysters (shell and meat) grew significantly better when reared "off-bottom" than "on-bottom." Reproductive effort was almost double in "off-bottom" reared oysters, compared with those "on-bottom"; thus, reproduction cannot be directly related to mortality in this summer mortality event. Low glycogen content recorded for both "on" and "off-bottom" reared oysters in summer, confirmed the probable lack of food and/or the overstocking in the Marennes-Oleron Bay, but did not discriminated among culture conditions. Whatever the immersion depth ("deep" or "shallow" conditions), "on-bottom" cultured oysters were adversely affected in growth, reproductive effort, and survival suggesting a direct effect of the mud (the so called "mud effect") on the biologic performance of oysters cultured on the bottom. Data from monitoring of sediment redox potential, organic content, and ammonium release did not support hypotheses that these parameters were alone responsible for the observed differences in mortality events.

KEY WORDS: Crassostrea gigas, Marennes-Oleron Bay, marine ecosystem, sediment, hydrology, summer mortality

INTRODUCTION

The Marennes-Oleron Bay, Europe's largest production area for cupped oysters (Crassostrea gigas, Thunberg), is situated on the French Atlantic coast, between La Rochelle and Marennes (1[degrees]10'W, 45[degrees]48'N; Fig. 1). It is bordered in the north by the Charente estuary, in the south by that of the Seudre and in the west by Oleron Island. Of the two rivers flowing into the bay, the Charente is the largest, with an output of 10-400 [m.sup.3] [s.sup.-1], whereas the Seudre has flow rates of only 1-40 [m.sup.3] [s.sup.-1] (Vouve 2000).

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In the Marennes-Oleron Bay, C. gigas standing stock reaches an average of 110,000 tons over an area of 2,600 hectares. The annual production, 30,000-50,000 tons, represents 25% to 40% of the total production in France, according to the specific year. The area is active in the collection of juveniles, partly reared on site and partly sold to other production areas. During the last 2 decades (1980-2000), the bay has experienced a significant increase in biomass of suspension-feeding species. From a previous stock of filter-feeders around 10,000 tons in 1971, it was estimated at more than 86,000 tons in 1984 to 1985. Finally during a third period (1993 to 1995) the biomass has been estimated to be more than 124,000 tons (Heral 1989, Sauriau 1992). Moreover, today, carrying capacity of the bay and economic productivity can be affected by anthropogenic pressure and global change (Soletchnik et al. 1998, Soletchnik 2001).

Since the beginning of the 1960s, abnormal episodes of C. gigas mortality have increased in the world, as reported in Japan by Imai et al. (1965), Mori et al. (1965) and in the United States by Beattie et al. (1980), Perdue (1983), Farley (1992), and Cheney et al. (2000 and 2001). In Europe, where the Pacific cupped oyster C. gigas was introduced from Japan around 1970 to replace C. angulata, summer mortality events were also reported. Significant mortality events (>30%) occurred in 1976 to 1977 (Parache 1989) and then in the 1980s on various oyster-rearing sites: (1) Arcachon Bay, south-west in 1982 to 1983 (Maurer et al. 1986); (2) Marennes-Oleron Sound in 1988 and 1993 (Lodato 1997); and (3) Brittany (west) and Normandy (north-west) in 1994 to 1995 (Goulletquer et al. 1998, Fleury et al. 2001).

Moreover, in the Marennes-Oleron Bay, oysters reared "on-bottom" using traditional culture methods are more severely affected by mortality than "off-bottom" cultures (Soletchnik et al. 1999). According to Cheney et al. (2000), mortalities occurring in C. gigas are the result of multiple factors or stressors including elevated temperature, low dissolved oxygen, xenobiotic stress, and physiologic stress associated with reproduction. Summer mortalities of oysters coincide with the gonad maturation period (Mori 1979, Perdue et al. 1981, Maurer et al. 1986, Soletchnik et al. 1997). For example, Japanese authors (Mori et al. 1965, Tamate et al. 1965) compared Onagawa Bay, where summer mortality was negligible, to Matsushima Bay, where summer mortality events were important, and they concluded that an over-maturation of oocytes in the eutrophic Matsushima Bay leads to "physiological disorder and metabolic disturbance" of oysters.

The aim of this study is to investigate if rearing conditions, including both culture techniques ("on" and "off-bottom" condition) and two depths, might affect the rearing performance of C. gigas. These two factors ("rearing method" and "depth") were crossed to create four rearing conditions. Biologic parameters, including mortality, shell growth, meat growth, and biochemical contents were monitored and analyzed for possible relationships with environmental conditions (both sediment and water column) to explore which parameters may contribute to differences in oysters reared under the varied conditions during the spring and summer period. Unlike classic studies focusing on single parameters, our combining of biologic performance with hydrologic and sedimentary parameters permitted us to test whether proximity to the sediment and depth of rearing have a direct (e.g., due to physicochemical quality of sediment) or indirect (e.g., by access to trophic resources) effect on the survival and biology of oysters and therefore on shellfish productivity.