Brooding pattern and larval production in wild stocks of the puelche oyster, Ostrea puelchana d'orbigny

Journal of Shellfisheries Research, Jan, 2005 by C. Castanos, M.S. Pascual, I. Agulleiro, E. Zampatti, M. Elvira

ABSTRACT Brooding pattern and fertility of the puelche oyster (Ostrea puelchana) were investigated in a native population from the San Matias Gulf (40[degrees]48'S; 65[degrees]05'W, Northern Patagonia, Argentina). Monthly samples of 100 oysters were randomly collected at the oyster ground during the period November 1999 to January 2000, and weekly in the 2000 to 2001 reproductive season. Complementary data were obtained from the literature (1976 to 1978) and samplings performed during the periods 1980 to 1984 and 1987 and 1998. The date at which oysters begin larval brooding may be placed somewhere within the period November 18 to January 11. In most seasons (77%), brooding starts within the period November 18 to December 5. The number of brooders is low at the debut (7.5% in 1999, 4% in 2000), peaking at the end of spring (20%), and gradually decreasing until reaching nearly 1% by middle February. Total number of larvae brooded on each sampling date showed a high correlation with brooding percentages ([r.sup.2] = 0.85). Mean fertility showed no correlation with the percentage of oysters brooding larvae ([r.sup.2] = 0.12). The highest value of mean fertility (2.7 million) was recorded at the beginning of the season when 4% of the oysters were brooding larvae. Mean fertility ranged from 900,000 to 2,700,000 larvae. Brood size ranges from 447,500 to 3,790,000 larvae (mean = 1,868,212; s = 813,808; n = 121). Individual fertility showed no correlation with neither size (total height) nor with internal volume of oysters within a size range of 60-115 mm. O. puelchana shows the highest fertility recorded in Ostrea species, a feature that seems consistent with its short incubation period, small egg size, long planktonic life, and small size of pediveligers.

KEY WORDS: oysters, Ostrea puelchana, brooding, larval production

INTRODUCTION

The puelche oyster, Ostrea puelchana, is a commercially valuable flat oyster species native to temperate waters of the San Matias Gulf (Northern Patagonia, Argentina). As with all other Ostrea species, O. puelchana broods the larvae and releases them as veligers, in this particular case after 6-7 days of incubation and at an initial size of 110-130 [micro]m (Pascual & Zampatti 1995).

This species has received a great amount of research effort clue to the biologic interest in its peculiar reproductive feature of "carriage" of epibiotic dwarf males (Calvo & Morriconi 1978, Fernandez Castro & Lucas 1987, Pascual et al. 1989, Pascual 1997, Pascual 2000). The commercial interest in its culture (Pascual & Bocca 1988, Pascual & Zampatti 1995) generated the oyster culture project that has been developed in Northern Patagonia since 1980.

Hatchery seed of this species was produced for the first time in France in 1988 (Pascual et al. 1991 ICES, Pascual & Zampatti 1995). Routine seed production in Argentina has been performed since 1997 in the state hatchery of Rio Negro Province.

From 1997 to 2003 research efforts were focused on reproductive studies and the application of the acquired knowledge on the improvement of broodstock conditioning to increase larval production.

As is the case with other flat oyster species (Perez Camacho 1987), hatchery seed production of the Puelche oyster has been constrained by the relatively low amount of larvae released by each individual oyster (fertility). Thus a large number of animals for broodstock conditioning in the hatchery is required. In consequence a detailed knowledge of brooding behavior in natural conditions is necessary to design a correct management plan for seed production.

In this study we present results gathered for several years, particularly in 2000 and 2001, regarding the annual pattern of brooding and larval production in Puelche oysters from one of the main natural oyster grounds of Northern Patagonia.

MATERIAL & METHODS

Condition Index of the Parental Stock

Monthly samples of 40 oysters of sizes ranging from 65-95 mm (total height) were randomly collected by scuba diving at the oyster ground of Las Grutas (40[degrees]48'S; 65[degrees]05'W; Fig. 1) during the period June 1997 to September 1998. Seawater temperature was recorded on each collection date.

[FIGURE 1 OMITTED]

Total height and length was recorded for each oyster. Internal volume was estimated in the following way: each live oyster was hang tied by a thread and sunk into a beaker filled with seawater and placed on a balance. Total live weight and shells weight were registered by this method, and internal volume (IV) was calculated as the difference between both weights. Condition index was calculated as: CI = IV/DMW x 100. Dry meat weight (DMW) was recorded a posteriori.

Brooding Pattern

A hundred oysters of size ranging from 60-100 mm (total height) were haphazardly collected on each sampling date from the population of Las Grutas (Fig. 1). Sampling was performed monthly during the 1999 to 2000 season (November, December, and January). In the 2000 to 2001 season, sampling frequency was increased and the collections were performed weekly (November to February) to assure an accurate coverage of brooding dynamics (brooding period lasts 5-7 days in this species, Pascual & Zampatti 1995).