. 1992 . The most extensive wild population and fishery for this oyster are spread over
1600 km
2
of Foveaux Strait in southern New Zealand, where oysters have been found in
y2
Ž . Ž
. densities up to 100 m
Fig. 1 Cranfield and Allen, 1977 . Over six years from 1985,
a protozoan parasite, Bonamia sp., destroyed most of the adult population in Foveaux Ž
. Strait Doonan et al., 1994; Hine and Jones, 1994 . This Bonamia species is thought to
be distinct from Bonamia ostreae, which has seriously affected the commercial produc- Ž
tion of the European flat oyster, O. edulis, in many parts of the world Elston et al., .
1986; Mialhe et al., 1988 . Bonamia species remain a significant threat to all fisheries and aquaculture of flat oysters and therefore a greater understanding of the relationship
Ž .
between the host and the parasite is needed Caceres-Martınez et al., 1995; Hine, 1996 .
´ ´
The pathogenesis of Bonamia sp. is closely related to the reproduction of O. Ž
. chilensis in Foveaux Strait Hine, 1991a, 1996 . Compared to other commercially
important oysters, such as O. edulis, the reproductive biology of the Chilean oyster is Ž
. poorly understood, especially in Foveaux Strait Jeffs and Creese, 1996 . Recent studies
have provided insights into the reproduction of O. chilensis in the warmer northern waters of New Zealand, but there are strong indications that the reproduction in these
populations is markedly different to that in colder southern areas, such as Foveaux Strait Ž
. Jeffs, 1998, 1999; Jeffs et al., 1996, 1997a,c . The recovery of stored samples of oysters
taken from Foveaux Strait in 1970–1971 provided a unique opportunity to describe the annual cycle of reproduction in this population 15 years prior to the mass mortalities
caused by Bonamia sp., which have greatly altered the population structure of these oysters. The aims of describing in detail the annual cycle of reproduction in this
population for the first time were to help elucidate the links between the reproductive biology of this oyster and Bonamia sp., and to provide an opportunity for comparisons
with descriptions of the reproductive biology of other populations of this oyster.
2. Materials and methods
The oysters for this study were collected at around monthly intervals from four sites Ž
. in Foveaux Strait from April 1970 to April 1971 Fig. 1 . The sites were located across
the Strait in the principal commercial oyster beds at depths of 23–35 m. The biological and physical features of Foveaux Strait have been well described and are characterised
Ž by eastward water flow Houtman, 1966; Nielsen, 1975; Bradford et al., 1991; Hine,
. 1996 .
Samples of oysters were collected from each site using a commercial oyster dredge. Oysters 19 mm in shell height were haphazardly sampled from the dredge hauls at
each site. Fifty oysters per sample were taken for the first 6 months, and 40 oysters per sample thereafter. The shell height of each oyster was measured to the nearest
millimeter. Oysters were carefully opened and the presence of larvae in the brood chamber noted. A 5-mm transverse section of the gonad was taken and fixed in Bouin’s
solution and then used for histological preparations on microscope slides, which were then put into storage racks. Over 20 years later, the stored slides were cleaned and some
were re-stained due to fading, and then remounted. The recovered specimens were examined under a compound microscope and analysed by a semi-quantitative technique
Table 1 Reproductive contents of the follicles of 1816 Chilean oysters, O. chilensis, sampled from Foveaux Strait,
New Zealand, from April 1970 to April 1971 Type of reproductive material
Number of Percentage of
Ž . present in the follicles
oysters total sample
Male and female 1305
71.9 Solely male
443 24.4
Solely female 60
3.3 None
8 0.4
Total 1816
100
Ž .
developed and verified for this oyster Jeffs, 1998, 1999 . This technique relies on scoring different features of the gametogenic cycle for each gonad. A visual estimate
was made of the percentage of all the male and female reproductive material over the entire gonad. The presence of ova and spermatozoa in each gonad was each scored on a
scale of 0 to 3, where 0 was the total absence of gametes from a gonad, 1 was a trace in the lumen of one or more follicles of the gonad, 2 was a small quantity in many
follicles, and 3 was abundance in most follicles. The abundance of haemocytes
Ž .
reabsorping reproductive material phagocytosis and the release of ripe gametes from Ž
. the lumen of the follicles spawning were scored in the same manner, where 0 was the
absence of gamete loss, 1 was a trace, 2 was a small quantity in many follicles, and 3 was abundance in most follicles. Thus, an individual score of 3 for gamete loss indicated
a major loss of gametes, i.e., a major spawning.
Only a small percentage of the Foveaux Strait oyster population incubate larvae, Ž
. hence only a few incubators were expected in the monthly samples Cranfield, 1979 .
Therefore, to establish the size range of oysters that brood larvae, about 1000 oysters were taken from each site on 6 December 1970, in the height of the brooding season. All
oysters were opened and the shell height of all brooding oysters was measured.
3. Results
