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Spawning and Development of the New Zealand Sprat, Sprattus Antipodum (Hector)

I. Eggs from the adult females

I. Eggs from the adult females.

Eggs stripped from the sprat ovaries were spherical and between 0.90 and 1.00mm in diameter. They were almost completely filled with translucent, closely packed yolk globules, giving the whole yolk a coarsely granular or segmented appearance. No oil globule was present. Samples dissected from the ovaries consisted of two kinds of cells: small (0.1-0.3mm), irregularly shaped, translucent cells, and large (0.5-1.0mm), rounded, yellowish cells. The small cells were very numerous and formed interstitial packing between the larger cells. The large cells contained yolk, and some were translucent like those stripped from the fish at sea (Fig. 2). Evidently the largest cells were eggs about to be spawned, for their diameters were almost the same as the fertilised eggs collected from the plankton. Ovary samples of the large eggs showed a progression of egg diameter modes for sprats at slightly different stages of maturity—samples from three fish are plotted in Fig. 1 for comparison with the diameters of the planktonic eggs.

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Fig. 1: Frequency of diameters for samples of Sprattus antipodum eggs from ovaries of three specimens (solid-line polygons, 1-3), and from plankton tow (broken-line polygon, 4).

Fig. 1: Frequency of diameters for samples of Sprattus antipodum eggs from ovaries of three specimens (solid-line polygons, 1-3), and from plankton tow (broken-line polygon, 4).

Counts of large eggs indicated that between 3,000 and 4,000 were present in each pair of ovaries.

Following the July catch, sprats with well developed ovaries and testes (stages 4-5, see Baker, 1972, p.19) were taken in the Sounds again in August, September, and October of 1966; and in January, 1967, one ripe/running female sprat was caught in Pelorus Sound. This indicates that the spawning season is a lengthy one, extending over seven months. Fishes with long spawning seasons are often fractional spawners, releasing their eggs intermittently throughout the season. Many clupeids spawn in this way (see Ivanova, 1949 and Naumov, 1956) and their ovaries are characterised by the presence of very small, non-developing eggs, and eggs in several different phases of development. Measured samples from such ovaries show multimodal egg diameter frequencies.

The New Zealand sprat, however, has only two size-groups of eggs in the ovaries–usually a characteristic of total spawning over a short period. While it is possible that the small size-group of eggs may develop to maturity after the large size-group and be released later in the spawning season, it is more likely that the long spawning season is due to different individuals, or broods of individuals, becoming mature at different times. The New Zealand sprat is, therefore, probably not an extended fractional spawner (nevertheless, the fishes could spawn intermittently over a short period of perhaps several days).