Reproductive biology of the blue sprat Spratelloides gracilis in the waters around Penghu, central Taiwan Strait

Abstract

The reproductive biology of the blue sprat, Spratelloides gracilis, is described based on 7737 specimens collected from Mar. 2001 to Feb. 2003 in the waters off Penghu (the Pescadores), off the southwestern coast of Taiwan in the central Taiwan Strait. The spawning season was estimated to be from Feb. to Sept. based on progressive changes in the size of the ovaries, histological examination of the ovaries, oocyte diameter measurements, and gonadosomatic index calculations. The body weight (BW)-fork length (FL) relationship was estimated to be BW = 6.06 x 10^(-6) FL^(3.087) for both sexes combined (n = 4042, p ＜ 0.05). Eight stages of oocyte development were determined based on the histological examination. Ovarian development could be divided into immature, maturing, mature, and spent stages. Mean fecundity of this species was 2606 ± 1678 eggs, and mean batch fecundity was 1204 ± 714 eggs. The sex ratio (0.45) significantly differed from 0.5 (p ＜ 0.05). Relationships of fecundity (F) and batch fecundity (BF) with fork length (FL) were estimated to be F = 0.0002 FL^(3.841) (r^2 = 0.58, n = 62, p ＜ 0.05) and BF = 0.001 FL3.806 (r^2 = 0.82, n = 60, p ＜ 0.05), respectively. Size at 1st maturity for females estimated from the logistic model was 54 mm FL, which corresponded to an age of 120 d old.