Reproductive biology of the Japanese butterfish, Psenopsis anomala, in the waters off southwestern Taiwan

The dramatic decline in annual yield of the Japanese butterfish, Psenopsis anomala, in southwestern Taiwan over the past decade suggests that this stock might have been overexploited. However, its fishery biology and stock status is poorly understood. This study therefore provides the first information on P. anomala reproductive biology based on 983 specimens collected by small trawlers in southwestern Taiwan waters between February 1999 and February 2000. The sex ratio, 0.44 (431/983), differed significantly from 0.5 and females predominated in fork length above 180 mm. The relationships between body weight (BW) and fork length (FL) were estimated as BW = 1.62 x 10-4 x FL2.637 (r2 = 0.74; n = 430, P < 0.01) for females and BW = 1.28 x 10-4 x FL2.671 (r2 = 0.82; n = 552, P < 0.01) for males. Oocytes were mature at 0.5 mm in diameter or larger, and transparent eggs were found at diameters of 0.85 mm. Mean fecundity was estimated at 171 900 +/- 61 700 and fecundity (F) was found to increase exponentially with gonad weight (GW) F = 5.4967GW0.511 (n = 132, r2 = 0.74). Mean batch fecundity was estimated to be 83 400 +/- 44 600, and relative fecundity from 377 to 2588 (mean 1040) per gram body weight. Logistic curves describing the relationship between proportion of maturity (Pr) at each length interval and fork length were estimated as Pr = 1/(1 + e11.4194-0.0749FL) (r2 = 0.99, n = 351) for females and Pr = 1/(1 + e11.5113-0.0732FL) (r2 = 0.99, n = 258) for males. Size at 50% maturity was estimated as 15.7 and 15.3 cm FL for females and males, respectively. A multi-spawning pattern was observed in the Japanese butterfish whereby it spawns throughout the year with a peak from February to July. A seasonal closure from April to July (spawning season for most fish species) can provide better breeding opportunities for adults and is believed to be a good fishery management measure for this species.