This study determined the effect of human chorionic gonadotropin (hCG) and handling stress on the spermiation and milt response of silver perch Leiopotherapon plumbeus based on the measurement of spermatocrit, sperm density, and milt production. Compared to saline-injected fish, the mean spermatocrit (or packed sperm) of hCG-treated fish was significantly lower at 18 h (47.9%) and 30 h (40.2%) post-injection while mean sperm density was significantly lower at 30 h post-injection (3.6 9 10 6 cells ll À1 ) but not at 18 h. At 18 h (1.8 ll g-BW À1 ) and 30 h (2.5 ll g-BW À1 ) post-injection, mean milt production of hCG-treated fish was significantly higher than in the saline group. Milt consistency was also thinner in the hCG-treated group. Mean sperm density of handled fish (18.0 9 10 6 cells ll À1 ) was significantly lower than control fish (23.4 9 10 6 cells ll À1 ). However, mean sperm density of handled plus saline-injected (16.2 9 10 6 cells ll À1 ) and handled plus hCG-treated fish (8.4 9 10 6 cells ll À1 ) was significantly lower than in the control goup. Having thicker milt consistency, mean spermatocrit and milt production of handled (77.5%; 1.1 ll g-BW À1 , respectively) and handled plus saline-injected fish (75.4%; 1.1 ll g-BW À1 , respectively) were not significantly different from the control fish (76.2%; 1.3 ll g-BW À1 , respectively). Handled plus hCG-treated fish had the lowest mean sperm density (8.4 9 10 6 cells ll À1 ) and spermatocrit (54.7%), but had the highest mean milt production (5.5 ll g-BW À1 ) among the treatment groups. These results demonstrate that the hCG injection effectively induces spermiation and milt expression and that handling-related stress negatively affects such responses. The spermatocrit method may be used to assess the spermiation and milt response of silver perch. U.S.

Summary Gonad development of the silver therapon Leiopotherapon plumbeus in two volcanic crater  lake  habitats (Sampaloc Lake, Taal Lake) in south Luzon, Philippines was examined during the annual reproductive cycle. The minimum body size‐at‐maturity of fish in these two lake habitats was also compared. Four gonad development stages were characterized as basis for the classification of ovarian (immature, maturing, mature, spawned) and testicular maturation (immature, maturing, mature) phases. The occurrence of all development stages in individual gonads suggest an asynchronous development whereby advanced stages are recruited continuously from a pool of younger stage germ cells to result in elevated female and male GSI throughout the annual cycle due to active gonadogenesis. Together with the increasing occurrence of advanced stage oocytes and spermatozoa from March until October, the elevated GSI of fish may indicate peak gonadal growth during the onset of the dry season (December–January) for eventual spawning from the beginning (May–June) until the end of the wet season (October–November). In both lake habitats, male fish were smaller than females but, regardless of sex, the minimum size‐at‐maturity of fish in Sampaloc Lake was significantly smaller than fish in Taal Lake. Overall, asynchronous development during oogenesis and spermatogenesis allows for year‐round reproduction of silver therapon, with elevated gonad growth in the dry season in preparation for spawning during the wet season. Compared with fish in Taal Lake, a smaller size‐at‐maturity of fish in Sampaloc Lake may be a response of the wild fishery stock to long‐term high fishing mortality and degradation of the lake habitat.