Search

The biology and population dynamics of Sardinella longiceps and S. fimbriata were studied around East Java, Indonesia. The total catch of Sardinella spp. in East Java was 56 000 tons in 1993, and more than 50% of this catch was landed by purse seiners. Samples of purse seine catches were taken from three landing sites around East Java, at the Madura Strait, the Bali Strait and at the Indian Ocean. Biological data were collected on a monthly basis, during a period of 2 years, from March 1990 to March 1992. Patterns in reproductive activity were determined on the basis of changes in geometric mean gonado-somatic index (GSI) of mature fish. Length-based methods were used to estimate growth and mortality. Peak catches of sardines from the Madura Strait and the Bali Strait were recorded at the beginning of the rainy season around November and December. The Indian Ocean produced peak catches during the dry season, from July through October. Considerable differences exist between monthly catches from peak seasons and monthly catches from off-seasons. Patterns in reproductive activity could be recorded for the fish from the Madura Strait and both species of Sardinella showed peak reproductive activity there around November and December. Both species were estimated to grow to lengths of around 12, 17 and 19 cm in their first, second and third year respectively. Based on the age structures in the catches from different sampling sites, in combination with the migration patterns of the fish, it was concluded that both S. longiceps and S. fimbriata stocks can be divided into northern and southern populations. The northern populations were sampled in the Madura Strait and were assumed to be from the Java Sea, Makassar Strait and/or Flores Sea. These stocks consist of three year classes and do not seem to be under excessive fishing pressure. The southern stocks, well known from the Bali Strait sardine fisheries, are concluded to be under very heavy fishing pressure. These stocks consist of only two year classes and hardly any fish reach the length of maturity. Regulations controlling fishing effort are needed urgently for these fisheries.

Pubertal female Clarias gariepinus (recipients) were exposed for 62 days to holding water from intact, completely castrated, seminal vesicles-extirpated or testes-extirpated adult male or from intact adult female conspecifics (donors). In operated male donor groups, levels of six androgens (measured by RIA) were lowest in testes-extirpaied and completely castrated males. However, even complete absence of the testes (GSI 0%) did not eliminate circulating androgens. Male holding water exerted primer pheromonal action in enhancing ovarian development of recipient females, irrespective of the male donor group. Holding water from female donors was ineffective, comparable to controls (fresh water). Thus, although the effect of holding water is linked to the male gender, the gonadal origin of ovarian growth-stimulating compounds in the holding water could not be demonstrated.

Stimulation and inhibition of gonadal development by intersexual contact was studied in pubertal African catfish, Clarias gariepinus. The effect of a possible interaction was studied by evaluation after a 98-day experimental period of gonadal development in combinations of intact and anosmic males and females. In addition, separate groups of males and females, respectively, were exposed to holding water from these combinations. A tentative model of stimulation of gonadal development by intersexual contact in pubertal fish was developed. Males stimulate ovarian development of females by both olfactory and tactile cues. In addition, males seem to enhance gonadal development of other males through olfactory stimulation via holding water. In contrast, females tended to inhibit male gonadal development, especially through tactile cues. It seems that although males are hampered by female tactile stimuli in their gonadal development and ability to stimulate male gonadal development, their ability to stimulate female gonadal development is not affected.

1. 1. Pubertal vitellogenic and previtellogenic female Clarias gariepinus were exposed to holding water from a mixed-sex population of adult conspecifics. 2. 2. Exposure to holding water caused an accelerated ovarian growth in both cases. 3. 3. Analysis of holding water by gas chromatography and mass spectrometry (GCMS) yielded the presence of several steroids and steroid glucuronides known to be produced in adult, prespawning C. gariepinus . 4. 4. The possible role of these compounds in stimulating ovarian growth is discussed in relation to earlier findings on their pheromonal properties.

Ovarian growth in pubertal African catfish was followed during a 100-day period. Different combinations of male sensory stimuli resulted in three distinct levels of ovarian development. A basic level (ovarian weight 5.3–7.5 g; GSI 1.9–2.7%) was reached in controls (no contact with males) and in females having visual and auditory contact with males. An intermediate level (ovarian weight 11 g; GSI 4%) was reached if olfactory contact with males was possible. A maximum level (ovarian weight nearly 20 g; GSI 7.3%) resulted from unlimited contact between the two sexes. The question is discussed whether the difference between maximum and intermediate levels of ovarian growth should be ascribed to a better water quality due to experimental conditions rather than to the presence of male tactile stimuli. The possible existence of male primer pheromones responsible for the observed stimulation of ovarian growth in pubertal African catfish by enhancing vitellogenesis is also discussed.