Egg/larval viability in fish: do hormones play a role?

Egg/larval viability (fertilizability, hatchability and/or subsequent larval survival) is adversely affected by two conditions: overripening (ovulated eggs) and loss of buoyancy (fertilized eggs, particularly in marine fish). Evidence suggests that overripening may result in part from a decline in steroid output (particularly progesterone) from postovulatory follicles. Progesterone replacement delayed overripening.

In at least two marine species, grouper (Epinephelus sp.) and Asian seabass (Lates calcarifer), non-buoyant eggs have a lower thyroid hormone content than buoyant eggs. Treatment of non-buoyant eggs (less viable than buoyant eggs) with thyroid hormones improved viability significantly, especially when obviously degenerating eggs were removed from the non-buoyant eggs.

The role of hormones will be further examined in relation to fertilization, embryogenesis, early larval growth and development (endotrophic, first-feeding, endo-exotrophic, and post-yolk sac/premetamorphic) and metamorphosis. Although the focus will be on progesterone, thyroid hormones and cortisol, other hormones will be mentioned.

(Department of Zoology, National University of Singapore, Kent Ridge, Singapore 0511)

The use of vaccines and immunostimulants in the culture of penaeid shrimps.

The increased incidence of diseases in cultured shrimps coupled with a growing awareness of the problems of the use of antibiotics in controlling such diseases has led to the development of alternative methods of disease control. Vaccines against several strains of luminous and non-luminous bacterial pathogens were tested for their efficacy in both small scale and commercial scale culture systems of P. indicus and P. monodon larvae and postlarvae. Formalin-killed bacterins and vaccines consisting of live attenuated strains of pathogenic bacteria produced by UV light mutagenesis gave a significant (P<0.5) degree of protection against subsequent infection by virulent pathogens when compared to nonvaccinated controls in small scale culture systems. Field trials of vaccines on a commercial scale using larval and early postlarval P. monodon were carried out in Indonesia. Following vaccination, vaccinated animals and non-vaccinated controls were grown on for a period of 17 weeks. The vaccinated animals showed a significantly higher survival rate (69.25%) than the non-vaccinated controls (27.5%) and a significantly enhanced growth rate. The incorporation of immunostimulants in artificial diets also increased the survival rate of larvae and postlarvae subsequently exposed to virulent bacterial pathogens indicating that both vaccines and immunostimulants offer a degree of protection against disease. To determine the mechanism by which vaccines confer protection against bacterial diseases juvenile P. monodon and P. indicus were vaccinated. The numbers and activity of haemocytes in the haemolymph of vaccinated animals was analysed and compared to that of non-vaccinated animals. Early findings indicate that the vaccines used in this study may confer a degree of protection upon inoculated animals due to their effect on the haemocytes in a similar fashion as has been suggested for immunostimulants such as beta linked 1-3 glucans.

(School of Ocean Sciences, University of Wales Bangor, Menai Bridge, Gwynedd, UK)

Bioeconomic evaluation of different fry production systems for milkfish (Chanos chanos).

Milkfish (Chanos chanos) is one of the most important brackish water finfish species being cultured in Southeast Asia. Annual total production from the Philippines, Indonesia, and Taiwan has been exceeding 300 000 metric tonnes since 1980. Wild-caught milkfish fry has been the only source for farming until hatchery fry became available in 1987.

In Taiwan, extensive larval rearing systems for milkfish were developed to meet the demand of the industry in both quantity and price. An extensive larval rearing system requires less energy input and labor, but provides less control of the system. On the other hand, intensive larval rearing systems developed in the Philippines and Hawaii, require higher capital investment, more labor and utility costs, but provide consistent and high unit production (10 fry.l-1 water volume).

This paper compares the operational procedures as well as production costs in both systems.

(The Oceanic Institute Makapuu Point, P.O. Box 25280 Honolulu, HI 96825, USA)

Larviculture of finfish and shellfish in Taiwan.

The larviculture of finfish and shellfish in Taiwan has been very successful. Currently, over 100 species are under aquaculture, over 90% of which are already being artificially propagated.

In finfish, the spontaneous spawning of grass carp (Ctenopharyngodon idella) and silver carp (Hypophthalmichthys molitrix) have been carried out routinely since 1993. Spontaneous spawning reduces the degree of stress on spawners and increases the fertilization and hatching rates. Taiwan has been self-sufficient in seed supply of all species of Chinese carps since 1964, and could even supply the export market.

The development of freshwater fry production had been hampered by the limited fish market. A few imported exotic species, such as largemouth bass (Micropterus salmoides) and hybrid striped bass (Morone saxatilis x M. chrysops), have been successfully cultured.

Larviculture techniques for marine finfish have rapidly advanced in recent years, especially for milkfish (Chanos chanos) and grouper (Epinephelus spp.). Nutritional deficiency, stress handling, and disease infestation are impediments in the development of grouper fry production.

The annual production of milkfish larvae from artificial propagation has exceeded 300 million, which largely covers the domestic needs. Therefore fry are no longer captured from the wild.

The success of mass seed-production can be attributed to several factors, including the production of rotifers in outdoor ponds, the widespread use of microparticulate feeds, the production of seed for large water volumes in outdoor ponds, highly-specialized secondary enterprises and well experienced aquafarmers. The supply of microalgae, such as Nannochloropsis sp. and Isochrysis spp., to hatcheries in Taiwan also contributed to the success of fry production.

Larvae of the Japanese eel (Anguilla japonica) have been successfully reared through day 31, in Taiwan. Considerable efforts have, however, to be made before eel larviculture is completely established.

With regard to shellfish, larvae of the hard clam (Meretrix lusoria), the small abalone (Haliotis diversicolor) and the freshwater clam (Corbicula fluminalis) have been mass produced from hatcheries since two decades, and the supply exceeds the domestic demand.

The culture of prawns, such as Penaeus spp., Metapenaeus ensis, and Macrobrachium rosenbergii, has been quite successful. Despite this, the seed production has been plagued by diseases in the grow-out. It was found that Penaeus monodon larvae fed microparticulate feed supplemented with 0.2% beta-1,3- glucan (Schizophyllan) can activate the immune system and thus significantly enhance the survival rate and stress and disease resistance.

In this paper, the status of larviculture of finfish and shellfish in Taiwan is discussed. Related problems and strategies to counter them are presented. Although Taiwan may not be able to compete with its neighboring countries in terms of land and labor costs, it can provide the technology and the seeds, thus becoming the regional center for larviculture in the Asian-Pacific region.

(Taiwan Fisheries Research Institute, 199 Hou-Ih Road, Keelung, Taiwan 202)

Fish gamete and embryo cryobanks - strategies in research and application.

The storage of viable gametes and embryos is potentially useful for aquaculture production management. While gametes or embryos may be stored for hours or days at temperatures ranging from 0-14C, effective long-term storage involves cryopreservation (CP) and storage in liquid nitrogen (LN2).

The concept of CP of germ plasm is not a new idea, but specific considerations for fish include the following:

1. The production of fish involves simultaneous fertilization and culture of a very large number of larvae. Spermatozoa are produced in large numbers and are viable in water very briefly before fertilization.
2. Cultivated fish only release relatively large yolk-containing eggs into the water prior to fertilization. Their embryos develop freely within these eggs and are protected by impermeable membranes. Hardening of the chorion occurs shortly after extrusion of the eggs into water.

Each of these steps is problematic and has to be determined empirically for spermatozoa or embryos and varies among species. For spermatozoa, important considerations include suspension of motility prior to freezing and high viability after thawing, thus reducing the volumes stored in LN2. For embryos, the stage of development that is most cold tolerant, the rate and extent of permeation of CP agents, the toxicity of these agents and the cooling strategy to LN2 have to be determined.

Suitable and simple methodologies will lead to the formation of cryobanks for future large-scale applications.

(Israel Oceanographic and Limnological Research, National Institute of Oceanography, P.O. Box 8030, Haifa 31080, Israel)