Abdul Halim Miah
(BSc. and MSc.)

Halim began a Masters degree with the the lab in 2001 examining morphology and RNA:DNA ratios in temperate marine fish larvae as tools to test ecological significance of condition indices.

To test the ecological significance of fish larval condition the changes in morphometric and biochemical indices of both healthy (fed) and lean (1-4 days starved) larvae were measured. Mulloway (Argyrosomus japonicus) larvae 15-20 d old were acclimated in four 100 litre tanks for 1 day prior to start. After 1 day food was not given to two tanks and the rest two tanks were supplied food for four days. Five larvae were captured from each fed and starved tanks on day 0, 1, 2, 3, and 4. Any dead larvae found in the tank were removed and total number was recorded. Biochemical index, RNA:DNA ratio; and morphometric indices, body area index (AREAINDEX) and anal body depth index (ABDINDEX) were measured to compare the condition of fed and starved larvae. RNA:DNA ratio, AREAINDEX and ABDINDEX were significantly higher in fed larvae than starved larvae after two days starvation. RNA:DNA ratio significantly decreased from starvation day 1&2 (ratio of 4.7 & 4.3) to day 3&4 (ratio of 3.7 & 2.4) and from starvation day 3 (ratio 3.7) to day 4 (ratio 2.4). Almost similar result was found for AREAINDEX and ABDINDEX. Starvation induced %mortality (31%) was higher in day 4 compared with day 3 (5.7%), day 2 (2%) and day 1 (>1%), whereas 0 mortality was found for fed larvae. The similar experiments were done with 18-23 d old snapper (Pagras auratus) larvae but samples haven’t analysed yet.

RNA:DNA ratio of individual larvae and the oxytetracycline (OTC) marks on larval otolith were used as a tool for larval survival prediction from a fish predator. Two days starved mulloway larvae (prey) and juvenile snapper (predator) were used in predation experiments to test the removal of poorer condition preys by a predator in three different sized -50, 100, and 750 litre tanks. Similar prey density (0.4 larvae per litre) and one predator were used in each tank. Either fed or starved larvae were bathed in 2 mgl-1 OTC solution for 24 h to mark their otoliths. Predation experiments were closed and predators were removed from each tank when 50% preys were disappeared. RNA:DNA ratios of remaining alive prey, and 2 and 3 days starved larvae (control preys) were measured using microplate techniques. Remaining alive preys were identified using RNA:DNA ratio of individual prey compared with the ratio of control (2 d starved for 50 and 100 tanks) and (3 d starved for 750 litre tank) larvae. Result showed that number of fed alive prey was higher in all of three sized tanks which indicated that fish predator were removed poorer condition larvae. The otoliths (both segittals) of each of remaining alive and dead preys haven’t been examined yet to find out OTC mark for classifying them as fed and starved. A series of predation experiments were also done in 100 litre tanks to know the predation rate with different prey and predator density, different sizes of preys and predators to develop a predation model. The similar predation experiments with snapper larvae (prey) and juvenile Australian bass, Macquaria novemaculeata (predator) were also conducted.

The similar condition indices will be used to determine larval condition in “recruitment hotspots” in estuarine areas for larval survival prediction.