Assessment of induced stress as schemes for selectively removing fish with opercular deformities in hatchery produced milkfish (<i>Chanos chanos</i>) seeds

Abstract

The effects of different types of induced stress were tested as schemes for the elimination of fish with opercular deformities in hatchery produced milkfish seeds. Salinity and pH were used as the main stressors for milkfish larvae of different ages. Induced salinity stress was initially tested on larvae at 15 days post-hatch (dph) by directly transferring the larvae from its rearing water salinity of 32 ppt to salinities of 10 ppt, 25 ppt, 35 ppt and 45 ppt. Almost all of the larvae subjected to salinity stress dies, hence, it was deemed necessary to examine the influence of handling stress and to determine the age of larvae that could tolerate handling. Larvae of different ages (from 10 dph to 22 dph) were thus subjected to handling stress by collecting them from the mass rearing tank and transferring them to 1000 ml beakers where survival was monitored. Results showed that milkfish larvae were able to withstand handling stress beginning 21 dph. Considering that milkfish fry (21 dph) are already known to tolerate direct transfers to various salinities, the next experiments were focused on pH stress. The suitability reduced or increased pH as stressors were initially tested by directly transferring larvae from normal seawater (pH 7.50) to acidic water (pH 6.5-7.0) and basic seawater (pH 8.0-8.5). Results revealed several advantages of reducing pH levels over increasing pH levels, which included the comparably higher larval survival, the lesser amount of chemicals required to change pH levels and the corresponding cost of these chemicals. A range finding test was then conducted to determine the optimum pH that could selectively remove fry with opercular deformities by directly transferring fry reared in normal seawater (pH 7.50) to seawater with reduced pH: Trt 1-pH 6.5; Trt 2-pH 6.6; Trt 3-pH 6.7; and Trt 4-6.8. Larval mortalities were recorded after 1 h, then 25% water change was done for all treatments which were monitored further for mortalities after 2h, 3h, 5h, and 24h. The surviving larvae from all the treatments were reared further for 18 days (until 40 dph) in order to monitor opercular deformities which can observed only when the larvae have metamorphosed into juveniles. Results showed that a sudden drop in pH of normal seawater to pH 6.5-6.7 selectively removed all the fish with opercular deformities in hatchery produced milkfish seeds; at such pH range, more than 90% survival of milkfish fry was obtained with this batch of larvae and all the surviving fish had no opercular deformities.