CHILLING TOLERANCE OF MUD CRAB Scylla olivacea JUVENILES

Geromiano, Merlyn*, Genodepa, Jerome
 
Institute of Aquaculture
University of the Philippines Visayas
Miag-ao Iloilo 5023, Philippines
memer852@gmail.com
 

Immersion of crabs in ice bath has been practiced to facilitate easy handling during harvest but no scientific evidence has been reported as to the extent of the low temperature that crabs can tolerate, as well as the period of immersion that will not compromise the crab's chances of survival. The tolerance of juvenile S. olivacea (27.2-44.2g) to 4˚C, 7˚C and 15˚C were thus determined based on the period of cooling that would result to mortality. Since previous reports have used different indicators of end point of dying crabs, these indicators were first tested for accuracy prior to the temperature tolerance experiment. Crabs were immersed in 7°C seawater until they were presumed dead (based on cessation of movement of claws, eyestalk, appendages, or the mandibles) and were then allowed to recover in tanks with ambient/ 29°C temperature. Results revealed that the cessation of movement of mandibles was the most accurate indicator of dead crabs (Table 1).

The cessation of movement of the other body parts were not sure indicators of endpoints of dying crabs since some of the crabs that were identified as dead were able to recover when returned to holding tanks.

The tolerance test showed that crabs immersed at 15˚C were able to tolerate this temperature and no mortality was recorded after 24h, while crabs immersed at 7˚C and 4˚C all died in less than 9 h. The tolerance of crabs immersed at 7˚C was twice longer than that of crabs immersed at 4˚C (Fig.1). The crabs immersed at 7°C started dying after 2h 19 min and reached 100% mortality after 8h 32 min while crabs immersed at 4°C started dying after 1h 39 min and had 100% mortality after 3h 6 min.

Crabs immersed in all the three temperatures showed slowing down of movement within 3 min and all the crabs also showed minimal resistance when they were caught and tied. These results suggest that crabs can be cooled in an ice bath to facilitate easier handling and 15˚C is the safest temperature.