Salinity tolerance of wild Poecilia reticulata (guppy) under laboratory conditions

Abstract

The present study was carried out to investigate the salinity tolerance of Poecilia reticulata (guppy) occurring in Sri Jayewardenepura canal system, in order to assess its suitability to control brackishwater breeding mosquito species such as Aedes sp. Guppies were exposed to different salinity levels directly and gradually under laboratory conditions in glass tanks. To test the direct effect of salinity, 9 different salinities (ranging from 0ppt as a control to35ppt) prepared in three replicates, each stocked with 10 fish were kept for a period of 12 weeks. To test the effect of gradual increase of salinity, three tanks containing freshwater were stocked with 10 fish in each and a gradual increase of salinity (by 5ppt once in 2 weeks up to 38ppt) was done. Mortality rates, growth (total length) and the breeding of fish were recorded in both experiments at regular time intervals. When the fish were exposed to different salinities directly, fish started dying at 10 ppt (10% mortality) after the fourth day of exposure and this mortality rate was observed in 10-20ppt salinity ranges within the 12 weeks (0% mortality was recorded in control tank). 50% survival was shown at 28ppt salinity level at the end of the 12 th week. Beyond 28ppt, there was 100% mortality. Fish showed an average of 5-7 mm growth in salinity levels up to 28ppt but they were not significantly different (p=0.886). Breeding was seen in salinity levels up to 20ppt producing 7-15 fry/female and the fry also could tolerate the salinity levels they were bred into with a mortality level of 20-30% within the study period. Contrast to the above results, the fish when exposed to gradual increasing of salinity levels could survive up to 38ppt with only a 20% mortality rate. They showed a length increase between 1-5mm, in all salinity levels but they were not significantly different (p=0.330). In gradually increasing salinity levels, fish were seen breeding even at 35 ppt (7-10 fry/female) with 10-20 % mortality rate. The results show that the guppy can tolerate and survive in salinity levels up to 28ppt when they are directly introduced from the wild and they can tolerate up to a level of 38ppt salinity if the salinity levels increase gradually. They can grow under saline conditions and breed successfully and the fry can survive well in the saline environment. This makes wild caught P. reticulata (guppy) a potential candidate that can be used as a bio-control agent in the control of species such as Aedes sp-mosquito that breed in brackish water environments of Sri Lanka.