'Belly sliders' caused by egg disinfectants, says study

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A new study on the effects of fish egg disinfectants on angelfish has shown that the chemicals can greatly increase the chances of fry developing swimbladder disorders - incorrect temperatures have similar affects.

Fish breeders regularly add disinfectants, including methylene blue, acriflavine, hydrogen peroxide and chloramine-t to batches of eggs - particularly those of egg-depositing species, such as cichlids - in an attempt to prevent the broods becoming infected by pathogenic fungi.

However, scientists now believe that the chemicals are responsible for a disorder in the developing fish that prevents their swimbladder inflating correctly. Incorrect temperatures are also believed to trigger the disorder.

The non-inflation of the swimbladder in fry results in fish that fishkeepers refer to as 'belly sliders' as they fail to become fully free-swimming.

Scientists from Israel, who are to publish the results of their study in the journal Aquaculture, provide evidence to suggest that raising angelfish (Pterophyllum scalare) fry at 28C resulted in significantly fewer belly sliders that when fry were reared at 26C.

The results also showed that methylene blue, acriflavine and hydrogen peroxide cause the condition.

The authors said: "Eggs hatched in the presence of 1, 2 and 5 ppm methylene blue exhibited significant increases in swim bladder non-inflation (11, 9 and 33%, respectively; none in controls). Time of exposure to methylene blue was a key factor.

"Exposure for up to 1 day post-hatch did not affect swim bladder non-inflation, but exposure from 2 days onwards significantly increased swim bladder non-inflation.

"Hydrogen peroxide at 250 ppm significantly increased swim bladder non-inflation (65% comparing to 27% in the control). Higher concentrations resulted in 100% mortality.

"Exposure to acriflavin at 2.25 ppm, but not 1.25 ppm, significantly increased swim bladder non-inflation (75 and 52% respectively; 20% in controls)."

Chloramine-t appears to be the safest egg disinfectant for use on angelfish and did not significantly affect swimbladder non-inflation:

"Chloramine-T did not significantly affect swim bladder non-inflation. Treatment with methylene blue and acriflavin, but not chloramine-T, produced significantly different frequencies of swim bladder non-inflation morphologies compared to control fish."

For more details see: C. Sanabriaa, A. Diamantb and D. Zilberga (2009) - Effects of commonly used disinfectants and temperature on swim bladder non-inflation in freshwater angelfish, Pterophyllum scalare (Lichtenstein). Aquaculture. In press.