Suitability of different formalin-containing fixatives for the eggs of freshwater Asian catfish Clarias macrocephalus (Gunther)

Abstract

The suitability of different formalin-containing fixatives was tested for the eggs of the catfish (Clarias macrocephalus) by comparing the osmotic pressure of several fixatives with catfish plasma and measuring the size of the eggs at several intervals after fixation. The test solutions consisted of Gilson fluid, 0.5% to 10% buffered formalin, 0.1% to 1% unbuffered formalin with 0.7% sodium chloride (NaCl) and 0.1% to 1% unbuffered formalin with 0.9% NaCl. The osmotic pressure of 1% phosphate-buffered formalin was not significantly different from the catfish plasma, hence it was considered to be the most suitable of all the fixatives tested. Measurement of catfish eggs can be done within 0.5 to 120 hours after fixation in this solution because the rate of swelling (4% to 11% of the size of fresh samples) has been determined. This information is important in induced breeding work of catfish, especially on studies involving the use of egg size.