Laboratory and field studies of the food selectivity and growth of imported mosquitofish, Gambusia affinis in Newfoundland.

Heng, Hock Heang (1972) Laboratory and field studies of the food selectivity and growth of imported mosquitofish, Gambusia affinis in Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

A series of laboratory experiments supplemented by field observations of caged mosquitofish (Gambusia affinis Baird and Girard) were employed to investigate the food selectivity of these predators. Various food organisms of approximately similar size were utilized. These included insect larvae, fish eggs, young fish and microscopic organisms. The probable effectiveness of this viviparous poeciliid as a predator on Newfoundland freshwater fish, was evaluated on the basis of these investigations. -- Variation in the forage organisms' external appearance, body covering, behaviour and activity affected Gambusia's ability to utilize different organisms. Also, certain organisms both with soft and hard body coverings, were not taken by the fish. -- Conditioning does not lead Gambusia to select fish eggs and young fish. However, the latter were readily eaten, especially in the absence of other food. Mosquitofish selected smaller eggs and young fish when available. The maximum sizes of eggs taken by small (33 - 36 mm in length) and large (38 - 46 mm) Gambusia approximated 4.2 mm and 4.7 mm in diameter respectively. Mosquitofish of these same size-ranges took young sticklebacks up to 18 mm and 20 mm in length respectively. However, the maximum size of soft-bodied tadpoles preyed upon in this fashion proved much larger than that of sticklebacks. Tadpoles were in fact preyed upon when approximately two-thirds as long as the largest female mosquitofish. Observations of Gambusia predation upon other food organisms were made as well. Cannibalism by female Gambusia upon both adult males and young was also noted. -- The ingestion of microscopic phytoplankton seemed to be on a completely random basis. No special preference for any particular algae over others was shown by the fish. That is to say, the higher the incidence of a given food organism in the tank, the higher its incidence in mosquitofish stomach contents. -- Lowered temperatures adversely affected the growth rate of mosquitofish both in tanks and in cage under stream conditions. Growth rate of fish in aquaria was fast at temperatures of 22.8 - 25.7°C, slower at 14.5 - 17.0°C and poor at 5.5 - 6.1°C. The amount of food taken also decreased with lowered temperatures. Under stream conditions, steady losses in weight and the amount of food eaten were also detected. This is considered to have been due to rapid diurnal changes of water temperature, especially towards the lower end of the range. However, lowered temperatures did not seem to be fatal to the fish, more than 50% of them surviving for 12 weeks.

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