Abstract
Researchers have documented that fishes breed in many different places and times, however, fishes are known to be particular when choosing a spawning site and no theory has been developed to predict when and where a particular species should spawn. Egg size has been proposed to be a key component in this decision. Temperature is also likely important because it affects growth rates, the efficiency of utilizing food energy, and the amount of dissolved oxygen in water. The purpose of my research is to determine how and why breeding fish chose a temperature at which to spawn. The Cichlidae is the third largest family of teleost fishes, and all members of the cichlid family exhibit some form of parental care. Substrate guarding is one of the most common forms of parental care, and because the eggs are adhesive, once they are laid, they will be committed to developing in that particular physical environment, i.e., water temperature, water velocity, etc; therefore spawning site choice is extremely important to the survival of the eggs and fry. Because larger eggs need more oxygen than smaller eggs, due to a higher surface to volume ratio, I hypothesized that when given a choice, a parent fish will chose its spawning temperature based on the oxygen needs of its size of egg, i.e., larger eggs will be laid in cooler water than smaller eggs. Egg size is typically fixed within a species, therefore all individuals within a species should make the same consistent spawning temperature choice. Prior to testing the main hypothesis, I tested an important assumption, namely that because of the importance of spawning temperature, choice of spawning temperature will not be influenced by prior exposure to different temperatures. In other words, fish will not be acclimated to spawn at a temperature different then their preferred spawning temperature, simply because they previously experienced warmer or cooler water. To test these hypotheses, I conducted two experiments using a temperature choice apparatus which allowed a pair of spawning fish to have four distinct temperatures at a time to choose from. I used substrate spawning cichlids because the location of the eggs, once laid, indicated the exact temperature at which the spawning took place. Temperature data loggers were placed in each compartment to record the temperature that was chosen. In Experiment 1, I conducted a set of trials to eliminate prior habitat temperature experience as a factor biasing future spawning temperature preference. To do this, I used ten pairs of convict cichlids subjected to two different temperature treatments. Five of the pairs used were housed in a tank set at 22ºC, and five of the pairs used were housed in a tank set at 32ºC, for two weeks prior to being put into the spawning temperature choice apparatus to allow for acclimation to that particular temperature. I predicted that these treatments would not affect future spawning temperature preference. The data was analyzed using a t-test, and no significant difference was found between the preferred spawning temperatures of the two treatments. This means that prior temperature exposure does not effect future spawning temperature preferences. Experiment 2 examined the effect of egg size on spawning temperature preference and consisted of collecting spawnings from five other substrate spawning species, namely Neetroplus nematopus, Hemichromis lifalili, Tilapia snyderae, Steatocranus casuarius, and Hypsophrys nicaraguense. The six species used (including A. nigrofasciatus) encompass a large range of the egg sizes found in the cichlid family. The results revealed that even with a spread of possible spawning temperature as great as 12.8°C, no species showed more than a 2.6°C variation from their particular mean preferred spawning temperature. This shows that each species tested had a consistent preferred spawning temperature. Comparing the average spawning temperature chosen by each species against one another, only T. snyderae against A. nigrofasciatus, and T. snyderae against H. lifalili showed a significant difference. An interspecies comparison of egg size and average preferred spawning temperature within a species was also conducted, and a linear regression showed no relationship between chosen spawning temperature and egg size. This demonstrates that the cichlids tested do not base their spawning temperature choices on egg size.