Effects of parasitism on the reproduction of common snook

The effect of parasitism on the individual, and on a population, is one of the least understood and poorly studied areas of fish ecology. Parasites compete for maternal energetic reserves required for the production of viable eggs and offspring; thus parasites can directly influence population dynamics by lowering the number of offspring that survive to produce. The goal of this work was to explore the effect of parasitism on the reproductive potential of fish. Traditional measures of somatic energy reserves and body condition were examined along with newer measures of fatty acids present in eggs to approximate reproductive potential. Eighty female common snook, Centropomus undecimalis, were collected during spawning season (mid April to mid October) from four spawning aggregations along the southeastern coast of Florida and examined for a suite of biological, reproductive, and parasite infection measures. General linear models were used to model somatic indices, body condition, fatty acid composition and the ratios of fatty acids in eggs as a function of parasite infection parameters, host age, capture location, capture month and year. All fish were included in the somatic indices and body condition analysis while a subset of 40 fish were used in the analysis on fatty acid composition and the ratios of fatty acids in eggs. Ninety five percent of the study population was infected with 8 genera of live parasites from 4 phyla including Acanthocephala, Arthropoda, Platyhelminthes, and Nematoda. Dead parasites were present in 55% of the population. Parasites had significant positive and negative effects on host body condition, liver size, percentage and weight of palmitic acid and eicosapentaenoic acid (EPA), and the ratios of n-3 to n-6 polyunsaturated fatty acids, docosahexaenoic acid (DHA) to EPA, and arachidonic acid (ARA) to EPA. Parasitism may lead to an increase in liver size and certain fatty acids due to increased feeding and a concomitant exposure to infection with parasites. Body condition and liver size decreased as the number of dead parasites increased, suggesting that the successful initiation of immune defenses is a costly process. Larval nematodes had the most prevalent effect on host reproductive potential, decreasing the weight and percent composition of EPA and therefore increasing the ratios of DHA to EPA and ARA to EPA. Eicosapentaenoic acid is crucial for larval development and modulation of the immune response; thus depletion in EPA suggests a negative effect on the survival of larvae. Overall, fish showed evidence of depleted energetic reserves and specific fatty acids as a response to infection with levels of parasitism found in the wild, which may decrease reproductive success. Future studies designed to further our understanding of parasitism on reproductive potential in fishes should pair captive testing with wild populations of fish and possibly focus on the mechanisms for energy depletion by parasites.