Dietary preferences of selected shallow-water herbivores

Abstract

Sarpa salpa and Sparisoma cretense are the two main herbivorous fish native to the Mediterranean Sea. The present study aimed to investigate the dietary preferences of these two species, determine any temporal trends in their feeding behaviour, and assess whether a dietary niche overlap exists between them. This was done through on field observations carried out at Anchor Bay, Għar Lapsi, Jerma, Xgħajra and Żonqor over the summer of 2019, and stomach content analysis procedures on specimens collected from these sites. During the on field procedures, cover estimates of phytobenthic species, fish density counts and grazing rate per dietary species at different times of day (morning, afternoon and evening) were done at each site for each fish species. From the data recorded, Ivlev’s Electivity Index were calculated for each phytobenthic species. Stomach content analysis was carried out on specimens of S. salpa (n = 48) and S. cretense (n = 4). Descriptive parameters and the percentage gravimetric composition of each phytobenthic species in the stomach were measured for each specimen. From the data recorded, the foraging ratios for each phytobenthic species were determined. Analysis of data shows that these two species have comparable diets. This is largely supported by statistical tests and Schoener’s Index results. However, grazing rates on different dietary species were statistically different, corroborating the fact that they are functionally different herbivores. For example, turf algae Cladophora sp. were mainly consumed by S. salpa. Coralline alga Jania rubens was consumed ubiquitously by both species while encrusting algae Lithophyllum incrustans and Hildenbrandia rubra were never fed upon by either species. A degree of diet-niche partitioning was noted. S. salpa selected Halopteris scoparia and Dictyota dichotoma when in sites where S. cretense populations were negligible or not recorded, but consumed these algae at a much lower rate in sites where both herbivores were abundant. The statistical tests carried out on the grazing rates show some support for the ‘Optimal Foraging Theory’ and the related ‘Diel Feeding Hypothesis’. Nevertheless, more research is required to assertively state if the nutritional content of phytobenthic species affects herbivorous fish feeding behaviour. Stomach content analysis for S. salpa specimens corroborated on field observations. Statistically no difference in dietary preferences was noted when comparing foraging ratios by fish maturity stage. However, males appear to prefer D. polypodioides / D. dichotoma and Posidonia oceanica.