Food selectivity and grazing responses of the Lawnmower Blenny (Salarias fasciatus) to nutrient enrichment and ammonia exposure.

Nutrient enrichment can alter the nutritional quality of the epilithic algal matrix (EAM) and modify the feeding environment of herbivorous and detritivorous reef fishes. However, wastewater inputs can also elevate ammonia concentrations, which may disrupt fish physiological processes, yet the effects of ammonia exposure on grazing activity and feeding selectivity remain poorly understood. In our nutrient enrichment experiment, the nutritional value of the EAM increased under higher nutrient levels, with enriched EAMs containing more lipids, carbohydrates and proteins. The Lawnmower Blenny (Salarias fasciatus) grazed more on these nutrient-enriched tiles, suggesting that feeding preferences were associated with differences in EAM nutritional composition. Ammonia exposure of up to approximately 10 μM did not significantly affect grazing rates, food selectivity or diet composition over the short term. Instead, grazing rates increased over the experimental period after accounting for measured NH3 concentration, suggesting behavioural acclimation to the experimental conditions and/or the offered food. These results indicate that, under short-term and sub-lethal exposure, feeding behaviour in S. fasciatus responded strongly to differences between nutrient-enriched and low-nutrient EAMs, while no detectable effect of ammonia on grazing rate was observed. This response may help explain why small benthic detritivorous fishes can persist under moderate nutrient disturbance. Further work is needed to determine whether these responses are maintained under longer exposure periods, higher ammonia concentrations and multi-stressor conditions.