Eric Gasset

Effect of chronic ammonia exposure on growth of European seabass (Dicentrarchus labrax) juveniles

Chronic effects of ammonia were studied in juvenile seabass, Dicentrarchus labrax (mean WEIGHT=11 g), exposed for 63 days to eight stable ammonia concentrations, ranging from 0.24 to 0.90 mg l−1 unionised ammonia nitrogen (UIA-N), respectively, from 6.1 to 22.3 mg l−1 total ammonia nitrogen (TA-N). Temperature (21.8 °C), pH (8.0), salinity (37.0 ppt), and oxygen concentration (over 80% saturation at the outlet) were maintained constant. Fish were fed using a self-feeder device, and they were starved during the last 8 days. Mortality of 28.9 and 42.6% occurred within the first 8 days at the two highest UIA-N concentrations, respectively, 0.90 and 0.88 mg l−1. From days 0 to 55, a 1.8- fold increase in weight gain was observed under the 0.90-mg l−1 UIA-N condition, compared to a 3.4- fold increase in the control. Weight gains were negatively correlated to ambient ammonia concentrations. Weight loss, or a transient period of growth stagnation, was observed from the onset of ammonia exposure to day 13 in seabass exposed to concentrations above 0.43 mg l−1 UIA-N. After day 13, weight gains were observed in all groups, indicating that the fish were able to adapt to increased ambient ammonia concentrations over time. By the end of the experiment, plasma ammonia levels were positively related to ambient ammonia concentrations, and oxygen consumption recorded in fasting fish was significantly dependent on ammonia concentrations. In seabass juveniles, the 0.26- mg l−1 UIA-N concentration, under an average pH of 8.0, can be considered as a safe long-term limit conditions in seawater.

Bacteria and nutrients—nitrogen and carbon—in a recirculating system for sea bass production

In a recirculating rearing system, different chemical compounds (nitrate, phosphorus and dissolved organic carbon (DOC)) are accumulated depending on the quantity of replacement water added in the system. Some of them can also be used as bacterial nutrients. The aims of the present study were (1) to focus on the different forms of dissolved carbon; and (2) to establish if there was a relationship between the dissolved forms of the nutrients (N and C) and bacterial concentrations. We observed that the nature of DOC changed as the volume of replacement water was decreased, and that humic substances (HS) were accumulated. The concentration of fixed and free bacteria was constant at 3.13×105±2.6×105 CFU g−1 of packing and 3.29×103±2.09×103 CFU ml−1 (M±SD), respectively, regardless of the quantity of replacement water added in the system. This shows that the quantity of biodegradable DOC was not the limiting factor for heterotrophic bacterial growth. The quantity of replacement water was not the most important factor to control heterotrophic bacteria population. What appears to be vital is the quantity of feces reaching the biological filter, which can be reduced by a good mechanical filtration and a good feed conversion index.

Effect of dietary lipid content on circadian rhythm of feeding activity in European sea bass.

In fish, dietary digestible energy (DE) content is a major factor controlling feed intake. It was therefore of interest to determine how circadian rhythm of feeding activity is influenced by the dietary DE levels. To that end, groups of European sea bass were fed on demand by means of self feeders, under light-dark and constant light conditions, with a fixed or an unlimited amount of feed with variable lipid contents. Daily total feed intake, but not the feeding rhythm, was adjusted in relation to the DE content of the diet regardless of the lighting conditions. We conclude that a satiation mechanism was likely responsible for the regulation of feed intake in relation to the dietary fat content but was not acting in itself on the mechanisms that drive the free-running rhythms of feeding activity. These results are giving additional evidence that a true endogenous clock is driving feeding activity rhythms in fish.

Comparative Allometric Growth of the Mimetic Ephippid Reef Fishes Chaetodipterus faber and Platax orbicularis.

Mimesis is a relatively widespread phenomenon among reef fish, but the ontogenetic processes relevant for mimetic associations in fish are still poorly understood. In the present study, the allometric growth of two allopatric leaf-mimetic species of ephippid fishes, Chaetodipterus faber from the Atlantic and Platax orbicularis from the Indo-Pacific, was analyzed using ten morphological variables. The development of fins was considered owing to the importance of these structures for mimetic behaviors during early life stages. Despite the anatomical and behavioral similarities in both juvenile and adult stages, C. faber and P. orbicularis showed distinct patterns of growth. The overall shape of C. faber transforms from a rounded-shape in mimetic juveniles to a lengthened profile in adults, while in P. orbicularis, juveniles present an oblong profile including dorsal and anal fins, with relative fin size diminishing while the overall profile grows rounder in adults. Although the two species are closely-related, the present results suggest that growth patterns in C. faber and P. orbicularis are different, and are probably independent events in ephippids that have resulted from similar selective processes.