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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 1.8fold increase in weight gain was observed under the 0.90-mg l−1 UIA-N condition, compared to a 3.4fold 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 0.26mg l−1 UIA-N concentration, under an average pH of 8.0, can be considered as a safe long-term limit conditions in seawater.

Around 2008, an ecosystem shift occurred in the Gulf of Lions, highlighted by considerable changes in biomass and fish mean weight of its two main small pelagic fish stocks (European anchovy, Engraulis encrasicolus; European sardine, Sardina pilchardus). Surprisingly these changes did not appear to be mediated by a decrease in fish recruitment rates (which remained high) or by a high fishing pressure (exploitation rates being extremely low). Here, we review the current knowledge on the population's dynamics and its potential causes. We used an integrative ecosystem approach exploring alternative hypotheses, ranging from bottom-up to top-down control, not forgetting epizootic diseases. First, the study of multiple population characteristics highlighted a decrease in body condition for both species as well as an important decrease in size resulting both from a slower growth and a progressive disappearance of older sardines. Interestingly, older sardines were more affected by the decrease in condition than younger ones, another sign of an unbalanced population structure. While top-down control by bluefin tuna or dolphins, emigration and disease were mostly discarded as important drivers, bottom-up control mediated by potential changes in the plankton community appeared to play an important role via a decrease in fish energy income and hence growth, condition and size. Isotopic and stomach content analyses indicated a dietary shift pre-and post-2008 and modeled mesozooplankton abundance was directly linked to fish condition. Despite low energy reserves from 2008 onwards, sardines and anchovies maintained if not increased their reproductive investment, likely altering the lifehistory trade-off between reproduction and survival and resulting in higher natural mortality. The current worrying situation might thus have resulted from changes in plankton availability/diversity, which remains to be thoroughly investigated together with fish phenotypic plasticity.

In two experiments, a computerized on-demand feeding system coupled with a PIT tag monitoring device was used to continuously record the triggering activity by ca. 50 individual sea bass for 55 days (Exp. 1, initial average body weight and coefficient of variation, CV: 299 g, 15%) and 69 days (Exp. 2, 157 g, 13%). Each group was stocked in 1 m3 tanks and exposed to a water temperature of 21 ± 1 °C and a light regime of 16:8 LD. Only 5% of the triggering activity was not associated with simultaneous PIT tag detection. Although each individual was detected via PIT tag during the experiment, only 67% and 74% of the fish actuated the trigger at least once in Exp. 1 and Exp. 2, respectively. Moreover, only two fish in Exp. 1 and one fish in Exp. 2 accounted for 82% (43 + 39) and 77% of the total triggering activity, respectively. These three, high-triggering individuals did not exhibit a higher specific growth rate or agonistic behavior as observed by video monitoring. Indeed, zero-triggering fish had either a slightly higher SGR (Exp. 1) or a higher final body weight (Exp. 2) compared with low-and high-triggering fish.

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