This study relied on the day-by-day analysis of bioenergetics and prey size preference inisolated cannibals of the African catfish Heterobranchus longifilis(13–57 mm standard length, 3–500 mg dry body mass, n = 153) that were offeredad libitum conspecific prey of adequate sizes in small-sized (2-L)environments under controlled conditions (12L:12D, 30 °C). In these conditions, cannibals ofincreasing body size selected preferentially prey of decreasing size relative to theirown, and increasingly closer to the optimal prey size (producing the highest grossconversion efficiency). The role of experience in cannibalism was found of secondaryimportance relative to body size, both as regards food intake and prey size selectivity.These results indicate that in environments that minimize the escape capacities of theirpotential victims, as applies to most aquacultural contexts, fish exercising cannibalismtend to forage optimally, which has rarely been evidenced for piscivorous behaviour. Thepresent study further highlights that H. longifilis possesses a very highcapacity for growth, which originates from the combination of high food intake and veryhigh conversion efficiency, and makes this species of utmost interest for aquaculturistswherever fast growth is desirable, but also extremely prone to cannibalism whereverfeeding schedules are inadequate.

Through the experiments presented here we wanted to test whether egg production of theblack-chinned tilapia Sarotherodon melanotheron heudelotii underexperimental conditions varies as a function of ambient salinity (fresh waters vs.sea waters vs. hypersaline waters, 0, 35 and 70, respectively)and whether these responses differ between fish acclimated within a few weeks from freshwater to saline and hypersaline environments (experiments E1 and E2, monitoring over 10and 18 weeks), and individuals born and raised all life long at the experimentalsalinities (E3, monitoring over 18 weeks). In total, 233 spawns were collected. In each ofthe three experiments, the reproductive investment (gram of egg per gram of female over 2weeks) did not differ between salinities of 0 and 35, whereas it was 2–3 times lower at 70than at 0–35, because of lower spawning frequency (E1-E3), smaller clutch size (E1) andlower spawn mass (E1-E3). Finally, fish acclimated to salinity from fresh water over a fewweeks and those maintained at a particular salinity all life long showed similarreproductive traits, thereby emphasizing the remarkable physiological plasticity of thisspecies.

The effect of dietary water content on Dicentrarchus labrax growthparameters and resistance to Vibrio anguillarum infection wasinvestigated using commercial pellets with identical energy contents and differentmoisture levels. The first experiment hypothesis was that moisturizing pelleted ration canhave an impact on Dicentrarchus labrax growth performance by theosmoregulation energy cost reduction. In a second time, the experiment explores the effectof water addition in pellets on the fish resistance to a disease. A specific device wasbuilt to uniformly moisturize dry pellets to different moisture levels, i.e. 8%, 20%, 40%and 60%. After an acclimation period and a 54-day rearing period, the control fish hadgrown from 72.7 ± 17.9 g to133.3 ± 29.4 g. Nosignificant differences were recorded for fish growth parameters. After the growth period,the tagged fish were mixed and challenged by bath exposure to live Vibrioanguillarum in triplicate. After 7 days, mortality was significantly lower inthe group of fish fed with pellets containing 60% water. Adding water toDicentrarchus labrax feed did not affect fish growth parameters butincrease its resistance to a Vibrio anguillarum infection. The moisturizing processcould be used to add specific compounds (such as probiotics or vitamins) to pellets justbefore fish feeding and could have a positive effect on fish rearing.

Vibrio harveyi is the main pathogen of the European abaloneHaliotis tuberculata, and recently caused important mortalities at theproduction sites of this marine gastropod in France. In the present work, the monthlyantibacterial activity of two red seaweed species from the French Atlantic coast, thenative Palmaria palmata and the introduced Grateloupiaturuturu, were investigated against the abalone pathogen Vibrio harveyistrain ORM4. Water-soluble extracts were screened using the microplate method.Grateloupia turuturu showed an antibacterial activity with a maximalgrowth inhibition in spring of around 16%. In contrast, Palmaria palmatawas inactive, as further growth of the bacteria was observed. Preliminaryone-dimensional proton nuclear magnetic-resonance (1H NMR) profiles identified thedifferences between the two water-soluble extracts.

Blue mussels (Mytilus edulis) and soft-shell clams (Myaarenaria) are both aquaculture species in east coast of Canada and US shellfishfarmers take advantage of the byssal threads production of mussels for suspension cultureand the burrowing behaviour of soft-shell clams for enhancement practices. It is importantthat these animals attach and burrow efficiently to minimize losses during rearing. Theaim of this work was to study two potential vitality indices on mussels (23.6 ± 0.1 mm) and clams (22.6 ± 0.1 mm) seeds following variousperiods of emersion: attachment strength of Mytilus edulis and burrowingability of Mya arenaria. The effect of emersion on energy content(proteins, lipids, glycogen) was also examined. We observed no significant decrease in theattachment strength of mussels after air exposure for 78 h or in the burrowing efficiencyof soft-shell clams after 54 h. Air exposure had no effect on different lipid classes,proteins, or glycogen content in either mussel or clam tissues. The stressful emersionevent induced in our study may not have been high enough to induce detectable behaviouralresponses. This can be explained by the bivalves’ ability to adapt their metabolism tominimize activity during air exposure. In doing so, they do not consume their energyreserves, which are then still available when specimens are reimmersed. Thus mussels areable to efficiently produce byssal threads and clams to burrow into sediments as soon asthey are back in the water.

Myxozoans are metazoan parasites which are highly pathogenic in commercially importantfish, particularly in aquaculture. Enteromyxosis is an economically significant diseasecaused by the myxosporean parasite Enteromyxum leei.Presently, early diagnosis of enteromyxosis in sharpsnout seabream was achieved bymonitoring fish on a time-scale scheduled basis in combination with a validated moleculardiagnostic technique. Two different successive fish batches of a commercial fish farm weremonitored starting from the first month of on-growing in sea cages until fish reachedcommercial size. Fish weight and water temperature differentially ranged between them.E. leei detection was performed both by classical microscopicobservation of fresh smears and by quantitative polymerase chain reaction (qPCR) whileprevalence, mean intensity and mortality rate were calculated. In total, 520 fish werescreened; 320 from the first batch and 200 from the second one. Parasitosis illustrated aseasonal occurrence pattern (spring and summer). When qPCR was used as a diagnosticmethod, enteromyxosis was diagnosed earlier with more increased prevalence compared to themicroscopic observation. In this case, the parasite was detected at the lowest watertemperature that has ever been recorded (14.7 °C), during the first month of on-growing in sea cages. The two-factorcombination of water temperature and fish weight proved to be the most significant factorthat could determine the emergence and development of parasitosis and smaller fish provedto be more susceptible. Mortality was influenced by both prevalence and intensity ofparasitosis. An earlier diagnosis and identification of all developmental stages of theparasite throughout its life cycle was verified with qPCR, significantly before anyoutbreak occurred. The latter could become a key diagnostic tool for the control ofenteromyxosis, allowing stakeholders and fish farmers to adopt all necessary precautionarymeasures minimizing economic loss in sharpsnout seabream intensive farming.