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a b s t r a c tTuna catches represent a major economic and food source in the Pacific Ocean, yet are highly variable. This variability in tuna catches remains poorly explained. The relationships between the distributions of tuna and their forage (micronekton) have been mostly derived from model estimates. Observations of micronekton and other mid-trophic level organisms, and their link to regional oceanography, however are scarce and constitute an important gap in our knowledge and understanding of the dynamics of pelagic ecosystems. To fill this gap, we conducted two multidisciplinary cruises (Nectalis1 and Nectalis2) in the New Caledonian Exclusive Economic Zone (EEZ) at the southeastern edge the Coral Sea, in 2011 to characterize the oceanography of the region during the cool (August) and the hot (December) seasons. The physical and biological environments were described by hydrology, nutrients and phytoplankton size structure and biomass. Zooplankton biomass was estimated from net sampling and acoustics and micronecton was estimated from net sampling, the SEAPODYM ecosystem model, a dedicated echosounder and non-dedicated acoustics. Results demonstrated that New Caledonia is located in an oligotrophic area characterized by low nutrient and low primary production which is dominated by a high percentage of picoplankton cyanobacteria Prochlorococcus (490%). The area exhibits a large-scale north-south temperature and salinity gradient. The northern area is influenced by the equatorial Warm Pool and the South Pacific Convergence Zone and is characterized by higher temperature, lower salinity, lower primary production and micronekton biomass. The southern area is influenced by the Tasman Sea and is characterized by cooler temperature, higher salinity, higher primary production and micronekton biomass. The dynamic oceanography and the complex topography create a myriad of mesoscale features including eddies, inducing patchy structures in the ecosystem. During the cool season, a tight coupling existed between the ocean dynamics and primary production, while there was a stronger decoupling during the hot season. There was little difference in the composition of mid-trophic level organisms (zooplankton and micronekton) between the two seasons. This may be due to different turnover times and delays in the transmission of primary production to upper trophic levels. Examination of various sampling gears for zooplankton and micronekton showed that net biomass estimates and

Spatial and temporal distribution of zooplankton off New Caledonia in the eastern Coral Sea was studied during two multidisciplinary cruises in 2011, during the cool and the hot seasons. Acoustic measurements of zooplankton were made using a shipborne acoustic Doppler current profiler (S-ADCP), a scientific echosounder and a Tracor acoustic profiling system (TAPS). Relative backscatter from ADCP was converted to biomass estimates using zooplankton weights from net-samples collected during the cruises. Zooplankton biomass was estimated using four methods: weighing, digital imaging (ZooScan), ADCP and TAPS. Significant correlations were found between the different biomass estimators and between the backscatters of the ADCP and the echosounder. There was a consistent diel pattern in ADCP derived biomass and echosounder backscatter resulting from the diel vertical migration (DVM) of zooplankton. Higher DVM amplitudes were associated with higher abundance of small zooplankton and cold waters to the south of the study area, while lower DVM amplitudes in the north were associated with warmer waters and higher abundance of large organisms. Zooplankton was largely dominated by copepods (71-73%) among which calanoids prevailed (40-42%), with Paracalanus spp. as the dominant species (16-17%). Overall, zooplankton exhibited low abundance and biomass (mean night dry biomass of 4.7 6 2.2 mg m 3 during the cool season and 2.4 6 0.4 mg m 3 during the hot season) but high richness and diversity (Shannon index $4). Substantially enhanced biomass and abundance appeared to be episodically associated with mesoscale features contributing to shape a rather patchy zooplankton distribution.