Analysis of the acoustic properties and vertical distribution patterns of mesoand bathypelagic fish, and their contribution to the biological pump

Abstract

This thesis presents novel results on carbon remineralization by the most abundant nonmigrator meso- and bathypelagic fishes on the earth (Cyclothone spp.), on the seasonality of migrant micronekton active flux, on the swimbladder properties of six Cyclothone species inhabiting the meso- and bathypelagic layers in the North Atlantic Ocean and Mediterranean Sea, and on the use of acoustic scattering as a proxy for mesopelagic abundance/biomass. Micronekton is mainly composed of cephalopods, crustaceans and fishes. These organisms represent an important resource for higher trophic levels and are a key component of the biological carbon pump, contributing through diel vertical migrations (DVM) in the case of migrants, and through remineralization in the case of non-migrants. The vertical migrators feed at shallow ocean layers and release carbon at depth through respiration, defecation, excretion, moulting, and mortality. Recent studies suggest that migrant micronekton transport similar amounts of carbon to migrant zooplankton. However, there is scarce information about biomass and carbon flux by non-migratory species in the mesopelagic and bathypelagic zones, and the seasonality of migrant micronekton active flux. In order to understand the contribution of carbon flux by non-migratory bristlemouth fishes (Cyclothone spp.) and partial migrator (Argyropelecus hemigymnus) we provide the first account of remineralisation by this community in the meso and bathypelagic zones from the oceanic upwelling off Northwest Africa to the south of Iceland (North Atlantic Ocean). We estimated that total carbon remineralisation in the meso and bathypelagic zone by these organisms was about 1%. We also studied the seasonal variability (June and October) of migrant biomass and respiratory carbon flux of numerically dominant pelagic crustaceans and mesopelagic fishes in the Gulf of California, a semi-enclosed sea (North East Pacific). Although this study included a fairly small fraction of marine life present in that zone, biomass and migrant fluxes values were similar or higher than in other regions. Acoustic and net sampling data are often used in combination to estimate fish and zooplankton biomass in the shelfs, but this technique is subject to several sources of errors when applied to mesopelagic species. The allocation of echoes to species for instance has often been biased by not considering Cyclothone spp. due to the use of nets targeting larger fishes. As expected, direct relationships between scattering and net sampling data were not significant. Comparisons made on depth-stratified data was only significant for the main non-migrant deep-scattering layer and Cyclothone braueri abundance as previously described in the literature. A significant correlation was also found for migrant fishes by manually selecting their contribution to the echogram based on previous knowledge. The acoustic properties of the target organisms must be well understood in order to allocate species to acoustic data and to convert acoustic density into numerical density. The gas content in swimbladdered fishes greatly determines their acoustic signature and thus it is of most importance to know the swimbladder morphology and gas properties. Cyclothone spp. vary in vertical distribution and thus in swimbladder strategies for buoyancy purposes. Our studies highlight the change in scattering properties of Cyclothone species from gas-bearing organisms (those that contain gas in their swimbladder) in the upper mesopelagic zone to the fluid-like scattering (with fat-filled swimbladders) of the deeper individuals. However, it also points out the necessity to further know about the regression process and to build more complex models that include swimbladders with both gas and lipids for intermediate lengths. Our results suggest the importance of the non-migratory bristlemouth fishes in the deep ocean according to their role in the biological carbon pump, the significance values of the seasonality active flux by migrant micronekton, and the importance of gaining knowledge on the acoustic properties of meso- and bathypelagic fishes for parametrizing acoustic backscattering models and inferring information from the combination of acoustic and net-sampling data.