Southern Ocean mesopelagic fish : scales, drivers and the effects of environmental variability

In the Southern Ocean, the mesopelagic zone 200 – 1000 m below sea level holds vast resources of fish, yet they remain one of the least investigated components of the Antarctic ecosystem. Sampling challenges have led to considerable uncertainty regarding mesopelagic fish biomass, limiting our ability to monitor populations or quantify their contribution to ecosystem function. Active acoustic methods, where pulses of sound are transmitted into the water column and the “backscattered” signal from organisms are detected by a transducer, enable us to sample the water column at greater spatial and temporal scales than conventional net sampling. However, to reliably interpret acoustic data we require information on the species present and their acoustic properties, which has been lacking for the Southern Ocean region. In this thesis I document the use of X-ray computed tomography scans to clarify swimbladder morphology of members of the mesopelagic fish community. I report a switch from fish possessing gas-filled swimbladders that contribute strongly to backscatter at low latitudes, to fish lacking gas-filled swimbladders at high latitudes. This contributes to the ubiquitous southward decline in acoustic backscatter that contrasts with latitudinal biomass patterns recorded from net samples. Patterns in acoustic data indicate that diel vertical migration of fish may be suppressed at high latitudes, which has implications for biogeochemical cycling. Acoustic backscatter was strongly related to sea surface temperature, daylight hours and sea ice extent. These relationships, coupled with taxon specific calculations of acoustic Target Strength, yielded Southern Ocean mesopelagic fish biomass estimates considerably greater than previous net based estimates. Overall, these findings indicate that knowledge of the acoustic properties of mesopelagic species, alongside core information on their relative abundance in the environment, can enable active acoustic data to become a powerful tool for researching, monitoring and managing the Southern Ocean ecosystem.