Distribution, population structure and trophodynamics of Southern Ocean Gymnoscopelus (Myctophidae) in the Scotia Sea

Gymnoscopelus braueri, Gymnoscopelus fraseri and Gymnoscopelus nicholsi are common in the Southern Ocean mesopelagic fish community. However, their ecology is poorly understood in the region. This study investigated spatial and temporal patterns in their abundance, population structure and diets at different times of year within the Scotia Sea to ascertain their functional role in the pelagic food web. G. braueri was the most abundant species (0.07-0.17 ind. 1,000 m(-3)) throughout the Scotia Sea. G. fraseri was absent from the sea-ice sectors and occurred mostly around the Antarctic Polar Front (APF), comprising densities of 0.01-0.04 ind. 1,000 m(-3). G. nicholsi occurred in low abundance (< 0.01 ind. 1,000 m(-3)) throughout the region. G. braueri and G. fraseri had a lifespan of 4 and 3 years, respectively, but spatial variation in their population structures was evident and recruitment appeared to occur only around the APF. G. nicholsi had a lifespan of > 4 years. There was evidence of seasonal variation in depth distribution, size-related sexual dimorphism and vertical segregation in size classes for each species. Overall, diets were dominated by copepods (Metridia spp., Rhincalanus gigas, Pleuromamma robusta) and euphausiids (Thysanoessa spp. and Euphausia superba), although G. fraseri did not predate E. superba. Regional, seasonal and ontogenetic patterns in diet were evident for all species. This study provides new insight into the ecology of these Gymnoscopelus species in the Scotia Sea. Such details contribute towards resolving how pelagic food webs are structured in the Southern Ocean and their sensitivity to ongoing environmental change.