Ontogeny versus environmental forcing off the Southwest Atlantic Ocean: Nutritional condition of Fuegian sprat (Sprattus fuegensis) early stages

Abstract

The Fuegian sprat, Sprattus fuegensis, plays a key trophic role in the Southwest Atlantic Ocean. Growth and survival of small pelagic fishes like sprat are strongly affected by environmental variability and can determine recruitment success. We estimated the nucleic acid composition and assessed with a standardized RNA/DNA index (sRD) the nutritional condition of 273 larvae, metamorphosing, and juvenile Fuegian sprat from two environmentally distinct habitats: the waters off Tierra del Fuego (TDF) and the Marine Protected Area Namuncurá/Burdwood Bank (MPAN/BB). A similar ontogenetic pattern, in which nucleic acid concentrations decreased and sRD increased with increasing standard length (SL) among larval stages was observed in both habitats. A higher percentage of preflexion larvae were under the sRD threshold for growth and optimal growth performance, with sRD values being significantly higher in TDF. Postflexion larvae in both habitats showed maximum sRD values, suggesting that both habitats are suitable nursery grounds. In contrast, metamorphosing and juvenile sprat (only captured in TDF) had consistently low nucleic acid concentrations and sRD values despite increasing SL. Environmental forcing (temperature, salinity, and station depth) was assessed over size-corrected larval sRD. The best model included a negative response to station depth, and the effect of processes associated with this factor are discussed. Although further analyses are needed to reveal underlying dynamics determining early development, these results comprise a baseline for future monitoring approaches on Fuegian sprat life traits and factors affecting their recruitment in this region.