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This study is based on the analysis of the stomach content and the morphology and morphometry of the three pairs of otoliths (sagitta, asteriscus and lapillus) of Lepidonotothen larseni (Lönnberg) collected at the South Shetland Islands and Antarctic Peninsula during summer, in order to find possible relationships between ontogenetic change of sagittal otolith shape and feeding ecology. Length-weight relationship resulted in a positive allometric growth, with juveniles and adults in good nutritional condition (Le Cren condition index . 1), and with a decreasing trend from noon to late evening of the stomach repletion index. The stomach content consisted of several prey, with copepods and amphipods more frequent and abundant in juveniles, whereas euphausiids were in adults. The morphometric analysis of otoliths enabled us to relate different measurements with fish size, and those contributing mostly to separate juveniles from adults were the otolith and rostrum length and their percentage (R index). Juveniles proportionally showed a shorter and wider sagitta than adults reflected in a major E index because of a rounded shape and a minor R index because of a less developed rostrum. This pattern can be tentatively linked to the different habitat of juveniles and adults of this species, being respectively pelagic and epibenthic, as also evidenced by the ontogenetic change of feeding habits.

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The North Patagonian gulfs, Argentina, support an important population of small pelagic fish (SPF) that play a key role in the marine ecosystem. Here, we assessed the seasonal SPF distribution in Nuevo gulf and San Matías Gulf concerning several environmental variables and the nautical area scattering coefficient (NASC) of Munida gregaria using generalized additive models (GAMs). We collected biological data by a 38/200 kHz echosounder along zigzag transects in all four seasons. The echoes of fish were separated from other targets using a dB difference algorithm. Then, we calculated the mean NASC of pelagic fishes every 0.5 nm. Satellite‐derived data were used to characterize the environment at study sites. GAMs were built in two stages for each gulf considering all season in a single model in one hand, and each season separately in the other one. In the first stage, we modeled the probability of presence as a function of predictors. In the second stage, we used the NASC of fish as the response for presence data only. In general terms, the probability of fish presence increases with bottom depth, and the fish density was higher in cold waters and zones with higher chlorophyll‐a concentration. The relative importance of the variables was different according to the season. The formation and rupture of the thermocline and its subsequent spatial heterogeneity observed in spring and autumn could be important drivers of SPF distribution. Squat lobsters’ distributions related positively with SPF in San Matías Gulf summer and negatively in Nuevo Gulf summer.