Oceanographic influences on the early life stages of a mesopelagic fish across the Chilean Patagonia

Environmental conditions, such as sea surface temperatures and salinities, may exert important influences on the life history traits of marine organisms, particularly on the early stages of fishes. We studied the interaction between spring environmental conditions, egg and larval abundances, and growth rates, of pearlside Maurolicus australis along a 10° latitudinal range in western Patagonia. Using onboard field experiments, otolith-based back-calculated daily growth rates, CTD data, together with sea surface temperature (SST, 2002–2019) and chlorophyll-a (Chl-a, 2017–2018) satellite images, we studied the effect of short-term mesoscale variability on larval growth as well as their interactions with environmental conditions. SST of 2017 and 2018 were similar, with colder than average (2003–2018) springs for both years. Onboard experiments indicated that egg size directly influenced the size-at-hatch, though this was independent of lipid reserves. Larvae were categorized into Oceanic (Oc), Inner Seas (IS) and off the Southern Ice Fields (SIF). On the shelf (Oc), larvae were more abundant with faster growth rates; a second maximum density was observed off the SIF. The latter individuals had the smallest otolith-at-size and showed the slowest recent growth rates, despite spatially coinciding with relatively large Chl-a concentrations (>8 mg m⁻²). Latitudinally, the decrease in sea surface temperatures and the rise in vertical salinity gradients, reduced the recent larval growth of the pearlside. However, daily fluctuations in SST did not trigger significant variations in the back-calculated detrend growth rates. Although low temperatures in the SIF lead to slow growth rates of M. australis, compared to the northern population, the area presents some benefits regarding food availability, retention and protection from predators. Moreover, it is possible that the low temperatures might affect the energy balance of fish larval stages by intervening in the allocation of energy for tissue formation and metabolism, resulting in slow growth rates in the southern population.