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It has been proposed that vertical movements of planktonic organisms coupled to a horizontally or vertically stratified circulation pattern could describe a retention mechanism, with ecological advantages such as favoring recruitment success. The Patagonian stock of the Argentine hake Merluccius hubbsi spawns mainly from January to February in relation to a highly productive tidal frontal system. Although retention of fish eggs and larvae has been previously hypothesized for this system, it has not been fully proven and its mechanisms have not yet been proposed for hake early stages. To better understand the physical and biological processes involved in the survival and distribution of hake larvae, we focused on transport features and associated larval behavior governing the retention of larvae in the spawning area and their subsequent distribution to settlement and nursery grounds. To test this hypothesis at appropriate spatial and temporal scales, we analyzed acoustic records to describe vertical and horizontal distribution patterns of hake larvae, discrete plankton samples to confirm the identity of acoustic targets, and outputs from a numerical circulation model to estimate current patterns in the region during the hake spawning months. Coinciding with the development of a functional swimbladder, hake larvae of 4 mm and larger showed a strong vertical distribution pattern associated with a sound scattering layer migrating from the thermocline during the night to near-bottom depths during the day. This diel vertical migration pattern was associated with the circulation structure, equivalent to a two-layer flow, indicating a recirculation pattern in the vertical plane. The retention of early larval stages in the spawning area was evident from the acoustic data, indicating a persistent location for the bulk of hake larvae at the main spawning ground during summer months. The distribution extends to the rest of the nursery grounds as the breeding season advances in agreement with the general pattern of middle shelf bottom circulation vectors produced by the numerical model. The results are discussed within the framework of Bakun’s fundamental triad, identifying its elements. In addition, a conceptual model for the main biophysical coupling processes during the early life history of M. hubbsi is proposed in order to gain insight about the recruitment mechanism of this species.

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Among those factors that determine the success of a fish cohort, trophic interactions play a key role, especially during the larval and juvenile stages. The Argentine hake Merluccius hubbsi is the most abundant demersal fish in the southwestern Atlantic and also one of the main commercial resources for Argentina. By employing C and N stable isotope analysis, we evaluated M. hubbsi changes in trophic niche and trophic position throughout their early life from early larvae to juveniles 2+ (8−330 mm total length, TL) during their drift from the spawning to the nursery ground. We analyzed 121 individuals and 7 possible resources in different sectors as fish move from the spawning to the nursery ground in the coastal region of northern Argentine Patagonia. Our results show that Argentine hakes occupy different trophic niches during their ontogenetic development. While larval stages (8−34 mm TL) feed almost exclusively on copepods, larger juveniles showed shrimps as their main prey. Individuals between 35 and 89 mm TL showed the most generalist diet, with a mix of both pelagic and epibenthic prey. Therefore, our results indicate that the change from a planktonic to a demersal habitat (settlement), which is of paramount importance in the early life history of hakes, is a gradual process.

During December 1999, a transect 260 km long was carried out along the major axis of the Río de la Plata estuary. Acoustic (120 kHz), oceanographic, and zooplankton sampling were performed to extract a continuous, quasi-synoptic acoustic view of the estuary's salt wedge. Three different zones were identified. The riverine and marine zones were characterized by vertically homogeneous water and low volume-backscattering strength (Sv). The mixohaline zone was characterized by a strong vertical salinity gradient and the highest Sv values. There were no relationships between the reflection value calculated from the salinity gradient and suspended matter (sediments) and measured Sv. The greatest abundance of acoustically detectable organisms was at the halocline depth, coincident with the echo recordings. The positions of the halocline and the two fronts were determined on the basis of aggregations of the different planktonic groups. The physical presence of the salt wedge, as shown by a “biological wedge”, was well described by the acoustic technique employed.

Food of Seriola lalandi was mainly composed of juveniles of horse mackerel (Trachurus lathami) (%IRI = 99.66) showing the species as carnivorous feeders predating on small pelagic organisms.