the humboldt current system of northern and central chile - figema

MARTIN THIEL ET AL.in these hydrodynamically variable but highly productive environments, coastal and offshoreorganisms have developed a series of reproductive strategies that enable them to counteract theeffects of offshore advection in the surface Ekman layer or take advantage of other oceanographicprocesses to return to the coastal zone (see also Coastal oceanography, p. 205ff.). For instance, on aseasonal timescale, small pelagic clupeiform fish tend to synchronise their reproductive timing withprocesses that induce shoreward transport and coastal retention. Probably the best-documented speciesin the HCS of central Chile are the anchovy (Engraulis ringens) and common sardine (Strangomerabentincki) that reproduce during winter, when the wind-driven Ekman layer is directed shoreward andthus eggs and larvae are retained in the coastal area (Castro et al. 2000, Castro 2001, Cubillos et al.2001, Hernández-Miranda et al. 2003). Shelf-break, slope-demersal and mid-water fish species, suchas Chilean hake (Merluccius gayi), big eye flounder (Hippoglossina macrops), and the mesopelagicMaurolicus parvipinnis, instead seem to prefer early spring reproduction when subsurface watersdrive their eggs and larvae to the coast during the upwelling season, where they develop in the seasonof higher production (Vargas et al. 1997, Vargas & Castro 2001, Landaeta & Castro 2002, Landaetaet al. 2006). This strategy, originally described for some large offshore copepods such as Rhincalanusnasutus, and more recently observed also in the galatheid Pleuroncodes monodon and the majidLibidoclaea granaria, is currently accepted as a common feature among several types of organismsthat have in common larvae inhabiting subsurface waters in central Chile (Castro et al. 1993, Yannicelliet al. 2006a,b). Other species of decapod crustaceans also synchronise their reproduction with theseasonal changes in hydrodynamics for their transport or retention (Yannicelli et al. 2006b). In thisgroup, however, several species reproduce during the upwelling season in spring and summer andtheir horizontal distribution seems to be associated with their behavioural ability for verticalmigration (i.e., Neotrypaea uncinata, Pagurus sp.). Other species with protracted larval periodssuch as Emerita analoga and Blepharipoda spinimana, which reproduce late in summer and earlyspring and then reside in the upper water column without signs of vertical migration, probably usemore than a single retention process, as yet unknown, because their reproduction occurs over periodsof contrasting hydrographic processes (Yannicelli et al. 2006b). Among molluscs, there is scarceinformation; for instance, the larval retention and shoreward transport mechanism of the gastropodConcholepas concholepas are the only information reported for the HCS. In this case, avoidanceof the surface Ekman layer by competent larvae appears to be accomplished by an inverse verticalmigration that reduces their time exposed to seaward flow and keeps the larvae between the coastand an upwelling front (Poulin et al. 2002a,b). Coastal oceanographic processes such as upwellingshadows have also been reported (Escribano et al. 2002) as larvae retention mechanisms and suchmight constitute an understudied coastal larval retention system along the HCS.Tidal transport of larvae, associated with frontal structures near the coast, the entrance ofestuaries or large bays, has also been reported recently for central and southern Chile. Internal tidalbores associated with semi-diurnal temperature changes coincided with bivalve, gastropod andcrustacean settlement, suggesting that coastward larval transport occurred during these events insummer (Vargas et al. 2004). In Corral Bay (~40°S), changes in larval fish distributions coincidentwith changes in the estuarine front position in different tidal phases have been reported by Vargaset al. (2003) as an indication of potential larval tidal transport. More recently, with the aid of fineresolutioncurrent profiles and stratified larval collections over 24-h cycles, semi-diurnal changesin water flow patterns and of decapod and larval fish fluxes in and out of the Gulf of Arauco havebeen estimated (Yannicelli et al. 2006a, R. Veas unpublished data). The overall larval fluxes weremodified by their vertical position in the water column, the diurnal vertical migration patterns andthe tidal cycle. Interestingly, although clearly associated with tidal phases, the larval fluxes experiencedby these decapod and fish larvae in the Gulf of Arauco did not correspond exactly toselective tidal stream transport (STST) as it has been reported at the entrance of other bays andestuaries in other coasts of the world (Forward et al. 1998).272