Abstract
Juvenile fishes were sampled every 15 days from September 2009 to April 2010 along the marine-estuarine gradient (surf zone, estuary and a freshwater stream) of the Mar Chiquita lagoon, Argentina. The temporal variations of juvenile assemblages in spring-summer and the environmental variables related to the spatial and temporal patterns were analysed. Four groups of sampling stations were defined, indicating differences in fish composition among zones during the spring–early summer period (Groups I to III), while the composition of juvenile fishes was homogeneous along the marine-estuarine gradient during the late summer–early autumn period (Group IV). Platanichthys platana and Ramnogaster arcuata (Group A) and Odontesthes argentinensis and Brevoortia aureaz (Group B) contributed most to the temporal differences observed. The three first species reached this estuarine system in spring, although with lower abundances than in early summer, while B. aurea was dominant in late summer–early autumn, showing different periods of recruitment of these species into the lagoon. After factoring out variation due to shared spatial-temporal-environmental factors (4.43%), canonical correspondence analysis (CCA) showed that temporal factors had an almost five times greater contribution (15.15%) than spatial factors (2.85%) and almost twice as great as the pure environmental factors (8.11%) to explaining the variation in abundance of the juvenile fishes. From the significant environmental variables incorporated in the CCA, wind direction contributed more than water temperature, salinity or transparency in explaining data variability. Indeed, most species were related to “onshore winds” and therefore the importance of wind in the successful recruitment of juveniles into this shallow and micro-tidal estuary is discussed.
Highlights
The majority of fishes found in estuaries represent those euryhaline marine teleosts that enter these systems in large numbers at particular intervals of their life cycles
Salinity varied from typical marine water in the surf zone (>30) to brackish water in the estuary (13-34) and the Vivoratá stream (7-34)
Salinity values recorded in the surf zone (S1-S4) were higher than those recorded at the two innermost stations of the estuary zone (E4 and E5) and the Vivoratá stream (V1 and V2) (Fig. 2B)
Summary
The majority of fishes found in estuaries represent those euryhaline marine teleosts that enter these systems in large numbers at particular intervals of their life cycles. The recruitment process into estuaries is presumably in response to certain environmental cues (odours, turbidity and/or salinity gradients) transported into the marine environment from either estuarine or freshwater sources (Whitfield 1994, Strydom 2003) and/or to tidal currents, in sufficiently deep estuaries where vertical movements of young fishes exploiting flood and/or ebb currents have been proposed (Boehlert and Mundy 1988, Harris et al 2001). These findings explain why fish larvae and juveniles congregate in the surf zones adjacent to estuaries before colonizing them Juveniles inhabit estuaries mostly throughout spring and summer, benefiting from suitable conditions for growth, namely high food availability and water temperature (Blaber and Blaber 1980, Potter et al 1990), though with some associated physiological cost due to natural (e.g. lower salinity) or artificial (e.g. pollution) stressors (Amara et al 2007)
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