Salt marshes do not present favourable conditions for cryptic fish species to inhabit these areas, mainly due to dryness and high temperatures at low tides, and scarce places to hide. Despite the harsh environmental conditions, we report, for the first time, the occurrence of two specimens of the half-naked goby Gobiosoma hemigymnum at high marsh, between oysters and Spartina alteniflora stems. In this paper we report evidence of a possible positive effect of autogenic ecosystem engineer and invasive species, the Pacific oyster (Magallana gigas), over the gobid, Gobiosoma hemigymnum, found 600 km away from its southernmost normal range.

Some biological parameters of Jenyns' sprat Ramnogaster arcuata in Bahía Blanca estuary, Argentina, were evaluated during a survey conducted on a monthly basis between September 2005 and August 2006. Jenyns' sprat specimens were observed in the estuary all through the year. Total length (TL; N = 721) ranged from 33 to 131 mm. No significant differences between the length–weight relationships of males and females were detected (P > 0.10). Positive allometric growth was observed in juveniles, males, females and sexes combined. Whole otolith analysis revealed four age groups (0–3), 1-year age being the most abundant. Gompertz growth parameters were L∞: 102.75 mm, k = 0.46 and t0 = 0.98 for males (r = 0.80), and L∞: 125.45 mm, k = 0.33 and t0 = 0.68 for females (r = 0.76). Males and females reached 50% sexual maturity (TL50) at 76 and 77 mm TL (age 1 year), respectively, and all individuals were sexually mature at a 110 mm-length at the age of 3 years. Macroscopic gonad inspection, gonadosomatic index and the high percentage of fish of the smallest size-class captured during spring and summer samplings indicate that the spawning season began in spring. Taken together, our results indicate that the biology of R. arcuata is fairly consistent with the biology of other temperate clupeoids.

The seasonal dynamics of Acartia tonsa and the invader Eurytemora americana were analysed in relation to the environmental variability occurring from April to November in the Bahía Blanca Estuary. Twice a month, the abundance of eggs, nauplii, copepodites and adults was examined and some environmental variables were recorded. Multivariate statistics (CCA) was applied to analyse the data of variables. Acartia tonsa eggs and nauplii diminished from April–May and they were almost absent between June and September, although a small larval peak could be detected from the end of July to October. All the stages of this species increased in number through spring. Eurytemora americana was registered as from June and only nauplii larvae were observed, with a peak increase during September. Copepodites and adults were observed as from July, increasing in number until peaking at the end of September. The number of all stages of this species decreased abruptly, the whole population disappearing from the plankton. The A. tonsa developmental stages were most positively correlated with temperature, photoperiod and other light variables whereas those of E. americana showed positive correlations with chlorophyll-a and salinity. The gradients of the main environmental factors likely give rise to a certain niche separation facilitating the coexistence of the two copepod populations within the period studied.