Does the quantity and timing of fresh water flowing into a dry tropical estuary affect year-class strength of barramundi (Lates calcarifer)?

Abstract

The influence of fresh water flowing into estuaries on biological processes, such as recruitment of juvenile fish, is poorly understood, but important if freshwater resources are to be managed sustainably. Typically, lagged correlations between freshwater flows and fisheries production (i.e. catch) are used to support speculation that flows affect the survival of fish (and thus year-class strength) during their first year of life. The present study compares the relative strength of year classes in an estuarine fish population with two indices of fresh water flowing into the estuary, river flow and coastal rainfall. Year-class strength was estimated from a subset of the age structure of commercially caught adult barramundi (Lates calcarifer), which were sampled at seafood processors for three consecutive years. Strong and coherent fluctuations in year-class strength were observed. Positive correlations were found between the abundance of year classes (accounting for age) and quantity of fresh water flowing into the estuary during spring and summer, when barramundi spawn and young-of-the-year recruit to nursery habitats. Regression analysis was used to explore the relationships between year-class strength and environmental variables. A possible, but unproven, causal mechanism for the relationship is that the quantity of fresh water flowing into the estuary during spring and summer influences the survival of early life-history stages of barramundi (i.e. juvenile recruitment) by altering accessibility, productivity and or carrying capacity of nursery habitats.