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.

Highly variable recruitment in an estuarine fish is determined by salinity stratification and freshwater flow: implications of a changing climate

“…For example, abundance and survival of early life stages (<1 yr old) of striped bass (Kimmerer et al 2001 are positively related to freshwater flow. For barramundi, strong year classes determined from age structure (Staunton- Smith et al 2004), and also lagged catch data (Robins et al 2005), are positively influenced by freshwater flow. Recruitment variability indicated by age structure for king threadfin in a drytropical estuary was also shown to be positively related to freshwater flow (Halliday et al 2008).…”

“…The advantage of using catch curve residuals based on age structure, however, is that a significant time period of recruitment variability can be generated based on only 1 or a few years of sampling. Although the methods of determining recruitment variability relied on assumptions such as relatively constant mortality across age classes (Staunton- Smith et al 2004), the very strong episodic variability in recruitment of bream meant that the patterns were likely to be clear even if the assumptions were not entirely met.…”

“…We assumed that deviation from the catch curve regression was a reflection of variable recruitment (Staunton- Smith et al 2004), with large positive and negative studentised residuals representing strong and weak year classes, respectively (Halliday et al 2008).…”

“…Recruitment variability was estimated from the age structure of the sampled bream using catch curve regression (the natural log of the number of fish classified to each year class regressed against age) following the methods of Maceina (1997) and Staunton-Smith et al (2004). We assumed that deviation from the catch curve regression was a reflection of variable recruitment (Staunton- Smith et al 2004), with large positive and negative studentised residuals representing strong and weak year classes, respectively (Halliday et al 2008).…”

Australia's tropical river systems: current scientific understanding and critical knowledge gaps for sustainable management

“…floodplains and estuaries), whereas in-stream environments are strongly affected by conditions over the rest of the year. Robins et al (2005) and Staunton-Smith et al (2004) have demonstrated the importance of freshwater inflows to tropical estuaries to sustain estuarine fisheries through enhancing habitat access, productivity or recruitment. Recent research on the perennial Daly River has demonstrated the importance of base-flow supported by groundwater inputs for ecosystem production and, consequently, food-web support of higher animals Webster et al 2005).…”