Distribution and population dynamics of larval fish off eastern Australia

Abstract

Since Johan Hjort first proposed his ‘critical period’ hypothesis in the early 1900’s, fisheries scientists have recognised the importance of the early-life stages of fish in determining year-to-year variation in recruitment. Despite this, our ability to forecast larval success remains limited. This is particularly true in the Australian context, where larval fish monitoring programs have only recently been established. This thesis aims to contribute knowledge regarding the population dynamics of larval fishes off eastern Australia. To achieve this, I utilise a new larval fish assemblage database, develop new modelling techniques, and conduct three sampling voyages to test my models on original data. The work is presented across 4 independent research chapters.

In Chapter 2, I characterise patterns in the distribution of larval fish over 15° of latitude with highly variable conditions driven by the East Australian Current, using a newly available larval fish database supplemented with recently collected samples. Along eastern Australia, generalized additive models reveal that larval abundance and diversity is higher in the north and decreases poleward for most of the year, establishing a baseline trend for the region. This pattern reverses in summer, when spawning events occur around Tasmania, which could change as the East Australian Current strengthens with climate change.

Existing methods for estimating mortality rates, such as catch-curves, require large sample sizes, as they work by grouping individuals into age bins to determine a frequency distribution. Yet, sampling enough larvae is often not possible at fine scales within the constraints of research projects. Drawing on size distribution theory, and improved computational techniques, in Chapter 3 I develop a novel method to simultaneously estimate growth and mortality of fish larvae which improves certainty in estimates with fewer fish available. Using Bayesian inference methods, I show how growth and mortality can be estimated from a continuous distribution of sizes removing the need to bin data. Furthermore, these new models are flexible and can estimate non-linear growth and mortality functions.

The ratio of larval growth to mortality, or its inverse, is used to indicate changes in cohort biomass and is referred to as a cohort’s “recruitment potential”. Most studies observing this metric have focused on variation at survey-level or annual scales, which may ignore crucial variability in cohort success at finer spatiotemporal scales relevant to individual larvae. By sampling larval Pacific sardine (Sardinops sagax) off southeast Queensland, in Chapter 4 I demonstrate that vital rates of cohort success vary at scales smaller than features that are generally considered to be oceanographic habitats (i.e., eddies and shelf waters). While growth rates are more consistent among features, rates of mortality are highly variable, highlighting the importance of incorporating mortality estimates in attempts to understand cohort success in field studies.

Building on the understanding that size-spectra are remarkably consistent in marine ecosystems, in Chapter 5 I test the hypothesis that the slope of the plankton spectra provides a valuable diagnostic tool for rapidly assessing growth and mortality of larval fishes in situ. To test this, I sampled larval Pacific sardine and plankton simultaneously on three voyages along eastern Australia, using a bongo net with an interior finer-mesh net. Contrary to expectations, results from this study demonstrate a negative relationship between the slope of plankton size spectra and recruitment potential of larval Pacific sardine of equivalent size. This may result from high susceptibility of larval Pacific sardine to predation, owing to the thin body shape and poor swimming ability characteristic of clupeid larvae. Several stronger relationships between larval growth and mortality, and physical oceanographic parameters were evident. Together, these results indicate that years where spawning occurs predominantly in shelf areas with a lower concentration of large predatory zooplankton, and, the East Australian Current is meandering further from the shelf break than normal, could provide the necessary conditions allowing for exceptional levels of Pacific sardine recruitment off eastern Australia.

My thesis suggests plankton size spectra are unlikely to reflect recruitment potential directly, at least for Pacific sardine. However, incorporating some size-based aspects of the plankton community into a broader modelling framework could further our ability to determine how larval success varies across a seascape. Improvements in the resolution of vital rate estimates, and of spatially explicit modelling procedures, might accurately predict larval success although data is currently unavailable. Therefore, continued sampling larval fishes and estimation of vital rates is required. While data from international programs such as CalCOFI provide a strong starting point, continuation of larval fish monitoring programs in Australia will provide an avenue to further explore forecasting of larval success