Age structure of the Australian lungfish (Neoceratodus forsteri)

Abstract

The Australian lungfish has been studied for more than a century without any knowledge of the longevity of the species. Traditional methods for ageing fish, such as analysis of otolith (ear stone) rings is complicated in that lungfish otoliths differ from teleost fish in composition. As otolith sampling is also lethal, this is not appropriate for a protected species listed under Australian legislation. Lungfish scales were removed from 500 fish from the Brisbane, Burnett and Mary rivers. A sub-sample of scales (85) were aged using bomb radiocarbon techniques and validated using scales marked previously with oxytetracycline. Lungfish ages ranged from 2.5-77 years of age. Estimated population age structures derived using an Age Length Key revealed different recruitment patterns between river systems. There were statistically significant von Bertalanffy growth model parameters estimated for each of the three rivers based on limited sample sizes. In addition, length frequency distributions between river systems were also significantly different. Further studies will be conducted to review drivers that may explain these inter-river differences.

Fig 1. Map of fish sampling locations.

The map shows the lungfish and mussel sampling locations as well as major impoundments.

Fig 2. Scale sample collection.

A) Dried and flat scale showing primordium as the black dot. B) Scale after removal of upper squamulae and lower elasmodin surfaces, cleaned area becomes transparent. C) Scale cut to prepare for sampling. D) F¹⁴C samples showing the first 11 samples removed in 1mm increments, then 2mm increments to the primordium.

Fig 3. Radiocarbon results (F¹⁴C) from selected fish.

A) Representative F¹⁴C from fish smaller than 800mm in length vs. distance from the outer edge of the scale B) Representative F¹⁴C from selected fish between 800-1000mm in length vs. distance from the outer edge of the scale. C) Representative F¹⁴C from fish >1000mm in length vs. distance from the outer edge of the scale. The radiocarbon bomb pulse is evident in the data from fish >1000mm in length.

Fig 4. F¹⁴C reference curve to age Australian lungfish.

Curve generated using the pre-bomb (pre-1950) ¹⁴C from old fish, the maximum ¹⁴C measured in any of the scales, and the small OTC tagged fish ¹⁴C from their outer slice of the scale and small fish collected in 2014.

Fig 5. OxCal age results from all fish.

A) OxCal probability distributions for lungfish ages from Burnett River overlaid on lungfish calibration curve. B) OxCal probability distributions for lungfish ages from Mary River overlaid on lungfish calibration curve. C) OxCal probability distributions for lungfish ages from Brisbane River overlaid on lungfish calibration curve.

Fig 6. F¹⁴C of lungfish food.

F¹⁴C values for mussel tissue and shell material from 7 locations. The highlighted box shows the range (2012–2015, due to mussels living for several years) in the atmospheric F¹⁴C CO₂ for comparison.

Fig 7. Oxytetracycline stained fish scales and F¹⁴C.

A) Original fish 745 mm long, on recapture fish 1010 mm long, OTC mark visible on scale. B) Original fish 1036 mm long, on recapture fish 1060mm long, OTC mark not visible on scale.

Fig 8. Length frequency histogram of Australian lungfish sampled from each river.

Fig 9. Size at age data for Australian lungfish.

The results from the Burnett River (a), Brisbane River (b) and Mary River (c). The 3 parameter VBGF curves (solid lines) for each river are displayed. For the Burnett River, a previous 2 parameter VBGF mark–recapture curve [13] (dashed line) and a 2 parameter VBGF curve (dotted line) based on the current data are plotted for comparison.

Fig 10. Age histogram of sub–sampled Australian lungfish.

Results are shown from the Burnett River (a) (n = 28), Mary River (b) (n = 27) and Brisbane River (c) (n = 30). The 5 year bins end in the year labelled on the x axis.

Fig 11. Histogram of estimated ages for the Australian lungfish populations.

Results are shown from the Burnett River (a), Mary River (b) and Brisbane River (c). The 5 year bins end in the year labelled on the x axis.

Fig 12. Comparison of the actual age frequency of lungfish with those derived from the ALK.

Results from fish <850 mm in length from the Burnett River derived from radiocarbon aging and that derived using the ALK of the sub–sampled population.