Cuttlefish population survey
The Point Lowly Giant Australian Cuttlefish population is an iconic species of South Australia, and a robust assessment of its status is conducted on an annual basis to inform fishery management.
Population surveys are undertaken by the South Australian Research and Development Institute (SARDI) using the standard survey methodology described in Steer et al. (2013) () to estimate Giant Australian Cuttlefish abundance and biomass of the Point Lowly spawning aggregation in 2020.
Similar to 2019, the 2020 survey was completed in June to coincide with the peak spawning period.
Cuttlefish population estimates for 2020 spawning season
Annual estimates of Giant Australian Cuttlefish abundance remained relatively high from 2015-2019, consistently exceeding 110,000 cuttlefish per year. Abundance increased 116% from 2019 to 2020, and at 247,146 cuttlefish, the 2020 spawning population size was the highest on record (Figure 1).
The biomass of the spawning aggregation remained above 70 t since 2015, with peaks of 165.2 t in 2016 and 140.5 t in 2020 (Figure 1). While high, unlike abundance, the 2020 biomass estimate was not the highest on record because the population consisted of relatively smaller animals. In 2020, the average size (mantle length) of female (141 mm) and male (172 mm) cuttlefish was below the long-term averages of 172 mm and 196 mm, respectively (Figure 2). The sex ratio in 2020 remained dominated by males, with approximately 5 males for every 1 female (~16% females; Figure 2).
In analysing the results, greater emphasis is placed upon cuttlefish abundance as a population estimate rather than biomass, which is dependent on size records. Biomass estimates are considered less robust than abundance estimates because cuttlefish sizes are estimated in-situ by divers with varying levels of experience and estimates are no longer verified by capturing individual cuttlefish.
It is important to recognise that the Giant Australian Cuttlefish population strength is intrinsically linked to environmental processes that are highly variable and impact both development and growth.
This is reflected in the last 13 consecutive survey years where both abundance and biomass have fluctuated considerably over short time scales. The recent estimates of abundance indicate that the population has increased substantially from historic low observed in 2013, with the 2020 estimate of 247,146 cuttlefish the highest on record.
Figure 1: Annual peak estimates (June survey) of total abundance and biomass (± SD) of Giant Australian Cuttlefish aggregating around Point Lowly during peak spawning from 1998 to 2020. * Population was heavily fished. Historic data obtained from Hall and Fowler (2003).
Figure 2: The population sex ratio presented as the percentage of females (top). The average size of Giant Australian Cuttlefish (± SE) for males (middle) and females (bottom) from 1998 to 2020. The red lines represent the overall average.