The curious case of the giant cuttlefish
By Rose Yeoman
Photo: Sean D. Connell
The decline of Australian giant cuttlefish (Sepia apama) populations in the upper Spencer Gulf, South Australia, is a mystery researchers are keen to solve.
During the past decade, one of the most spectacular examples of spawning in a marine species has been that of the Australian giant cuttlefish.
It is the largest cuttlefish species in the world, and each year between early May and late August hundreds of thousands would aggregate in water less than 10 metres deep off the coast of Point Lowly, near Whyalla, SA.
This spawning spectacle attracted 30,000 spectators each year.
Today, the breeding aggregation is markedly less dense. Cuttlefish numbers declined from 183,000 recorded in 1999 to 18,500 in 2012. An accelerated decline raised concern about cuttlefish sustainability and highlighted the paucity of information.
A new two-year research project aims to fill those gaps, working to determine the movement patterns and population structure of the cuttlefish at a finer scale, and to also assess how the cuttlefish respond to environmental and human factors.
The University of Adelaide is leading the project in partnership with the South Australian Research and Development Institute (SARDI), SA Museum, the Department of Primary Industries and Regions SA and the SA Department of Environment, Water and Natural Resources (DEWNR). The FRDC and DEWNR are jointly funding the research, to run from March 2013 to February 2015.
Bronwyn Gillanders, from the marine biology program at the University of Adelaide, acknowledges there are clear knowledge gaps about the movement of cuttlefish on and off spawning grounds, whether they spawn elsewhere in the upper Spencer Gulf and the taxonomic status of the species.
“A study on population structure has shown that the breeding aggregation in the area forms a distinct and separate population from other populations in the species, but there’s a lack of information regarding finer-scale movements. For example, the dispersal of hatchlings away from the spawning site and their residence as sub-adults is unknown,” she says.
“The upper Spencer Gulf population is genetically isolated from others in the area and may constitute a separate species. This isolation imparts a higher conservation focus on the species, as well as highlighting the need to identify ecological and environmental factors driving the decline. We must also ask whether some life-history stages are more vulnerable than others.”
Bronwyn Gillanders also leads an FRDC-funded project in the Spencer Gulf that is using modelling approaches to investigate food web relationships, or ‘trophodynamics’, of key species of conservation and commercial significance in the region, including the Australian giant cuttlefish.
Linked to this is a SARDI study led by Simon Goldsworthy into the impact of the increasing fur seal populations on the seafood industry in SA. Seal faecal samples collected near cuttlefish breeding sites will help determine the importance of cuttlefish in seal diets.
Simon Goldsworthy says the fur seal population of the upper Spencer Gulf is relatively low and he considers the link between seals and declining cuttlefish population numbers to be “tenuous”.
A report published in 2013 by Mike Steer and colleagues at SARDI and the SA Environmental Protection Authority considered a number of factors that may be associated with declining cuttlefish numbers, including water temperature, weather, disease, pollution, tourism, increased predation pressure and fishing pressure.
“Of the variables we’ve looked at, only rainfall showed an inverse correlation with peak cuttlefish abundance and biomass,” Mike Steer says.
“However, it’s unknown whether rainfall affected coastal salinity, localised pollution through coastal run-off or water clarity – all of which may deter aggregating cuttlefish. Although we know Australian giant cuttlefish in the upper Spencer Gulf are physiologically adapted to high salinity, we don’t know how sensitive they are to fluctuations in salinity.”
He points out that while no specific cause or set of factors has been identified for the decline, there is a lack of historical evidence for large Point Lowly spawning aggregations prior to 1986.
“High population numbers in the mid-1990s may represent a population explosion and therefore be atypical, so the current decline may be part of a natural process,” Mike Steer says.
“Historically, Point Lowly was considered the main spawning area for cuttlefish, but there is a possibility that they’re spawning elsewhere in the upper Spencer Gulf. A key area of interest is the movement of the Australian giant cuttlefish on and off the spawning ground and how that relates to smaller spawning grounds in the region.”
Members of the public can assist researchers in tracking Australian giant cuttlefish populations by reporting sightings of aggregations of 10 or more in the Spencer Gulf via Redmap.
FRDC Research Codes: 2011-054, 2013-010
Bronwyn Gillanders, 08 8313 6235
Simon Goldsworthy, 08 8207 5435