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Title:

Giant Australian Cuttlefish in South Australian waters

Project Number:

2013-010

Organisation:

University of Adelaide North Terrace Campus

Principal Investigator:

Bronwyn M. Gillanders

Project Status:

Completed

FRDC Expenditure:

$367,638.00

Program(s):

Environment

Need

Giant Australian cuttlefish, the largest cuttlefish species in the world, is widely distributed across southern Australian waters. Through most of its range it breeds on inshore rocky reefs, but it is renowned for forming the single largest known breeding aggregation of any cuttlefish species in the world. Each winter thousands of cuttlefish aggregate at Point Lowly in Upper Spencer Gulf (USG) to spawn. Estimates of abundance and biomass of spawning cuttlefish have indicated that population density at the aggregation site has declined through time with the most dramatic decreases occurring in 2011 and 2012. The magnitude of this decline has raised concerns about the sustainability of cuttlefish and highlighted the paucity of information relating to the fine-scale population structure and dispersal of this species within USG, as most studies have concentrated on the adult component of the population. The dispersal of hatchlings away from the spawning site and residence as sub-adults is currently unknown. Recent research investigating the broader-scale population structure suggested that the USG population was genetically isolated from the rest of the State, and may constitute a separate species (Gillanders and Donnellan, ARC Linkage project). This degree of isolation imparts a greater conservation focus on the species and highlights the importance of identifying the ecological/environmental factors that are driving the observed population decline and whether certain life-history stages are more vulnerable than others.

Objectives

1. Determine the movement throughout the life history and finer scale population structure of the giant Australian cuttlefish in Upper Spencer Gulf.

2. Resolve the systematic status of the USG giant Australian cuttlefish to determine the extent of its geographic boundaries.

3. Develop an integrated model that assesses and evaluates the response of the USG population to environmental and anthropogenic factors and thereby assess population viability.

Final Report - 2013-010-DLD - Giant Australian cuttlefish in South Australian waters

Final Report
ISBN:978-0-646-95420-2
ISSN:
Author(s):Bronwyn M. Gillanders
Date Published:December 2016
Principal Investigator: Bronwyn Gillanders
Key Words: Giant Australian cuttlefish; Sepia apama; Point Lowly; Whyalla; Upper Spencer Gulf; South Australia
Summary: The iconic giant Australian cuttlefish, Sepia apama, is the largest cuttlefish species in the world, and forms an extraordinary breeding aggregation on a small stretch of rock reef in Upper Spencer Gulf (USG), South Australia. This research, coordinated by the University of Adelaide, was undertaken to address key knowledge gaps relating to movement and population structure, as well as factors contributing to population viability and commenced following dramatic declines in abundance at the breeding aggregation location. The research used a range of approaches including statolith and cuttlebone chemistry, next generation nucleotide sequencing, and population viability modelling. Results confirmed that within Spencer Gulf there are two highly resolved genetic clusters with significant levels of divergence and no evidence of recent hybridisation or introgression. The southernmost extent of the northern genetic cluster was -33.828° latitude and the northern most extent of the southern genetic cluster was -33.502° demonstrating some overlap. Our data suggest that these two clusters can be considered two separate species. A spatial age-structured population model was used for scenario testing of different potential threats to the USG cuttlefish population. This model indicated that by-catch due to prawn trawling and cuttlefish harvesting away from the breeding aggregations pose negligible risks to the population, but the population’s viability could be compromised by increased mortality of embryos along the aggregation site prior to hatching and increased adult mortality if there is an increase in harvesting at the breeding grounds. Consequently, the continued closure of the aggregation site to cuttlefish harvesting is expected to improve the viability of the northern Spencer Gulf population.