The Great Australian Bight Trawl Sector (GABTS) of the Southern and Eastern Scalefish and Shark Fishery (SESSF) targets two main species, Deepwater Flathead (Neoplatycephalus conatus) and Bight Redfish (Centroberyx gerrardi). Industry-based fishery-independent resource surveys of the Great Australian Bight (GAB) have been conducted since 2005 with the primary goal of obtaining robust annual indices of relative biomass of these two main species. These indices are incorporated into formal stock assessments, which were previously hampered by input data with little contrast.
The Great Australian Bight Industry Association (GABIA) supported implementation of the industry-based fishery-independent resource survey of the GABTS, driven largely by industry’s desire for a better understanding of the extent of shelf resources of their main target species. Surveys are conducted during February–April each year using a ‘standard’ research net. Relative biomass estimates are calculated using swept area calculations, avoiding the need to make assumptions regarding the catchability and efficiency of the gear. Industry observations, supported by preliminary analysis of data from the 2005 survey, showed large diurnal effects on catch rates of Bight Redfish, but not Deepwater Flathead. Consequently, only data from night shots (when catch rates are higher) are used in calculations of relative biomass estimates of Bight Redfish, but data from both day and night shots are used in calculations for Deepwater Flathead.
This report details the results of the 2015 GABTS resource survey — the seventh survey.
The two trips of the GABTS resource survey are generally conducted during the week leading up to the full moon in March and April. During 2015, the occurrence of the full moon was on 5th March and 4th April. Seventy-five valid survey shots were completed over the two surveys; Deepwater Flathead and Bight Redfish occurred in 100% and 76% of the shots respectively.
Due to the age and fatigue of the original and backup survey nets, the footlines had been removed subsequent to the 2011 survey and a new survey net was constructed for the 2015 survey based on the original survey net plans, using an old net as a backup. During Trip 1, catch rates appeared abnormally low. On inspection, there appeared to be problems with under-weighting of the footline in the central part of the net that effected its fishing efficiency. Following completion of the first trip, the footline was completely rebuilt to ensure it replicated the original survey ground gear based on archived pictures and input from the designer. The net performed as expected during Trip 2, but catch rates of the main species were still very low.
It is also relevant that two seismic surveys were being conducted in about the same region and time that the GAB surveys were being undertaken. The TGS-NOPEC Nerites Multi Client 3D Marine Seismic Survey was being conducted close by the Central 2 stratum and the PGS Ceduna 3D seismic survey was being conducted near Central 1 stratum. Seismic impacts on fish behaviour and it is not clear to what extent these seismic surveys may have impacted catch rates.
For the above reasons, results in this report are presented combined for both trips and also for each trip separately.
Relative biomass indices with CVs<0.2 were obtained for Deepwater Flathead and many other main species within the survey area, using swept area estimates from trawl shots in a stratified random survey design. Using data from night shots only, the CV of the relative biomass for Bight Redfish was 0.2 when data from both trips were used, but was 0.28 when only data from Trip 2 was used.
The effect of using data from only Trip 2 on relative biomass estimates varied by species, resulting in higher estimates for Deepwater Flathead, Common Sawshark, Gummy Shark, Jackass Morwong, Latchet, Ornate Angelshark, Spikey Dogfish and “other species”, but lower estimates for Bight Redfish, Ocean Jacket, Yellowspotted Boarfish and Knifejaw. Using data from both trips, the relative biomass estimate of Bight Redfish for 2015 was 3,633 t (CV = 0.20), which is 72% lower than the 2011 estimate and 75–87% lower than 2005–2009 estimates. Estimates of relative biomass of Bight Redfish in 2005, 2006, 2007,
2008, 2009 and 2011 were 20,887 t (CV = 0.13), 25,380 t (CV = 0.16), 25,713 t (CV = 0.16),
14,591 t (CV = 0.11), 27,610 t (CV=0.13) and 13,189 t (CV=0.18) respectively. When only data from Trip 2 is used, the relative biomass estimate of Bight Redfish for 2015 was 2,573 t (CV = 0.28), which is 80% lower than the 2011 estimate and 82 –91% lower than 2005 –
Using data from both trips, the relative biomass estimate of Deepwater Flathead during 2015 was 4,657 t (CV=0.07). This is 50% lower than the 2011 estimate of 9,227 t, and 40% – 62% lower than 2005–2009 estimates. Estimates of relative biomass of Deepwater Flathead in 2005, 2006, 2007, 2008, 2009 and 2011 were 12,152 t (CV = 0.05), 8,415 t (CV = 0.06),
8,540 t (CV = 0.05), 7,725 t (CV = 0.06), 9,942 t (CV=0.05) and 9,227 t (CV=0.05)
respectively. When only data from Trip 2 is used, the relative biomass estimate of Deepwater Flathead for 2015 was 5,065 t (CV = 0.09), which is 45% lower than the 2011 estimate and 34
–58% lower than 2005 – 2009 estimates.
During 2005–2008 surveys, Bight Redfish had been the most commonly caught species during the survey (20%–26% of the catch composition). Bight Redfish comprised 19% of the catch during 2009 (second to Ocean Jacket), then in 2011 comprised only 11% of the total catch. During the 2015 survey, Bight Redfish comprised only 7% of the catch and was the fifth most commonly caught species behind Wide Stingaree (20%), Latchet (18%), Deepwater Flathead (11%) and Ocean Jacket (10%).
Length-frequency measurements were made on 1,062 Bight Redfish and 1,337 Deepwater Flathead during the 2015 survey. The modal length of Bight Redfish was 31 cm, and has stabilised at 30-32cm over the last four years of the survey after displaying a declining trend from around 35 cm recorded from the 2005 inaugural survey to 31cm during the 2008 survey. Modal length of Deepwater Flathead from 2015 samples (43 cm) was similar to the previous surveys.
Otolith samples of 229 Deepwater Flathead and 196 Bight Redfish were also collected during the survey. Data from the ageing of these otoliths will be used during future assessments to be conducted for Bight Redfish and Deepwater Flathead.
The results of this survey provide the seventh year of a fishery-independent index of abundance for both Deepwater Flathead and Bight Redfish. The biomass index for both Deepwater Flathead and Bight Redfish appeared much reduced during the 2015 survey compared to previous surveys. This time-series now provides a fishery independent estimate of relative stock biomass that is incorporated into the stock assessments in conjunction with commercial CPUE and length and age data. In addition to these two important target species, we now have a six-year time-series of relative abundance indices for many of the important bycatch and byproduct species in the GABTS including Ocean Jacket, Common Sawshark, Gummy Shark, Yellowspotted Boarfish, Jackass Morwong, Knifejaw, Latchet, Ornate Angelshark and Spikey Dogfish.