Barramundi - Gulf of Carpentaria monitoring
Biological monitoring update
Commercial and recreational fishers provide the samples from their harvested fish for the collection of length, sex, and age. This biological data provides us with a time series of length and age-structured information. We primarily use this data in stock status evaluations and stock assessments. This report summarises the biological data collected from commercially harvested barramundi.
Recreational catch and effort is collected from periodic phone surveys, fisher diaries and boat ramp surveys . Boat ramp surveys commenced in 2017 in the southern Gulf of Carpentaria. Recreational catch trends will be released when a time series of data is available.
The stock of barramundi in Queensland’s southern Gulf of Carpentaria was assessed as depleting in 20181 under the national stock status framework . This stock status determination was based on a combination of fishing pressure and biomass indicators. Biomass may have been impacted by low river flows between 2013 and 2016, reducing survivorship and recruitment of young fish, and slow fish growth. A more recent stock assessment model indicated the 2017 biomass was an estimated 39%, which is below the target levels of 40–50% by 2020 and 60% by 2027 Sustainable Fisheries Strategy target levels. However, the biomass shows an increasing trend and the egg production ratio is showing a slow recovery.
Data from 2014 to 2018 shows large fish are still being harvested from the stock. Large barramundi are important as they produce more eggs per unit of body weight than smaller fish.
The harvest of many fish species is dominated by the youngest age group fully available to the fishery (i.e. susceptible to fishing methods and of legal size). For barramundi, this is around three years old.
Barramundi recruitment depends on favourable environmental conditions and therefore, the annual age structure of this species shows variable recruitment. Stronger year classes are obvious in successive years as they age (Figure 2).
Barramundi spawned in late 2010 to early 2011 have dominated the fishery since 2014 to 2018 (as three to seven year olds respectively). This year class is relatively strong compared to those spawned in subsequent years (i.e. those spawned between 2012 and 2015).
Individual fish growth is quite variable, shown by the wide spread of ages for each length class in the graphs below. For example, barramundi 100–104 cm long are likely to be about seven years old, although they are usually between four and twelve years old (Figure 3). However, slow growing individuals have been sampled that were up to 22 years old (Figure 4).
Barramundi (Lates calcarifer):
- have six genetic stocks in Queensland
- utilise freshwater reaches, particularly as juveniles
- aggregate in estuarine areas to spawn
- change from male to female at about 6 year old
- harvested by the indigenous, commercial, charter, and recreational fishing sectors
- commercial harvest from the southern Gulf of Carpentaria stock was 544 t in 2018, this was 72% of the total Queensland wild harvest
- have a highly variable growth rate
- usually reach legal size at 2 to 4 years old
- the ages of 9992 barramundi from southern Gulf of Carpentaria have been estimated since 2000
- oldest fish sampled within the southern Gulf of Carpentaria stock was 22 years old (caught in 2018 at 1 m long)
Support and assistance
This monitoring is only possible due to the generous assistance of recreational and commercial fishers, and fish wholesalers and retailers who allow their catch to be measured, donating frames and providing length measurements.