Barramundi: Gulf of Carpentaria update

Biological monitoring

Sampling region for the Gulf of Carpentaria Barramundi stock which comprises estuarine areas from Mornington Island to Aurukun.

Fisheries Queensland conducts biological monitoring for three of Queensland's six genetic stocks of barramundi in; the Gulf of Carpentaria, north east coast and central east coast. Commercial and recreational fishers and seafood wholesalers assist with the collection of length, sex and age data from fish retained in the fisheries.

In 2013 and 2014, the commercial harvest of barramundi in the Gulf of Carpentaria was much lower than the ten year average. Despite a reduction in the number of active operators and days fished, the catch rate is low compared to other years. For barramundi, high rainfall and river flows can increase:

Recent wet seasons in the catchments that flow into the Gulf of Carpentaria have not had extensive rainfall that favours the species.

Using the biological data

Fisheries Queensland uses the national stock status assessment criteria to routinely determine the exploitation status of individual fish stocks. The most recent assessment has classified each of Queensland's six barramundi stocks as sustainably fished. The biological data collected through this program provides a time series of length and age structured information, and estimations of total mortality for the evaluation of the Gulf of Carpentaria stock.

Fish length

Commercial and recreational fishers retain a similar size range of fish. However, small fish (less than 66 cm total length) account for a higher percentage of recreational catch (Figure 1). More importantly, the results show that large fish are present in the stock.

  • Bar charts of the annual length frequency for Commercial (blue) and recreational (yellow) harvest of barramundi from the Gulf of Carpentaria.
    Figure 1. Length structure (percentage of fish in each 4 cm length class) of the Gulf of Carpentaria barramundi harvest (2012-2014) shown by fishing sector.

Fish Age

The age of many fish species can be determined by examining their otoliths (ear bones). Fish age, in years, can be estimated by identifying and counting the number of opaque bands, like growth rings in a tree (Figure 2).

  • ear bones of barramundi photographed under a microscope – Photo on left is whole otolith with cut indicated and Photo on the right is a sectioned otolith with annual rings marked.
    Figure 2. – A) Whole barramundi otolith; the line marks where a thin section is cut. B) A thin section of a barramundi otolith, dots indicate the annual growth bands.
  • 	Figure 3. How old is your fish? Estimate the age of a barramundi from the Queensland Gulf of Carpentaria by using this age-at-length graph. Measure the total length of your fish and find the corresponding age on the graph. For example, at 83 cm barramundi are likely to be five years of age, although they could easily range between three to seven years old. From 2008, the oldest fish sampled was caught in 2013, measured 1.14 m long and was 20 years old.
    Age at length relationship for the Gulf of Carpentaria stock of barramundi – shaded area shows the range of ages usually encountered for each size class, symbols marks the most common (median) age.

From 2008, the oldest fish sampled was caught in 2013, measured 1.14 m long and was 20 years old.

For many fish species, the harvest is dominated by the youngest age group when it is fully available to the fishery (i.e. susceptible to fishing methods and of legal size). For barramundi this is around three years old. However, due to their dependency on favourable environmental conditions, the annual age structure shows variable recruitment. Particularly strong year classes will be seen in successive years as they grow older (Figure 4).

  • bar chart of annual age frequency from the commercial (blue) and recreational (yellow) sector caught barramundi.  Arrrows  highlight strong cohort.
    Figure 4. Age structure of commercially and recreationally harvested barramundi in the Gulf of Carpentaria from 2010 to 2014. Fish over ten years old are grouped into one category. Arrows follow a strong year group of fish spawned late 2008 (or early 2009) seen as two year olds as they start to enter the fishery in 2010, three in 2011, through to six in 2014.

Further Information

Support and assistance

This monitoring is only possible due to the generous assistance of recreational and commercial fishers as well as fish wholesalers and retailers who assist by donating frames, providing length measurements or allowing Fisheries Queensland staff to measure their catch.

Last updated 21 December 2015