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Continuation of the monitoring of the recruitment success of the Black Bream population in the Vasse-Wonnerup

Tweedley, J.R., Cottingham, A. and Beatty, S.J. (2018) Continuation of the monitoring of the recruitment success of the Black Bream population in the Vasse-Wonnerup. Centre for Fish and Fisheries Research, Murdoch University



Non-technical summary

The Vasse-Wonnerup is a shallow intermittently-open estuary located near the city of Busselton, Western Australia and is listed under the Ramsar Convention as being an internationally significant area for birds. However, despite its ecological importance, this system and its catchment have been highly modified and the system suffers from excess nutrients and periodic episodes of low oxygen levels (hypoxia), which can lead to fish kills. A major constituent of the fish that die during these events are individuals of the iconic recreational species Black Bream (Acanthopagrus butcheri). Because Black Bream complete their life cycle within the estuary and typically do not leave, depleted populations of this species cannot be replenished from stocks in the marine environment (e.g. Geographe Bay) or from other nearby estuaries.

Following a major fish kill in the Vasse-Wonnerup in April 2013, there was very little evidence of recruitment (i.e. an increase in juveniles following the birth of new fish) of Black Bream later in that year (2013) or in the following year (2014). It was unclear, however, whether this was due to the environment at the time of spawning (i.e. winter-early spring) not being suitable for the survival of larval fish or to a lack of spawning stock (sufficient numbers of sexually mature (adult) fish), and thus whether recruitment would continue to fail in the future. The results of the current study and data collected one year earlier by Tweedley et al. (2016a), demonstrate that recruitment of Black Bream in the Vasse-Wonnerup Estuary was successful in two successive years (2015 and 2016), thus indicating that recruitment failure in previous years was most likely related to adverse environmental conditions at the time of spawning.

The mean density of juvenile Black Bream in November 2016 of 1.8 fish 100 m-2 was less than that recorded in the same month in previous years (i.e. 2012, 2014 and 2015; 5.6-18.0 fish 100 m-2). Conversely, a greater average number of juveniles was caught in February 2017 6 (11.0 fish 100 m-2) than in previous years (i.e. 1.3 and 8.6 fish 100 m-2 in 2014 and 2015, respectively). The total lengths of juvenile Black Bream collected in November 2016 were significantly less than those collected in that same month previously. These differences may have been caused by poorer growth or delayed spawning, brought about by the prolonged and cooler winter in 2016 than normal. Air temperatures this during winter were 1.2°C cooler than average and only increased by 0.5°C between July and September 2016 as opposed to the average increase of 1.5°C.

Although there were no significant differences in catch rate of adult Black Bream in the offshore, deeper waters of the Wonnerup Inlet and Deadwater in November 2016 (i.e. 8.8 fish h-1) with the catch rate in November in previous years (i.e. 7.5-9.1 fish h-1), the numbers of adult Black Bream in offshore waters of in February 2017, i.e. 3.0 fish h-1 were considerably lower than those recorded previously in any month. The low catch rates may have been related to the opening of the bar prior to sampling and the unseasonal rainfall that occurred immediately before sampling as freshwater discharge can sometimes lead to Black Bream temporarily leaving the estuary. Alternatively, the freshwater discharge flowing downstream through the surge barrier may have afforded Black Bream the opportunity to move upstream.

The results of this study demonstrate that recruitment of Black Bream has now occurred in two successive years, which is very encouraging. However, as this population continues to be under extreme pressure from ongoing environmental perturbations and climate change, continuation of the monitoring of this Black Bream population is essential in order to (i) track the ongoing recovery of the population, (ii) better understand the factors influencing recruitment and (iii) to assess the relative severity of any subsequent fish kill events.

Publication Type: Report
Murdoch Affiliation: Centre for Fish and Fisheries Research
Publisher: Centre for Fish and Fisheries Research, Murdoch University
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