Within the Murray Darling Basin (MDB), experts have estimated a decline in native fish abundance of ~90% since European settlement. Things such as changes to the hydrological regime, habitat degradation, river regulation and infrastructure, over-fishing, and impacts from alien species are all contributing to the ongoing decline (Koehn and Lintermans 2012; Lintermans 2013).
Ongoing monitoring of native fish populations is vital to securing their long-term survival. However, limited amounts of funding often prevent research from being long-term, meaning that it is difficult to report on changes over time. Additionally, the focus of most survey work is done to gather a ‘snapshot’ of spatial patterns in diversity or abundance, often overlooking biomass and size changes over time.
In the MDB, monitoring of fish populations began as stock assessment of commercial fisheries by the collation of catch records (Reid et al. 1997). However, as commercial fisheries across the MDB were progressively closed, monitoring effort moved towards fishery-independent assessments using electro-fishing methods. This practice has been consistently happening since the 1990’s and the fish monitoring data gathered over the last three decades has great potential to provide timescale context for adaptive management and policy decisions.
A recent 2023 study aimed to find out how fish have changed over time, by examining the MDB data between 1994 to 2022 with a focus on several large bodied fish in the NSW MDB and river catchment areas (valleys). These areas make up ~57% of the MDB, a sizable portion of the whole Basin. The relative abundance, biomass, and size structure were studied to provide a more detailed look at five native species (Murray cod, Maccullochella peelii; Golden perch, Macquaria ambigua; Silver perch, Bidyanus bidyanus; Macquarie perch, Macquaria australasica; Freshwater catfish, Tandanus tandanus) and one alien species (Common carp, Cyprinus carpio).
The research found that there was a strong change in relative abundance, biomass and population structure for all species observed occurring within the timescale. Within the NSW Basin scale, the relative abundance of Murray cod, Golden perch and Common carp increased across the time series, with no clear trends for silver perch, Macquarie perch or Freshwater catfish. However, patterns in relative abundance, biomass, and population structure were variable among valleys for most species.
Although these findings are positive, it is important to note that the scope of the study was limited to a few large-bodied species that are commonly found during electrofishing surveys. The decline in small-bodied fish is well documented and not studied in this research. Additionally, recovery of native fish was not consistent across all valleys, with recent, localised declines occurring in different regions of the MDB (e.g. large-scale fish kills in the parts of the Lower Darling–Baaka River in 2018–2020; Ellis et al. 2022).
The study proved that the overall status of some native fish has improved over recent decades at the NSW Basin scale. This finding, however, does not deny the fact that native fish populations are facing ongoing decline across the MDB.
Although native fish populations in the MDB remain degraded and face escalating threats, the increase in numbers and size of some native species is an encouraging sign that adaptive restoration actions can improve the outlook for native fish.
Featured image: Murray cod.
Photo credit: NSW DPI