Understanding fish movement is a crucial component of the knowledge required for protecting and restoring native fish in the Murray-Darling Basin (MDB). A priority need for the Basin Plan is to further understand fish movement metrics. The development of the Murray-Darling Basin Fish Monitoring and Evaluation Strategy has identified analysis of the Passive Integrated Transponder (PIT) tag data base (FishNet) as an important step in increasing the Murray-Darling Basin Authorities ability to monitor and report on movement-based native fish outcomes.

Recently, Ecology Australia (EA) was engaged to tag fish that were collected as part of The Living Murray (TLM) program monitoring sites in Northwest Victoria. The project focused on four iconic large bodied native fish species: Silver perch (Bidyanus bidyanus), Murray cod (Maccullochella peelii), Freshwater catfish (Tandanus tandanus) and Golden perch (Macquaria ambigua). The project was undertaken between 18 March and 30 May 2024 with the main objective being to increase the population of PIT tagged fish in the MDB for increased fish movement data. This project builds on the tagging already undertaken in the 2020–2023 period during TLM and other projects, with over 1300 fish tagged in the region over that period.

Methods

The majority of the tagged fish were captured using boat electrofishing, a method that creates an electrical field within a 3–4 m radius of anodes held on extendable booms at the front of the boat, momentarily stunning fish for capture and processing. Electrofishing is a commonly used method worldwide, is undertaken by highly trained personnel and is relatively harmless to fish provided appropriate settings and approaches are used. The remaining fish were captured fyke nets that were deployed overnight. This method uses an extended ‘wing’ to direct fish into the tubular body of the net, where a series of funnels net trap fish in the ‘cod’ end chamber for collection the following morning.

Native fish including Golden perch, Silver perch, Murray cod and Freshwater catfish, were tagged internally with a PIT tag. Depending on fish size and condition, an external PDL dart tag with a unique identifying number and a reporting phone number was also fitted. Smaller native fish (120–300 mm in length) were only tagged with a small internal PIT tag. Aside from the small size and weight of a PIT tag compared to other tags used for monitoring fish movements, the primary advantage of a PIT tag is that it requires no battery and offers the opportunity to collect data over the remaining lifetime of the fish.

The unique identifying numbers of both tags were recorded and carefully checked to ensure accurate transcription. Tagging only occurred if prior scanning did not reveal an existing PIT and if the overall visual condition of each fish was deemed to be appropriate for tagging. Fish considered to be in poor condition (e.g. severe fin rot, significant lesions, heavy infestations of external parasites) were not tagged due to the risk of additional stress potentially compromising that individual’s survival.

So, what did they achieve?

291 fish were tagged, majority during TLM surveys.

• 5 Silver perch (339–420 mm)  

• 60 Murray cod (210–1180 mm)  

• 7 Freshwater catfish (387–517 mm)  

• 219 Golden perch (182–585 mm)

This adds to the 1,363 fish tagged in the region by EA over the 2020–2023 period:

• 127 silver perch (122–439 mm)  

• 376 Murray cod (120–1186 mm)

• 6 freshwater catfish (235–502 mm)  

• 853 golden perch (122–575 mm)

• 1 spangled perch (164 mm)

Angler call ins have been received for a small number of the Autumn 2024 dart tagged Golden perch, with most recaptured in the same area as they were tagged, and all kept by the anglers. Based on call in data received to date for the dart tagged fish over the 2020–2024 period, we know that release rates are high for Murray cod (~90%) and low (~30%) for Golden perch, although the presence of a visible dart tag may potentially influence an angler’s decision on whether to release a captured fish. Although a proportion of tagged fish (particularly Golden perch) will be retained by anglers, and a proportion will die due to natural causes, these species are relatively long-lived (~26–27 years for Golden perch and Silver perch and ~48 years for Murray cod). Even accounting for the proportion of fish that ‘reject’ their tags, a large proportion of the tagged fish are expected to be active in the system for many years to come. These fish will contribute movement data to the FishNet database when they pass through a fishway with an installed PIT reader system.

The detection antennas at these systems scan continuously for PIT tagged fish. For further information please click here. A third way that data can be collected from PIT or dart tagged fish is via subsequent recapture by researchers during future monitoring surveys, thereby allowing for comparisons to be made in terms of growth (comparing length and weight data) and in some instances, improvements in physical condition (e.g. recovery from moderate fin rot, lesions, parasite infestations).

None of the 291 fish PIT tagged during Autumn 2024 as part of this project have been detected moving through fishways just yet, however movements would typically be expected to commence in Spring 2024, particularly following rises in river flow and temperature. 309 (~23%) of the fish tagged by EA in 2020–2023 period have been subsequently detected by the PIT reader systems, with two Murray cod detected a combined total of over 546,000 times since being tagged in 2021 at 410 and 502 mm in length (FishNet 2024). This suggests these two cod have likely been residing in the fishways themselves (regularly passing by the detection antennas!) where passing small fish might be targets for an easy meal. Several Golden perch and Silver perch tagged during in previous years have been detected (by PIT reader systems, angler call ins or both) as having moved upstream over 870 km to Torrumbarry Weir on the Murray River, or over 670 km to Menindee Weir on the Darling River. While large scale movements are well known for these species, migratory movement information is limited for juvenile Silver perch. Five of the fish detected at Torrumbarry Weir were juvenile Silver perch (209–229 mm) tagged in Autumn 2022, contributing towards filling this knowledge gap.

Routine fish tagging with PIT and dart tags during fish monitoring surveys can be a cost-effective way to contribute important data on the movements and migration patterns of some of the most iconic native fish species in Australia. Using this data, we can better inform decision makers about the best management actions for native fish, and the ideal time for environmental water releases.

I've found a tagged fish - what now?

If you find a fish with a tag by its fin, call it in!

Each of the fish tagged by EA have a distinct orange tag with the number printed with ‘EA Fish’, a unique identifying number, and the phone number: 1800 077 001 to provide the capture location, capture date, length and release status. Anglers contributing this information are provided with a certificate acknowledging their contribution and containing details about when and where the fish was originally tagged, when and where it has been recaptured and how far it has moved and grown between recaptures.