Key words: Managed flows, regulation, fish spawning
Natural recruitment (survival of fishes from eggs to reproductive stage) of native fish is vital for sustainable populations, however unfortunately the conditions required for successful recruitment of many freshwater fish species in the Basin remain unknown. This is especially true with respect to the natural timing and environmental conditions required for spawning, and to ensure the subsequent survival of larval and juvenile fish.
Research in the southern Murray–Darling Basin (MDB) suggests that many fish species still breed under modified flow conditions, but that high numbers of juveniles do not always result. This suggests that recruitment failure, rather than lack of spawning, may be the limiting factor behind low population size and restricted distributions of a number of species in MDB.
Figure 1. Mullaroo Creek one of the six rivers surveyed in this study. (Photo courtesy Paul Humphries.)
Broad aims and methods:
The specific aims of this study were to determine:
- whether the level of recruitment in Australian Smelt (Retropinna semonii) is proportional to the abundance of newly hatched larvae;
- whether spawning and recruitment are similar between flow-managed and non flow-managed rivers for fish in general;
- whether spawning and recruitment are similar between years for fish in general;
- whether timing of peak recruitment is similar for protracted and short spawners; and,
- if environmental variables or food availability are associated with peak recruitment of Australian smelt in a subset of rivers sampled.
Sampling of larvae and juvenile fish using seine nets, drift nets and throw nets was conducted within six rivers in the MDB (some flow managed and some non-flow managed) (Fig 1). Water quality parameters and food availability were also sampled. All larval and a subset of juvenile fish were aged, and length/age relationships were explored to calculate hatch date. Daily flow data was used to investigate relationships between discharge, temperature and biomass of zooplankton and epibenthos (organisms which live on the surface of the riverbed or other structure in the river).
Figure 2. An Australian Smelt otolith showing increments used to estimate age. (Photo courtesy Paul Humphries)
Findings:
A total of 19 species — 14 native and 5 alien — were collected from the six rivers, and most showed evidence of spawning and recruitment in both years of the study. Because of small and patchy abundances, only Australian Smelt could effectively be compared among all rivers (Fig 2), whilst a small suite of species could be compared for a subset of rivers.
The project found that overall species presence differed depending on region. However, substantial differences in species composition were also found between pairs of rivers (flow-managed versus non flow-managed) within regions. In addition to this, the study found that in the Southern and Northern regions, common species tended to begin spawning earlier, and spawn for longer, in flow-managed rivers compared to their non flow-managed counterparts. In the Lower Region the opposite was the case. The results point to the fact that timing and duration of breeding of fish in rivers are likely influenced more by local differences in flow than broader climate and geographical influences.
The study identified three types of spawning strategies: ‘brief’ spawners (only spawn for one or two months), ‘flexible’ spawners (spawning period varies by river and by year), and ‘protracted’ spawners (spawning consistently spans more than two months). Only Gambusia (Gambusia holbrooki) fitted the last criteria. Australian Smelt were found to be a ‘flexible’ spawner, and its success cannot be purely because of its supposed protracted spawning behaviour.
Most species spawned over a long enough period to indicate that recruitment was not proportional to larval production. This was shown best for Australian Smelt — in most cases the early hatching events resulted in relatively poor recruitment, and later hatchings resulted in relatively good recruitment.
Whereas timing and duration of spawning seemed to be influenced by local factors, timing of recruitment tended to be similar for rivers within regions, except for the Ovens/King and the Goulburn systems. Preliminary comparisons of environmental variables with food availability suggested that temperature (positively) and discharge (negatively) influence production of the smallest size classes of zooplankton and that the first significant rise in zooplankton coincided with peak Australian Smelt recruitment in the Ovens River.
Lessons learned and future directions: This research will lead to better management of flows released from dams to facilitate spawning and recruitment of native fish species. This is likely to result in increased recruitment and enhancement of native fish populations.
Stakeholders and Funding bodies: This project was funded through the Murray-Darling Basin Authority’s Native Fish Strategy, and delivered by a research team from the Murray-Darling Freshwater Research Centre.
Contact: Dr. Paul Humphries, Charles Sturt University, (02) 60519920, phumphries@csu.edu.au.
EMR Project Summaries 