Fish Movement and Migration
The fact that native fishes move within and between habitats is well known. Barriers to these movements have long been recognised as major contributors to the decline in numbers and distributions of native fish species. Under the Murray-Darling Basin Authority’s Native Fish Strategy, numerous research projects and management interventions have been undertaken that have significantly increased our knowledge of fish movement and opened up large lengths of river to unimpeded passage (e.g. the Sea to Hume Fishway Program).
Types of Movement
Movements range from small scale predator avoidance bursts to large scale spawning migrations. The different types and scales of fish movement are summarised in the table below (modified after Koehn and Crook, 2013).
|Short, non-sustained bursts
|Escape from predators, prey capture etc.
|Short term but sustained, within normal home range
|Feeding, avoidance of local poor conditions, deil movements, spawning etc.
|Prolonged, long-term, large scale movements between habitats
|Migrations, exploration, feeding, spawning, avoidance of larger scale poor conditions etc.
|Upstream and downstream
|Migrations, spawning, feeding, genetic mixing etc.
|In channel, on to floodplains, in/out of lake literal zone
|Dispersal, recolonisation, feeding, larval rearing etc.
Movement may occur entirely within freshwater (e.g. Golden perch, Murray cod) and fish that exhibit this type of movement are called Potamodrous. Other species move between freshwater and marine environments (e.g. Tupong in the Lower lakes) and are referred to as Diadromous.
Fishes of the Murray-Darling Basin are highly mobile (some migrating up to 100km a day) and exhibit all the modes of movement outlined above. Connectivity within and between aquatic ecosystems is essential to the long-term rehabilitation of most native fish populations. Consideration must be given to the upstream, downstream and lateral movements of not only the adults of large bodied fish such as Golden perch and Silver perch but all life history stages as well as small bodied fish such as Gudgeons, Spangled perch etc.
The terms “movement”, “migration” and “dispersal” are often confused and require clarification. Here the definitions used by Koehn and Crook (2013) are used. They are:
Refers simply to the change in location of a fish and should be used when not referring to specific kinds of movement (i.e. migration and dispersal).
Movement that occurs with some periodicity involves most of the population and movement from one habitat/environment to another (e.g. upstream movement of Golden perch to spawn followed by downstream drift of larvae)
Movement of fish from areas where they are concentrated to areas where they are less concentrated (e.g. movement of fishes from drought refugia after the drought has broken)
Barriers to Movement
The Murray-Darling Basin is a highly modified and regulated system and there are many impediments to fish movement. The most obvious are the numerous artificial barriers such as dams weirs, locks, culverts and road crossings throughout the Basin. Unless such structures regularly drown out, they form significant barriers to fish movement. Fishways can be retrofitted to existing structures and significant advances have been made in fishway design for native fishes of all sizes over the last 20 years. Nevertheless much remains to be learnt and good monitoring programs are essential. While the number of artificial barriers across the Basin is in the thousands and the task may seem daunting, strategically placed fishways on main channel structures can open up many kilometers of river to fish passage (e.g the Sea to Hume Fishway Program). Downstream passage, particularly of larval fish must also be considered with recent data indicating that significant mortalities may result when larvae pass downstream through some weir designs.
Lateral movements into regulated waterways and onto floodplains may also result in mortalities of native fishes as fish may become trapped behind regulating structures when seeking to return to main stem watercourses on receding water levels. There is also growing evidence that large numbers of fish are lost from Murray-Darling Basin rivers through water abstraction via irrigation pumps. The implementation of a basin-wide program to fit appropriately designed screens to pump intake pipes could significantly reduce these mortalities.
Modification of natural flow regimes will further exacerbate the impacts of barriers to fish movement in a number of ways. Specifically, flow modification may reduce fish passage by reducing flows in the main channel, or by reducing river connectivity to floodplain wetlands. Environmental cues which stimulate migrations such as water temperature may also be impacted by changes in flow patterns and cold water pollution.
A fishway, fish ladder, fish pass or fish steps, is a structure that enables fish to pass artificial barriers (dams, locks, weirs etc.) by swimming and leaping up a series of relatively low steps into the waters on the other side.
If native fish populations are to be rehabilitated it is essential that water management throughout the Basin is sensitive to their movement requirements and that both hard engineering solutions, such as the retrofitting of fishways, and more sensitive operational procedures, such as more appropriate flow releases, are implemented. Over the last 10 years much has been learnt about the movement requirements of native fishes and significant progress has been made in mitigating the impacts of barriers to movement. It is essential that this foundation continues to be built upon. For example, replicating the Sea to Hume Program in the north of the Basin would restore fish passage to thousands of kilometers of main stem river.
Together with management interventions, it is essential that research into fish movement continues. There is still much we don’t know, but the following projects provide insights into the knowledge that has been generated on this issue through the Native Fish Strategy and how it can be used to better manage fish movement and migration.