It makes sense, if you live in rivers, to make use of river currents to get around. After all, it saves energy, and energy is something that is critical to the young stages of any animal. The phenomenon of ‘drift’, or downstream movement using currents, is well known for macroinvertebrates, and a wealth of literature exists on the subject. Good reads include Noel Hynes’ The Ecology of Running Waters (1970) and J. David Allan’s Stream ecology: structure and function of running waters. But ‘drifting’ is also very widepread in the early stages of fish, yet we know comparatively little about what motivates them to start, the paths that they take and for how long they stay in the drift.
Many species of riverine fish drift, using river currents to sweep them from areas where their parents have spawned, to areas more suitable for growing and maturing. For some species of riverine fish, broadcast spawning (throwing eggs and sperm into the water column and hoping for the best) results in eggs and newly-hatched young, being swept downstream as a matter of course. But for others, where eggs are laid or attached to the bottom of rivers, newly hatched (and even older) young move up into the current and drift downstream, seemingly deliberately. We are, however, mostly talking about fish larvae, or fish in the first few weeks of their lives, and often less than 15 mm in length. Drift largely is a way of dispersing from a location where there is a concentration of young fish all potentially competing for the same resources or potentially wanting to eat you, to locations where this is less likely to happen.
There has been an ongoing debate for many decades about whether river fish do deliberately drift or are swept into currents unwittingly. There are, of course, current speeds that fish cannot resist, because they are too strong for them to swim against. Stacey Kopf, a PhD student at Charles Sturt University (CSU), is currently studying aspects of the swimming ability of several species of river fish from the Murray-Darling Basin. But for riverine fish, flow is what they do, and even the smallest are amazingly adept at getting out of the current if they have to. But, when they are in the current, do they have control over the distance they drift and where they end up?
Dmitrii Pavlov and colleagues from the Institute of Ecology and Evolution, Russian Academy of Sciences, have studied drifting fish for many years and have come up with a 3-way classification of drifters. There are ‘passive drifters’, who drift downstream in an un-orientated sort of way and apparently have little control of how they drift and where they end up. ‘Active drifters’ swim with the current, facing downstream and apparently can influence how far they drift and for how long. And finally there are ‘active/passive drifters’ that face upstream, and have some control during their time in the drift. It is a little bit like driving a car down a hill. Passive drifting is like having no steering: you will career down the hill and be lucky if you don’t crash – you’ll certainly find it hard to get to where you want to go. Active drifting is like having the engine on, being able to steer and facing down hill: not only can you steer, but you will actually be able to go faster than just coasting and you should make it down OK and get to where you want to go. Active/passive drifting is like having brakes, being able to steer, but only being able to face uphill. You can slow yourself down and have some control, but it’s not easy, and will be touch-and-go if you make it down in one piece.
In the vast majority of cases of drifting young fish, we do not know to which group they belong. But it is important that we understand these things, because with so many of the world’s rivers regulated and dammed, drifting is not what it used to be. A nice paper by Dudley and Platania (2007) has highlighted the importance of connectivity, and that dams and associated non-flowing areas upstream in normally free-flowing rivers have likely been responsible for the demise of several species of fish that have drifting eggs and larvae.
Studies of drifting fish larvae in Australia are relatively few and far between, but more are being done each year. My own work, and that of John Koehn and Alison King from the Department of Sustainability and Environment in Melbourne (DSE) on the larvae of Murray cod and other species, show that for some of our best-known species, drifting is almost certainly a necessary part of their life cycles. For others, it may be that some individuals do and some don’t. A recent Honours study at CSU by Tim Kaminskas showed convincingly, that Murray cod larvae ‘choose’ to drift, and that they do so when they get to a certain stage of development. Tim’s study also showed that most larvae drifted only at night-time, confirming what we have seen in the wild. A study by Zeb Tonkin and colleagues at the DSE have shown that the distribution of silver perch eggs is not uniform , they drift towards the shore and near the river bottom and they also tend to be more abundant in the drift at night.
In 2008, I collaborated with Hubert Keckeis and Lissie Schludermann, (University of Vienna) as part of Lissie’s PhD study, when we released 40,000 nase larvae (Chondrostoma nasus) into the River Danube, below Vienna, and collected them as they drifted downstream. We were interested in working out if they just went with the flow or if they could influence when and where they drifted. Our results showed conclusively that, despite high current speeds, the larvae did not just drift passively, and some took considerably longer to move through the study reach than predicted from hydrological modelling that had been done by Michael Tritthart (BOKU University). We are now following up on that work and, together with colleagues from the BOKU University, have released more larvae at different locations and at different stages of development. Ultimately, we want to know what processes are involved in the dispersal and survival of young fish from spawning grounds to where they settle and grow.
What is becoming increasingly clear, is that rather than simply acting as passive particles swept where the current takes them, fish larvae have a lot more control over how fast they move, if they move at all, how long they drift for and where they end up. However, there is much still unknown about the patterns and processes associated with drifting fish in rivers around the world. The description above only scratches the surface. What is certain is that we need to know more about this interesting phenomenon if we are to conserve riverine fish species and manage our rivers effectively.
Allan, J. D. (1995). Stream ecology: structure and function of running waters, Kluwer Academic Pub. Brittain, J. E. and T. J. Eikeland (1988). “Invertebrate drift—a review.” Hydrobiologia 166: 77-93. Dudley, R. K. and S. P. Platania (2007). “Flow regulation and fragmentation imperil pelagic-spawning reiverine fishes.” Ecological Applications 17: 2074-2086. Humphries P and Lake PS (2000) Fish larvae and the management of regulated rivers. Regulated Rivers: Research & Management 16, 421-432. Humphries P (2005) Spawning time and early life history of Murray cod, Maccullochella peelii peelii (Mitchell) in an Australian river. Environmental Biology of Fishes 72, 393-407. Humphries P and King AJ (2004) Drifting fish larvae in Murray-Darling Basin rivers: composition, spatial and temporal patterns and distance drifted. In ‘Downstream movement of fish in the Murray-Darling Basin. Statement, recommendations and supporting papers from a workshop held in Canberra 3-4 June 2003.’ (Eds M Lintermans and B Phillips) pp. 51-58. Murray-Darling Basin Commission, Canberra, Australia. Hynes, H. B. N. (1970). The Ecology of Running Waters. Toronto, University of Toronot Press. Koehn JD and Harrington DJ (2005) Collection and distribution of the early life stages of the Murray cod (Maccullochella peelii peelii) in a regulated river. Australian Journal of Zoology 53, 137-144. Pavlov, D. S., V. N. Mikheev, et al. (2008). “Ecological and behavioural influences on juvenile fish migrations in regulated rivers: A review of experimental and field studies.” Hydrobiologia 609: 125-138.Tonkin Z, King A, Mahoney J and Morrongiello J (2007) Diel and spatial drifting patterns of silver perch Bidyanus bidyanus eggs in an Australian lowland river. Journal of Fish Biology 70, 313-317.