OVER millions of years of evolution, fish have evolved a huge variety of life histories that involve migrations. Some species, like northern hemisphere salmon, start their lives in tiny freshwater streams in the high mountains. They then migrate downstream for hundreds or even thousands of km out to sea, only to return to the same stream to lay their own eggs before they die. Fish species which perform these amazing upstream spawning migrations are called anadromous fishes.
Others, like our own barramundi and Australian bass, do the exact opposite. These so called catadromous fishes move downstream to spawn in saltwater, then the juvenile fish swim back upriver into the freshwater nursery reaches of rivers where they spend much of their lives. Then there are the amphidromous species that spawn in estuaries with adult fish living offshore, and the potamodromous and oceanodromous species that migrate large distances whilst spending their entire lives in freshwater and seawater, respectively.
Indeed, virtually all of the most famous and spectacular fish migrations are for the purposes of spawning, with the migratory behaviour providing an evolutionary advantage. For example, the multiple hundred km upstream migrations of the potamodromous Murray cod and golden perch in the Murray-Darling River system. It seems they do this to compensate for the natural environmental flows of this once mighty river system, which otherwise would result in downstream movement of their eggs and larvae out to sea. Similarly, the oceanodromous tailor undertake a compensatory migration 1000+ km northwards along Australia’s east coast during the winter months, to spawn in South East QLD (and more lately due to climate change, in northern NSW). They probably head north in winter because the eastern Australian current drives their eggs and larvae southwards in spring. However there are other types of fish migrations too, undertaken for the purposes of accessing particular feeding areas, or escaping seasonal extremes of hot and cold water, and so on.
Despite the massive variety of hows and whys of fish migrations, all migratory fish species have one thing in common. They all developed this behaviour in the absence of human intervention or modification of their habitats. Because it’s impossible for fish to switch off instincts that have been honed over millions of years, this means that even apparently minor changes that humans make to rivers, streams and other water courses can have major impacts on the life cycles of fish that live within them.
It’s obvious that blocking a river by building a large dam for water storage and/or hydro power is bad for migratory fish. Indeed, it is thought that the threatened status of virtually all northern hemisphere salmon populations is due to humans stuffing up their migrations. Whether this is by diversion or complete destruction of spawning streams, increased death rates of juvenile fish as they pass through one or more hydropower tunnels on their way to the sea, or reduced success of adult fish moving back upstream (even with fish ladders and other fish passage promoting technologies that bypass dams), the result is the same.
Similarly, in Australia increased river regulation, particularly in the form of barriers such as locks and weirs which regulate river flow, together with over allocation of water for irrigation, has greatly reduced the spawning potential of iconic native fish like Murray cod and golden perch by preventing fish passage and environmental flow events. Of course, the many water storage dams on our east coast completely prevent upstream movements of catadromous species like barramundi and Aussie bass, which (depending on the location of the dam) can result in localised extinctions of wild fish populations in the affected river. Stocking of the water impounded upstream of the dam can, of course, compensate for this loss somewhat, as shown by QLDs famous stocked impoundment fisheries for both barra and bass. But these are strictly “put and take” fisheries as both species require access to brackish water to breed successfully.
But what many people don’t realise is that even when water flows freely past a human-made obstacle, even slight alteration of stream flows can still wreak havoc with native fish. This was bought home to me recently during the recent heavy rainfall events along Australia’s east coast in early 2022. With widespread river flow events and flooding in many places, these episodes mostly benefit migratory native fish as these inundations are usually the only times these days that native fish can get upstream or downstream past obstacles like dams and weirs. Unfortunately, around where I live in SE QLD, flooding also has negative effects, such as allowing dispersion of exotic pest species like Tilapia into virtually every suitable waterway. The massive volumes of mud, nutrients and pollutants bought down by the floods also destroys inshore coral and shellfish reef habitats throughout the Moreton Bay Marine Park (which by the way not only fails to protect these habitats from damage, actually has legislation which prevents restoration of the damaged habitats – go figure).
In any case, I was walking with a group of family and friends along an access track past a small freshwater coastal stream in a state forest on Bribie Island when saw an amazing sight. The track crossed over the stream on a small 2 metre wide earthen bridge with a couple of 400 mm pipes used to funnel water underneath. The others made a couple of brief remarks as the crossed the bridge then moved on, but I had a closer look and found a significant piscatorial tragedy was unfolding before my eyes. The stream was flowing nicely and was pushing a fair volume of water down the pipes. You’d think all would be well, but on the downstream side of the bridge the water was packed with many millions of native gudgeons swimming in a near solid mass. Upon closer inspection it was clear the pipes under the bridge were the problem – the water velocity coming through the pipes was far too high for the small gudgeons to be able to swim against it. Their upstream migration back into the wetland where their parents had spawned following the rainfall event was being prevented by the innocuous looking earthern bridge with its poorly designed pipes, which were trapping millions of fish that were slowly starving to death.
Once I alerted the walking group as to exactly what was going on, they became concerned. Which is a lesson in itself because they walked right past it, showing that unless we point these things out, most people tend to be unaware of what is going on underwater. Determined to do something about it, I got some buckets and dip nets together and the family returned the next day to move millions of gudgeons past the barrier (10,000 per bucket!), releasing them on the upstream side to continue their journey back to the wetland above. Within a couple of minutes each bucketful of fish disappeared upstream, where they would complete their lifecycle, help keep the mosquitoes in check and provide food for other fishes, birds and aquatic animals. Nice.
The upshot is, a simple, well designed culvert would fix this problem for good, but you can rest assured such is the extent of coastal development, this sort of silent drama is occurring in many tens of thousands of places along our coastal and inland waterways every day. In any case, I alerted my local government authorities and used the issue as a case study to show why fish passage needs to be assessed whenever any construction is being done on or near waterways in the region. We can only hope they listen, but I am not holding my breath.
