Friday, 20 May 2016

Why migratory fish are an important food supply and livelihood for millions of people around the world

For a huge number of fish and other aquatic animals, migration is a part of life. Atlantic salmon return to the same river they were born to lay eggs, whales swim from frigid Antarctic waters to the warmer climes to calf, and whale sharks never stop looking for food (covering thousands of kilometres in the process). There is a belief that fish migration only occurs between salt and fresh water (as in the case of our salmon) but fewer than 1% of species change habitat so drastically. Rather, the vast majority of migrating fish species feed in one place, then migrate to another to breed, all within the same system.

Tuna fishing in Indonesia. Photo: ACIAR
Within river systems, migrations usually occur in conjunction with increased flow. Spring thaws bring sudden inundations that signal a time to move. This has two advantages: firstly, greater flow means that it is easier to swim over barriers (such as natural rocky cascades); and secondly, new habitat becomes available. Flooding of river banks into grassland and forest provides important refuge for fish to breed in safety. A large number of migrating fish don’t swim upstream at all, instead they swim ‘sideways’ into recently flooded land. This is one of the reasons that the building of dams and smaller weirs is so damaging to migrating fish species. Not only are fish unable to move upstream, but the reduction, or removal, of these floods can remove the signal to breed entirely!

So why do we care? People need fish. In areas such as the Mekong, fisheries contribute immensely to people’s lives. In 2008 there were 3.9 million tonnes of fish caught in the Mekong, which added 12% to Cambodia’s GDP and 7% to Lao PDR’s. In many South East Asian and Pacific countries they are the primary source of animal protein and important micro-nutrients such as calcium and Vitamin A. Fish also provide important ecological functions such as regulating food webs and nutrient balances (mosquito control for example), seed dispersal, and regulating carbon fluxes. Not to mention that they can be good fun to catch!

Fish farming. Photo: ACIAR
Dam building, as well as smaller road crossings and water control are major threats to fish migration. “Fishways” and “fish ladders” are useful inventions developed to help fish navigate over and around low level barriers, but they aren’t a fix for high dams. A large proportion of fish don’t have the swimming power to climb these distances over dams (they work comparatively well for powerful north American fish like the salmon), and larval or newly hatched fish are often killed coming back down through turbines, and they still regulate the flooding that is so important to the fish life cycle.

A fishway during construction. Photo: ACIAR

Completed fishway under high river levels in Lao PDR. Photo: ACIAR
So what is ACIAR doing in the area of fish migration? ACIAR has a number of projects working on upstream and downstream migration past low level barriers on flood plains, such as irrigation structures, weirs, flood control devices and the like. These are generally less than six metres in height, about the maximum height that non-salmonid fish can swim up. See projects FIS/2009/041 and FIS/2014/041 for more details. These projects are not focusing on “big” dams (over 30 metres) as this is too high for almost all fish to climb. The only solution to these? Don’t build them on the main river channel.

World Fish Migration Day is celebrated globally on 21 May and this year's theme is 'connecting fish, rivers and people'. For more information, visit the event's website.

By Lachlan Dennis, Graduate Officer, Livestock Production Systems, ACIAR

No comments:

Post a Comment

Thanks for taking the time to comment!

Note: only a member of this blog may post a comment.