portfairy_bustupOne of the reasons we have such as rich and productive ocean off the southwest coast is due to the productive season phenomenon of the Bonney Upwelling!  Although this cold water nutrient upwelling current occurs over a distance of a couple of hundred kilometres it is second only to the East Australian Current (EAC) in Australian waters.

In the warmer South-eastern Australian summer months a combination of physical factors combine to create a seasonal phenomenon that occurs along the Southwest Victorian coast and the nearby coast of south eastern South Australia.  This stretch of coastline is often referred to as the ‘BonneyCoast’ and generally ranges from Robe in South Australia to Portland in Victoria. This is the home of the ‘Bonnie Upwelling’.

The Bonney Upwelling is natural occurring physical process which fuels a productive food chain and attracts a range of high order pelagic fishes, including Southern Bluefin Tuna on their annual migration.  Although this is not the only upwelling that occurs in Southeast Australia it is the most prominent and its effects and processes are worthy of a closer inspection.


A school of tuna smashing baitfish off the southwest coast

Upwelling is best described as the movement of water rising to the surface from depth.   Upwelling can result in increased localized productivity of ocean waters and even though upwellings make up only a few percent of the world’s ocean surface area they are responsible for a majority of the world’s fish production.

Classic upwelling plumes are regularly observed along the BonneyCoast usually between November/December and March/April.  The productivity of the southwest coast booms as deep cold nutrient rich water upwells from offshore bringing nutrients to the surface kick starting an amazing pelagic food chain.


The coastal waters are productive for a number of pelagic fish species

The upwelling creates an opportunistic feeding ground for seabirds, whales, seals and of most interest to anglers the high order pelagic fish including sharks, tuna and kingfish. The area is also a significant aggregation area for Blue whales which feed in the krill rich waters.


Large pods of dolphins are commonly found off the coast

The upwelling is the result of the prevailing south easterly winds acting on surface waters and effects of the Flinders Current (which flows north past western Tasmania, western Bass Strait and South Australia-Victoria Border).  As the water moves along the coast the warmer surface waters are drawn offshore and these are replaced by the cooler nutrient rich deep ocean waters.  As a result of the frictional stresses that exist between ocean layers, surface water is transported at a 90 degree angle to the left of the prevailing wind direction in the southern hemisphere.


An diagrammatic example of the upwelling process

During the warmer months prevailing winds follow a predominantly NE-SE pattern which provides favourable conditions for this process to occur. As the cooler westerly winds of the NW-SW cycle arrive in late autumn the process is prevented from occurring.

The duration of these upwelling events are thought to last as long as a few days to a month or more.  Characteristics of these events include rapid decrease in temperature, a decrease in salinity, reduced surface oxygen concentrations and large increases in surface nutrients. As a result surface waters along the Bonney Coast can decrease to as low as 12.5 degrees Celsius after upwelling events, but can reach in excess of 20 degrees Celsius in the height of the warmer months in the absence of this process occurring. Several factors can influence the upwelling process these include topography and slope of the continental shelf, stratification of water layers and strength and persistence of favourable winds.  Even with all of the favourable factors occurring there is still no guarantee that the process can occur or will be intense.  This is where satellite sea surface temperature maps can be of great benefit for anglers to study before you go.


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