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The mixing zone

Posted on: July 6, 2012

Photo: Kylie Hordyk.

Estuaries are an integral part of the aquatic environment in Western Australia.  They are the mixing zone between our freshwater rivers and the salty ocean. They may be either permanently or seasonally open to the sea. Many fish species found in the marine environment, such as sea mullet, King George whiting and pink snapper, utilise estuaries at certain times during their life cycle. Black bream on the other hand, complete their entire lifecycle in the estuary. Why not investigate the lifecycles of different species of fish common to your local area, to find those that utilise the estuary during some stage of their life. Discuss with your students why those species may utilise the estuary during that stage also. Check out our poster – Estuarine fish in the mixing zone – to discover how fish can be grouped according to where they breed.

Estuaries are a dynamic habitat. In the wetter months of the year, the proportion of freshwater increases as rainfall increases and enters the estuary. Also during the wetter months, estuaries that may be periodically closed by sandbars are now opening. During the drier months, estuaries become more saline as the water evaporates. Under such conditions, estuaries may become hypersaline (more salty than seawater) meaning few fish can survive.

As some estuaries are closed to the ocean for a period of time (up to years), humans can have a profound impact on the health of an estuary.  Obtain photos of your local estuary and discuss with your students how humans could have (or have had) a negative impact on the estuary – the poster may provide you with some ideas for discussion also.  Better still, check with your local newspaper office or maybe even your council to find some archive photos of the estuary to be compared with present day photos.

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