Saline Wedges in Estuaries

An interesting event of some significance to anglers happens in deep estuaries such as the Hopkins River. During winter and spring, very high flows down the rivers push-out the salt water replacing it with all fresh water; or mix with any remaining seawater resulting in salinity becoming relatively uniform from the bottom to the surface.

Once the freshwater flood flow decreases, the incoming tidal seawater slides in under the fresh water and remains separate from it. The thickness of this salt wedge and its distance upstream will fluctuate depending on differences in the amount of fresh or salt water entering the estuary. The salt wedge and freshwater upper layer tend to remain intact with a very clear boundary between the two water bodies.

The location of this boundary (called the halocline) can be determined by lowering a salinity meter down from the surface to the substrate and recording the significant change in salinity as the meter passes from the less dense freshwater layer on top into the denser salt water layer on the bottom. If the estuary has some deep holes or pools, these can fill with seawater and form a stable saline wedge or salty pool which will remain there until freshwater flows increase again in the winter.

As this saline water does not easily mix with the fresh water above, it has no means of getting oxygen from the surface and over time, biological activity within the wedge will use up the available oxygen and it will become totally anaerobic. This means that, although from the surface, this stretch of estuary appears normal, healthy and productive, fish and other biota providing food for fish cannot survive in the bottom metre or so of water. The same process occurs when the estuary is closed-off from the sea by a sand bar.

The trapped seawater also becomes stagnant and eventually anaerobic. Fishing on the bottom in the deepest water, is therefore not always the best, particularly during summer. The fish will have moved into shallower water where oxygen is present, so try fishing in these areas.

The free movement of freshwater downstream and the tidal movement of seawater into an estuary not only maintains good water quality conditions but also produces different levels of salinity throughout the estuary. This influences the distribution of fish species but is particularly important during the spawning season of fish which spawn in the estuary. For example black bream prefer to spawn in water with a salinity of around 40,000EC.

Extensive flooding of rivers during the spawning period of estuarine fish or drought situations with little freshwater flow can both affect salinity in an estuary and is one reason why spawning success of a particular species can vary significantly in different years.