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Estuarine Fish
Estuaries are extremely productive systems, in productivity it ranks right up there with coral reefs and tropical forests as the most productive of natural ecosystems (see article on Productivity in Estuaries). Yet, it is also a very harsh environment with conditions less than optimal for either plant or animal species.
Some of the factors that contribute to this harsh environment are the following:
Salinity variations – at any one point in the mid region of an estuary, this can change from virtually completely fresh water to seawater over a period of six hours.
Temperature variations – shallow waters can heat up to the surrounding air temperature on a hot day, but deeper waters remain cool. Cooler or warmer water can also flow in from the sea during tidal action or from the river.
Oxygen level variations – levels of oxygen in the water also vary considerably. Where eutrophication is taking place it can drop right down, eg in salt marshes and in deeper water areas. Right next to such an area in the mainstream of the estuary the oxygen levels can be normal.
Tidal currents – tidal currents changing direction every six hours mean that pelagic animals (ie free-swimming animals) are continuously swept up and down the estuary; and have to spend a lot of energy on trying not to be swept out to sea or too far up the estuary.
Plants and animals normally seek a reasonably stable environment to establish themselves. They do not like extreme fluctuations in environmental conditions, and where this does occur would usually take evasive action to keep their own immediate environment as stable as possible (eg animals in the desert will usuallly burrow underground to escape the extreme heat of the day). Yet, in spite of extreme fluctuations in several environmental conditions, the high productivity in an estuary is a major attraction to plants and animals. We thus find that although an estuary have relatively few species living there, these often occur in very large numbers.
Our estuaries are popular fishing spots, but have you considered how these fish species have managed to adapt to these harsh conditions and even thrive.
All fish living in estuaries are of marine origin. Most breed out at sea, but a few (like sea horses and some smaller klipfish species) may also breed in the estuary if the conditions are right. Some species can live their whole life-cycle in the estuary, but these were originally marine species that have adapted over time. There are some freshwater species (eg salmon and trout) which migrate through an estuary to the sea, but they are not typical estuarine species, even though they have the same physiological adaptations.
Fish species found in estuaries fall into one of two categories:
Opportunistic – these use the estuary for protection and feeding during the juvenile stages (as a nursery), and some adult species will make use of the highly productive estuary for feeding purposes. Sometimes we find that older and larger fish may make the estuary a more or less permanent home (eg kabeljou), probably as protection from large predators at sea (sharks). With opportunistic users it is usually only a relatively small proportion of the total population that actually enters an estuary.
Essential – some species have an essential estuarine phase, which means that a major proportion of the population will enter estuaries at some stage of its life-cycle. One well-known example is the so-called freshwater eel, which is actually of marine origin, but spends most of its adult life in freshwater; however it must return to its breeding grounds in the sea to reproduce. It is also thought that some species like steenbras (and possibly other species in that group) must have an estuarine phase to become sexually mature.
The extreme conditions means that life is not easy for a fish in the estuary, the most important physiological adaptation is to be able to adapt to salinity changes.
The body fluids of a fish has salt and mineral content lower than sea water – this means that in the sea a fish will actually lose water and tend to dehydrate. To compensate the fish extracts the salt from the water it swallows and excretes it in highly concentrated urine – in the process retaining water in its body.
Sea water       Fresh water
In fresh water the biogenic salt content is higher than the surrounding water – this means that water will tend to enter the fish’s body and make it swell up and burst!  Here the fish must extract as much salts from the water as possible and retain it in its body, in the process excreting as much water as possible in the urine.
Estuarine fish have adapted to use both mechanisms – in seawater they excrete salts and retain water, and when the water gets fresher, they excrete water and retain salts. This remarkable physiological adaptation allows these species to exploit both marine and freshwater (or semi-freshwater) environments.
The change-over from one "system" to another does not take place very quickly, it can take several days for a fish to move from a completely marine to a freshwater environment, slowly adapting as it moves up the estuary.
This adaptation is not all that common, and is the main reason why relatively few fish species have been able to adapt to life in estuaries. In South Africa out of the total of approximately 2000 marine fish species, there are only about 20 species that regularly occur in estuaries, a few more may at times enter estuaries but these normally stay in the lower region of the estuary where it is mainly seawater conditions.
So next time you see fish swimming around in the estuary, or catch it on your line, think of the amazing processes in its body to ensure that it not only survives, but thrives under such difficult conditions.
Written by Niel van Wyk. Based on talk given to Friends of Rietvlei in 2007.
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