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Catadromy - an endangered lifestyle in South African fishes.

Catadromy - an endangered lifestyle in South African fishes.
By Mike Bruton, Anton Bok & Martin Davies.

Fishes are among the most mobile of all animals and frequently undertake long migrations in order to complete their life cycle. These migrations may be wholly within one waterbody, such as the ocean or a lake, or they may take place between different waterbodies. Fishes which migrate between the ocean and freshwater as a normal part of their life cycle are called diadromous, and their migrations may take several forms. Catadromous fishes feed and grow in freshwaters and migrate downstream to the sea to breed, whereas anadromous fishes live most of their lives in the sea and migrate into freshwater to breed. Amphidromous fishes migrate between the sea and freshwaters to both feed and breed. These migrations are typically undertaken because habitats which may be optimal for feeding by adults may not be optimal for breeding, or for feeding by juveniles.

Although southern Africa has a relatively diverse fish fauna - over 2200 marine and 245 freshwater fish species - there are relatively few diadromous fishes in the subcontinent. There are several reasons for this, and there is also evidence to suggest that those few species which do occur here may be endangered.

How many diadromous fishes?
Present knowledge is there are no truly anadromous or amphidromous fishes in southern Africa. At least six fishes (the estuarine round herring Cilchristella aestuatia and five gobies) are on the fringe of diadromy, as they have established separate breeding populations in both estuarine/marine and freshwater habitats and may migrate to the other biome to feed. However they do not habitually migrate between marine and freshwaters and are therefore at best facultatively diadromous.

The Zambezi shark has been reported to release its young in Lake St Lucia in Zululand, although mating takes place at sea. An additional 25 species of marine and estuarine fishes sporadically or frequently enter fresh- waters to feed, but these movements do not constitute an essential part of the life cycle. These species include the Zambezi shark, tarpon, needlefish, pipefish, grunter, moony, river snapper, glassy, mullet and gobies. Small numbers of introduced brown and rainbow trout appear to exhibit anadromy in the western Cape. These fishes have sporadically been reported in the sea in False Bay and probably return to the Eerste River to breed. The lack of anadromous and amphidromous fishes in southern Africa is in stark contrast to the situation elsewhere. Many salmonid fishes, such as salmon and charrs, migrate into freshwaters to spawn in the northern hemisphere, and amphidromy is common in the tropics.

The five species of catadromous fishes in southern Africa include four anquillid eels and one of the fifteen mullet species known from the sub-continent. A second mullet, Mugil cephalus, commonly enters fresh-waters, but is more common in estuaries and the ocean. The total number of truly catadromous species (5) represents only 0,2% of the total number of fish species known from the region, which is below the world average of 0,6%.

The freshwaters of southern Africa
Before we look in detail at the catadromous fishes of southern Africa let us first examine their freshwater habitats. In southern Africa the proportion of mean annual precipitation (rain, snow, mist etc.) which is converted to river flow varies from less than 25% in high rainfall areas to nearly zero in deserts, with an average of less than 10%. This is far lower than in the high rainfall areas of the northern hemisphere where the range is 25 to 75%. Moreover, in the more and continents such as ours, rain is less frequent, the sun's heat is more intense and the soil is generally drier. Any rain which does fall has to first restock underground water resources before surface runoff takes place. Furthermore, evaporation rates exceed precipitation over much of the subcontinent. As a result of all these factors river flows in southern Africa are extremely variable, and rivers are subject to floods followed by long periods of low flow. Storage reservoirs built on these rivers need to be very large in order to provide a reliable water supply and to store large inputs from occasional floods. Flow regimes downstream of these reservoirs are consequently drastically altered, and may be totally dependent on the operating criteria of the reservoirs. Clearly fishes which migrate to and fro as an obligate part of their life cycles in our rivers will have difficulty surviving under the above circumstances.

The freshwater eels
All true eels have an elongated, compressed larva known as a leptocephalus, and breed in the open ocean. Southern African anguillid eels are reported to breed in the western Indian Ocean east of Madagascar, but this is not as yet well-established. We do know, however, that their leptocephali are carried southwards by the Mozambique and Agulhas Currents. The leptocephalus changes into a glass eel just before it enters fresh- water, and the glass eel becomes an elver and then a pigmented young eel as it migrates up the river. The glass eel migrations can be massive, involving hundreds of thousands of young eels on a single night in a given river. The largest migrations occur in association with high spring tides during above average river flows and at night. The longfin eel Anguilla rnossambica is the most common eel in eastern Cape, Ciskei and Transkel rivers, whereas the mottled eels (A. bengalensis and A. marmorata) are more common in Natal.

The small eels are able to negotiate rapids and climb up moist rock surfaces adjacent to waterfalls during their migrations, and to shelter from predators in the gravel on the beds of rivers. As they grow larger they feed on aquatic insects, crabs and small fish. When the eels reach a length of about 25 cm the urge to swim up- stream decreases and they become more sedentary. At this stage they are known as 'yellow eels'. Male longfin eels reach a maximum length of about 60 cm and spend up to 1 0 years in rivers whereas females reach 120 cm in about 20 years. The mottled eels reach a larger size (1 85 cm and 18 kg) whereas the shortfin eel only reaches about 60 cm and 500 g.

Before migrating back to the ocean the yellow eels undergo marked changes. Their colouration changes from green/brown to silver and the size of the eye more than doubles. In addition, the head elongates and substantial fat reserves accumulate. The eel is now ready for the long ocean journey back to the breeding grounds several thousand kilometres away.

The freshwater mullet Myxus capensis.
The freshwater mullet is endemic to the south and south-east coasts of South Africa and occurs commonly in the freshwater reaches of rivers and coastal lakes. Recent reports have indicated that the upstream ranges of the freshwater mullet have been drastically reduced by man-made changes to rivers. In the more northern parts of the range of M. capensis in Natal there is evidence that some parts of the population spend their whole life cycle in the ocean and estuaries.

M. capensis breeds in the sea beyond the estuary mouth in winter and spring (usually June to September). The juveniles migrate into freshwater when only a few centimetres long and swim strongly upstream. The freshwater mullet feeds on dia- toms and minute algae and reaches a length of about 30 cm after 3 years. Females grow larger than males, reaching about 45 cm after 7 years, although most fishes die after 4 or 5 years. The gonads do not develop in fresh water but begin developing in the estuary during the downstream migration to spawn. Ripe-running fishes are only found at sea.

The effect of man-made changes to rivers.
The catadromous life cycles of the freshwater mullet and the four anquillid eels in the erratic rivers of southern Africa have made these species particularly vulnerable to man-induced changes to the freshwater environment. The most detrimental impact is the construction of barriers to migration and the regulation of water flows downstream of impoundments. These actions interfere with the ability of the fishes to colonise the niches for which they are adapted, and force them to live in the sub-optimal reaches of rivers near the estuary, where they have to compete with specialised estuarine fishes. Because they are unable to re- colonise large areas of riverine habitat, their abundance is reduced and, at least in the case of the freshwater mullet, their survival is threatened. Further threats to our catadromous fishes include increased sediment loads in the water, due mainly to increased soil erosion rates, and predation by alien fishes.

There are several management strategies which could reduce the impact of these changes. In order to ensure adequate water depths and flow regimes for mullet and eel migrations, base flow levels and simulated peaks need to be established in regulated rivers. Controlled releases of relatively large volumes of water in the middle of the migration season (July-August for mullet and December/January for eels) would be most effective. Juvenile eels and mullet can easily be caught and translocated above barriers to migration. This can either be done using sophisticated trapping and transport equipment or more simply by locally-based water bailiffs who could translocate the fishes over the barriers using hand nets and buckets or an apparatus resembling an Egyptian shadoof. The translocation of elvers is especially easy as large numbers of them congregate below barriers to migration.

The construction of fish ladders on the many man-made barriers to migration of eels and mullet would be very costly, but could be achieved at a relatively low cost on small weirs and low dams. These ladders should be designed to take into account the swimming abilities and behaviour of the fishes concerned. The entrance to the ladder should be at the base of the barrier for ease of discovery, which usually requires a zig-zagging stepwise structure. The location of the fish ladder on a river will also influence its design, as smaller fish will have to be catered for nearer the estuary and larger fishes in the upper reaches.

Mullet ladders take the form of a series of obstructions and hollows which allow the fish to swim rapidly from one rest point to another. Eel ladders take advantage of the eels' ability to climb, and consist of an intricate matrix of bristle or sacking through which the eels wriggle against the current. As fish ladders are useless without water, they must be positioned in such a way that they receive water for most of the year. An additional conservation method is to artificially propagate eels and mullet and restock them into depleted rivers. However, all the catadromous fishes in southern Africa are difficult to cultivate and this method does not therefore hold immediate promise.

Discussion
The basic strategy of the catadromous fishes of southern Africa is to make use of two different waterbodies, the ocean for breeding and freshwaters for feeding, but their need to migrate between the two makes them vulnerable to alterations to their migration routes. The reason for the relatively low numbers of catadromous fishes in southern Africa must be that the inland waters do not provide reliable resources which improve on conditions in the ocean. The lack of anadromous fishes (which migrate into freshwaters to breed) may, however, be due mainly to historical events as the salmon-like fishes did not have the opportunity to invade southern Africa. Those species which have been introduced (like trout) have thrived and are already showing signs of anadromy. The fact remains though that Southern Africa possesses only a few indigenous catadromous fishes, and they appear to be severely threatened by man-made changes to their migration routes and to their freshwater habitats. The migratory life style itself may therefore be threatened with extinction unless we do some- thing about it.

Acknowledgement
The authors are grateful to Alan Whitfield for comments on the manuscript