Interesting new record and further notes on the occurrence of marine fish in Nyamithi Pan , Ndumo Game Reserve , South Africa

ISSN 0075-6458 123 Koedoe 45/2 (2002) Nyamithi Pan, situated in Ndumo Game Reserve, South Africa, is a floodplain pan near the confluence of the Usuthu and Pongolo rivers. It lies approximately 75 km from the Indian Ocean. The floodplain and its fish have been extensively surveyed (Coke & Pott 1970; Kok 1980; Merron et al 1993, 1994, 1994a, 1994b, 1994c, 1994d; Pooley 1975) and there are many records of the occurrence of marine fish in this and other pans of the Pongolo and Usuthu rivers. These are, however, usually isolated instances of individual fish being caught and attracting attention.

Nyamithi Pan, situated in Ndumo Game Reserve, South Africa, is a floodplain pan near the confluence of the Usuthu and Pongolo rivers.It lies approximately 75 km from the Indian Ocean.The floodplain and its fish have been extensively surveyed (Coke & Pott 1970;Kok 1980;Merron et al 1993Merron et al , 1994Merron et al , 1994aMerron et al , 1994bMerron et al , 1994cMerron et al , 1994d;;Pooley 1975) and there are many records of the occurrence of marine fish in this and other pans of the Pongolo and Usuthu rivers.These are, however, usually isolated instances of individual fish being caught and attracting attention.
Many of the records (Table 1) are old and the Pongolo and Usuthu river systems have been markedly modified in the last few decades through the construction of dams, particularly the Pongolopoort dam, agriculture and water abstraction.Fishing, mainly gillnetting, has intensified greatly and several exotic fish, particularly Cyprinus carpio (carp) have become well established in many Pon-golo floodplain pans (pers.obs.) in the last few years.
More recent, although erratic, fish monitoring in Nyamithi Pan by EKZN Wildlife staff has yielded some interesting records (Table 2) confirming the continued immigration of some species into the system and also added a new species record for the pan and river systems.More formal, though intensive, fish sampling (Merron et al. 1993(Merron et al. , 1994a(Merron et al. , 1994b(Merron et al. , 1994c) ) failed to show the presence of any marine fish in Nyamithi.In 1999, tourists reported a dead shark in Nyamithi Pan and on investigation it was found to be a Carcharhinus leucas of almost two metres.Shark fin sightings had been reported for several months but the reason for its death was not clear.There are often well over one hundred Crocodylus niloticus (Nile crocodile) in the pan but the dead shark was mostly intact and it was reported that an adult hippo appeared to be guarding the car- Natural History Notes cass and was seen to drive away any crocodiles which approached the dead fish.
One Megalops cyprinoides was caught in 1996 while, early in 2002, nine more were caught and many more were seen breaking the surface in the manner typical of the species (Skelton 1993).In 1998, one Acanthopagrus berda, of 420 mm (fork length), was caught, tagged and released and another, smaller, A. berda was caught in 2000 (Tables 1 & 2).
In early 2002, a total of nine Elops machnata, of fork lengths 650-800 mm, were caught and released.Several more were seen and the fish were in good condition.These fish represent the first records of this species in these river systems.This species has often been recorded in estuaries, but a survey of the literature ( Bruton & Kok 1980;Skelton 1993;van der Elst 1998;Whitfield 1998) yielded no mention of E. machnata appreciably upstream of estuaries, despite their ability to survive in fresh water.
All the recent Nyamithi records were made during relatively low water levels, several months after any flooding episodes.It seems likely that the fish enter the pan during high flood levels and then stay there for extended periods.The fish were released alive and so not examined internally for gonad condition.
As the M. cyprinoides and E. machnata recorded recently were in good condition and the species were still apparently abundant several months after they had immigrated, it appears that they compete well with the Hydrocynus vittatus (tigerfish) which were abundant in the pan at the time.The most recent records are the first evidence of large scale sustained immigration of M. cyprinoides and E. machnata far up rivers.Using Nyamithi as an indicator of the system's fish species composition, it appears that despite their modifications, the immigration of marine species to the Usuthu and Pongolo river systems has not stopped and may have increased in some instances.

Table 1
Historical records of the occurrence of marine fish in the Pongolo and Usuthu river systems They also appear to survive well in spite of high densities of large C. niloticus.No C. carpio have yet been recorded in Nyamithi, though they are abundant in Namanini Pan, just outside the reserve.It is possible that they fall easy prey to C. niloticus.

Table 2
Recent records of marine fish in Nyamithi pan, Ndumo Game Reserve