A Snapshot of the Fish Fauna in Lake Sebakwe, Zimbabwe: A First Record  

1. Arecent paper in this journal (Utete & Tsotsonga, 2013) gave the results of a short survey of fishes inLake Sebakwe,Zimbabwe. Whether or not this paper is of more than local interest is the editor’s decision but my comments concern its scientific quality, which is far below the standard that one would expect in an international journal.

2. This paper has three major problems. The first is a lack of the basic data that we would expect from a survey of this kind. The authors write that fish were “… counted, measured to the nearest millimeter and weighed to the nearest gram. The catch per unit effort was standardized according to the sampling method (numbers per net day for gill nets). Fish data that is species present [sic], their abundance and sex were collected for each net and the total figures for each sampling station collected” which suggests that a fairly detailed data set might be available. None of it is given in the paper, however, and all we have is the frequency (%) of each species; one assumes this is percentage by numbers although this is not stated. The authors failed even to give the total number of fish from which these percentages were calculated. This is an inadequate record of the work that was done and does not give a useful picture of the fish stocks in the lake.

3. If the authors had given the catch per unit effort it may have been possible to evaluate their assertion that “the lake has a huge potential for fisheries.” Attempts to establish a commercial fishery on this lake in the 1960s were unsuccessful as the catches were too low to justify the effort. As a government Fisheries Officer I visited the lake in the early 1970s and it appeared to be very unproductive. At that time its water was a curious greyish colour, apparently because of a heavy load of suspended particles that may have inhibited light penetration and reduced primary productivity. It could have changed over the years although the cryptic comments about “white still waters” in this paper suggest that it might not have done. It is a pity that the authors did not examine the catches made by the subsistence fisheries as this may have shed some light on its potential productivity.

4. The second major problem is that the bulk of the paper consists of a series of general statements about the fish species that occur in the lake but without any direct relevance to the findings of their survey. Most of these statements are based on citations from two books, namely Skelton (2001) and Marshall (2011) - cited incorrectly throughout the text asMarshall(2010). The authors have misquoted these two works so extensively that these general statements have little value. Some of the more egregious examples will indicate the scale of this problem.

In writing about Labeo cylindricus the authors say “This species is also commercially important throughout the African continent, having contributed significantly to various fisheries (Skelton, 2001).” Skelton makes no such statement and would not have done because L. cylindricus is of no fisheries importance anywhere in Africa owing to its small size and relative scarcity. They go on to say that “It is a cryptic species that hides under rocks and is most active at night” citing Gratwicke et al. (2003) even though these authors never made such a statement.

They then go on to discuss Marcusenius pongolensis, which they incorrectly claim is the first time this species has been reported from Zimbabwe. The species of Marcusenius that occur in Zimbabwe are nearly indistinguishable morphologically and are separated primarily by differences in their electrical discharges (Kramer et al., 2007). It is unlikely that the authors were able to measure these and one can only rely on geographical distribution to separate the species. Marcusenius pongolensis is restricted to the Limpopo River system in the south of the country while Lake Sebakwe is located in the middle Zambezi River basin, which drains the northern half of the country. Here a similar species, M. macrolepidotus is widespread and sometimes abundant in most of the larger rivers. The distribution maps in Marshall (2011) show this clearly and there is no excuse for such a misidentification. The authors go on to say “This fish is usually found in main channels but lower catchments [sic]. It does not enter still lagoons and backwaters in the lower floodplains. It feeds primarily on benthic insects and prefers white still waters.” This statement is unattributed and is largely incorrect because this species occurs in almost any habitat (Gratwicke et al. 2003; Marshall, 2011).

Similarly inaccurate statements are made about Mormyrus longirostris, the other mormyrid species in the lake. They write that “It is a demersal and potamodromous [sic] [species that] lives in caves and muddy areas, and it also hides among weeds (Marshall, 2010). It prefers quite deep waters with soft muddy bottoms and is rarely caught in marshy areas” which, again, bears no resemblance to comments about this species inMarshall (2011).

The next species they discuss is the African catfish Clarias gariepinus. They note, correctly, that it is a hardy species that can withstand high turbidity and desiccation, and is often the last species to survive in drying pools but these comments have no relevance toLakeSebakwe where such conditions do not prevail. Similarly, the discussion of this species’ value as an indicator of environmental health serves no purpose because the authors provide no evidence to suggest that water quality inLakeSebakwe is poor or that the lake is polluted in any way.

In their remarks on Tilapia rendalli, the authors write “Its natural habitats are fresh water lakes and fresh water marshes (Skelton, 2001)” once again misquoting Skelton who noted that it occurs in estuaries in Mozambique and South Africa and can tolerate salinities of 19 parts per thousand. They go on to say “It is a tolerant of a broad range of temperatures and salinity as well as high levels of silt (Marshall, 2010)” thereby also misquoting Marshall (2011) who made no such comment.

In the final paragraph the authors say that “The underlying pattern was that we sampled freshwater fish that favour clean still white waters, a factor which points to the nutrient status and quality of the water in LakeSebakwe.” This is nonsense because they give no information on other habitats in which the fish they collected might occur. They went to great lengths to point out that species like C. gariepinus and T. rendalli are hardy species that can tolerate a wide range of environmental conditions, which is at odds with the conclusion that they favour “clean, still white waters.” Moreover, there is no evidence that the authors attempted to measure even elementary physico-chemical variables such as temperature, Secchi disc transparency or conductivity, so it is difficult to see how they could have reached any conclusions about the nutrient status and water quality in the lake.

5. The third major issue is the host of smaller problems, some of which are elementary, that occur throughout the text. For example, the work is undated so no one knows the year or month in which it was done. This is an important omission because gill net catches inZimbabweare highly seasonal. There is little evidence that it was proof-read with any care because of its numerous spelling mistakes, grammatical errors, omissions, and so on. It would be tedious to enumerate them all but some examples will indicate the scale of the problem.

To begin with, the preferred citation at the top of the paper is incorrect because it gives only the first name of the first author and omits the second author entirely. Then the Abstract says that the work was done over five days but the text says that it was done over four. The discussion of L. cylindricus includes is a baffling statement that reads “This species is widely distributed throughout Africa and consist [sic] of at least 80 species which comprise 16.4% of the African cyprinid ichthyofauna.” This has no obvious meaning and no connection to the preceding or following sentences; how a “species” could consist of “80 species” remains unknown. Finally, Serranochromisrobustus should not be written as a single word and Skelton (2001) is referred to as “Skeleton (2001)” [page 36, right-hand column, line 1].

The authors conclude by expressing the hope that this paper will “… form the basis for future ichthyology [sic] studies.” Unfortunately, Zimbabwean ichthyology has progressed far beyond the level of this paper and this hope is unlikely to be realised.

References

Gratwicke, B, Marshall, B.E. & Nhiwatiwa, T. 2003. The distribution and relative abundance of stream fishes in the upper Manyame River, Zimbabwe, in relation to land use, pollution, and exotic predators. African Journal of Aquatic Sciences 28: 25-34.

Kramer, B., Skelton, P., van der Bank, H, & Wink, M. 2007. Allopatric differentiation in the Marcusenius macrolepidotus species complex in southern and eastern Africa: the resurrection of M. pongolensis and M. angolensis and the description of two new species (Mormyridae, Teleostei). Journal of Natural History 41: 647-708.

Marshall, B.E. 2011. The fishes of Zimbabweand their biology. Smithiana Monograph No 3: 290 pp. South African Institute for Aquatic Biodiversity, Grahamstown.

Skelton, P. 2001. A complete guide to the freshwater fishes of southern Africa. Struik Publishers,Cape Town: 395 pp.

Utete, B. & Tsotsonga, C.S. 2013. Asnapshot of the fish fauna in Lake Sebakwe, Zimbabwe: a first record. International Journal of Aquaculture 3: 35-37.

B.E. Marshall

Department of Biological Sciences, University of Zimbabwe, PO Box MP 167, Mount Pleasant, Harare, Zimbabwe.

Present address: 9/4074, Great North Road, Auckland 0602, New Zealand.