Trewavas (1966) describes the differences between T. nilotica and T. aurea. and Daget and Iltis (1965) the differences between T. zillii and T. dageti. Welcome (1964) defines differences in juvenile specimens of T. zillii and T. nilotica. McConnell (1966) and Burchard (1967) are general references for Kainji Lake Tilapia. taking due note of species name changes as outlined.
Dr. Trewavas (personal communication) has a paper in press proposing the separation of Tilapia at generic level into mouthbrooders (Serotherodon) and substrate brooders (Tilapia). Thus T. galilaea and T. nilotica become Serotherodon galilaeus and S. niloticus; T. aurea will also join this genus. T. zillii and T. dageti remain as Tilapia.
Dorsal and anal fin counts were made on a substantial number of T. galilaea and a few T. nilotica. with the following results:
| Species | Standard length range, cm | Dorsal fin formula | Anal fin formula | ||
| T. galilaea | 12.8 – 32.5 | XV | - 13 | (1) | III - 11 (14) |
| XV | - 14 | (6) | III - 12 (26) | ||
| XV | - 15 | (1) | |||
| XVI | - 13 | (15) | |||
| XVI | - 14 | (16) | |||
| XVI | - 15 | (1) | |||
| T. nilotica | 26.2 – 32.8 | XVII | - 13 | (3) | III - 11 (5) |
| XVII | - 14 | (2) | |||
Note: Number of samples in brackets
The five Kainji Lake specimens of T. nilotica all had 17 dorsal spines, while the T. galilaea have either 15 or 16 spines. This appears to be one method of separating these two species, a method supported by counts of dorsal spines in small specimens-However, caution should be employed in using this criterion as in the Sudan area, Sandon (1950) found dorsal spines of T. nilotica to range from 15–18 and of T. galilaea from 15–17. Trewavas (1966) gives the range for T. nilotica as 16–18, presumably a composite of fish from the Jordan Valley, coastal Palestine, and the Nile and Niger Rivers. Burchard (1967) gives the dorsal spine count (presumably Nigerian specimens) of T. galilaea as 15–17 and of T. nilotica as 17–18.
A problem was encountered in separating small T. nilotica from T. galilaea in dipnet samples. A number of characters were tested for differences between specimens 32 mm total length (T.L.) or larger which could be easily identified, including:
Dark pigment bars on the tail of T. nilotica. These can be used for separation from T. galilaea. down to a size of 25 mm standard length (S.L), (32 mm (T.L.). Among a large number of fish below this size sampled, either there were no T. nilotica (considered unlikely), or bars on the tail are not formed until a length of 32 mm T.L. is reached (probable). The bare on the tail could be accentuated in live specimens by placing them in a clear dish over a black background.
Bars on flanks. T. nilotica was found to have from 8–11 bars and T. galilaea from 7–10. This is too great an overlap to be useful in separating the species.
Tilapia mark. This tends to be more rounded and lies anterodorsal on the dorsal fin of T. nilotica and more elongate tending to lie in a posterodorsal inclination in T. galilaea. However, the overlap was too great to be of much assistance as a separation character.
Bars on the dorsal fin behind the Tilapia mark. In T. nilotica these number two or three, are well-pigmented on the posterior tip, and tend to lie anterodorsal. In T. galilaea these generally number three, have less pigment at the dorsel end, and Tend to lie vertically or posterodorsal. Again, however, there was too much similarity to allow positive separation.
Pectoral fin length differences as indicated by Sandon (1950). The pectoral fin reaches between the anus and anal fin insertion in both species and was found to be useless for identification of snail specimens.
The lower pharyngeal teeth can be used to separate T. nilotica and T. galilaea down to a size of 25 mm S.L. In smaller fish, however, this structure has not assumed a typical shape and so cannot be used as a separation characteristic (the shape and orientation of the teeth themselves were not examined in the smaller specimens - this may possibly be a distinguishing feature).
Number of dorsal spines. In a sample of 43 T. nilotica between 32 and 52 mm T.L. with definite pigment bars on the tail, the number of dorsal spines was 17 in all cases. Among 87 T. galilaea of a size range between 32 and 90 mm T.L. the dorsal spine count was 16 in all fish. A dorsal spine count of 79 Tilapia less than 32 mm T.L. included 76fish with 16 spines and three fish with 17 spines, indicating that T nilotica was rare but not recognized by visual appearance in the sample.
The standard length-weight relationship of 506 T. galilaea from Kainji Lake is recorded in Table 1. No tendency for a difference between males and females was noted in the limited sample available. A curve is fitted to the data for combined sexes in Figure 3, in which is also plotted the points for this relationship of T. galilaea from Volta Lake (Reynolds, Adetunji and Ankrah, 1969). The length-weight relationship is similar in Kainji and Volta Lakes to a size of about 230 mm S.L.; above this size the Volta Lake fish tend to weigh less.
The standard length total length of 416 T. galilaea is recorded in Table 2 by millimetre length groups. The data when plotted (Figure 4) suggests a slight curve, which was not tested mathematically; it has been used, however, in converting these two measurements in the present study where necessary. Table 3 and Figure 5 present the standard length-total length relationships for 95 T. nilotica.
Dr. E. Trewavas (personal communication) examined two T. zillii and two T. dageti from Kainji Lake and forwarded the following observations:
| T. zillii | T. dageti | |
| Soft dorsal rays | 12 or 13 | 14 |
| Body depth | Shallower, 41.5 and 41 % S.L. | Deeper, 49.6 and 48.8% S.L. |
| Pectoral fin | Shorter, 35.8 and 32.8 S.L. | Larger, 39.5 and 37.9% S.L. |
| Mouth as measured by | Larger, 37.8 and 40% of | Smaller, 33.7 and 34.9% of |
| length of lower jaw | head length | head length |
| Pharyngeal bone | Narrower, 32% of head length (1 specimen) | Wider, 38.8% of head length (1 specimen) |
Differences between these two species are given by Daget and Iltis (1965, not seen). Dr. Trewavas has summarized these as follows: “In T. dageti the soft rays of the dorsal fin number 11–16, in T. zillii 10–13, the soft anal rays in T. dageti 8–11, in T. zillii 7–10. T. zillii generally has two longitudinal dark bands on the body as well as The vertical. In T. dageti the vertical predominate. The anal fin and lower half of the caudal are generally coloured red in W. African T. zillii; T. dageti is without this colour, the vertical fins being usually pale, the caudal sometimes bluish. Both species may have pink or red colour at the sides of the chest and lower flanks.
The bluish cast of the tail is the character by which the Kainji Lake Research Project workers have, to date, separated T. zillii from the former T. melanopleura (now T. dageti).
