Morphological variability of the 11 th generation strain of nile tilapia , ( Oreochromis niloticus ) and traditional genetically improved farmed tilapia

To investigate the morphological variations 12 morphometric and 11 meristic characters were studied for 100 individuals of the same age of traditional strain GIFT (5 generation) and recently developed 11 generation strain (GIFU) of Nile tilapia, Oreochromis niloticus. No significant difference was found between total length and other body measurements at 5% level. However, GIFU showed faster linear growth on body depth, maximum body circumference and minimum body circumference where GIFT showed faster linear growth on standard length and head length. On meristic characters, notable variations were observed in case of scale above and below the lateral line, where GIFU individuals showed greater number of scales in both cases. The total length and body weight relationship of both the strains were found to be straight line in logarithmic scales. The mean values of condition factor (K) have been found to be 1.671 for GIFT and 1.711 for GIFU and the mean values of relative condition factor (Kn) were 1.001 and 1.002 for GIFT and GIFU respectively. Findings of the present study suggested the variability of strain GIFU and GIFT in some important parameters.


Introduction
Oreochromis niloticus, popularly known as 'Nile Tilapia' all over the world, is an important commercial fish species in Bangladesh and contributes a major share in the culture fisheries.The cultivation of tilapia is becoming more and more popular due to its higher growth rate.A total of 35 seed production hatcheries have been established which are producing 10-12 billion fry every year and a number of commercial farms have also been established, which are producing roughly about 0.02 million tons of marketable size fish (Hussain, 2008).Mossambique tilapia (Oreochromis mossambicus) was the first species, which was introduced in Bangladesh from Thailand in 1954.The fish did not flourish due to its early maturation and prolific breeding habits in the ponds.As a result, producers and consumers regarded the fish as "nuisance fish" (Hussain, 2004).Genetically improved farmed tilapia (GIFT) was introduced by Bangladesh Fisheries Research Institute (BFRI) from the Philippines in 1994 and according to BFRI study the growth was found at least 40% higher than the existing stock of Tilapia (Rahman, 2005).GIFU, the 11 th generation of tilapia is a recent introduction in Bangladesh.It has 20% higher growth rate, higher fillet ratio and attractive fillet color (light pink) than other available strains of Oreochromis niloticus (Zubin Agrobased Industries Limited, 2010).No published data is yet been obtained about the general biology, growth or breeding behavior of this strain.This research work is therefore designed to identify the morphometric and meristic variations between the 5 th generation GIFT and 11 th generation GIFU strain of nile tilapia, (Oreochromis niloticus).

Materials and Methods
100 individuals of each strain of the same age and total length ranging from 115 mm to 257 mm were collected from earthen ponds of Zubin Agrobased Industries Limited, Noakhali.Total length (TL), standard length (SL), head length (HL), pre-orbital length (PreOL), eye diameter (ED), post-orbital length (PostOL), inter-orbital length (IOL), snout length (SnL), mouth gape (MG), maximum body circumference (MaxBC), minimum body circumference (MinBC) and body depth (BD) of fish were measured to the nearest mm using fish measuring board.The fishes were weighed on tanetag, KD-160 balance having one gm precision.Meristic characters which were studied are dorsal fin spines (DFS), dorsal fin soft rays (DFSR), pectoral fin soft rays (PecFSR), pelvic fin spines (PelFS), pelvic fin soft rays (PelFSR), anal fin spines (AFS), anal fin soft rays (AFSR), caudal fin rays (CFR), branchiostegal rays (BrgR), scale above lateral line (SALL) and scale below lateral line (SBLL).The body characters viz.SL, HL, PreOL, ED, PostOL, IOL, SnL, MG, Max BC, Min BC, BD were expressed as percent to total length of the fish as done by Carlender and Smith (1954) and Hile (1948).Regression of various body parts against TL of fish were drawn by least square method.
Length-weight relationship was calculated by cube law as given by Le Cren (1951).
log W = log a + b × log L Where, W is weight, L is length of fish and 'a' and 'b' are constants.
Where, K is condition factor, W is observed body weight of fish and L is observed length of fish.

′ =
Where, Kn is relative condition factor, W is observed body weight of fish (g) and W' is calculated body weight of fish (g).Regression of morphometric characters between GIFT and GIFU were compared by Snedacor (1956).

Results and Discussion
From morphometric characters it was observed that, the mean (±SD) total length of GIFU (176.73±30.37mm) was slightly larger than GIFT (173.88±33.61mm) whereas, the mean (±SD) body weight of GIFU (103.63±50.16g) was also larger than GIFT (97.03±48.98g).Other morphometric measurements for GIFT and GIFU were standard length 137.No significant (p<0.05)difference was found between total length and other variables viz.standard length (Fig. 1), head length, pre-orbital length, eye diameter, post-orbital length, inter-orbital length, snout length, mouth gape, maximum body circumference, minimum body circumference and body depth of both GIFT and GIFU (Table 1).Percentage values (Fig. 2) of standard length of GIFT (78.99) was slightly higher than the standard length (78.73) of GIFU.Again, the percentage of body depth at pectoral fin-base in GIFU (30.92) was higher than that of GIFT (30.36).A close examination of values of the characters revealed a strong heterogeneity between GIFT and GIFU.The GIFU were broader in anterior part of the body at pectoral fin than the GIFT where GIFT (28.65%) having longer head than the GIFU (27.41%).The maximum body circumference and minimum body circumference of GIFU (76.7% & 27.33% respectively) were higher than that of GIFT (74.71% & 26.46% respectively).Thus it may be inferred that GIFU showed faster linear growth on body depth, maximum body circumference and minimum body circumference where GIFT showed faster linear growth on standard length and head length.Devi et al. (1991) reported the value of head length (24.91%) of males to be higher than that of females (22.91%) and the value of depth of body at pectoral fin-base in females (21.09%) to be higher than that of males (19.50%) in Rita rita.This finding is similar to the present study.Such phenomenon was also reported by Khumar (1985).

Conclusion
Strain GIFU showed faster linear growth on maximum and minimum body circumference along with body depth whereas strain GIFT showed faster linear growth on standard length and head length.Notable variations were found in case of scales above and below the lateral line.Condition factor and relative condition factor were found higher in GIFU during the study period.For further study, genetic level works like allozyme electrophoresis, DNA based markers i.e. microsatellite, PCR-RFLP, RAPD etc. should be conducted to identify the actual reason of such variations.

Fig. 1 .
Fig.1.Relationship between standard length (SL) and the total length (TL) of (A) GIFT and (B) GIFU

Fig. 2 .
Fig. 2. Growth of different morphometric body parts of GIFT & GIFU when considering the total length (TL) as 100%