The Feasibility of Using Eye Lens Diameter and Weight as an Age Indicator in the Indian Mackerel Rastrelliger Kanagurta (Cuvier, 1817) Collected from the Sea of Oman

Laith Jawad1, Juma AL-Mamry1, Luqman AL-Hassani1 and Nadir AL-Abri1




1 Marine Science and Fisheries Centre, Ministry of Fisheries Wealth, Sultanate of Oman, P.O. Box 427, Postal Code 100 Muscat; e-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it




Total 395 specimens of Rastrelliger kanagurta have been collected from the coasts of Muscat City at the Sea of Oman. The ocular lens diameter and weight were tested as an additional age indicator to that using operculum and preoperculum bones. The results showed that this technique could be adopted for determining the age of the species Rastrelliger kanagurta when the specimens are in the second year of age in case of eye lens diameter and weight. The method is especially useful for age determination when otolith or scale ring are not visible or when false rings give erroneous reading.

Keywords: Eye lens diameter, eye lens weight, ageing, Rastrelliger kanagurta




Eye lenses as an age indicator have been applied to a wide variety of animals since proposed by Lord (1959). Friend (1967) used this technique for birds and animals other than fish. Teska and Pinder (1986) used eye lens weight to determine the effect of nutrition on age determination in vertebrates.

The application of this technique is so limited in these animals, it being only possible to distinguish between juveniles and adults. Several authors concluded that both eye lens parameters (lens diameter and weight) can be used to estimate the age of fishes (Carlton and Jackson, 1968; Burkett and Jackson, 1971; Crivilli, 1980; Saleem et al., 1990; Douglas, 1987; Al-Hassan et al., 1991, 1992; Al-Hassan and Al-Sayab, 1994; Conides and Al-Hassan, 2000; Jawad, 2001, 2003, 2004; Jawad et al., 2001).

Age determination is an important step in the process of studying growth in fish species. The method involves counting of scale or otolith annuli and usually requires the measurements of a large number of specimens (Fletcher, 1991). Otolith and scale readings require a variable and considerable effort to prepare each specimen and even then the readings are subjected to both systematic and random errors in interpretation and require independent validation (Beamish, 1979).

Thus, a considerable time is needed to acquire the skill necessary for consistent interpretation of the materials. In addition, extra readings are usually needed in order to verify the age assigned to a specimen (Sanderman, 1969).

The aim of this study is to determine the validity of the eye lens diameter and weight as age indicators in the Sea of Oman fish, Rastrelliger kanagurta, and to establish a faster method for ageing fishes beside the conventional methods of scale and otolith.


Material and methods

Specimens of Rastrelliger kanagurta (395) were collected from the coasts of Muscat City at the Oman Sea during the period February 2010-May 2010. Fishes were taken to the laboratory and the diameter and weight of the eye lens were taken to the nearest mm and g respectively following the procedure of Jawad (2004). The lenses were extracted, dried at room temperature (25°C). The measurement of the lens in each side of the animal was kept separate. The large bone such as operculum and preoperculum were used to determine the age following Al-Hassan and Al-Sayab (1994). The bones on both left and right sides were twice read independently, using an ordinary dissecting microscope for verification. One way analysis of variance followed by Duncan's multiple range test (Harraway, 1997) were applied to test the differences between the total length of the fish and its age.


Results and Discussion

The age of Rastrelliger kanagurta samples ranging from less than one year (young of the year) (180 specimens) to two years (35 specimens) however sufficient number of two years old fish was not obtained to provide an adequate sample. The total length observed in different age classes of the species in question showed that body size is variable within an age class and considerable overlap exists between these age classes (P> 0.05). This is considered as one of the reasons for using eye lens diameter as an age indicator (Figure 1).

The average lens diameter showed a considerable increase with age for the species under consideration (Figure 2). This increment is obvious in fishes belonging to age class I & II (P>0.05). The overlap in lens diameter between young of the year class and Classes I & I+ invalidates any accurate age determination for fish samples younger than two years of age. Carlton and Jackson (1968) and Jawad (2001) reached the same conclusion with carp and tilapia respectively when working on a small sample size and with fish not older than five years. Thus, only two years old can be effectively separated from the remaining age groups on the basis of lens diameter (P<0.05). On the other hand, it is possible to differentiate fishes belonging to 0+ from those of one and two years old on the basis of eye lens weight (Figure 3).

Jawad et al. (2001) have tested the eye lens diameter and weight technique for aging R. kanagurta collected from the Red Sea coasts of Republic of Yemen.

Their results showed that this technique cannot be adopted for this purpose as the ranges of eye lens diameter and weight of the different age groups tested are overlapped. These results differ significantly from those of the present study. The growth rate of R. kanagurta from the two different areas might be the reason behind such differences. Al-Hassawi and Mohammed (2004) studied the growth rate of R. kanagurta from the Omani and Yemeni waters. They concluded that R. kanagurta has higher growth rate in Omani waters than in the Red Sea coasts of Yemen in particular and Yemeni waters in general.

Gerking (1966) showed how different environmental factors could alter the growth rate in the bluegill sunfish (Lepomis macrochirus). Rafinesque (1819) and Swedberg (1965) summarized the various growth rates of drum (Aplodinotus grunniens), and Rafinesque (1820) from different areas in the United States. Environmental conditions must be considered in applying the lens technique (Burkett and Jackson, 1971). Crivilli (1980), working on carp, stated that in the reproductive period energy is transformed from somatic to gonadal growth. Since the increment in lens diameter and weight is closely correlated with somatic growth, the variation in individual reproductive development could result in an increased variation in lens weight within an annual group. In other words, the growth rate during the reproductive period drops down due to the concentration of body on the reproductive metabolism. This drop in growth rate will affect the different parts of the fish body including the eye lens. This will end up giving variable results not in accordance with the general growth rate of the individual (Wootton, 1990).



Figure 1: Total length vs age (determined from opercular bone) of Rastrelliger kanagurta. Vertical bars represent range of fish total length and horizontal lines represent mean fish length



Figure 2: Lens diameter vs age (determined from opecular bone) of Rastrelliger kanagurta. Vertical bars represent total range of lens diameter and horizontal lines represent mean diameter



Figure 3: Lens weight vs age (determined from opercular bone) of Rastrelliger kanagurta. Vertical bars represent range of lens weight and horizontal lines represent mean lens weight.



We would also like to thank the Ministry of Fisheries Wealth, the Agriculture and Fisheries Development Fund and Marine Science and Fisheries Centre for giving us the opportunity to work on the fish samples within the qualitative and quantitative distribution of marine organisms in Sultanate of Oman and to provide the appropriate financial support.