INTRODUCTION
The stargazer, Uranoscopus scaber (Linneaus, 1758) is one of the most important demersal fish species in the Black Sea. Unfortunately there are no catch and landing statistics for this species in the Black Sea countries. According to the latest documents prepared by Yankova et al. (Reference Yankova, Raykov, Ivanova, Diasamidze, Mgeladze, Radu, Nicolaev, Agapov, Grinchenko, Oral, Düzgüneş, Öztürk, Bat, Karpova, Shlyakhov and Boltachev2011), it is in the Red Data Book of the Bulgarian Black Sea, while it was notified as ‘vulnerable’—a high risk of endangerment in the wild—in Turkey. There is insufficient data to make an assessment of its risk of extinction in Romania, there is a low risk in Georgia and Ukraine and no data are available from Russia. On the other hand, there is no direct fishing pressure on the stargazer population in Turkey. All of the catch comes from various fishing gear (mainly from trawl nets and gillnets) as by-catch.
The age determination process of rare fish species is a serious problem for scientists working on population dynamics (Echeverria, Reference Echeverria1987; Panfili & Tomàs, Reference Panfili and Tomàs2001). In most cases the relationship between fish size, otolith size and the age of the fish is highly correlated, which is very useful for estimating some population parameters (Pawson, Reference Pawson1990; Cardinale et al., Reference Cardinale, Arrhenius and Johnsson2000; Yan et al., Reference Yan, Wu, Lu, Li and Jin2009). The relationship between the age of fish and otolith length and weight is a good example for the estimation of the age of U. scaber (Kasapoglu & Duzgunes, Reference Kasapoglu and Duzgunes2012). In order to exploit the existing stocks in a sustainable manner, having information on age and growth of fish is vital (Chalanchuk, Reference Chalanchuk1984; Das, Reference Das1994). Although there are several studies on the relationship between fish size and otolith size for Atlantic horse mackerel (Polat & Kukul, Reference Polat and Kukul1990; Bostanci, Reference Bostanci2009; Kasapoglu & Duzgunes, Reference Kasapoglu and Duzgunes2012), there is no study on the relationship between fish size, otolith size and the age for stargazer in the Black Sea region.
Otoliths are also very important in stomach content research for understanding prey–predator relationships, because they are difficult to digest (Pierce & Boyle, Reference Pierce and Boyle1991; Pierce et al., Reference Pierce, Boyle and Diack1991; Granadeiro & Silva, Reference Granadeiro and Silva2000; Battaglia et al., Reference Battaglia, Malara, Romeo and Andaloro2010).
In this study we aimed to determine all of the relationships between body size, otolith size and the age of stargazer as one of the least studied species in the Black Sea.
MATERIALS AND METHODS
A total of 268 Uranoscopus scaber samples were collected from commercial trawling and gillnetting operations. Trawl gears were usually used at 5 to 120 m depth in the area from Samsun Province (Yakakent town, Çayağzı cape: 41°41.040'N 35°25.193'E) to Ordu Province (Ordu: 40°59.288'N 37°53.430'E) between September 2011 and March 2012 (Figure 1).
Fig. 1. Sampling area in the survey.
Samples were carried in ice boxes to the laboratory of the Faculty of Marine Science, Ordu Province for further studies. Total length and weight of fish were measured and weighed to within 1 mm and 0.01 g precision, respectively. After the removal from each fish, the otoliths were cleaned and stored in labelled envelopes. Both measurement and age estimation of the otoliths were done in the laboratory after washing with 50 ml ethyl alcohol (90%) in a Petri dish of 10 cm diameter. Otolith length and width were measured at maximum vertical and horizontal axes, respectively, with a Vernier caliper to within 0.01 mm precision (Figure 2). Otoliths were weighed by PRESCIA balance to within 0.0001 g precision. A Nikon 8MZ800 binocular microscope was used for age reading after otoliths were dried for 1 h in an oven at 100°C.
Statistical analyses (regression, χ2, Student t-test) were done where necessary (Sokal & Rohlf, Reference Sokal and Rohlf1973). Microsoft Office® Excel software was intensively used for the graphs and regression analyses.
RESULTS
The ages of the stargazer were determined between 0+ to 4+. The majority of the samples were found in age group 1 + , followed by groups 2+, 3+, 0+ and 4+, due to selectivity of the fishing gear. Mean otolith weight of the total sample was found to be 44.04 ± 1.62 mg, and it was 10.23 ± 1.25 mg, 27.28 ± 0.96 mg, 51.46 ± 1.84 mg, 93.79 ± 3.47 mg and 137.20 ± 10.60 mg for ages from 0+ to 4+, respectively (Table 1).
Table 1. Otolith weight and age—frequency distribution of stargazer samples in the Black Sea in 2011–2012 fishing season.
Although length, width and weight of right otoliths are bigger than the left otoliths, the difference is statistically insignificant (Student t-test, α = 0.05) (Table 2). The same statistical conclusion is consistent for males and females with the same parameters. Thus we preferred to use right otoliths for further analyses and mean values of males and females. Mean length, width and weight of right otoliths were calculated as 6.46 mm, 3.54 mm and 44 g, respectively.
Table 2. Otolith length (mm), witdh (mm) and weight (g) of stargazer by sex.
Number of females in the sample was greater than males (F:M = 1.39:1), but there was no significant difference between sexes according to the χ2 test.
Mean length and weight of fish samples were found to be 150.48 ± 1.97 mm (86–251.00 mm) and 66.24 ± 0.83 g (10.88–246.06 g). Same parameters for age groups 0+ to 4+ were calculated as 90.67 mm and 12.21 g, 124.65 mm and 32.69 g, 166.91 mm and 81.55 g, 210.02 mm and 163.75 g, and 244.50 mm and 233.64 g, respectively (Table 3).
Table 3. Fish length and weight, otolith length, width and weight by age groups of stargazer in the south-eastern Black Sea.
The relationships between otolith dimensions and fish size are given in Table 4. Actually, this table contains all types of relationships derived from samples in this survey, that is, between fish length and otolith length (FL-OL), fish length and otolith width (FL-OWi), fish length and otolith weight (FL-OW), otolith length and otolith width (OL-OWi), otolith length and otolith weight (OL-OW), otolith length and fish weight (OL-FW), otolith width and otolith weight (OWi-OW), otolith width and fish weight (OWi-FW), otolith weight andfish weight (OW-FW), age and fish length (A-FL), age and fish weight (A-FW), age and otolith length (A-OL), age and otolith width (A-OWi) and age and otolith weight (A-OW). The relationships between weights and lengths are shown in exponential form (y = axb), and the rest of the relationships are shown in linear form (y = a + bx). The lowest correlation coefficient was found in relations between age vs otolith width (r = 0.778) and age vs otolith weight (r = 0.809), while the highest correlation coefficient was calculated for the relations between otolith width vs otolith weight (r = 0.965) and otolith length vs otolith weight (r = 0.949). Maximum values were observed in relationships, from highest to lowest, between otolith width and otolith weight, otolith length and otolith width, fish length and otolith length, otolith length and otolith width, otolith length and fish weight, age and fish length, fish length and otolith width, otolith width and fish weight and otolith weight and fish weight (range of r is from 0.965 to 0.884).
Table 4. Relationship between fish size and otolith size (linear: y = a + bx, power: y = axb) (abbreviations are given in the text)
DISCUSSION
Five age groups from 0+ to 4+ were identified in this research. Similarly, five age groups were reported by Demirhan et al. (Reference Demirhan, Can and Seyhan2007), but the main difference is that there was no fish in age group 0+ in their survey. On the other hand U. scaber in the Egyptian Mediterranean waters off Alexandria have six age groups starting from age 0+ (Rizkalla & Bakhoum, Reference Rizkalla and Bakhoum2009). The reason for the difference from the first reference may be attributed to missing rings regarding the age group 0+, and regarding the latter reference, it can be said that the warm waters of the Mediterranean Sea are a better habitat for stargazer compared to the Black Sea habitat. Additionally, habitat loses due to costal degradation and overfishing might be reasons for the absence of the 6+ age group in the Black Sea.
Although there is a slight difference between the female and male stargazer populations in the present survey, this difference was found to be insignificant. On the other hand, Demirhan et al. (Reference Demirhan, Can and Seyhan2007) found that the number of females was about twice the number of males (1:1.98) which may be due to unrepresentative samples obtained in their study. Apart from mean ratios of total females to males, the difference is much higher in age group 2+ with 1:4, age group 3+ with 1:21.5 and age group 4+ with 1:24. All these figures are evidence of irregularities in their study, which was carried out more than three years ago (January 2002–May 2005). In our survey we were not able to sample males in the 4+ age groups, whereas Demirhan et al. (Reference Demirhan, Can and Seyhan2007) were able to sample males in 5+ age groups.
Regression analyses show that there are high correlations between fish size, otolith size and age of stargazer. The most important output of the present study is that the age of fish can be estimated from otolith length more precisely than from otolith width and otolith weight, due to a high correlation rate (0.912). Similarly high correlations between fish size and otolith dimensions were found between fish length and otolith length (r = 0.936), otolith length and otolith weight (r = 0.949), otolith length and otolith width (r = 0.930), otolith length and fish weight (r = 0.927), fish length and otolith width (r = 0.897) and otolith width and fish weight (r = 0.897).
Due to lack of surveys and data on stargazer in the Black Sea, we were not able to compare our results with others. However, we believe that the equations with high correlation rates derived in this research will be useful for further studies on population analyses of Uronoscopus scaber in the Black Sea region.
CONCLUSION
The output data of this research might be a valuable baseline for researchers who pursue research on this species in the future. The findings of this research may serve as a first record regarding the relationship between age and otolith size as well as fish and otolith size in the Black Sea region.
There is no legal measure regulating the stargazer fishery in Turkey, such as minimum legal size, fishing season or gear characteristics. This fish is usually found in the market and in restaurants, where most come from by-catch during trawling operations for whiting as the main target. Authorities should take measures to regulate the stargazer fisheries by means of minimum allowable catch size and season for the sustainability of this population in the region.
The authors believe that the findings from this research will be helpful for researchers who will work on stargazer in the Black Sea region.
