1 Introduction

Population characteristics and age composition of a fish stock are important parameters used in stock assessment models in fisheries management. The age profile of a stock gives an indication on how healthy the stock is Metin and Ilkyaz (2008). Age determination in fish can be carried out with direct (observation on hard structures) or indirect (marking and recapture, length–frequency analysis, etc.) methods (Morales-Nin 1992). Age determination in bony fish can be carried out using the sagittal otoliths which are the largest otoliths in most bony fishes (Platt and Popper 1981; Gauldie 1988). Because of their accretionary growth and species (sometimes population) specific shape, they can be used in fish aging (Karlou-Riga 2000), determination of stock relationships and stock identification (Cardinale et al. 2004). Depositions of annual growth rings formed in this tissue are caused by seasonal changes in the environment (İlkyaz et al. 2011). By determining the age of a large number of individuals, it is possible to build up a picture of the age structure of the whole population. Knowledge of the age structure provides an indication of how the stock is measuring up to exploitation (Metin and Ilkyaz 2008).

Fishes of the family Sparidae, commonly known as seabreams, are among the most important fishes and contain many species of commercial and recreational importance throughout their range and some that are used for aquaculture (Smale and Buxton 1985; Foscarini 1988; Kailola et al. 1993; Sommer et al. 1996; Ingram et al. 2002). The king soldier bream Argyrops spinifer (Forsskal 1775) is a large sparid fish that is distributed throughout the Western Indian Ocean extending eastward to the Indo Malayan archipelago and northern Australia (Sommer et al. 1996). Although some data on population characteristics and age of A. spinifer have been published in the last three decades (El-Sayed and Abdel-Bary 1995; Grandcourt et al. 2004; Al Mamry et al. 2009; Al-Kiyumi et al. 2013; Ghanbarzadeh et al. 2014a, b), currently, very little is known about this species in the northern Persian Gulf. The objective of this study was to determine the population characteristics of A. spinifer such as growth parameters and age composition to provide information required for their management in this region.

2 Materials and Methods

This study was carried out in the northern Persian Gulf along the Bushehr coasts, Iran (28°55′19″N and 50°49′59″E) by monthly sampling intervals from June 2010 to May 2011. A total of 622 specimens of A. spinifer were collected from the fishermen just after the fishing and transferred to the laboratory on ice. Total length (TL) was measured to the nearest 0.1 cm and the total weight (TW) to the nearest 0.01 g for small specimens and 1 g for larger specimens. The individuals were sexed by macroscopic examination of the gonads. A cut was made just behind the eye and the Sagitta was removed with forceps. The otoliths were cleaned and stored dry in code-numbered envelopes. The age was determined by reading growth rings on the otoliths. First, otoliths were polished and then immersed in a solution of glycerin and read whole under a compound microscope using reflected light. Opaque rings from the nucleus to the margin were counted along the longest axis of the otolith. As a rule, each otolith was read three times and only coincident readings were accepted. Since the otoliths of A. spinifer are thick, to validate this method, some of them were sectioned and re-examined. For sectioning of otoliths, one otolith from a random subsample of 50 individuals was embedded in epoxy resin, acid (as fixing material) and cobalt by a proportion of 25:1:0.5, respectively. After cutting otolith sections, the specimens were mounted for polishing to reach approximately 0.5 mm in thickness. Otolith sections were then immersed in a solution of glycerin and read under a compound microscope at 12.5× magnification using reflected light (Campana and Neilson 1985). Results did not show differences between the two reading methods, thus age was estimated from whole otoliths for the rest of the specimens. Also, the age reading was performed by three investigators to assure the accuracy of the readings.

The condition factor was measured using Eq. (1)

$$C = \, W/L^{3} \; \times \;100.$$
(1)

Growth was modeled using the Von Bertalanffy (1938) growth equation (Eq. 2)

$$L_{t} \; = \;L_{\infty } \left[ {1 - e^{{ - K\left( {t\, - \,t_{0} } \right)}} } \right],$$
(2)

where L t is the fish total length at age t, \(L_{\infty }\) the asymptotic total length, K is the growth coefficient and \(t_{0}\) is the hypothetical age when fish would have been at 0 total lengths.

Parameters of the length–weight relationship were calculated for both sexes and for the whole sample, by fitting the power function to length and weight data (Eq. 3).

$${\text{TW}}\; = \;a \times {\text{TL}}^{b} .$$
(3)

Pauly (1984) t test was used to determine if the slope of relationships was significantly different from 3. Statistical differences between the means were determined by independent-sample t test. To test for possible significant differences between the sex ratio from the 1:1, Chi-square test was used in SPSS.18 software.

3 Results

A total of 622 fish were collected for this study during a full year (Table 1), among which 231 (37%) were males, 301 (49%) females, 20 (3%) immature and 70 (11%) undetermined sex. The overall sex ratio was 1 M: 1.3 F which was significantly different from the expected ratio (Chi-square test, p < 0.05). The number, mean, maximum, minimum and SD values of the total length and weight for males, females and immature are presented in Table 2. No significant differences were found between males and females in mean length and weight (p > 0.05). The highest number of individuals in both sexes was observed in 20.1–25 and the least in the 45.1–50 size groups (Fig. 1).

Table 1 Mean observed length and weight-at-age (±SD) for different age classes of males and females of A. spinifer off the northern Persian Gulf coast (June 2010–May 2011)
Full size table
Table 2 Month sample size, mean length and mean weight for all fish of Argyrops spinifer off the northern Persian Gulf coast (June 2010–May 2011)
Full size table
Fig. 1
figure 1

Size group frequency distributions of males, females and immature Argyrops spinifer off the northern Persian Gulf coast (June 2010–May 2011)

Full size image

Mean total length and weight of each age group is presented in Table 3. A pair-wise comparison of the mean length and weight-at-age of males and females showed that there was a significant variation between mean length and weight of age classes 1–6 in the two sexes (p < 0.05) (Table 4).

Table 3 Number, range and SD values of the total length and total weight for males, females and immature Argyrops spinifer off the northern Persian Gulf coast (June 2010–May 2011)
Full size table
Table 4 Mean observed length and weight-at-age (±SD) for different age classes of males and females of Argyrops spinifer off the northern Persian Gulf coast (June 2010–May 2011)
Full size table

Only 532 sexed male and female fish were aged. The resulting sections showed a ring pattern that was very clear and the fish could be aged easily and the use of sagittal for age estimating of the king soldier bream proved straightforward, with distinct opaque and translucent bands surrounding a white opaque nucleus. The distance between bands became smaller from the core towards the outer margin of the otoliths. The ages of A. spinifer ranged from 2+ to 25+ years, with male fish attaining the oldest age (25+ years for males and 21+ years for females). The majority of fish were within a restricted age range of 2+ to 8+ years (Fig. 2).

Fig. 2
figure 2

Age-frequency (n = 532) distributions of male and female of Argyrops spinifer off the northern Persian Gulf coast (June 2010–May 2011)

Full size image

4 Discussion

The total length of males and females ranged from 15–61.2 cm to 13.5–64.2 cm, respectively. Total length of males and females of A. spinifer in the Arabian Sea was reported as 20–68 cm and 24.5–70 cm, respectively (Al Mamry et al. 2009), slightly higher than those of present study. The maximum observed fork length reported by Mcilwain et al. (2006) in the Arabian Sea was 57.6 cm in males and 58.5 cm in females.

Mcilwain et al. (2006) reported the weight in males of A. spinifer from 169 to 3940 g and in females from 157 to 4020 g. The highest number of individuals in both sexes was observed in the 20.1–25 cm and the least in 45.1–50 cm size groups. Similar finding was recorded in the Arabian Sea for A. Spinifer (Mcilwain et al. 2006). The most frequent size class of males reported by Al Mamry et al. (2009) was in >60 cm size class and in females was in 50–54.9 size class. It appears that observed differences in linear distribution of these populations are related to differences in applied fishing gears, mesh size, skewness in sampling, different ecological conditions, different seasons, different habitats and interspecific differences, as observed in other fish species (Keivany et al. 2012, 2014; Alavi-Yeganeh et al. 2011; Daneshvar et al. 2013; Ghanbarifardi et al. 2014; Hasankhani et al. 2013, 2014; Asadollah et al. 2017).

The overall sex ratio was in favor of females as reported in previous studies and explained as a result of protogynous hermaphroditism (Grandcourt et al. 2004; Al Mamry et al. 2009). Differences in sex ratio might be related to the interspecific differences in adapted population of a species to different ecological conditions, different in the date and time of capture, fishing gears, location, different growth rates, different mortality in males and females, migration of mature fishes from the region and different behavior in male and female fish species (Qasim 1966; Pitcher and Hart 1982; Rajaguru 1992; Keivany and Soofiani 2004; Soofiani et al. 2006; Asadollah et al. 2011; Abaszadeh et al. 2013; Tabatabaei et al. 2014; Keivany and Daneshvar 2015).

In many sparid species, age determination is difficult since their otoliths are very thick and cloudy so that light cannot pass through. This difficulty is also related to a consequence of the phenomenon of stacking zones towards the otolith edge, especially in older fish (Van Der Walt and Beckley 1997). Sparid fishes, in general, are long lived, displaying a steady and slow rate of growth (El-Sayed and Abdel-Bary 1995; Ghanbarzadeh et al. 2013, 2014a). The oldest age estimated in this study was 25 years as observed in Oman waters by Al Mamry et al. (2009). The maximum reported age for this species from Qatari waters as 18 years is more likely a mistake as suggested by El-Sayed and Abdel-Bary (1995), because ages were not validated and fish 70 cm in length were excluded. Al-Kiyumi et al. (2013) in studying growth, mortality and yield per recruit of the king soldier bream A. spinifer (Sparidae) in the Oman coast of the Arabian Sea refuted the maximum age determination of Al Mamry et al. (2009) as 25 years, however, their specimens hardly exceeded 61 cm in total length. The annuli formation pattern of otoliths of the king soldier bream of the northern Persian was consistent with those observed in other sparids and teleosts. Seasonal growth cycles might be related to physiological changes produced by the influence of temperature, feeding regime and reproductive cycle (Morales-Nin 1989). Longhurst and Pauly (1987) hypothesized that the formation of growth checks in structures used for ageing fishes may require seasonal changes in water temperature of at least 4 °C. The evidence presently available suggests that a seasonal temperature difference of 6 °C might be sufficient to cause ring formation (Nevman et al. 2000). Seasonal deposition of alternative rings has been demonstrated for this species in other regions such as the Arabian Sea by Al Mamry et al. (2009) and in the southern Persian Gulf by Grandcourt et al. (2004) and also in other sparid species such as Diplodus sargus (Pajuelo and Lorenzo 2002) and Diplodus vulgaris of the Canary Islands (Pajuelo and Lorenzo 2003).

In conclusion, the king soldier bream is a long-lived species, females are dominant, live longer and reach higher lengths and weights and the growth pattern in both sexes is negative allometric.