Aspects of the life history of Acipenser stellatus (Acipenseriformes, Acipenseridae), the starry sturgeon, in Iranian waters of the Caspian Sea.
The starry sturgeon, Acipenser stellatus, (Pallas 1771), limited to the Black, Azov and Caspian Seas, is one of the most important sturgeon species. The largest populations are now concentrated in the Caspian Sea, from which they migrate into the Volga, Kura, Ural, Terek, Sulak and Samur rivers (Berg 1948). They also live in the Iranian rivers Sefid-Rud and Gorganchaii (Rostami 1961). This species is commercially one of the most common sturgeons in catches from Iranian coastal waters of the Caspian Sea, comprising about 45% of the total sturgeon catch (Moghim 2003).
However, official reports of the sturgeon catch in the Caspian Sea show that landings have sharply decreased from 24.8 thousand tons in 1975 to 0.74 thousand tons in 2004, as was confirmed by the Caspian Research Institute of Fisheries (Casp-NIRKH) (Pourkazemi 2006). The starry sturgeon is a highly valuable species, a long-lived fish that grows and matures slowly with a low rate of natural mortality (Billard & Lecointre 2001). These characteristics, coupled with high and unregulated commercial fishing, habitat loss and environmental degradation (such as the accumulation of pollutants in sediments, the damming of rivers, and the restriction of water flows) have negatively influenced the migration and reproduction of these fish (Birstein et al. 1997; Billard & Lecointre 2001). Their stocks have been reduced, as have some other marine and freshwater fish species world wide (Myers & Worm 2003; Pauly et al. 2003; Safina et al. 2005). Quantifying the nature and magnitude of the differences in the life history parameters of the starry sturgeon, as an example of a fish species that is under fishing pressure, warrants further investigation into the age structure and mortality rates. The period between spawning is shorter for males than for females (Makarov 1970; reviewed by Holcik 1989) and males reach sexual maturity 2-3 years earlier than females (Holci k 1989). Hence, because males appear more in inshore and estuarine waters for the spawning activity than females, they are more susceptible to fishing. The late age at maturity and the 2-year spawning interval in the starry sturgeon probably inhibits population recovery (Pikitch et al 2005). Effective management of the starry sturgeon fishery would be sensitive to biases in the estimation of their life history traits (Brennan & Cailliet 1989; 1991).
The estimation of sturgeon age is commonly made from cross sections of the pectoral fin rays (Koch et al. 2008). These bony structures provide the greatest precision for age estimation, and unlike other structures such as opercles, clavicles, cleithra, and medial nuchals, they can be obtained without killing the fish (Brennan & Cailliet 1989). Pectoral fin sections of the starry sturgeon are suitable for aging since they are easily collected, processed and have legible, precisely interpretable growth zone (Brennan & Cailliet 1989; Stevenson and Secor 2000).
The objectives of this study were to estimate ages at size, somatic growth, population age structure and mortality of starry sturgeons collected in 2008-2010 to determine whether changes in life history parameters have occurred during this recent period of increased fishing mortality.
MATERIAL AND METHODS
Study sites and sampling
A total of 69 specimens of the starry sturgeon, Acipenser stellatus, were obtained from Iranian coastal waters of the Caspian Sea between October 2008 and June 2010 (Fig. 1). Sixteen samples were obtained from commercial catches of Iranian fisheries (for the restocking program) using anchored gill nets. Additionally, 53 specimens were obtained from beach seine fisheries to provide a broader range of fish. The gill nets used to collect the samples measured 18 m long by 2.1 m deep with a mesh size (stretched knot to knot) of 100 mm.
Gutted weight (W) to the nearest kilogram, and total length (TL) and fork length (FL) to the nearest centimeter were recorded after sacrificing all samples. Fish were sexed and staged by macroscopic examination of the gonads. The gender of three fish could not be determined and therefore they were classified as immature. The right pectoral fin rays were removed for the purposes of age analysis (Chugunova 1959; Rien & Beamesderfer 1994).
Processing of the pectoral fin ray
The fin rays were placed in water at 60-70[degrees]C for 10 min (Jearld 1992) to separate soft tissues. They were then defleshed with a stiff brush and placed on filter papers to dry. Transverse sections were obtained using a fret saw and were polished with 250 and 400 grit sandpapers successively until a thickness of between 0.3-0.6 mm was achieved. Samples were cemented to a glass slide with clear nail polish to keep them immobilized. Glycerol was used to enhance the differentiation between the rings and to aid in the examination of growth increment formation under transmitted light using a microscope system and a camera (Motic China Group Company) that displayed the image on a video monitor. The final magnification used varied from 10-40x.
Bias and precision analyses
All sections were read twice by the first reader (the first author) and 17% of the samples were randomly chosen for a second reading by an expert on sturgeon aging to determine the percentage agreement (PA). All sections were read blind without reference to fish size. Age estimates were accepted if counts were identical between readings. If counts differed by 1 year, fish were allocated the higher age on the basis of the conservative assumption that a reader was more likely to underestimate age by overlooking a partly obscured section than to overestimate age by counting an anomaly like false bands (Brennan & Cailliet 1989). If counts differed by 2 years or more, sections were read again until agreement was within one year.
To assess the precision of ring counts of the fin ray sections provided using images, the ages of all 69 samples were estimated by the first reader from the one section provided, using both the direct reading and estimates from the images. The average percent error (APE) between the ring counts using the two techniques was calculated using the following formula (Beamish & Fournier 1981):
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
where N is the number of fish aged in the subsample, R is the number of times the age of each fish was estimated, Xij is the ith determination for the jth fish, and Xj is the average estimated age of the jth fish. A paired sample T-test was used to determine if there was a difference between the ages estimated using the two techniques.
In addition, the precision of age estimates was also assessed between the two readers by the coefficient of variation (CV) as indices of precision (Chang 1982). The CV is described as the ratio of the standard deviation of age estimates to the mean:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
where Xij is the ith determination for the jth fish, Xj is the mean age of the jth fish, and R is the number of times the age of the fish was estimated. The CV was averaged across the samples for each reader to derive a mean CV.
The Chi-square test of independence was used to examine differences in age frequency distributions between the sexes. The data from four seasons were pooled for age frequency distributions between the sexes.
Growth estimates and curves
Parameters of the length-weight relationship were achieved by fitting the power function to length and weight data (Ricker 1975):
W = a [FL.sup.b]
where W is the gutted weight, a and b are regression constants and FL is the fork length. The condition factor (CF) was calculated by [CF=W/FL.sup.b], and the comparison of CF between the genders was performed by applying the ANOVA test (Saborowski & Buchholz 1996).
The length and weight of the starry sturgeon estimated in this study were compared with the mean length and weight obtained from a previous study (Taghavi Motlagh 1996) in the south Caspian Sea, using the Student s one sample t-test (Zar 1996). Linear, logarithmic, power, Von Bertalanffy, and exponential equations were tested to acquire the best fit of size-at-age data. Finally, growth was characterized using the von Bertalanffy growth function, fitting to size-at-age data using standard nonlinear optimization methods (least square method). The von Bertalanffy growth function is explained as:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]
where [L.sub.t] is length at age t, [L.sub.[varies]] is the asymptotic length, K is the growth coefficient and [t.sub.0] is the hypothetical age at which length is equal to 0. The comparison of von Bertalanffy growth curves of the two genders was done using an analysis of the residual sum of squares (ARSS) (Chen et al 1992). Shabany et al (2003) revealed that there is only one population of the starry sturgeon in Iranian coastal waters of the Caspian Sea and therefore data from different locations in the south Caspian Sea were pooled in this study.
For an estimate of the annual instantaneous total mortality coefficient (Z), we used the formula (Gulland 1983):
Z = K(L.sub.[varies]] - L)/(bar.L - L') where Z is the instantaneous total mortality coefficient, K is the shape parameter from the von Bertalanffy growth equation, [L.sub.[varies]] is length at infinity from the von Bertalanffy growth equation, L is the mean fork length at capture and L' is length for which all fish of that length and longer are under full exploitation. Following the calculation of Z, annual mortality can be calculated through the formula A = 1 - [e.sup.-z]; survival rate through the formula S=1-A (Ricker 1975).
All statistical analyses were performed with the SPSS (Version 13) software package.
Starry sturgeon females had a higher fork length and body weight (Table I) compared to males. The relationship between length and weight was best expressed using a geometric mean functional regression (Fig. 2) as described by Ricker (1973). The condition factor (CF), expressing the feeding activity during the year, was not significantly different between the sexes (F = 0.49, df 1, P = 0.83).
Table I. Descriptive length and weight composition of the male and female starry sturgeon from Iranian waters of the Caspian Sea in 2008-2010. Size Sex N Mean S.E. of mean Minimum Maximum FL (cm) Female 31 126.03 2.89 91 173 Male 38 110.46 1.83 83 125 W(kg) Female 31 7.17 0.25 5 10 Male 38 4.93 0.24 2 8
Bands were present in all the fin sections examined. Growth bands (Cailliet & Goldman 2004) were more widely spaced near the origin and in older fish were usually more tightly grouped toward the outer edge. Tight groupings of growth rings were a common problem in age determination, and they occurred even in the youngest samples. Moreover, incorporation of secondary fin rays into the posterior lobe of the first ray could potentially lead to errors in age determination. Two independent readers agreed on the same age 58% of the time, and differed within [+ or -] 1 band counts 17% of the time. Because of high agreement and precision between the two readers on the subsample (PA = 75%, CV= 4.55), only one reader continued with the age estimates for all the samples. There was no significant difference between the counts taken by the two techniques (the direct reading and readings from the images) (t=-1.33, df 68, P=0.188). The average percent error (APE) was very low (0.95%), suggesting no difference between the profiles of pectoral fin rays and their images. An APE of less than 5% is indicative of consistent interpretation of age (Morison et al 1998).
The estimated ages for females caught in 2008-2010 ranged from 9 to 29 years with a median age of 17 and for males the ages ranged from 9 to 17 years with a median age of 13 (Fig. 3). Males and females had dissimilar age-frequency distributions in Iranian coastal waters of the Caspian Sea ([x.sup.2]=36.63, df 14, P>0.001) The Iranian coastal waters of the Caspian Sea starry sturgeon had a higher proportion of females in the 18 and 19 year classes as compared to the males. Approximately 68% of females and 97% of males were less than 17 years old. The frequency of the starry sturgeon younger than 12 years is low (Fig. 3), suggesting that these age classes were under sampled, and the maximum age recorded for the starry sturgeon was 29 years.
Growth trajectories were significantly different between the sexes for the starry sturgeon (F=3.79, df 3, 67 P > 0.05). The growth curves for both sexes therefore were plotted separately (Fig. 4). The predicted length at age for females was greater than for males (Fig. 4).
For the starry sturgeon in Iranian coastal waters of the Caspian Sea, the mortality rates (Z) for females and for males were calculated as 0.79 and 1.08 per year and the annual mortality (A) as 55% and 66% respectively. Additionally, the survival rates of the starry sturgeon in Iranian coastal waters of the Caspian Sea were found to be 45% for females and 34% for males.
Life history characteristics can be used to classify the vulnerability of a species to fishing pressure and to judge the level of productivity within a population (Musick 1999; Roberts & Hawkins 1999). The slow growth and high longevity of the starry sturgeon in combination with their big size and the late achievement of sexual maturity cause much variation in their life-history characteristics. The age estimate precision from pectoral fin ray for the starry sturgeon is acceptable compared to the Atlantic sturgeon (Acipenser oxyrinchus) (Mitchill 1814) (Stevenson & Secor 2000) and the white sturgeon (Acipenser transmontanus) (Richardson 1836) (Rien & Beamesderfer 1994) that reported higher CV for these long-lived species. Nevertheless, difficulties in reading the fin sections were caused by damaged sections, abnormal or compressed bands on the anterior fin ray margin of older fish.
It is assumed that the size and associated age structure were representative of the study populations because samples were obtained throughout the year. Part of the existing starry sturgeon population in the south Caspian Sea originated from hatcheries, but due to their long life, these fish may be considered to represent a wild population. The maximum recorded age for the starry sturgeon in a study made in 1976-1978 (Pirogovskii & Fadeeva 1982) was 35 years. Makarova and Alekperov (1988) found a similar age structure to that in the present study which was same as the record previously by Taghavi Motlagh (1996). The ages of the starry sturgeon captured in the Iranian coastal water of the Caspian Sea fishing sites were 4 to 27 years, in which the highest proportion of age classes for males and females were 11 to 14 (76%) and 9 to 12 (75%) years old respectively (Taghavi Motlagh 1996). The predominance of females in the older age classes accords with observations made by Pirogovskii and Fadeeva (1982) in the whole Caspian Sea, and appears to be a life history strategy (Roff 1984) because of the bigger maximum length and probably longer life span of females.
The number of females aged 18 years and over exceeds that of males (Fig. 3) which can be a result of the more frequent spawning migrations of males to inshore waters (Scarnecchia et al 2007). This result seems to indicate indirectly that males, which migrate to spawn at a younger age and return to spawn at more frequent intervals, have a higher mortality coefficient. The combination of the higher mortality rate of males and the greater years of vulnerability to fishing is reflected in the much higher prevalence of females aged 18 years and more.
The mean fork length of females was the same as that reported for females in 1990-1994 (Taghavi Motlagh 1996). However, males showed a decreased fork length in 2008-2010 and also the mean fork length of both females and males was lower than that reported by Veshchev and Novikova (1986) in the Volga. These differences might be related to the capture of predominantly the mature migrating fish in this area, while the catches in the southern part contained both mature and immature fish (Taghavi Motlagh 1996). Moreover, the average weight of starry sturgeon of both sexes showed a decrease com-pared to the fish studied in 1990-1994 (Taghavi Motlagh 1996) and were lower in weight than the larger, mature fish which enter the Volga river (Veshchev 1979). The weight-length relationship of the starry sturgeon appears to be dependent on age, sex, and the feeding conditions in the habitat (Babushkin & Borzenko 1951; Putilina 1981). Although the type of sampling gear may introduce a bias in the size structure of the fish, in this current study the use of both gill nets and beach seine nets allows for representation of all exiting sizes of the starry sturgeon.
The length and weight-at-age relationships were asymptotic in form, with the majority of growth being achieved early in life. There was a comparatively small increase in size after that, similar to the growth scenario for the white sturgeon, Acipenser transmontanus (Brennan & Cailliet 1989). The reduction in growth rate coincided with the age at sexual maturity, suggesting a physiological shift from somatic growth to reproductive development. The von Bertalanffy growth parameters estimated in this study (2008-2010) were different from those in the study made in 1990-1994 (Taghavi Motlagh 1996). Bias in individual L' and K estimates can be over-come by considering the parameter estimates jointly (Sainsbury 1980). In both time periods, the predicted length at age in different size classes and asymptotic length (Fig. 5) for females was greater than for males. This difference has been associated with the faster growth of females of a brood year compared with males of that brood year which have undergone sexual maturation (Rosen et al 1982; Alexander et al 1985; reviewed by Stamps 1993) and the greater age at maturity of females (Russell 1986; Scarnecchia et al 1996). Estimates of the asymptotic length for females in 2008-2010 was much lower than for females in 1990-1994, and there was an increase in the growth coefficient, indicating a decrease in size both overall and at similar ages. This may be a result of increased fishing pressure on the large (female) component of population, as males in the 2008-2010 study approached a smaller asymptotic size with an increase growth rate compared to the males of the 1990-1994 study. Harvesting pressure could generate an evolutionary response towards slower growth and smaller size; this has been studied mainly by quantitative genetic methods (Favro et al 1979; Law & Rowell 1993) in the effects of harvesting pressure reported in plaice (Pleuronectes platessa L.) (Linnaeus 1758) (Rijnsdorp 1989; Barot et al 2005), Atlantic cod (Gadus morhua) (Linnaeus 1758) (Barot et al 2004; Olsen et al 2005) and in small mouth bass (Micropterus dolomieu) (Lacepede 1802) (Dunlop et al. 2005).
However the effect of fishing on the starry sturgeon population can be determined where estimates of the age at first sexual maturity exist, and there may be a minimum threshold age below which sexual maturation does not occur. Overfishing, which specifically increased after the break-down of the ex-Soviet Union (Pourkazemi 2006) could be one factor influencing the severe depletion of the starry sturgeon stock. In addition, several environmental conditions such as pollution (from factories and oil rigs) can be influential on life history variables (Ivanov 2000) since pollutants affect both the gills and liver of the starry sturgeon (Halajian et al. 2006).
The mortality coefficient of starry sturgeon females (Z = 0.65 per year) and males (Z = 0.92 per year) in 1990-1994 (Taghavi Motlagh 1996) was consider-ably lower than for the period 2008-2010, for both females and males, which indicates that intensive commercial fishing may be a causative factor in the sharp decline of the starry sturgeon stock. The increase in the annual mortality and decrease in the survival rate of the starry sturgeon force the conclusion that there is indeed fishing pressure in the Caspian Sea.
The current fishing pressure on the starry sturgeon, as one of the most precious species in the Caspian Sea, is high and with its life history characteristics makes this species vulnerable. For a better understanding of the magnitude and mechanisms of the processes affecting demographic parameters, analysis of fish stocks by continual sampling is necessary to assess the status of the stock of starry sturgeon, with an intent to rationalize and to restructure state fishing regulations so that fish mortality may be reduced.
We would like to thank R. Nahrevar for a second reading of fin ray sections. We also thank R. Rastin for his assistance in the preparation of fin ray sections. Sampling authorization was given by the Madar Khaviari sector of the Iranian fisheries organization. We would also thank H. Dadari, A. Alinejad, M. Ataee, A. Qavidel, F. Shakori, A. Kor and G. Salehi for their assistance in the collection of fish. We greatly appreciate the constructive comments and suggestions provided by G. Cailliet. We also wish to acknowledge Dr. Margaret Nelson for her assistance in editing the English language text of this manuscript. We also thank K. Bakhshalizadeh, S. Bakhshalizadeh and O. Arshad for their spiritual support.
Received: 27 August 2011 - Accepted: 19 January 2012
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Shima Bakhshalizadeh (1)*, Shahram Abdolmalaki (2), Ali Bani (1)
(1.) Fisheries Department, Faculty of Natural Resources, University of Guilan, PO Box 1144, Sowmeh-Sara, Iran
(2.) Inland Water Aquaculture Research Institute, PO Box 66, Bandar Anzali, Iran
* Corresponding author: Shima Bakhshalizadeh; Fisheries Department, Faculty of Natural Resources, University of Guilan, PO Box 1144, Sowmeh-Sara, Iran Phone: +98 182 3223599 - Fax: +98 182 3223600. E-mail: email@example.com
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|Author:||Bakhshalizadeh, Shima; Abdolmalaki, Shahram; Bani, Ali|
|Publication:||aqua: International Journal of Ichthyology|
|Date:||Apr 15, 2012|
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