[Research]

Stock assessment of juvenile sturgeons in the Iranian water of theCaspian Sea by bottom trawl survey

M. Tavakoli¹*, M.R. Behrooz Khoshghalb¹, D. Kor², H. Ghadirnejad³

1.  International Sturgeon Research Institute, Agricultural Research, Education and Extension Organization (AREEO), Rasht, Iran.

2.  Iranian Fisheries Science Research Institute (IFSRI), Caspian Sea Ecology Research Center (CSERC), Agricultural Research, Education and Extension Organization (AREEO), Sari, Iran.

3.  Iranian Fisheries Science Research Institute (IFSRI), Inland Waters Aquatics Research Center, Agricultural Research, Education and Extension Organization (AREEO), Gorgan, Iran.

* Corresponding author’s E-mail: M_tavakoli_e@yahoo.com

(Received: Dec. 31. 2016 Accepted: May 28. 2017)

The sturgeon stock assessment was performed to aim at estimation of absolute and relative abundance and determination of species composition at lower 10 m depths using the Si-Sara2 RV vessel in the Iranian coasts of the Caspian Sea in Guilan, Mazandaran and Golestan provinces during 6-30 September 2011-2012. In this study, 40 stations were selected on the basis of stratified random sampling design and then the stock estimation was performed using the swept area method. The study was carried out using bottom trawling with 9 m head rope. The time and speed of trawling in each station were 30 minutes and 2.5 knots respectively. The Catch per Unit of Effort (CPUE) in 2011–2012 were 7.03 and 6.96 individuals per trawling, respectively. The catch per unit of area in these years were found to be 1662 and 1644 fish in nm², respectively. Total abundance of sturgeon juveniles was 13,327,164 individuals in 2011. So, the species composition included A. persicus (87.8%) and A. stellatus (12.2%). Total abundance of sturgeon juveniles was found to be 14,364,882 individuals in 2012 and the species composition comprised A. persicus (61.4%) and A. stellatus (38.6%). In 2011 the biomass of sturgeons in Iranian coastal water of the Caspian Sea was 295 tons and the composition of biomass included A. persicus (81.5%) and A. stellatus (18.5%), respectively. In the cruises conducted in 2011, this amount was estimated to be 217 tons comprising A. persicus (54.2%) and A. stellatus (45.8%), respectively. The results of this study in 2011–2012 showed remarkable abundance of juvenile sturgeons in Iranian coastal waters of the Caspian Sea in late summer and early autumn. So, by conserving these valuable stocks, the number of spawners will be increased in the future.

INTRODUCTION

Sturgeons of the Caspian Sea are the most important and valuable resources.  Only five out of 27 species in the world (Pikitch et al. 2005) including beluga (Huso huso), stellate sturgeon (Acipenser stellatus), ship sturgeon (A. nudiventris), Persian sturgeon (A. persicus) and Russian sturgeon (A. gueldenstaedtii) inhabit in the Caspian Sea. These species migrate to the major rivers for natural spawning (Holcik 1989). Stock assessment of sturgeons and evaluation of the changes in their stocks over time could have an important role in the fisheries management as well as planning the conservation and sustainable exploitation of their resources.

The sharp decline in sturgeon stocks has led to establishing some conservation programs by the Caspian littoral states as well as some international organizations. Since 2000, sturgeon endangered species were listed in Appendix II of CITES (Pourkazemi 2008), while restrictions were imposed on the fish’s trade from 2001. According to the Caspian Sea Bio-resources  Commission approvals and also the CITES Convention and the International Workshop for assessment of sturgeon stocks in Astrakhan, Russia, in order to unify methods of study on sturgeon resources, it was decided that all Caspian states should participate in the cruises for sturgeon stocks assessment and use bottom trawl surveys for this assessment.  So that one of the most important factors in determining caviar export quotas was the countries participation in the cruises   for the sturgeon stocks assessment.

According to this legislation adopted in a joint meeting of experts of the Iranian Fisheries Science Research Institute and KaspNIRKH Russia with the participation of FAO experts prepared project proposals on the sturgeon stock assessment. These cruises were started with the participation of all five Caspian littoral states from 2001.

Thereafter, some studies were conducted to assess the sturgeon stocks in Iranian waters of the Caspian Sea from 2003 independently.

The sturgeon stock assessment trawl surveys were also conducted from 2011 through 2012 in order to determine the species composition, relative abundance, distribution, stocks estimation and absolute abundance of different species of sturgeon in Iranian waters.

MATERIALS AND METHODS

The marine survey for sturgeon stock assessment was conducted in September 2011 and September 2012 to estimate the abundance of each species and their composition (%). The study area located in the Iranian waters of the Caspian Sea (lying below Imaginary line from Astara city (longitudinally 48º 52' and latitudinally 38º 26') to Hassan Gholi (longitudinally 53º 55' and latitudinally 37º 22') in Guilan, Mazandaran and Golestan provinces (Fig. 1). The Sisara 2 R/V with engine power of 250 hp and 9 m bottom trawl was used for sampling sturgeons at depths less than 10 m. Trawling was carried out in 40 stations selected using a stratified random sampling design (Table 1). The number of stations in each stratum was based on the area of the strata in terms of the total study area (772.7 nm²). Trawling were carried out in the daytime, by the trawling velocity kept at 2.5 knots and   lasted 30 minutes in each station. At each station, geographic location, depth, trawling duration, distance traveled along the coast were recorded. The total length of fish was measured with an accuracy of 1 cm and weight with an accuracy of 1 g.

Fig. 1. Map showing trawling stations in the Iranian waters of the Caspian Sea.

Table 1. Number of trawl stations in the study area (2-10 m depth).

Specimens with total lengths >86 cm were considered to be adults in the case of A. persicus, A. stellatus, A. nudiventris and A. gueldenstaedtii, while those with total lengths > 180 cm considered in the case of H. huso (Khodoreskaya & Krasilov 1999). Biomass of sturgeons was calculated using the swept area method (Sparre & Venema, 1998) as agreed upon the Commission on Aquatic Bio-resources of the Caspian Sea.

Relative abundance of sturgeons in terms of species was calculated on the basis of CPUE which was number of sturgeons per trawls towed for half an hour (Sparre & Venema 1998).

Formulas and equations used in sturgeon stock assessment

Calculation of swept area: a = D*h*X₂

where      a = swept area

                  D = distance traversed

                   h = length of upper rope

                   X₂ = coefficient of bottom

Trawl opening (0.644 for 9 m trawl)

Catch per unit area for each trawl = (cw/t)/ (a/t) = cw/a

where     cw = catch amount per trawl

                 a = swept area per trawl

                 t = swept time (h)

Biomass per unit area = b/a = (cw/a)/x₁

where      b = biomass per unit area

                   x₁ = vulnerability coefficient (0.04 for H. huso, 0.1 for A. persicus and 0.07 for A. stellatus for 9 m trawl) 

                 (Cw/a) = mean catch per unit area for all trawls

Biomass for total study area B = [(∑cw/∑a) *A]/ x₁

where     B = Biomass in total study area

                  A= total area studied (surface area of Iranian waters from 2–10 depths is calculated as 772.7m².

Results obtained were analyzed using Excel and SPSS.

RESULTS

The number of caught individual

In 2011, the total number of caught sturgeons was 281 individuals comprising 256 Persian sturgeons and 25 stellate sturgeons, while in 2012, the total number of caught sturgeons was

278 individuals comprising 193 Persian sturgeon and 85 stellate sturgeon. The highest number of fish was caught in Mazandaran

province while the lowest number was in Golestan Province. In these cruises, only A. persicus and A. stellatus were caught (Table 2).

Species composition

In 2011, the sturgeon species composition of the trawl catch in all studied areas included Persian sturgeon (91.1%) and stellate sturgeon (8.9%), while in 2012 were 69.4% Persian sturgeon and 30.6% stellate sturgeon (Tables 3-4, Fig. 2).

Relative abundance

The CPUE of sturgeon was 7.03 individual per 30 minutes trawling in 2011, while was 6.96 in 2012 (Table 5).

Catch per unit of Area (CPUA)

In 2011 and 2012, catch per unit of area were 1622 and 1646 individuals per nm²,respectively including 1514 and 1141 individuals per nm² for Persian sturgeon and 148 and 503 individuals for stellate sturgeon in 2011 and 2012, respectively (Table 6).

Table 2. The number of sturgeons caught in different provinces.

Table 3.  Sturgeon catch composition in different provinces (%).

Table 4. Species composition of sturgeons caught in different provinces (%).

 Table 5. Catch per unit of effort (CPUE) of sturgeons caught (depths < 10 m) in Iranian coastal water of the Caspian Sea in 2011-2012(numbers of sturgeon per trawl).

Fig. 2. The sturgeon catch composition in different provinces.

Table 6.  Catch per unit of area (CPUA) of sturgeon species in 2011-2012 (numbers in nm²⁾.

Length and weight measurements

The mean total length of Persian sturgeon caught in 2011 and 2012, were 17.7 ± 3.2 ( ± SD) and 16.3 ± 3.6cm, while the mean weight were 20.5 ± 13.8 and 16.7 ± 11.7g, respectively. These values for stellate sturgeon were 26.0 ± 5.7cm, 21.9 ± 2.3cm, 33.6 ± 20.9g and 22.4 ± 5.3 g, respectively (Table 7).

Total abundance and biomass

Total abundance of sturgeons in 2011 was calculated 13,327,164 individuals, of those, 87.8% belonged to the Persian sturgeon, while

12.2% to the stellate sturgeon. In 2012, the total abundance was calculated to be 14,364,882 individuals, of those 61.4% belonging to the Persian sturgeon, while 38.6% to stellate sturgeon.

The total biomass of sturgeons in 2011 was calculated 295 tons, of those, 81.5% and 18.5% belonged to Persian sturgeon and stellate sturgeon, respectively. In 2012, the estimated total biomass was 271 tons of which, 54.2% and 45.8% belonging to the Persian sturgeon and stellate sturgeon, respectively (Table 8).

Table 7.  The mean, minimum and maximum total length and weight of sturgeons caught in 2011-2012.

Table 8. Total abundance and biomass of sturgeons caught in 2011-2012.