Biological study on Gorganrood River was performed from November 1990 until October 1991. Fortheen sampling stations along the 200km distance from Chai Qushan to estuary. Fish distribution was not similar in all parts and was more dense in lower parts. Density of common carp, roach and white fish were more considerable particularly in spawning s-eason. Density of benthic organism was very low and blood worms (Chironomidae) and shells of dead mollusca were obtained. Frequency of phytoplankton maximum in September and minimum in January. Bacilliarophyceae, Chlorophyceae and Mynophyceae had more biomass during studying period. Zooplankton had maximum biomass in September and minimum in February and March. Among zooplankton, Protozoa and Rotatoria had maximum biomass.

A study was conducted on 2 species of Gobiidae N. fluviatilis and N. melanostomus in the Madarsoo Stream, an upper reach of Gorganrud river in the province of Golestan situated in the southeast Capian basin, from February 1995 to February 1997. 316 fish speciemens were captured by electrofishing of which N. fluviatilis and N.melanostomus 95% and 5% of the catch respectively. In the catch of N.fluviatilis, there were 22.2% male and 77.8% femalet. The Ivlev index was used to determine food selection in different months of sampling. The selected food in .February 1995 and July 1997 were Chironomidae and Ephemeroptera with 0.2 and 0.21 respectively. Shannon index of species diversity were highest in Habitat and stomach contents in spring with 0.84 and 0.56 respectively. Horn index of nich overlap showed that Apr. 1997 with 0.68 and Feb. 1997 with 0.19 had the maximum and minimum niche overlap respectively.  

A study was conducted to compare 28 morphometric and 12 meristic characters of Anzali Lagoon roach (Rutilus rutilus caspicus natio Kurensis Berg) and Gorgan-Rud estuary roach (Rutilus ruti/us caspicus natio Kniiopowistschi Pravdin) from late february to late march 2000. Coefficients of variation for morphometric and meristic characters were 17.48 and 5.55 for Anzali lagoon and 27.6 and 5.56 for Gorgan -Rud estuary respectively. One-way ANOVA and multivariate analysis show that there were significant differences between the roach of the two areas in most absolute and relative morphometric and meristic characters. Factor analysis revealed that the followings were the most important and independent characters; snout length, caudal peduncle length, anal fin height, eye diameter, head length/interorbital distance, standard length/body depth, standard length/pectoral fin length, anal fin rays, predorsal scale number and total body vertebrate. Discriminant function analysis showed that the two populations in Anzali lagoon and Gorgan-Rud estuary are distinct from each other.

This study was carried out in two periods: 1377 and 1378. During two periods of this study in which Persian sturgeon juveniles were released, the results showed that those juveniles which selected the estuary for their living for a short period of two days, depending on the water salinity, they started to feed on the gammaridae and chiromidae a day after the released. The mentioned feed items are used as the main bait by these juveniles generally. At this time stomachs and guts of the juveniles have an average weight in the order of 4.30±1.70 and 6.90±4.90 mg approximately. The estimated condition factors of juveniles were 0.347±0.03. Stomachs and guts weights as well as the condition factors of the juveniles were higher at the beginning of the spring at the time of release than the end of the releasing period, and as closing to the end of the period the trend of decrease in weight was higher than the starting period. Considering the average water salinity that measured to be about 6.75±3.02 g/l, it seems that the time needed for juveniles to reach the sea from the point of their release, it takes about 6 hours or something near 3 hours per km. approximately.

Genetic variation of stellate sturgeon, Acipenser stellatus, from the North (Volga river) and South (Gorgan river) Caspian Sea was investigated using NADH 5/6 gene and PCR-RFLP analysis. In this study, a total of 60 samples of stellate sturgeon from the North and 14 samples from the Southern Caspian sea were collected. MtDNA ND 5/6 gene was amplified by polymerase chain reaction (PCR). PCR-product digested using 31 end nucleases restriction enzyme. Five of the 31 enzymes showed polymorphism (RsaI, HinfI, HpaII, HaeII/, MboI). Totally 18 composite heliotypes among 74 samples were detected. Haplotypes AAAAA, BAAAA, ABABA from the Vo]ga river showed 28.33%, 11.66% frequency, respectively, and from the Gorgan river heliotypes AAAAA, BAAAA showed 64.28% and 14.28% frequency, respectively.Average haplotype and nucleotide diversity within the populations in Volga river was 0.88±0.027 and 0.0122 and in Gorgan river was 0.5934±0.144 and 0.00398, respectively. Average nucleotide diversity and nucleotide divergence among populations of North and South Caspian Sea was 0.008644 and 0.000542, respectively.Monte-Carlo simulation using 1000 replication showed no significant difference between sampling site in the southern and northern Caspian Sea (X2=0.926±0.0083).

A survey has been conducted on the metazoan parasitic infections of juvenile sturgeons in southeast of Caspian Sea during 1997-1999.239 specimens were obtained from beach seines in Miankaleh Peninsula and Gorgan - rud estuary. Ten and eight parasite species were recovered from 126 juvenile A. persicus and 113 juvenile A. guldenstaedti respectively. Anisakis larvae with 19.8% and 13.3% had the highest prevalence of various parasites were observed in A. persicus and A. guldenstaedti respectively, too. Age and diet of samples determined. Prevalance, frequency, range, intensity and dominant of parasites were determined. Effects of seasons on parasitic infections and parasitic infection in different ages of fishes were studied. The result of this study was compared with results of other investigators in other parts of Caspian Sea.