A scientific lecture with the title of "Evaluation of the effect of northern pike on fish population and size in the Chitgar Lake" was delivered by Dr. Alireza Mirzajani at the conference hall of the National Inland Water Aquaculture Institute. This investigation was conducted in the context of studies for the comprehensive study of Lake Chitgar in Tehran. Due to the increase in weed fish in the Chitgar Lake and the increase in water eutrophication, there was a need to control the population of these fish. In this regard, the total number of 2000 northern pike was released in the lake in 2016 and 2017. In order to investigate the effects of release of this fish on weed fish, populations of each fish including northern pike as predatory fish and weed fish including crucian carp,
Hemiculter lucisculus
as well as economic economically valuable carp as prey were estimated. Various fishing methods including fishing by hook, different fishing traps (with 10 different net meshes), cast net and purse seine were used. Different methods of estimating fish population were also used for each
fish. For northern pike, plaque marking and signal tracking were performed using the Jolly method whereas for crucian carp, marking through cutting off fish fin and Peterson's modified method were performed. The measure of Catch Per Unit Effort (CPUE)was used for
Hemiculter lucisculus
. Plaque marking was performed for carp and both Jolly and CPUE methods were applied for this fish. The data obtained from specimens and plaque marking indicated that there were 575 northern pike and the absolute growth rate of fish was 7.3 grams per day from 2017 to 2019. The decrease in crucian carp population was observed by 59 % over different time periods. The
Hemiculter lucisculus
fish population declined 94 % during the same period. During the study period the final number of carp decreased and its total biomass increased. The study of northern pike nutrition also showed that a high percentage and large volume of
Hemiculter lucisculus
, crucian carp and
Pseudorasbora parva
were eaten by this fish. The final evaluation of this study showed that northern pike have been able to properly control the weed fish population the Chitgar Lake.