The effects of northern pike, Esox lucius, in controlling coarse fish and other pest organisms and enhancing harvest size in carp ponds was investigated. We used two treatments and one control each with three replicates. The pike density was 200 and 500 fish individuals per hectare for the first and second treatments, respectively and the control pond had no pike. The carp density in ponds was 3500 individuals per hectare and  stocking  rates  were  55% silver carp, 20% common carp, 10% bighead carp and 15% grass carp. Survival rate of the northern pike in treatments  1 and 2 was 43.8 and 40%, respectively.

The results showed that the mean (±SO) biomass of unwanted fishes (13 species) in the control treatment was 358.8±55.4kg/h and in the first and second treatments were 74.2±6.3 and 92.2±10.9kg/h, respectively in the end of the trial period. So unwanted fishes biomass reduced by 79.3% and 74.3% in the two treatments, respectively. Stomach  investigation showed that the pike fed mostly on frog juveniles and some pest fishes like Hemiculter leucisculus, Pseudorasbora parva, Carassius auratus, and few Macrobrachium and  aquatic insects. Also, the average biomass of unwanted fishes like Carassius auratus, Hemiculter leucisculus, Alburnus alburnus and Pseudorasbora parva of the first and second treatments in comparison to control showed a reduction of94, 88.9, 62.4 and 56.82 percent, respectively. A significant difference was found in the final mean weight of common carp between treatments and control ponds (P<0.05), as the weight of common carp in treatments ponds was 220% more than the control pond. Also, Esox lucius with controlling coarse fish increased cultured fish (such  as common  carp, grass carp, silver carp and  bighead  carp) production  by  about

17.9%.