Primary productivity and related fish yield in intensely manured fishponds

Studies of various parameters associated with productivity were conducted in experimental manured ponds with the object of estimating the effects that organic manure produce on fish yield.Photosynthesis—depth profiles showed autoshading by the plankton resulting in compensation depths of approximately 70 cm. Net photosynthetic rates were found to be only slightly higher than those obtained under chemical fertilization. Their variation seemed to be somewhat independent of the total standing crop of plankton but was more directly related to the composition of the plankton.Several fish yield prediction models based on primary productivity were applied to the experimental data. In every case the results show that the actual yields considerably exceeded the predicted yields.This work gives support to the notion that primary productivity is not sufficient to account for the high fish yields attained in a manured system. Thus it is concluded that manuring of fishponds may not only be a practice of fertilization but also a means of promoting the growth of fish through some other alternative pathway.