Secondary production in brackish water culture ponds

Abstract

Secondary production of zooplankton and zoobenthos (macrobenthos and meiobenthos) of two brackishwater ponds, A and B, each having 0.042 hectare area and connected to the Cochin backwaters, in the instructional fish farm of College of Fisheries, Panangad, Cochin have been studied for three culture periods. Viz, culture I.C. chenos (120 days, during August to December, 1986), culture II. P.indicus (60 days, during January to March, 1987) and culture III.P. monodon (45 days, during mid May to June, 1987).

Fortnightly fluctuations in the biomass of zooplankton, meiobenthos and macrobenthos both group wise and total, their percentage dominance and frequency of occurrence have been studied during each culture period. Attempts have been made to correlate the fortnightly biomass of zooplankton, meiobenthos and macrobenthos with the fortnightly growth increment of C.chanos, P. indicus and P.monodon separately.

Since zooplankton production is mainly dependent on primary production, fortnightly estimations of net and gross primary production have been carried out. The physic-chemical parameters of the pond water and soil have been studied fortnightly since both primary and secondary productivity of a culture pond depends mainly on these variables.

The zooplankton groups are constituted by rotifers copepods and crustacean nauplii. The total biomass of zooplankton ranges from 22.388 mg/m3 to 5476.950 mg/m3 and 5.085mg/m3 to 1316.832 mg/m3 in ponds A and B respectively. The zooplankton biomass shows three peaks i.e., 1) in November 2) in March and 3) in June in pond A during culture I,II and III respectively while in pond B, it does not show such pronounced peaks. Neverthless, small zooplankton peaks are apparent in pond B in the former half of September, February and May during culture I ,II and III respectively. The relationship between the fortnightly biomass of zooplankton and that of growth increment of C. chanos is mostly an inverse one which could be because of the grazing effect of the latter on the former.

An inverse relation is also observed between the biomass of zooplankton and growth increment values of P.indicus and P.monodon. Since the prawns are benthic feeders such an inverse relation cannot be attributed to direct consumption of zooplankton by them. However, zooplankton on dying sink to the bottom and form part of the detritus, constituting direct food for the prawns.

The meiobenthos in both the ponds is constituted by nematodes and copepods. The total biomass of meiofauna ranges from 36.018 µg/10cm2 to 2539.322 µg/10cm2 in pond A and from 57.882 µg/10cm2 to 1556.616 µg/10cm2 in Pond B. A direct correlation is observed between the biomass of meiofauna with growth increment values of C.chanos whereas an inverse relation is noted between the former and the growth increment value of P.indicus and P.monodon. This suggests that meiofauna may not from direct food to C.chanos while it may be the contrary to the prawns.

The macrobenthos is composed of amphipods, tanaids, polychaetes and mollusks. The total biomass of macrofauna groups ranges from 0.035 g/m2 TO 43.074 G/M2 in pond A and from 0.144 g/m2 5.712 to g/m2 (excluding villorita cyprinoides var.cochinensis) in pond B. One peculiarity observed in pond B is the presence of thick bed of black clam V. cyprinoides var. cochinensis during all the culture periods. A direct correlation between the fortnightly biomass of macrofauna with growth increment of C.chanos during the former half of culture period and inverse relation during the latter half of culture period are observed . this may be attributed to the utilization of macrofauna by c.chanos during the later stages of their growth. This supports the previous views put forward by several authors. An inverse relation is observed between the fortnightly biomass of macrofauna and growth increment values of P.indicus as well as P.monodon. This is in agreement with the views of several authors which highlight the utilization of macrofauna by the prawns.

Fortnightly observations on primary productivity of phytoplankton as well as physic – chemical parameters of pond water and soil have also been discussed in general.