Productivity Relations in Seawater Fish Ponds: A Comparison of Stocked and Unstocked Ponds

Abstract

The interactions between nutrient load, fish activities and developing phytoplankton were studied in a stocked seawater fish pond as compared to an unstocked seawater control pond. Both ponds received the same nutrient load from a common inflow (2.7 g at [atoms] N d-1 and 0.065 g at P d-1); they were flushed at a rate of 0.6 d-1. Apparent growth rates of the phytoplankton varied from 0.62-1.7 in the ponds. The stocked fish pond received an extra nutrient load from the fish food (1.9 g at N d-1 and 0.14 g at P d-1); this was responsible for the higher standing crop observed in this pond, 10-53 mg chlorophyll a m-3 compared to 4-28 mg chlorophyll a m-3 in the unstocked control pond. The average daily primary production was 3.5 g C m-2 d-1 and 2 g C m-2 d-1 in the stocked fish pond and control pond, respectively. Remarkably, both ponds showed the same pattern of monthly periodicity in phytoplankton blooms. The high N:P ratios (18-80) in the inflow water and their periodical fuctuations that correlate with the blooms suggest that both systems are P limited. The tidal pattern in the gulf seems both to induce the periodicity of the inflow water N:P ratios and to govern the monthly cycle of the blooms.