Oxygen Limits Pond Production - Courtesy - ROEL H
The principal constraint in pond production is oxygen availability. Oxygen is needed for fish respiration, waste decomposition and algal respiration. Part of the primary production is available for the fish growth.
The addition of feed to the pond increases oxygen demand. The chemical oxygen demand (COD) of 1 kg of feed is about 1400 g O2. About 45% of the feed COD is retained in the fish and is harvested as fish gain (Heinsbroek et al., 1990). The remaining 55% of the total COD or 770 g O2 (kg feed)−1, consists of non-fecal (which includes fish respiration) and fecal loss, and needs to be provided to the pond. Assuming a feed conversion of 1.5, 1150 g O2 (kg gain)−1 will be consumed, of which about 1/3 is for fish respiration, and 2/3 for the degradation of the waste (Verdegem et al., 1999).
Nile tilapias were grown from 50 to 250 g on a 30% protein diet receiving a ration of 10 g kg−0.8 d−1(Verdegem and Bosma, 2009). Of the COD in the feed, 30% was retained in fish gain, 42% became non-fecal loss and 28% fecal loss. Of the dietary nitrogen 48% was retained in fish gain, 42% was excreted as non-fecal loss and 10% as fecal loss. More nutrients would be retained with a high protein diet, but farmers prefer to use a 30% protein diet, because nutrients not used by the fish contribute to the production of natural food organisms (algae, periphyton, zooplankton, and detritus) which in turn complement the fish diet. This is illustrated when in ponds fish are confined in cages, resulting in 10–30% less growth than in ponds with the same biomass roaming freely (Schwedler et al., 1989). The free-swimming fish have access to more natural food than confined fishes and therefore grow better.
Under optimal circumstances, surface aeration can support a production of up to a maximum of 3500 kg ha–1 d–1 (Brune and Drapcho, 1991). In such a well-fed tropical pond, primary productivity will be about 4 g C m–2 d–1 which is equivalent to 8 g organic matter m–2 d–1, representing a COD of 104 kg O2 d–1. The COD demand of feed wastes is much less: on average such ponds have a COD for feed wastes of 7 kg O2 d–1. As such, the COD resulting from primary production is 15 times larger than the COD resulting directly from feeding. Generally, fish production in stagnant ponds is lower, in which case the relative weight of COD demand resulting from primary production will even be higher (Verdegem et al., 1999). The excess oxygen produced by algae during light hours is needed to support algal respiration and decomposition. In consequence, only oxygen provided through surface aeration is available to compensate the direct oxygen demand due to feeding.