Threats
The high economic value of T. niloticus because of its nacre- lined shell used in mother- of pearl products, as well as its edible meat, has lead to its exploitation as a fishery resource in the Indo- Pacific. According to Pakoa et al (2008) the T. niloticus shell has a high unit value compared to fin- fish and is also non- perishable. This makes it an attractive source of income in isolated island communities (Pakoa et al, 2008).
The most severely depleted stocks are on reefs surrounding Fiji, the Solomon Islands and Papua New Guinea (Crowe et al, 2002). In one province in the Solomon Island densities of T. niloticus were recorded at less than 20 per hectare, whereas previously they had been at 100- 300 per hectare (Foale, 1998). This level of exploitation has lead to developing techniques to replenish these stocks and create a sustainable fishery.
One of the most promising methods under development is cage rearing the animals until after hatching and releasing them as juveniles. The older and therefore larger they are when released gives them a better chance of survival, as they are more able to move around and feed, as well as protect themselves from predators (Crowe et al, 2002). However rearing the animals for too long is expensive, so the most economically viable option is to release high densities of juveniles to large areas (Crowe et al, 2002). The lab environment is designed to mimic the wild so as to make the transition as smooth as possible and aid survival (Crowe et al, 2002). The densities of juveniles while inside the cages can cause growth problems. An equilibrium must be found between density and time spent rearing the juveniles. With higher densities the animals take longer to develop (Amos and Purcell, 2001).
Measuring the effectiveness of these replenishment programs is difficult. The mark and recapture technique is used to measure the survival of juveniles released to the wild. However environmental factors, such as currents and crypsis make recapture a hard task, so success rates are hard to quantify (Crowe et al, 2002).
In some areas poaching is also a major threat to T. niloticus, again for the valuable nacre- lined shell. It was reported that in the 1980's between 4000- 6000 tonnes of the animal was harvested, for a retail value of around US $28 million (Hahn, 2000 cited in Dolorosa, 2010). In some areas in the Philippines the average size of T. niloticus shells show a downward trend over the years, which is believed to be directly linked to poaching (Dolorosa et al, 2010). Calls are being made for better prevention methods against and harder persecution of poachers (Dolorosa et al, 2010).
T. niloticus are harvested in island communities by picking them up off the reef flat at low tide, as well as skin diving in shallow water (Pakoa et al, 2008). This can cause damage to the reef, as well as depleting T. niloticus stocks. |