Shrimp yields and harvest characteristics of mixed shrimp-mangrove forestry farms in southern Vietnam: factors affecting production

Shrimp yields and harvest characteristics were monitored at mixed shrimp-mangrove forestry farms in two state forestry-fisheries enterprises, Tam Giang 3 and 184, in Ca Mau province, southern Vietnam. The aim was to identify key factors contributing to poor and declining shrimp production in the region over recent years. Shrimp yields were highly variable between farms but were generally low with mean annual yields of 286 +/- 106 kg ha-1 yr-1 and income of 388 +/- 146 USD ha-1 yr-1. Secondary fisheries products, such as fish and mud crabs, increased total farm production by 24% (54 kg ha-1 yr-1) and income by 14% (71 USD ha-1 yr-1). Shrimp yields peaked between July-October and March-May, which is consistent with the traditional "Mua" and "Tong" harvest seasons, respectively. Yields were significantly higher at enterprise Tam Giang 3 and were generally higher at extensive farms than traditional farms. A correlation analysis of water quality and technical parameters revealed that pond depth (r = 0.62), maximum fluctuation in pond depth (r = -0.55) and ammonia concentration (r = -0.63) were significantly correlated with shrimp yields (P < 0.05). Stepwise multiple regression revealed the model: Yield = 1.73-7.8 * NH3-N + 0.03* Pond Depth (r2 = 0.63). Metapenaeus ensis and M. lysianassa are the dominant shrimp species cultured, representing 48-50% and 31-32% of harvests, respectively. Penaeus indicus is the third most important species, although it represents a much smaller proportion (6.7-9.7%) of total harvest. All three species vary in abundance with season, with M. ensis being dominant in the wet and M. lysianassa dominant in the dry. The size of shrimp harvested is small, with a mean total length of 49.7 +/- 0.18 mm and 50.6 +/- 0.2 mm for M. ensis and M. lysianassa, respectively. Key factors contributing to poor and declining shrimp yields in Ca Mau province include inappropriate management techniques and pond design, poor wild seed recruitment, and reliance on small, low-value Metapenaeids as culture species. Recommendations to improve current farm production are presented.