AQUACULTURE DEPENDENCY ON WILD SEED

Generally, groupers spawn on offshore reefs where they form aggregations of hundreds to tens of thousands of individuals, in a few specific locations (Johannes et al., 1999; Rhodes and Sadovy, 2002). They produce pelagic larvae that may disperse over hundreds of kilometres in the course of 30–45 days and experience high density-independent mortality.

However, recent research suggests that groupers and other reef fishes may have greater control over their distribution than previously thought, and that at least some proportion of the gametes spawned may be retained near their natal reef (Jones et al., 1999; Jones, Planes and Thorrold, 2005; Swearer et al., 1999). Larvae, transported to near-shore nursery habitats settle as juveniles in sea-grass beds, mangroves, algal beds, coral rubble, oyster reefs and marshes (Coleman et al., 1999; Tupper, 2007). For this reason grouper seed is mainly caught in coastal areas, particularly around sea-grass, mangrove and shallow brackish water areas near river mouths and estuaries, as well as in tidal pools, tidal channels and around reefs.

The peak grouper seed season is often associated with the relatively wet months in the year (e.g. monsoon seasons); in several areas, grouper seed collectors have claimed that their best catches were associated with strong onshore winds (Johannes and Ogburn, 1999). This is consistent with a number of recent studies into recruitment pulses of settlement-stage reef fish – including groupers – that accompanied cyclonic storms, which apparently caused the fish to be transported shoreward (Shenker et al., 1993; Dixon, Millich and Sugihara, 1999).
Because grouper are particularly difficult to culture in closed systems, full-cycle culture of most grouper species is not yet possible. For this reason, approximately 66–80 percent of all grouper culture involves the capture and grow-out of wild seed (Sadovy, 2000). The volume of seed caught each year exceeds hundreds of millions of individuals (Sadovy, 2000). The greatest catches tend to be of the smallest size classes (1–3 cm); during peak seasons a catch can be of tens of thousands by a single unit of gear, in a single night, by one fisherman (e.g. using a fyke net). Even larger sizes of fish are being captured in massive numbers region-wide each year. It is important to realize that the equivalent of the typical annual amount of seed produced in the hatcheries in the whole of Southeast Asia (excluding Taiwan Province of China), i.e. 20 000 to 80 000 fry, can be caught by one fisherman in one night (Sadovy, 2000).
When seed catches are compared to the numbers of marketable fish produced, the results strongly suggest crude and wasteful culture practices. Sadovy (2000) estimated that about 60 million seed fish are needed produce the regional total of 23 000 metric tonnes of table-size live fish from culture annually.