ENVIRONMENTAL IMPACTS OF THE JUVENILE GROUPER FISHERY

Mous et al. (2006) conducted a pilot study in Indonesia of artificial shelters (gangos), to determine the sizes and capture rates of species of interest to the live fish trade, and to determine the likely environmental footprint of a gango type of capture method. From the results of the 15-month study, they drew inferences regarding the sustainability of this fishing method and requirements of space, fish and materials for a viable grow-out operation.

The results showed that gangos were unselective for either species or size. Only 1.4 percent of the total fish catch (by number) were target species, mainly the grouper Epinephelus coioides, and most were large enough (mean total length was 13.6 cm) to have bypassed the early high mortality phase. Moreover, there were large non target catches that included many food fish species too small to be useful in catches. Assuming that a soak-time of 3 months results in an average catch per gango of 6.6 E. coioides (as was observed for this species in Terang Bay, the most productive of the four sites), yearly production per gango would amount to 26.4 fish. Even a small local grow-out industry with a capacity of 25 tonnes would require an annual supply of 80 000 fish, assuming a grow-out weight of 0.6 kilogram and 50 percent mortality from fingerling stage to market-ready product. This would require deployment of an estimated minimum of 3 000 gangos.

With such figures in mind, Mous et al. (2006) estimated of the space needed to accommodate sufficient gango deployment and suggested that a sizeable fish culture industry based on capture of fry, fingerlings and juveniles from the wild would have a large ecological footprint. For example, the 3 000 gangos estimated to support a 25 tonnes grow-out operation, would require approximately 300 000 m2 (assuming that each gango requires a plot of 10 x 10 m) or 30 kilometres of coastline (assuming that gangos are deployed in a single line following the optimum depth contour). In other words, juvenile supply would require 1.2 hectares of shallow coastal waters for each tonne produced.
Given the large number and area of gangos needed for a viable operation, and that many groupers captured could probably have survived to reproduce, the ecological footprint of this approach could be substantial (Mous et al., 2006). These results, and literature on other juvenile fisheries, suggest that CBA sources of seed such as gangos may often need management, have important links to other capture fishery sectors, and require careful evaluation of potential costs and benefits before introduction or development.
Support for grouper CBA is often based on the assumption that the natural morality of early juvenile grouper is very high, so that the fishery is not adding substantially to this natural mortality and therefore not affecting adult population size to any great extent. This assumption remains untested for grouper species. As Sadovy (2001b) points out, the critical question is how early do juvenile mortality rates decline to adult levels? If early mortality is high, then removal of some post-settlement fish for culture may have little impact on adult numbers, since the probability of survival of any individual fish is low. However, recent research suggests that the period of very high mortality occurs during and immediately after settlement, and that fish surviving more than a few days have a much higher chance of survival. Tupper (2007) estimated the cumulative mortality of early juvenile (2.5–5.0 cm TL) Plectropomus areolatus and Epinephelus polyphekadion in their preferred nursery habitats to be around 50–75 percent over the first 3 months post-settlement. Assuming an exponential rate of decline in mortality, the instantaneous mortality at 3 months post-settlement would be much lower than 50 percent. Indeed, mortality rates of post-settlement juveniles may not be substantially greater than adult mortality (estimated at 20–30 percent for most groupers, e.g. Posada and Appledoorn, 1996) and are likely much lower than the estimates of >90 percent mortality often suggested for newly settled reef fishes. If each individual has a 50 percent chance of surviving the first 3 months after settlement, then removal of large numbers of juveniles will almost certainly have an impact on adult population size. This could result in direct conflicts with the adult capture fishery and could accelerate overfishing of groupers.
In addition to problems of bycatch, wasteful mortality, and overfishing, cage and net culture can create other environmental problems, most notably point-source pollution which can have adverse effects on coastal waters, and particularly on coral reefs. For example, in 1994, researchers in Barbados noted complete bleaching and eventual death of coral patch reefs in the vicinity of a cage culture operation for dolphin fish (Coryphaena hippurus). Disease transfer is another problem exacerbated by the complex and extensive trade in live fish between Asian countries.