ENVIRONMENTAL IMPACTS OF CAPTURE-BASED AQUACULTURE

Impact of juvenile and fry fisheries

Pangasiid catfishes

The capture of Pangasiid catfish juveniles has largely been replaced by hatchery reared fry for the main catfish industry in the Mekong Delta of Viet Nam. However, operations in Cambodia, Lao People’s Democratic Republic and, to a lesser extent Thailand, still use wild-captured juveniles as seeds for cage and pond culture.

The collection of Pangasiid larvae from the Mekong delta has generally been perceived as unsustainable and detrimental to the target species as well as to the many other species caught as bycatch in the fisheries. For instance, Trong, Hao and Griffiths (2002) cited information from Donh Thap Province that the capture of Pangasiid larvae had declined tenfold during the past decade, “due to over-fishing”.
The bycatch from the fishery is significant. Phuong, 1998 (cited in Trong, Hao and Griffiths, 2002) estimated the bycatch at between 5 to 10 times (by weight) the catch of the targeted Pangasiid larvae. Trong, Hao and Griffiths (2002) concluded that the fishery for catfish fry was “highly destructive” for both the catfish populations themselves as well as for other species caught as bycatch.

 

Although it is easy to draw the conclusion that the catch and bycatch levels are and were detrimental, no data exist to support this claim. In general, most fish species of the Mekong are adapted to high larvae and juvenile mortalities as a result of living in the versatile, but productive floodplain habitats.
The Mekong River Commission (MRC) has facilitated several studies over the past 6–7 years on larvae and juvenile drift in the lower Mekong in both Viet Nam and Cambodia. Data from these surveys do not indicate any reductions in numbers of larvae in recent years.
Genetic studies of Pangasianodon hypophthalmus do not suggest any recent declines in genetic diversity and/or population sizes (So, Volckaert and Srun, 2006). They attribute the high genetic diversity of Pangasianodon hypophthalmus to the large and productive feeding habitats associated with the Mekong floodplains.
Although existing information is inconclusive, fisheries catch data (e.g. Tonle Sap Dai fisheries, described in Lieng, Yim and Van Zalinge, 1995), larvae sampling and recent genetic studies suggest that Pangasianodon hypophthalmus and Pangasius bocourti have not suffered recent population size declines. Any negative impacts of juvenile fishing seem to be negligible and the annual recruitment appears to have been able to absorb the fishing pressures on all life stages of the species.
However, there may be some impacts of these fisheries at the sub-species population level. Although such impacts are currently little understood, genetic studies on the larval drift of Pangasianodon hypophthalmus have revealed that up to five sub populations exist, which are temporally separated in the drift and therefore probably represent distinct spawning populations of the species. Two of those populations were not found downstream of Phnom Penh and in the Mekong delta, where three relatively common populations were identified. Interestingly, studies on the larval drift in the Mekong delta in Viet Nam also identified three temporally separated peaks of Pangasianodon hypophthalmus larvae, corresponding to these three genetically distinct populations (Nguyen et al., 2006).
The two other populations appear to be rare and could potentially be impacted negatively by juvenile fisheries at certain times and/or places. The reason why two out of five populations appear to be comparatively rare is not currently understood. Further studies, combining ecological and genetic methods, will be needed to shed light on this issue and possibly suggest management implications (So et al., 2006b).
Population genetics of migrating Pangasiid catfishes in the Mekong is extremely complex and genetic research is only just beginning to reveal some of these subtle characteristics that nevertheless may have important management implications. For instance, one interesting observation coming out of recent genetic population studies is that one of the ecological drivers of the high number of sympatric intra-species populations may be the disproportionate availability of productive nursery and feeding habitats compared to spawning habitats (So, Maes and Volckaert, 2006). Different populations use the same spawning sites, but at different times of the spawning season, and all the off-spring are subsequently mixed and distributed throughout the vast nursery areas on the floodplains. As a consequence, the management priority for sustaining these species and populations should be the conservation of their spawning sites.
Potential genetic impact of hatchery-reared fry on wild populations is another issue related to genetics that is little understood and not documented for the Pangasiid catfishes. Such impacts may occur if hatchery-reared fish escape to the natural environment and interbreed with wild populations. If broodstock are taken from the local environment, as is generally the case for the catfish industry in the Mekong delta in Viet Nam, such impacts would be minimal.
However, if broodstock are transferred to other areas, particularly if those are in different river basins, such impacts could be significant. For instance, broodstock used in the aquaculture industry in the upper Mekong in Thailand probably originate from the Chao Phraya River basin and are genetically different from the wild populations of the Mekong basin. Chao Phraya broodstock have also been imported to Cambodia for use in aquaculture. This practice was subsequently stopped, because of concern that fish might escape and breed with the wild populations (So and Haing, 2006).
In Viet Nam, broodstock from wild populations of Pangasianodon hypophthalmus are taken in on a regular basis to maintain genetic diversity of aquaculture broodstock – a practice that effectively ensures that the genetic integrity of the wild populations will not be compromised by hatchery-reared material.
Ish and Doctor (2005) ranked the risk to wild Pangasiid stocks from escaping cultured Pangasiid catfishes as “low” because most cage structures are floating and sited in relatively sheltered areas. In addition, because most Pangasiid broodstock are from wild populations, and to date there has been minimal selection and improved breeding programme work conducted the genetic diversity and make-up of hatchery and wild Pangasiid fish populations are essentially the same. Hybridization of Pangasius bocourti and Pangasianodon hypophthalmus, while possible, has been banned by MOFI and the ban is being enforced.

Snakeheads

As with the Pangasiid catfishes, there is no data to suggest that juvenile snakehead fisheries have negative impacts on the species. Again, the large annual recruitments appear to be able to counteract any potential negative impacts.
Channa striata is one of the most common species of the lower Mekong basin and one of the most frequently encountered fish at markets throughout the basin. It is well adapted to living in rice-field habitats and therefore, may in fact have benefited from anthropogenic impacts, including the conversion of natural habitats to paddy fields.
Due to its conspicuous spawning behaviour including parental care of the larvae, adults and juveniles are easily captured by fishers. Parents guarding snakehead seed are easily identified in shallow waters and can simply be scooped up by net together with the entire larvae shoal. Therefore, the potential impact on populations of other species appears to be minimal, although no data exist to confirm this.

Aquaculture feed

Since the culture of both catfishes and snakeheads is based on the use of low value and/ or trash fish, concerns have been raised about the environmental impact of the practice. For instance, earlier estimates suggested that up to 300 000 tonnes of trash fish are used as fish feed for the river catfish and Pangasius bocourti industry in Viet Nam annually (Sverdrup-Jensen, 2002). This figure must now be significantly higher due to the recent increase in Pangasiid catfish production from this area.
When the production of cultured fish was based on the traditional capture-based system (i.e. the use of a seasonally abundant, low value fish resource to produce a high-value product that could be marketed outside the peak season) the practice could probably be carried out in a sustainable manner. However, as the Pangasiid industry, in particular, has developed into a large export industry, the demand for trash fish has exceeded local supply. As a result, marine trash fish are now also used as a feed for the culture of Pangasiid catfishes and snakeheads. This raises environmental concerns, not only locally, but for the marine environment and fisheries that supply the trash fish.
The use of wild-caught trash fish for aquaculture feed is also practiced for other capture-based aquaculture activities in the Mekong basin. Large quantities of the indigenous cyprinids Cirrhinus siamensis and Cirrhinus lobatus are used throughout the basin, but particularly in Cambodia. An unknown (but significant) amount of the Cambodian catch of Cirrhinus sp. is transported across the border to Viet Nam to satisfy the need of the expanding Pangasiid catfish culture industry.
Feed for snakehead culture in Lao People’s Democratic Republic and Thailand is mainly based on the capture of the small freshwater clupeid Clupeichthys aersarnensis, from reservoirs (e.g. Nam Ngum Reservoir in Lao People’s Democratic Republic). In Thailand, slaughter-house waste, particularly from chicken processing, is also used as a feed supplement for snakehead culture (Simon Funge-Smith, personal communication).
The use of wild fisheries resources for feed is the main environmental sustainability challenge that currently faces the Pangasiid catfish industry in the Mekong Delta of Viet Nam. Research efforts aiming at reducing the use of feeds based on trash fish are on-going.