Being a predator, rainbow trout has been considered detrimental to native fish fauna in many places where it has been introduced (for example, McDowall 1989). Due to the lack of native fishes in the altitudinal range where trout occur (or could be stocked), this seems to be a minor problem in Papua New Guinea.
There are two native species in the Purari catchment which might have been effected by the introduction of rainbow trout. The goby, Glossogobius brunnoides and the highlands gudgeon, Mogurnda sp.3 (see: Allen 1991).
G. brunnoides occurs in mountainous headwaters of the Purari River system, e.g. tributaries to Asaro River near Goroka and to Waghi River near Mount Hagen (Allen 1991). The effect of trout on G. brunnoides is probably very minimal, since the upper altitudinal limit of the goby (1800 m) seems to correspond to the lower altitudinal limit of rainbow trout. In this study, G. brunnoides was found at three sampling stations, the highest altitude was 1720 m.a.s.l. (see Povlsen 1992a,b).
Mogurnda sp.3 occur around Mendi, Southern Highlands, in streams where trout has established (e.g. Anggura River). It is impossible to scientifically estimate the effect of trout on the gudgeon, since no data are available from the period before introductions occurred. According to local people the gudgeon has decreased in numbers in recent years, but 20 years after the trout-introductions it still occurs in the river. As mentioned earlier, no fish specimens were found during stomach content analysis of trout from Anggura River. Although this is not a proof that trout doesn't prey on the gudgeon, it indicates that the detrimental effect on the gudgeon resulting from predation by trout is minimal. Spending most of the time hiding underneath rocks and stones, the gudgeon might avoid becoming easy prey for trout.
The effect of trout on coldwater fish species recommended for introduction by Sepik River Fish Stock Enhancement Project is, of course, of major interest to the project.
Four fish species have been recommended by the project for stocking mid- and high-altitude streams in the Sepik-Ramu catchment: Schizothorax richardsonii, Tor putitora, Acrossocheilus hexagonolepis and Labeo dero, all belonging to the cyprinid family native to the Himalayan region (Coates 1991). Povlsen (1993b) discussed their suitability for the upper Purari and recommended the snow trout, Schizothorax richardsonii, for stocking high-altitude rivers in the Purari catchment.
Snow trout lives in a temperature range of 8°C to 22°C (Coates 1991). Consequently, rainbow trout may have some effect on snow trouts in areas where they overlap, but will not be a hindrance to its establishment in Papua New Guinea.
In the native range of the four species in India, rainbow trout is not considered detrimental to any of them (Sehgal, personal communication).
During my work, I have several times heard villagers report that the trout has decreased in recent years. In some places it was even reported that the trout has disappeared (streams along the Kundiawa-Keglsugl road, southern part of Gembogl District, Simbu Province). The most important reason for that is probably overfishing. Being the only fish species in most areas, trout is subject to a high fishing pressure.
As opposed to the disappearance of trout in some streams in Gembogl District, the trout population in Anggura River, Ialibu District, Southern Highlands Province, seems to be able to cope with the fishing pressure. According to people from Kireni Village nearby, trout was introduced in the early seventies, and there are no signs of a decrease in numbers.
In the Gembogl District the human population density is 47 per km2; in Ialibu District it is 21.6 per km2 (1980 census).
This is a clear indication that trout has problems establishing in densely populated areas due to high fishing pressure.
Other parts of the tropics have had the same experiences with rainbow trout. In Nyanga National Park in Zimbabwe, rainbow trout have been stocked for many years, but even though wild breeding is very successful in many streams, the resulting recruitment is not sufficient to keep up with the angling pressure (Bell-Cross and Minshull 1988). Thus, continual stocking will always be necessary in those streams.
The trout fishery in Zimbabwe is mainly a sport fishery, and it is strictly controlled to avoid overfishing. A solution which is not applicable under PNG conditions, where trout is mainly caught by local villagers for subsistence purposes.
The vulnerability of trout to overfishing in Papua New Guinea is, partly, due to the fact that trout currently is the only exploitable fish species within its altitudinal range, and the problem may be reduced if other coldwater fish species are introduced.
The rainbow trout has some drawbacks with regards to suitability for stocking purposes, as also mentioned by Coates (1989). These are:
The preference of trout for streams in the altitudinal range of 2000–2500 m, where relatively few people live.
The lack of dispersal ability. Due to its coldwater-preference trout seems to stay in the area where it was introduced.
Vulnerability to high fishing pressure.
The predatory nature of rainbow trout.
On the other hand, this study indicates that trout has some advantages:
In cold-water, low-fertile streams trout may be more productive than other coldwater species.
People living in the altitudinal range where trout occur, although relatively few, they have nutritionally less options than people living at lower altitudes.
Furthermore, the argument that few people live in the altitudinal range of rainbow trout is true for the Sepik/Ramu catchment, but may not apply for the Purari. No exact data exists on altitudinal distribution of people outside the Sepik/Ramu, but based on 1990-census figures (1990 National Population Census. Preliminary figures, Census Division Populations), it seems that the number of people living above 1800 m.a.s.l. (including both Sepik/Ramu and Purari) is equal to the number of people living below 1000 m.a.s.l. in the Sepik/Ramu catchment. And fish stocking has already been justified in that area (Coates 1990a). Socio-economic data on the current catch of rainbow trout in the highlands is urgently needed to evaluate its importance.
This study indicates that there may still be streams and rivers in the highland area, which could be stocked successfully with rainbow trout. However, the aim of a stocking programme should be to establish a sustainable, self-reproducing fish stock. Consequently, fish stocking is a temporary activity. When the stocking programme has finished and a self-reproducing stock has been established, further stocking should be unnecessary. In fact, further stocking will not result in any significant increase in returns (Davies et.al. 1988).
The question then rises, why, after more than 30 years of trout stocking practices in Papua New Guinea with more than 300,000 trout fingerlings stocked, there may still be a need for further stocking of trout ?
There are two main reasons for this:
Vulnerability to high fishing pressures. In densely populated areas, it may not be possible to establish a self-sustainable trout stock without continual stocking (or stocking of additional fish species). This is probably the main reason why rainbow trout have never been able to support a fishery for the majority of the people in some of the areas where it was stocked.
The lack of dispersal ability. Trout will not move between different sub-catchments, and consequently will have to be stocked in each catchment area. This problem is intensified by its vulnerability to high fishing pressures.
Any future plans for stocking of rainbow trout should be considered in the light of possible introductions of other coldwater species. Trout seems to be a suitable species for altitudes above 1800 m. At altitudes lower than 1800 m, where the majority of people live, other species are more suitable as suggested by the Sepik River Fish Stock Enhancement Project.
This project, as well as the FISHAID-project for stocking higher altitudes, emphasizes that the stocking programme should improve the fish stock for the majority of people. Compared to the coldwater species recommended by SRFSEP, rainbow trout is not an optimal species (i.e. it has a more narrow altitudinal range). On the other hand, in many areas it may a nutritionally important supplement to people, and in these areas it may prove to be a good complementary species to those recommended by SRFSEP.
In general, there is an urgent need for more knowledge on biology and ecology of rainbow trout in tropical high-altitude environments.
This study of rainbow trout under New Guinea conditions can be regarded as initial, and further research should be carried out on a more regular basis (i.e. regular sampling at selected sites). This should lead to more accurate measures of production and reveal any seasonal differences in growth and production.
Research on breeding habits of rainbow trout in Papua New Guinea should be undertaken, including identification of breeding sites and establishment of the breeding season (if any).
Finally (but not least important), a socio-economic study on the importance of rainbow trout in the highlands should be undertaken.
Further research on rainbow trout could be initiated as part of the FISHAID project and in cooperation with fisheries officers from the highland provinces.