The Importance of Competition for Food Resources in the Interaction Between Brook Trout (Salvelinus fontinalis) and Rainbow Trout (Salmo gairdneri)

The last 80 years have seen a drastic reduction in the range of the brook trout (Salvelinus fontinalis) the only salmonid native to the Southern Appalachians. Much of this range reduction is directly correlated with increases in the range of the introduced rainbow trout (Salmo gairdneri). The purpose of this study was to determine if competition for food resources plays a significant role in this interaction.

Stomach contents were obtained from sympatric and allopatric populations of brook and rainbow trout during June, July, August and September of 1987. Terrestrials comprised the majority of food items in the stomachs of adult fish from all populations, the mean percentage relative wet weight ranging from 59% in sympatric rainbow trout to 74% in sympatric brook trout. Comparisons using Schoener's Index of Dietary Overlap indicated that there was significant dietary overlap between all populations during the majority of the sampling dates. Analysis of variance indicated that there were no significant differences in the mean relative weight of stomach contents of adult brook trout in sympatry or allopatry, or between adult brook trout and adult rainbow trout living in sympatry. However, the mean relative weight of stomach contents in adult rainbow trout in allopatry was significantly lower than that in adult rainbow trout in sympatry or adult rainbow in allopatry. Calculated caloric intake using the mean observed stomach values was never sufficient to meet the estimated metabolic demands of adult fish at any of the sites during any of the sampling periods, despite the fact that there seemed to be sufficient food resources in the stream drift. Fulton-type condition factors of sympatric and allopatric brook trout were not significantly different, but condition factors of sympatric rainbow trout were significantly higher than the condition factor of allopatric rainbow trout. Population estimates taken in July and October indicated all populations suffered losses in biomass as standing crops decreased from 48% in allopatric rainbow trout populations to 24% in sympatric rainbow trout populations.

In summary, despite data that seemed to indicate that these populations were food limited, there was no evidence to support competition for food resources in areas where the two species coexist.