Black flies are ubiquitous members of freshwater streams. These dipterans are Ecological important as food for other invertebrates and a number of vertebrate predators, and anthropogenically important as a significant pest species to humans and livestock. Reliant on fast flowing water to facilitate filter feeding these larvae are well known to act as living flow probes, locating and congregating in the fastest micro-habitat flow conditions on a given bed element. Intra-specific competition usually leads to the largest, oldest larvae claiming the best, fastest flow regions. This competition, in addition to other factors such as crowding, often leads to displacement or active dispersal of younger larvae. As there are serious risks involved in dispersal (starvation and predation) it is assumed that the benefits of finding faster flow must outweigh the risks, however, the link between flow conditions and growth rates have, till now, not been directly tested. This study used controlled laboratory conditions to determine the growth rates of neonate and older, 4^th instar, larvae under two flow conditions relevant to their natural riffle habitat. Flow conditions were monitored with laser-doppler velocimetry to assure consistent flow, both spatially and temporally across the three-day trials. Larval size was determined in-situ with non-invasive image analysis.

Results/Conclusions

The results showed that at both 40 and 60 cm/s flow rates the smaller neonates grew significantly faster than their older counterparts, even when larval size was accounted for. Further, the 50% increase in flow led to a proportional 50% increase in growth rates for the neonates, but only a 30% increase for the older larger larvae. Since faster growth, and larger size can positively impact survival and reproduction, the ability of neonates to receive such a large benefit from faster flow may explain why dispersal is such a commonplace behavior for them. This work also has implications for future modeling efforts to predict changes in demography and population size for larvae living in systems where in-stream flow is a consideration.