Drifting or walking? colonisation routes used by different instars and species of lotic, macroinvertebrate filter feeders

Many views of stream invertebrate populations centre on drift as the major route of larval dispersal, but few studies have presented unambiguous information about the role of drift. We present the results from an experiment designed to determine whether the major route of colonisation of substrata by hydropsychid larvae (commonly found in the drift) is by drifting directly onto substrata or by walking along the stream bottom. The experimental design contained four treatments: substrata open to drifters and walkers; fenced substrata open to drifters only; and two treatments open to drifters and walkers that provided forms of fence controls. Fifteen replicates of each treatment were set out at random locations within a riffle at each of three sites, with each site on a different river (the Little River, the Steavenson River and the Acheron River) in the Acheron River catchment. The experiment was run twice, once during autumn (April 1999) and once during early summer (January 2000). Both experiments were colonised by three species of hydropsychids, Asmicridea sp. AV1, and Smicrophylax sp. AV1 and AV2. We found that 2nd/3rd instars of Asmicridea sp. AV1 walked as well as drifted, whereas all others primarily drifted. No relation between numbers of recruits and water speed was found when substrata were open only to drifters, whereas substrata open also to walkers gained more recruits in faster flows. Additionally, larvae more frequently abandoned nets in slow than fast flows, indicating that drifting into unfavourable flow environments may result in mortality or redispersal of larvae. These findings demonstrate that, although drift is important, it is not necessarily the only method used by hydropsychids to colonise substrata. Larvae may have more capacity to choose substrata in fast flows when they colonise substrata by walking. Spot measures of hydropsychid distribution cannot distinguish between these explanations. The finding that walkers can sometimes comprise significant numbers of recruits raises the prospect that hydropsychids can be sourced locally and have not inevitably drifted in from upstream locations.

The central-place territorial model does not apply to space-use by juvenile brook charr Salvelinus fontinalis in lakes

1. Territoriality is widely accepted as the mechanism responsible for density-dependent mortality, emigration, and 'self-thinning' of populations of juvenile salmonine fishes in streams. Numerous studies have focused on territoriality exclusively in stream (lotic) environments and thus have fostered a stereotyped view of juvenile salmonines as sedentary and territorial. We term this behavioural paradigm the central-place territorial model (CPTM).

2. We tested predictions characterizing the CPTM for young-of-the-year (YOY) brook charr (Salvelinus fontinalis) in two Canadian lakes to determine if territoriality may also potentially limit space and population size of brook charr in lakes.

3. Our findings were not consistent with the CPTM. Fish in both lakes were not central-place forages. Maximum displacement distance did not increase with body length as predicted by the general salmonine model of Grant and Kramer (1990). Net displacement distanced increased with the proportion of time spent moving. Aggressive frequency was greatest for fish which spent large proportions of time moving and did not defend from a central-place.

4. Fish in both lakes were rarely aggressive, highly active, and often moved back over the same areas. However, lake fish which migrated to a tributary stream had no net displacement (central-place foraging) illustrating the immediate effects of current on foraging tactics and space-use.

5. The effect of hydrodynamic environment (flowing vs. still water) on fish behaviour needs to be explicitly considered in future models of salmonine behaviour.

Limitations to the feasibility of using hypolimnetic releases to create refuges for riverine species in response to stream warming

Given predicted increases associated with human-induced climate change, stream temperatures are likely to approach upper tolerance limits of aquatic biota within the coming decades. Little information is available regarding thermal tolerance limits of lotic fauna or mechanisms allowing fauna to persist following high temperature events (e.g. use of thermal refuges). Cold-water refuges can facilitate survival of fish in the Northern Hemisphere, but little evidence of similar refuges exists elsewhere.Planned releases of hypolimnetic, or a mixture of top and bottom, waters from reservoirs have recently been touted as a novel method to potentially ameliorate extreme temperature events. However, the feasibility of this technique has not been fully discussed in the published literature. Therefore, we present a literature review, an analysis of thermal data for some large dams in southern Australia in relation to known thermal tolerances of native fauna, and an assessment of current management practices regarding the technique. We show that hypolimnetic releases have variable impacts on water temperatures downstream of a dam, depending on size, off-take infrastructure and management practices but, even where there is an effect, knowledge gaps are too numerous for this technique to be currently feasible. Furthermore, hypolimnetic releases generally evoke negative connotations among natural resource managers, due to the occurrence of cold-water shock in some species. If knowledge gaps and limitations can be addressed, it is possible that the technique may be considered in future, so we present potential tools for future assessment, capacities and limitations and discuss potential scenarios where environmental managers might consider this technique.