Ecology of standing waters

The impact of introduced trout on invertebrate communities in inland Canterbury lakes

Brown and rainbow trout have been introduced to many inland waters in New Zealand, but research on the impacts on native communities has focused mainly on streams. In collaboratin with FERG members, Scott Wissinger's research (Wissinger et al. 2006, pdf, 460kb) compared the benthic communities of inland lakes with (26 lakes) and without (17 lakes) trout, in Canterbury and Marlborough (Fig. 1).

Figure 1. Lakes Sarah and Grasmere in the Cass Valley, Inland Canterbury. Most of the large lakes in this area contain both rainbow and brown trout.

This study indicated the benthic community composition and diversity of lakes with and without trout were nearly identical (Fig. 2) and biomass was as high or higher in the lakes with as without trout (Fig. 3).

Figure 2. Bray-Curtis ordination on presence-absence of 77 taxa in 43 lakes. Invertebrate communities in lakes with (open symbols) and without (closed symbols) trout were very similar, whereas some separation in communities occurred between lakes across different geographic clusters.

Figure 3. Mean (±1 SE) ash-free dry mass (AFDM) of benthic invertebrates in the three subhabitats in trout and troutless (NoT) lakes on the South Island of New Zealand. Emergent = shoreline emergent vegetation zone; open = littoral zone of sparse or no vegetation; submergent = submergent vegetation zone.

There was no evidence that trout have caused local extinctions of benthic invertebrates. Although the proportional abundance of large-bodied aquatic was slightly lower in lakes with than without trout, the abundance of several groups of large-bodied benthic taxa (dragonflies, caddisflies and water bugs) did not differ.

Our findings are in contrast to those in North American and Europe where trout introductions into previously troutless lakes have led to declines in the abundance of benthic invertebrates, especially large-bodied taxa. The modest effects of trout in New Zealand could be explained by: i) the high areal extent of submergent vegetation that acts as a benthic refuge; ii) low intensity of trout predation on benthic communities and/or; iii) characteristics of the benthic invertebrates that make them invulnerable to fish predation.

Subsequent experiments indicate predatory invertebrates respond to fish chemical cues by reducing foraging rates. This behavioural plasticity (consistent with the third hypothesis) may contribute to the weak effects of trout on benthic communities.

The relative importance of the hypotheses requires further experimental investigation. Nevertheless, our results emphasise that the same invertebrates occurred in all of the lakes, regardless of size, elevation and presence of trout, suggesting habitat generalists dominate the benthic fauna in shallow New Zealand lakes. This observation is also consistent with Hamish Greig's PhD research, where nested community patterns were observed along pond permanence gradients in Canterbury Ponds (Greig 2008, pdf, 150kb).

People involved in this project

University of Canterbury
Angus McIntosh
Hamish Greig

Allegheny College
Scott Wissinger

Publications
Greig 2008. Community assembly and food web interactions across pond permanence gradients. PhD thesis, University of Canterbury (pdf, 150kb).
Wissinger, McIntosh & Greig 2006. Impacts of introduced brown and rainbow trout on benthic invertebrate communities in shallow New Zealand lakes. Freshwater Biology 51:2009-2028 (pdf, 460kb).