Do trout respond to riparian change? A meta-analysis with implications for restoration and management

Abstract

There are strong conceptual links between riparian zones and freshwater fish via riparian influences on water quality, habitat quality and availability, and trophic dynamics. Many of the world's riparian zones are, however, severely degraded, and the key functions they provide for fish are lost or compromised. In response to their ongoing degradation, extensive works are underway globally to restore the structure and function of riparian zones. Despite intense effort, we lack clear empirical evidence of how fishes respond to changes in riparian zones. We conducted a systematic review and meta-analysis to explore how trout (specifically brook, brown, cutthroat, rainbow and steelhead), fish with globally important social, cultural, economic and ecological value, respond to key drivers of riparian alteration. We also identified where and with which species current research is being undertaken and examined the broad characteristics of different studies (e.g. location, focal species, length of study, study design) to better understand potential knowledge gaps in our understanding of how trout respond to changes in riparian zones. ISI Web of Science and Google Scholar were searched for relevant peer-reviewed studies, and from an initial 6514 papers, 55 were included in the formal meta-analysis. From these, we extracted data to calculate response ratios comparing biological attributes at sites with altered riparian characteristics to suitable unmanipulated control sites. We used linear mixed effects models to assess general and species-specific trout responses to eight key ‘drivers’ of change in riparian condition. Most studies were undertaken in North America using control-impact designs. We found little evidence for species-specific responses to riparian change, and surprisingly, many drivers deemed important in the literature (e.g. revegetation, managed canopy removal, grazing, and forestry clearing) did not consistently influence trout population- or individual-level metrics. Nonetheless, trout populations did respond positively to increasing woody debris and livestock exclusion (+87.7 and +66.6% respectively), and negatively to bushfire and afforestation (−67.4 and −88.2% respectively). We found some evidence that positive riparian changes may just attract fish (i.e. increased abundance or density) rather than enhance actual population production (i.e. individual size and growth). While this conclusion necessarily needs to be interpreted with caution, it does suggest that targeted research on the ‘production versus attraction’ hypothesis would be beneficial. Several key drivers of riparian change, such as revegetation activities, have been the focus of only limited research. More generally, long-term data are lacking for most drivers. Both of these key information gaps limit our ability to predict the likely timing and trajectory of responses to riparian management. Robust monitoring programmes in areas with altered riparian zones – particularly using BACI designs to allow changes to be attributed to management – are required. The knowledge gaps present for fishes as ecologically, socially and environmentally important as trout are likely to be even more pronounced for the majority of less-studied freshwater fish species.