Ecological Applications, 22(3), 2012, pp. 828–842 Ó 2012 by the Ecological Society of America Invasive salmonids and lake order interact in the decline of puye grande Galaxias platei in western Patagonia lakes CRISTIAN CORREA 1 AND ANDREW P. HENDRY Redpath Museum, Department of Biology, McGill University, 859 Sherbrooke Street West, Montreal, Quebec H3A 2K6 Canada Abstract. Salmonid fishes, native to the northern hemisphere, have become naturalized in many austral countries and appear linked to the decline of native fishes, particularly galaxiids. However, a lack of baseline information and the potential for confounding anthropogenic stressors have led to uncertainty regarding the association between salmonid invasions and galaxiid declines, especially in lakes, as these have been much less studied than streams. We surveyed 25 lakes in the Ayse´n region of Chilean Patagonia, including both uninvaded and salmonid-invaded lakes. Abundance indices (AI) of Galaxias platei and salmonids (Salmo trutta and Oncorhynchus mykiss) were calculated using capture-per-unit-effort data from gillnets, minnow traps, and electrofishing. We also measured additional environmental variables, including deforestation, lake morphometrics, altitude, and hydrological position (i.e., lake order). An information-theoretic approach to explaining the AI of G. platei revealed that by far the strongest effect was a negative association with the AI of salmonids. Lake order was also important, and using structural equation modeling, we show that this is an indirect effect naturally constraining the salmonid invasion success in Patagonia. Supporting this conclusion, an analysis of an independent data set from 106 mountain lakes in western Canada showed that introduced salmonids are indeed less successful in low-order lakes. Reproductive failure due to insufficient spawning habitat and harsh environmental conditions could be the cause of these limits to salmonid success. The existence of this effect in Chilean Patagonia suggests that low-order lakes are likely to provide natural ecological refugia for G. platei. Finally, pristine, high-order lakes should be actively protected as these have become rare and irreplaceable unspoiled references of the most diverse, natural lake ecosystems in Patagonia. Key words: AIC c ; Aplochiton; Chile; constraint to salmonid invasiveness; deforestation; Galaxiidae; information-theoretic approach; invasive trout; path analysis; Yulton Lake. INTRODUCTION Threats to native biodiversity are acute in fresh waters, and invasive species are considered a primary cause (Clavero and Garcı´a-Berthou 2005). However, direct confirmation of these effects has proven problem- atic owing to the absence of baseline data and the potential effects of other disturbances, such as habitat degradation, that took place concurrently to species invasions. This common inferential problem in invasion biology makes it difficult to firmly establish whether or not invaders are the root cause of problems for native species, and thus what might be the most effective management tools to achieve conservation goals (Did- ham et al. 2005, Light and Marchetti 2007, Hermoso et al. 2011). The effects of salmonid fishes (family Salmonidea) on native galaxiid fishes (family Gala- xiidae) in temperate, Austral lakes pose precisely these challenges. The salmonids rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta), native to the northern hemisphere and regarded among the most common and disruptive invasive fishes worldwide, are now wide- spread in southern cold-temperate freshwaters (Cam- bray 2003, Casal 2006). One taxonomic group thought to be strongly impacted by these invaders is galaxiids, the most speciose taxon in the otherwise sparse freshwater fish fauna of the austral region (McDowall 2006). Although galaxiids have weathered a series of major environmental changes throughout their evolu- tionary history (geological activity, glaciations, and climate change), they are now seriously threatened (Cussac et al. 2004, McDowall 2006, Ruzzante et al. 2008). In an exhaustive review, McDowall (2006) found evidence of salmonids having adverse effects on 29 (58%) galaxioid species; but quantitative analyses of population-level impacts are rare, particularly for lake ecosystems and especially in Patagonia. This difficulty of robust inference stems largely from a general lack of pre- invasion baseline information and a rarity of known lakes that have not been invaded (Milano et al. 2002, Pascual et al. 2002, McDowall 2006, Lattuca et al. 2008a, Arismendi et al. 2009). In addition, the salmonid invasions were concurrent with other environmental changes, such as habitat degradation, and this increases uncertainty as to whether or not salmonids are the main Manuscript received 29 June 2011; revised 18 November 2011; accepted 23 November 2011. Corresponding Editor (ad hoc): J. M. Moore. 1 E-mail: cristiancorrea@gmail.com 828