Quantifying Predation on Galaxiids and Other Native Organisms by Introduced Rainbow Trout in an Ultraoligotrophic Lake in Northern Patagonia, Argentina: A Bioenergetics Modeling Approach

Abstract

Exotic rainbow trout Oncorhynchus mykiss support an economically valuable recreational fishery in Patagonia but also create concern for impacts on native organisms. These concerns are intensified by the possibility of hatchery release programs in this region. We estimated losses of different prey from predation by rainbow trout in Lake Moreno, Río Negro Province, Argentina, using a bioenergetics model combined with input data from directed sampling on growth, seasonal diet, distribution, and thermal experience. The fish community was sampled seasonally using gill nets, hydroacoustics, and ichthyoplankton [Article] nets. Pelagic galaxiid larvae and benthic juvenile and adult small puyen Galaxias maculatus were the most important components of the diet. Bioenergetics simulations showed that over a 6-year life span in the lake (ages 1-7), rainbow trout attained a body mass of 2.3 kg and consumed 74.7 kg of food, of which 20% consisted of galaxiid larvae and 16% consisted of adult small puyen. Based on an estimated abundance of 29,000 rainbow trout of ages 1-7, this predator exerted significant but sustainable mortality on the native prey populations, consuming 44 metric tons or an estimated 23% of the annual larval galaxiid production and 35 metric tons of adult small puyen, which represented an unknown fraction of the postlarval population. Galaxiids supported the estimated predation demand under current conditions. However, simulations of stocking strategies normally proposed for this region showed that consumption demands on prey would increase to unsustainable levels, reducing native fish populations and likely reducing growth of rainbow trout. It is also probable that the fish community composition would shift further in response to the increased demand for prey by stocked predators. This implies that in some cases, stocking could jeopardize sport fisheries; stocking strategies should be evaluated on a case-by-case basis to be consistent with specific objectives for native fish conservation and sustainable food web interactions. 1406 VIGLIANO ET AL. 1408 VIGLIANO ET AL. 1416 VIGLIANO ET AL.

Extensive diel fish migrations in a deep ultraoligotrophic lake of Patagonia Argentina

“…The presence-absence catch data (Table 1) showed that all species described for the Moreno system (Vigliano et al, 2009) were caught in our sampling, except O. hatcheri. This species appears to use only the shallow littoral zone (0-3 m) with high macrophyte vegetation cover (Vigliano et al, 2009), and is thus excluded from hydroacoustic surveys.…”

“…This species appears to use only the shallow littoral zone (0-3 m) with high macrophyte vegetation cover (Vigliano et al, 2009), and is thus excluded from hydroacoustic surveys. The small number of O. viedmensis caught were always in close association with the near-shore lake bottom and were thus masked within the bottom hydroacoustic signal.…”

“…Galaxiids smaller than 6 cm (larval and post metamorphic stages) play an important ecological role because they are the main and most abundant planktophagous organisms in these lakes (Barriga, 2006). These galaxiids suffer high predation pressure from most of the remaining fish species in the lakes (Milano, 2003;Macchi, 2004;Milano et al, 2006;Macchi et al, 2007;Vigliano et al, 2008Vigliano et al, , 2009). Owing to these trophic interactions, galaxiids exhibit DM patterns that would allow them to decrease predation risk, thus influencing the lower trophic levels (Barriga et al, 2002;Milano, 2003;Reissig et al, 2003;Reissig, 2005).…”

Marine effect of introduced salmonids: Prey consumption by exotic steelhead and anadromous brown trout in the Patagonian Continental Shelf

“…The effects of resident salmonid populations on receiving freshwater communities have been studied in some depth in places around the world (McDowall 2006), including Patagonia (Aigo et al 2008;Vigliano et al 2009). Much less is known about the ecological effects of anadromous salmonids in marine habitats in general, and no information is available for populations inhabiting the southwestern Atlantic Ocean.…”

“…This tool has been used to evaluate conservation and management questions regarding factors limiting the production of fish populations (Hartman and Margraf 1993), including the effects of nonnative species (Ruzycki et al 2003;Vigliano et al 2009). Because these models are based on energy balance equations, they are frequently used to estimate consumption or growth for fish while accounting for changes in body mass, thermal experience, diet, and energy density of prey (Chipps and Wahl 2008) and are less time consuming than traditional methods.…”