The importance of escaped farmed rainbow trout (Oncorhynchus mykiss) as a vector for the salmon louse (Lepeophtheirus salmonis) depends on the hydrological conditions in the fjord

Abstract

The objectives of the study were to see if escaped rainbow trout (Oncorhynchus mykiss) spread rapidly or not from fish farms, and to test whether the hydrological conditions in a fjord influence their vertical distribution and importance as vector for the salmon lice (Lepeophtheirus salmonis). Fifty farmed rainbow trout were tagged with acoustic transmitters including depth sensors and released from two of 11 fish farms in the fjord system. In addition, unintentionally escaped rainbow trout were recaptured for analysis of salmon lice and stomach content. Dispersal out of the fjord system was limited. Most fish stayed in the vicinity of and moved between the fish farms but fed primarily on a variety of indigestible items. They moved in the warm relatively fresh surface layer from late spring until early autumn where the risk of being infested with salmon lice was low. They swam gradually deeper and became much more infested with salmon lice as the surface layers cooled and salinity and temperature gradients became less distinct over the course of the winter. The observed postescapement behavior may challenge the control of the spread of diseases and parasites between neighboring farms and to wild fish, but also increases opportunities for recapture.

Native and exotic fishes in a Patagonian reservoir with rainbow trout cage culture: spatial and trophic resource use

“…the exotic rainbow trout (both farm escapees and wild) and the native P. trucha, the former dominating at shallow depths and the latter at greater depths. Escaped (ERT) and wild (WRT) rainbow trout each accounted for c. one third of the total littoral capture (<4 m), coinciding with other studies on depth distribution of farm escapees (Chittenden et al, 2011;Skilbrei, 2012;Patterson and Blanchfield, 2013). Diet differences between species were marked, paralleling the observed segregation by depth.…”

“…Two studies in Norwegian sea fjords reported numerous indigestible items in ERT stomach contents with less than a year in the wild, but not farm pellets or wild food (Rikardsen and Sandring, 2006;Skilbrei, 2012). Wild food and farm pellets, but not indigestible items, were found in ERT with probably more than a year since escape time in Chilean sea fjords (Soto et al, 2001) and lakes (Arismendi et al, 2009).…”

“…Ecological impacts include predation, competition and species replacement (Habit et al, 2010;Johnston and Wilson, 2014). A variable degree of escaped fish fidelity to farms has been reported, but dispersal does occur eventually, as does the major use of surface layers (Bridger et al, 2001;Skilbrei, 2010Skilbrei, , 2012Chittenden et al, 2011;Patterson and Blanchfield, 2013).…”

“…native species, or previously introduced exotic free-living species; Soto et al, 2001;Naylor et al, 2005;Johnston and Wilson, 2014). However, cases have been reported where escapees feed heavily on farmed pellets (Abrantes et al, 2011), or consume only indigestible items floating on the surface (Rikardsen and Sandring, 2006;Skilbrei, 2012). Feeding behavior seems to be time-dependent on the escape, and age at escape (Rikardsen and Sandring, 2006), but might also depend on interactions with wild fishes (Skilbrei, 2012).…”

“…However, cases have been reported where escapees feed heavily on farmed pellets (Abrantes et al, 2011), or consume only indigestible items floating on the surface (Rikardsen and Sandring, 2006;Skilbrei, 2012). Feeding behavior seems to be time-dependent on the escape, and age at escape (Rikardsen and Sandring, 2006), but might also depend on interactions with wild fishes (Skilbrei, 2012). Interaction between escaped and wild fishes is likely to be context-dependent (Podemski and Blanchfield, 2006), with more intense effects (e.g.…”