Experimental challenges of A tlantic salmon S almo salar with incremental levels of copepodids of sea louse C aligus rogercresseyi : effects on infestation and early development

Over the last decade, the native parasite which has worst affected salmonids reared in Chile (Atlantic salmon and Rainbow trout) is Caligus rogercresseyi (Gonzalez & Carvajal 1994; Gonzalez, Carvajal & Medina 1995; Boxshall & Bravo 2000; Rozas & Asencio 2007). The financial costs associated with treating Caligus have been estimated at US$0.30 kg 1 or US$160 million annually, principally from intensive use of antiparasitic drugs (Carvajal, Gonzalez & Nascimento 1998; Costello 2009). Consequently with the economic impact caused by this parasite to the Chilean salmon aquaculture, several studies have described significant biological aspects of Caligus species along with their interaction with wild and cultured hosts (Gonzalez, Carvajal & George-Nascimento 2000; Gonzalez & Carvajal 2003; Pino-Marambio, Mordue, Birkett, Carvajal, Asencio, Mellado & Quiroz 2007). Gonzalez and Carvajal (2003) described the life cycle of C. rogercresseyi under laboratory conditions and cultured hosts, which comprised eight developmental stages lasting between 30 and 45 days, and which are strongly dependent on temperature and salinity. Furthermore, field evaluations have shown that rainbow trout is the most susceptible species, followed by Atlantic salmon and Coho salmon (Zagmutt-Vergara, Carpenter, Farver & Hedrick 2005). Coinciding with such evaluations, Gonzalez et al. (2000) showed through experimental infestations that Atlantic salmon is less suitable to be colonized by infective copepodids, while the Coho salmon is heavily colonized by copepodids although only a low proportion of the parasites reaches the adult stage. The interaction between salmonid fish and sea lice has been widely studied with Lepeophtheirus salmonis (Kroyer) under laboratory conditions (Bron, Sommerville & Jones 1991; Johnson & Albright 1992; Dawson, Pike, Houlihan & McVicar 1997; Finstad, Bjorn, Grimnes & Hvidsten 2000; Treasurer & Wadsworth 2004; Kolstad, Heuch, Gjerde, Gjedrem & Salte 2005; Gjerde & Saltkjelvik 2009). Patterns that emerged from this parasite– host system related to settlement and early development may be applied to other sea lice parasites and its salmon host. A positive relation between infection pressure (number of copepodids per fish) and settlement may be construed from these studies despite high variability observed, where low infection pressure (28–36 copepodids per fish) produce a low abundance of parasites on fish during settlement (1.66 ± 1.15 and 29.8 parasites per fish) (Bron et al. 1991; Johnson & Albright 1992; Gjerde & Saltkjelvik 2009); intermediate infection pressure (74–145 copepodids per fish) cause mean abundance of parasites on fish (25.1 ± 16.4 and