The tolerance of Caligus rogercresseyi to salinity reduced in southern Chile

A study was carried out to obtain information about the tolerance of Caligus rogercresseyi to salinity. Samples of C. rogercresseyi were collected from Atlantic salmon (Salmo salar) in four areas of southern of Chile (Lat 42°S), which were exposed to different concentrations of salinity. From the results obtained it was evident that Caligus displayed sensitivity to salinity lower than 20‰. However differences of tolerance were observed in the parasites collected from Puerto MonttCastro region compared with parasites collected from Quellón and Hornopiren, on these latter sites freshwater had a major influence and reduced the intensity of infection. * Corresponding author’s E-mail: sbravo@uach.cl Introduction Caligus rogercresseyi Boxshall & Bravo is the most important parasite of farmed salmon in the south of Chile since it was recorded for the first time infecting Atlantic salmon (Salmo salar) in the spring of 1997. The most susceptible hosts are Atlantic salmon and rainbow trout (Oncorhynchus mykiss). However in the last few years’ juvenile stages of Caligus have also been recorded severely infecting smolts of coho salmon (Oncorhynchus kisutch) following transference to seawater. Females of Caligus rogercresseyi are smaller (0.5 mm, total length) compared with Lepeophtheirus salmonis (12 mm). The life cycle is dependent on temperature and, at 10°C; the development of the parasite is 45 days while at 15°C it is 26 days (Gonzalez & Carvajal, 2003). As Caligus displays a lower tolerance to salinity, some farms have been located in estuarine zones to avoid the high load of parasite over the fish. The economic implications of the severe infections by caligids are mainly associated with the mechanical damage caused by the parasite. Fish show numerous petechial haemorrhaging over the entire body, and these may provide an entry portal for other pathogens. The parasite does not kill the fish, but weakens them, and this leads to diseases such as Infectious Pancreatic Necrosis (IPN) or Salmon Rickettsial Syndrome (SRS). The low condition exhibited by the infested fish is the result of the inappetence shown by the fish because of the high stress imparted by the parasite, and this is also exacerbated by osmotic problems. A variety of environmental and biological factors, husbandry and managements practices that may influence the abundance and impact of sea lice on farmed salmonids have been identified (Pike & Wadsworth, 1999; Rae, 2002; Johnson et al., 2004). These factors and husbandry practices have been Bull. Eur. Ass. Fish Pathol. , 28(5) 2008, 199 used to develop management strategies for sea lice on farmed salmonids (Johnson et al., 2004). Besides temperature, which has been shown to have an important influence in the life cycle of the sea lice, controlling the development times of parasitic copepods (Tully, 1992; Tucker et al., 2000), salinity has also been shown to be important in the abundance of the parasite. Tucker et al. (2000) reported that Lepeophtheirus salmonis had a higher growth rate and rate of settlement at a salinity of 34‰ when compared with 24‰. Salinity has also been shown to affect hatching and development of the free-swimming stages of L. salmonis (Wootten et al., 1982). Johnson & Albright (1991) found that nauplii were not produced at salinity below 15‰ and that at salinity levels of 25 ‰ and below, less active nauplii were produced with copepodids dying on moult. In British Columbia it was reported that sea lice larvae do not consistently develop to an infectious stage at salinities <30‰ (Brooks, 2005). Pike & Wadsworth (1999) reported that adult lice die rapidly at salinities <12‰ and that, while eggs hatched successfully at salinities as low as 15‰, survival was negligible. Survival improved at 20–25‰, but development to the copepodid stage remained low at salinities <30‰. Heuch et al. (1995) reported that copepodids of L. salmonis avoid water with less than 20‰. There is no documented information about the tolerance of salinity in Caligus rogercresseyi, only the records of some farms located in estuarine areas where caligus is not present with salinity lower than 15 ‰ (personal observations). This report gives information about the tolerance of C. rogercresseyi to different levels of salinity recorded in the laboratory trial, under controlled conditions. Methodology Between May and June 2006 samples of adults caligids were taken from four farms located in the south of Chile. One farm was selected from the areas of Puerto Montt, Castro, Quellón and Hornopiren (Figure 1). The first two farms are located in sites with a high level of salinity and the last two in sites with influence of freshwater. Fish were anaesthetized with 15 ppm of benzocaine and parasites were gently removed, stored in a container supplied with aeration and kept at low temperature with ice pack in a box and transported to the laboratory at the Aquaculture Institute of the Universidad Austral de Chile in Puerto Montt, where the salinity challenges were carried out. At the laboratory, five males and five females were exposed to different gradient of salinity (0; 5; 10; 15; 20 and 31‰) and maintained in containers supplied with constant aeration. Mortality was recorded at 30 min, 1 hour, 12 Figure 1. Location of sites from where caligus were collected. Bull. Eur. Ass. Fish Pathol. , 28(5) 2008, 200 hours; 18 hrs and 24 hrs respectively. The mix of salinity gradient was obtained using seawater at 31‰ previously filtered and mixed with fresh water obtained from a river near the sites. Salinity concentrations were measured with a digital salt meter YSI Mod. EC300CC-04. Temperature was also recorded during the period of challenge. Confidence limits of 95 % were estimated for the parameters. Analysis of variance and the Multiple Range Test, (LSD) Fisher was used for estimating differences between sites. Results After 30 minutes of exposure at 0‰, survival was 0% in all the samples of C. rogercresseyi obtained from the four areas, while all parasites survived at 31‰ after the 24 hours of exposure (Table 1; Figure 2). Parasites obtained from the area of Ancud-Castro showed lower tolerance to salinity of 5‰ than the parasites collected from the other three areas. At 15‰ the survival of parasite collected from Hornopiren was 80% after 24 h of exposure, while for parasites collected from Ancud-Castro this was 20%. Hornopiren was the area where Caligus showed major tolerance to low salinity (Table 1) and also was the area with the higher fluctuation of salinity during the year, while in comparison the Ancud-Castro area showed less fluctuation at both depths (Figure 3). At 20‰, females showed more tolerance to salinity than males (Figure 5). However, significant differences in salinity tolerance between males and females (Table 3) were not recorded in the different areas (p<0.33). The salinity gradient through the full year showed a similar pattern of distribution at 0.5 and 5.0 m, with significant differences between both depths (p<0.05). Hornopiren showed the lower gradient of salinity with respect to the other areas and the lowest prevalence of C. rogercresseyi was recorded between June to November and the low intensity of infestation (Figure 6). The higher prevalence was recorded in the areas of Ancud-Castro and Puerto Montt, However the highest rate of infestation was recorded in the area of Quellón (Table 4; Figure 6). All areas showed a decrease of temperature from May to September, a period which comprises 0% 20% 40% 60% 80% 100% 120% 30 m in 1 hr 12 h rs 18 h rs 24 h rs 30 m in 1 hr 12 h rs 18 h rs 24 h rs 30 m in 1 hr 12 h rs 18 h rs 24 h rs 30 m in 1 hr 12 h rs 18 h rs 24 h rs 30 m in 1 hr 12 h rs 18 h rs 24 h rs 30 m in 1 hr 12 h rs 18 h rs 24 h rs 0 ‰ 5 ‰ 10 ‰ 15 ‰ 20 ‰ 30 ‰