However, infections with this sea louse have not been reported as a problem on farms Ž
raising coho salmon Associacion Chilena de Productores de Salmon y Trucha, pers.
´ ´
. com. .
C. teres and C. flexispina have low host specificity and are found on several species Ž
. of wild hosts commonly present around salmon farms Carvajal et al., 1998 . In addition
to both species, seven other species of Caligus have been reported from Chilean wild Ž
. fish Fernandez and Villalba, 1986 . Seven of these species have been found in coastal
´
Ž .
waters and others in offshore areas. Nevertheless, Gonzalez and Carvajal 1994 first
´
reported the presence of C. flexispina on farmed salmonids in 1992, even though the species had been previously reported in offshore areas such as the Juan Fernandez
´
Ž .
archipelago and Easter Island, as parasites of non-salmonid fish Carvajal et al., 1998 . The aim of this study was to determine whether there are differences in the
Ž .
Ž .
susceptibility of rainbow trout Oncorhynchus mykiss , atlantic salmon Salmo salar Ž
. and coho salmon O. kisutch to C. flexispina infection under both field and controlled
laboratory conditions. The development rate of the parasite in the three hosts was assessed to determine host preference.
2. Materials and methods
2.1. Farm obserÕations Ž
X Y
X Y
. Sampling trips to a site located at Quinchao island 42831 00 S, 73827 40 W Chiloe
´
Ž .
archipelago, Chile Fig. 1 , were carried out in April, July, September and November 1996 and January 1997 to determine if under field conditions there were differences in
Table 1 Summary of parameters for the experimental infestation in tanks
Trial Tank
Host species Number of fish
Weight Ž .
number number
examined g
1 1
Rainbow trout 3
300 Coho salmon
14 80
2 1
Rainbow trout 14
150 Atlantic salmon
16 74
2 Rainbow trout
12 150
Atlantic salmon 19
74 3
1 Rainbow trout
6 140
Atlantic salmon 13
58 Coho salmon
2 138
2 Rainbow trout
5 140
Atlantic salmon 14
58 Coho salmon
4 138
4 1
Rainbow trout 17
62 Atlantic salmon
17 99
Coho salmon 17
34 2
Rainbow trout 18
62 Atlantic salmon
18 99
Coho salmon 17
34
infection levels between the three host species. Rainbow trout, atlantic and coho salmon are farmed at this site. Each time 7 to 14 specimens of each host species were collected
using a hand net, anesthetized with BZ20 and placed in individual plastic bags. Samples were kept cool during transport to the laboratory, in Puerto Montt where parasite counts
were taken.
Caligids were collected from fish skins using a dissecting microscope and stored in 70 alcohol prior to microscope determination of ontogenic and reproductive stages.
Taxonomic identification was done according to the descriptions and keys of Wilson Ž
. Ž
. Ž
. 1905 , Lewis 1964 and Fernandez and Villalba 1986 .
´
2.2. Experimental studies Ovigerous C. flexispina were obtained from farmed rainbow trout. Eggs were
hatched and reared at the laboratory, to the infectious copepodid stage, in 1 l bottles with
Table 2 Infection of the three salmonids species farmed at the same site in southern Chile with adults, pre-adults and
juvenile C. flexispina. Samples were collected in autumn, winter and spring 1996 and summer 1997. Std: standard deviation
Host species Number of
Prevalence Mean
Ž .
Ž . number examined
copepods Abundance
S.D. 4r 96
Ž .
Rainbow trout 10 94
100 9.42.1
Ž . Atlantic salmon 9
16 67
1.80.5 Ž
. Coho salmon 15
30 47
2.00.8 7r 96
Ž .
Rainbow trout 10 104
100 10.41.8
Ž . Atlantic salmon 7
1 14
0.10.1 Ž
. Coho salmon 10
2 20
0.20.1 9r 96
Ž .
Rainbow trout 10 28
80 2.80.7
Ž .
Atlantic salmon 10 3
30 0.30.1
Ž .
Coho salmon 10 1
10 0.10.1
11r 96 Ž
. Rainbow trout 10
10 60
1.00.4 Ž
. Atlantic salmon 10
Ž .
Coho salmon 10 2
20 0.20.1
1r 97 Ž
. Rainbow trout 10
18 70
1.80.5 Ž
. Atlantic salmon 10
1 10
0.10.1 Ž
. Coho salmon 10
All data Ž
. Rainbow trout 50
254 82
5.14.5 Ž
. Atlantic salmon 46
21 24
0.50.7 Ž
. Coho salmon 55
35 22
0.50.8
1 mm filtered seawater and gently aerated. The seawater at ambient temperature was changed daily.
Four laboratory trials were conducted to determine differences in susceptibility to infection amongst the three salmon species. The number and species of fish used in each
trial varied depending on the freshwater hatchery-reared salmonids available at the time. Smoltation of all fish used in these trials was done at the laboratory to ensure no
previous exposure to sea lice and the fish were previously kept for at least a week in 500-l tanks with sand-filters and circulating seawater. Infestation took place in shaded
and aerated tanks with no water flow and reduced volume of ca. 200 l. The exposure time was from 1 to 3 h after which, seawater flow and rearing volume of 500 l were
Ž restored. The water salinity was 29–31 ppt and the mean temperatures, 13.68C range:
. Ž
. Ž
. Ž
11.3–14.7 , 11.18C 11.9–18.5 , 14.28C 11.9–18.5 and 16.38C range: 13.9–19.3, .
total: 2458C-days for the first, second, third and fourth trial, respectively. Table 1 summarizes experimental procedures of the four trials. In the first trial,
rainbow trout and coho salmon were placed in a tank, and fish were examined for copepods 14 days after copepodid exposure. In trial 2, the fish were placed in two
replicate tanks where rainbow trout and atlantic salmon were challenged with copepo- dids and examined for sea lice 14 days post-infection. In trial 3, rainbow trout, atlantic
and coho salmon were tested in the two replicate tanks. Each tank received three inoculations of copepodids at 1 to 6 days intervals and fish were examined for copepods
at 22 days post-infection. In trial 4, the 3 salmonid species were also tested in two replicate tanks. Each tank was infected with 1540 copepodids and five fish of each
species were sampled from each tank on the second day post-infection to determine initial levels of infection. The remaining 74 fish were sampled at 9 days post-infection to
determine the number of the copepods and their developmental stages.
The terms prevalence, abundance and density were calculated according to Margolis Ž
. Ž
. et al. 1982 and Bush et al. 1997 definitions. Copepod density is defined as the
Fig. 2. Density of total parasites of C. flexispina in netpens from April 1996 to January 1997.
Table 3 Infection of rainbow trout, coho salmon and Atlantic salmon with C. flexispina under laboratory conditions.
Ž .
Fish were maintained at 158C–208C and ambient salinity 29–31 ppt Host species
Number of Prevalence
Mean Ž .
copepods abundance
Ž .
found S.D.
Trial one: sampled at 14 d.p.i. Rainbow trout
58 100
19.21.5 Coho salmon
1 8
1.00 Trial two: sampled at 14 d.p.i.
Rainbow trout 569
100 20.63.2
Atlantic salmon 283
100 7.20.7
Trial three: sampled at 22 d.p.i. Rainbow trout
352 100
33.12.5 Atlantic salmon
106 93
3.00.6 Coho salmon
2 17
0.20.3 Trial four: Sampled at 2 d.p.i.
Rainbow trout 120
100 9.92.7
Atlantic salmon 51
100 4.41.1
Coho salmon 125
100 11.12.2
Sampled at 9 d.p.i. Rainbow trout
425 100
16.11.2 Atlantic salmon
99 100
3.60.4 Coho salmon
142 100
5.00.7
number of copepods per 100 g host weight in order to control differences in the size of the various host species. Statistical analyses were carried out on intensity and percentage
Fig. 3. Density of total parasites of C. flexispina in the four experimental infections in tanks.
U
Only rainbow trout and coho salmon in trial 1 and rainbow trout and atlantic salmon in trial 2.
of larval stage data. Kruskal–Wallis and Mann–Whitney U-tests were used to compare the mean number of lice within and amongst the three salmonid species as well as the
differences in the developmental stages of C. flexispina between species.
3. Results
