* Material and methods

* Results and discussion

* Bibliography

ABSTRACT: The study was conducted in a center of salmon farming on the island Guar, X Region, Chile (50 ^0 44′S., 74 ^0 30′W.). In an isolated cage remained salmon (Salmo salar) from May 1993 to August 1994. Every three months were sampled at random 500 fish and determined the number and size of parasites Ceratothoa gaudichaudii located in the mouth and gills. The prevalence of parasites increased from 33.4% to 98.8% in the 16 months of observations. Infective forms (Manco) of the parasite is present in greater numbers in late spring and summer, which resulted in increased small-size specimens in the autumn and winter. The number of medium and large parasites increased during the summer months. We discuss the importance of horse mackerel (Trachurus murphy) and the normal host of C. gaudichaudii and its role as a source of infection for farmed salmon.

Keywords: Isopoda, Ceratothoa gaudichaudii, infestation, farmed salmon.

SUMMARY: Variation of intensity infestation with infective stages of the Isopod Ceratothoa gaudichaudii in Farmed Salmon in the south of Chile

The study WAS Carried out at a salmon farm Located on Guar Island (42 ^0 41′S., 73 ^0 01 E.) Xth Region, Chile. 500 salmon (Salmo salar) Were randomly sampled from an isolated cage Every three months. Each occasion on the number and the size of the parasitic isopods Ceratothoa gaudichaudii Settled in the oral cavity and gills Were determined.

Prevalence ranged from 33.4% to 98.8% in the 16 months of the assay. The Highest number of infective stages (Manco) of the parasite Were found in spring and DURING the summer, this produces The increment of small size parasites in autumn and winter. The number of medium and large size parasites INCREASED DURING the summer months.

The Importance of wild fish, and Specially of Trachurus murphy, as the normal host of C. gaudichaudii and Its role as a main source of infestation for cultured salmon is discussed.

Key works: Isopod, Ceratothoa gaudichaudii, infestation, Farmed Salmon.

INTRODUCTION

In Chile we have described several pathogens affecting cultured fish (Reyes, 1983; Leal, 1988, Bravo, 1988). Among them is the isopod Ceratothoa gaudichaudii, which normally parasitizes horse mackerel (Trachurus murphy) and other native fish (Jaramillo, 1977), and in 1984 was first detected in Coho salmon (Oncorhynchus kisutch) and Bravo (1988 .) From that time the parasite has increased in many farms Atlantic salmon (Salmo salar), found a prevalence of up to 100% in the centers and Guar Huelmo near the city of Calbuco (Inostroza et al., 1993 .)

C. gaudichaudii is a bloodsucking parasite that is located mainly in the oral cavity of fast swimming pelagic fish. Their geographical distribution extends from the Gulf of California to Cape Horn, being frequently found in fish off the coast of Ecuador, Peru and Chile (Brusca, 1981).

According to Brusca (1991) *, the life cycle of Lironeca vulgaris, another isopod parasite, would be homologous to the life cycle of C. gaudichaudii: in this species the fertilized adult female remains in her abdominal cavity from 300 to 600 eggs that develop through five moults to infective larvae or “Manco.” The maimed, that are released into the aquatic environment (Brusca, 1981), with three changes and actively swim to find a suitable fish. It sets in the oral cavity, but can change several times to host (parasitic optional). To settle permanently in a fish parasite life begins, their appendages atrophy swimming facilities and the parasite begins to grow. Initially all individuals are male and after 17 to 19 months of development, one of them becomes female. Intercourse and subsequent pregnancy was performed in the oral cavity of the fish hosts (Brusca, 1978, 1981).

C. gaudichaudii cause deep erosion of the oral mucosa and destruction of the salmon gill-infested, the greater the number of parasites per fish, the lower their body weight, which can cause a loss of weight at harvest up to 15% ( Sievers et al., 1996).

This paper describes and tries to interpret the changes over time in prevalence and intensity of infestation of the different sizes of C. gaudichaudii in farmed salmon.

MATERIAL AND METHODS

Between May 1993 and August 1994 he worked at a salmon farming on the island Guar (50 ^0 44′S., 74 ^0 30′W.), X Region, Chile. We used Atlantic salmon (S. salar), admitted in October 1992 to breeding in sea water weighing 900-1100 grams. The fish were kept in a cage exclusive and isolated until the time of harvest in August 1994.

Every three months, randomly reviewed 500 salmon and recorded the number and size of the parasites found in the mouth and gills. Only in May 1994 totaled 232 fish review due to inclement weather, for this opportunity, we calculated the proportionate amount of parasites. The parasites were classified according to the following criteria: a) Mancas: 0.3 to 0.5 cm (pigmented and swimming appendix developed flow). b) Small: 0.5 to 1 cm (white, swimming atrophied appendix). c) Medium: 1 to 2 cm, and d) Large: greater than 2 cm.

Every three months we determined the prevalence and intensity of infection with different sizes of the parasite.

RESULTS AND DISCUSSION

At the beginning of the experiment, in May 1993 were parasitized 33.4% of the salmon, this prevalence increased to 98.8% in August 1994 (figure 1). A similar trend was observed by Inostroza et al. (1993) in salmon farming center very close (Huelmo) between 1990 and 1991.

When analyzing the parasites was observed that Manco was found in greater numbers in the November, February and May 1994 (Table 1, Figure 2), coinciding with the months that were recorded higher water temperatures. It should be noted that in this study were classified as “maimed” the parasite pigment, but no differentiation was made of the three stages of development. Future work will be necessary to distinguish the state of manca 1 in order to determine more accurately the time of greatest risk of infection for salmon. Presumably development manca manca 2 and 3 requires one to two months when the water temperature is high, and longer than during the cold months. This can explain the presence of pigmented forms of the parasite for most of the year in farmed salmon, which also suggests that the infestation of salmon is checked constantly throughout the year.

Figure 1.

Trends in the prevalence of Ceratothoa gaudichaudii in farmed salmon in the center of the island Guar, X Region, Chile.

Evolution of the Prevalence of Ceratothoa gaudichaudii in Farmed Salmon on Guard Island, Xth Region, Chile.

Table 1.

Number of different developmental stages on farmed salmon Ceratothoa gaudichaudii

a center of the island Guar, X Region, Chile, reviewed every three months from May 1993 to August 1994.

Number of Different Stages of Development Ceratothoa gaudichaudii

Save on Farmed Salmon in Island, Xth Region, Chile, examine Every three months from May 1993 to August 1994.

We sampled 46.4% of the fish and it was calculated proportionally.

Figure 2.

Variation in the intensity of infection with Manco and small specimens Ceratothoa gaudichaudii

and average water temperature in the Guard Island, X Region, Chile.

Variation of the intensity of infestation Ceratothoa gaudichaudii, maimed and small parasites,

and mean water temperature on Save I sland, Xth Region, Chile.

All stages of the parasite and set (small, medium and large) have increased in number in May 1993 to August 1994 (Table 1), but highlights the increasing number of young parasites in the months of May and August 1994 , indicating that the time of greatest risk of infection is during the summer. The number of medium and large parasites increased from February 1994, indicating that the parasite grows in the summer.

To be harvested and checked all the fish in August 1994, no females of parasite eggs or larvae. This suggests, surely, it is a parasitic disease that, under the management conditions of the farming center, fails to complete its life cycle in salmon and that the source of infection is external to the cage. As the horse mackerel (T. murphyi) the main natural hosts of C. gaudichaudii (Jaramillo, 1977), strengthening the hypothesis Inostroza et al. (1993), where these fish would be the main source of infection for farmed salmon.

In previous observations made in the farms and Huelmo Guar found in late summer, females of parasite eggs in some salmon broodstock. Very rarely, and only fish that have spent more than two years at sea, have found females with larvae inside.

C. gaudichaudii has a relatively long development and gestation of about 500 infectious forms can be considered very low if compared with most of the parasites that cause long periods much higher amount of eggs or larvae. This fact can be concluded that C. gaudichaudii is a very efficient parasite in its ability to infect new hosts, such as S. salar (Inostroza et al., 1993) and Oncorhynchus kisutch (Gonzalez and Carvajal, 1994), they can accomplish in their life cycle. If we add to their low host specificity (Jaramillo, 1977) is explicable their successful adaptation to farmed salmon.

The shoals of mackerel enter the channels and fjords in late spring (November-December) and the drop in the fall. During this seasonal migration are numerous of salmon farms (which does not exist just 15 years) and are concentrated around the floating cages to feed on the leftovers. The concentration of jacks in a confined space increases the chances of the lack of the invasion to the same jacks and, incidentally, the salmon in captivity. It would be interesting to determine whether increasing the prevalence and intensity of parasitic infestation in the horse mackerel. This could explain the respective steady growth of the parasite in farmed salmon (Bravo, 1988; Inostroza et al., 1993; Lobos, 1994).

REFERENCES

BRAVO, S. 1988. Register of parasites detected in salmon farms in Region X.. Updated Information Bulletin Pfizer. Santiago, Chile.

SHARP, R.C. 1978. Studies on the cymothoid fish symbionts of the eastern Pacific (Crustacea: Isopoda: Cymothoidae). II. Biology and Systematics of Lironeca vulgaris. Occasional Papers of the Allan Hancock Foundation (NS) 2: 1-19.

SHARP, R.C. 1981. A monograph on the Isopoda Cymothoidae (Crustacea) of the eastern Pacific, Zool. J. Linn. Soc 73 (2): 117-199.

GONZALEZ, L., J. CARVAJAL. 1994. Parasites in fish farming of salmonids in southern Chile, Invest. Pesq. (Chile) 38: 87-96.

INOSTROZA, R., G. SIEVERS, J. ROA, R. AGUIRREBE~NA. 1993. Infection prevalence and intensity of seasonal Ceratothoa gaudichaudii in salmon Salmo salar cultured in seawater in southern Chile, Arch Med Vet. 25: 173-179.

JARAMILLO, E. 1977. New hosts and geographical distribution of Meinertia gaudichaudii (Milne-Edwards, 1840) (Isopoda Cymothoidae), Environment 3: 132-134.

LEAL, J. 1988. Exploration of major parasitic genera found in salmon from three freshwater fish farming in the province of Llanquihue (X Region), Chile. Thesis, M.V. Universidad Austral de Chile, School of Veterinary Medicine, Valdivia, Chile.

LOBOS, C. 1994. Action Ceratothoa isopod gaudichaudii on the body weight of Atlantic salmon (Salmo salar). Thesis, MV, Universidad Austral de Chile, School of Veterinary Medicine, Valdivia, Chile.

REYES, X. 1983. Ictiocontagiosas and parasitic diseases of farmed salmon in Chile. In: International Symposium, Progress and Prospects of aquaculture in Chile, Universidad del Norte, Coquimbo, Chile, pp. 407-422.

SIEVERS, G., C. LOBOS, R. INOSTROZA, S. ERNST. 1996. The effect of the parasitic isopods Ceratothoa gaudichaudii, on the body weight of farm Salmo salar in southern Chile, Aquaculture. (In press).

G. SIEVERS1, M.V., Dr. med. vet., C. LOBOS1, M.V., R. INOSTROZA1, M.V.

1. Parasitology, Institute of Animal Pathology, Faculty of Veterinary Science, Universidad Austral de Chile, Casilla 567, Valdivia, Chile.

2. Marine Harvest Chile SA, Avenida Diego Portales 860, Puerto Montt, Chile.