Disease induced by the sea louse ((Lepeophteirus salmonis)) (Copepoda: Caligidae) in wild sockeye salmon (Oncorhynchus nerka) stocks of Alberni Inlet, British Columbia

Abstract

The occurrence of the marine ectoparasitic copepod Lepeophtheirus salmonis and the prevalence of lesions caused by its feeding activities were monitored on sockeye salmon (Oncorhynchus nerka) adults returning to the Sproat and Stamp rivers through Alberni Inlet, British Columbia, in 1990. All sockeye examined were infected with L. salmonis and had higher intensities of infection than previously reported. The presence of high numbers of early developmental stages of L. salmonis suggests a high rate of infection for sockeye in coastal waters. Lesions attributable to L. salmonis ranged from minor skin discoloration to large open lesions that exposed the musculature. In 1990, when escapement into these river systems was delayed, sockeye holding in the inlet developed severe lesions and suffered high mortalities. High percentages of fish with open lesions entered both river systems in 1990, but few fish with open lesions were observed on the spawning grounds, suggesting additional prespawning mortality. In 1992 and 1993, when escapement patterns were more normal than in 1990, the severity of lesions owing to L. salmonis was reduced and no mortalities were observed. Throughout the study, fish in the Sproat River escapement had more severe lesions than those in the Stamp River escapement.Résumé : Nous avons surveillé l'occurrence du copépode marin ectoparasite Lepeophtheirus salmonis et la prévalence des lésions causées par son activité alimentaire chez des adultes de saumon rouge (Oncorhynchus nerka) retournant aux rivières Sproat et Stamp en passant par l'inlet Alberni (Colombie-Britannique) en 1990 et en 1992-1993. Tous les saumons rouges examinés étaient infestés par L. salmonis, et présentaient des infestations plus intenses que ce qui avait été observé jusque-là. La présence d'un nombre important de parasites aux premiers stades du développement indique probablement un fort taux d'infestation des saumons dans les eaux côtières. Les lésions attribuables à L. salmonis allaient de simples altérations de la couleur de la peau à de grandes lésions ouvertes qui exposaient la musculature. En 1990, alors que la remonte vers ces cours d'eau a été retardée, les saumons rouges qui demeuraient dans l'inlet ont subi de graves lésions et une forte mortalité. Des pourcentages importants de poissons portant des lésions ouvertes ont pénétré dans les deux réseaux en 1990, mais rares étaient les individus atteints de lésions observés sur les frayères, ce qui permet de penser qu'il se produit une mortalité additionnelle avant la fraye. En 1992 et 1993, alors que les patrons des échappées étaient plus normaux qu'en 1990, la gravité des lésions dues à L. salmonis a baissé, et on n'a observé aucune mortalité. Tout au long de l'étude, les poissons de l'échappée de la rivière Sproat présentaient des lésions plus graves que ceux de l'échappée de la rivière Stamp. [Traduit par la Rédaction]

Development of the salmon louse Lepeophtheirus salmonis and its effects on juvenile sockeye salmon Oncorhynchus nerka

“…Although comparative studies have shown that branchial infections do not occur with the same severity in juvenile pink and chum salmon (Jones et al 2007), their occurrence mainly in laboratory trials indicates a limited value in predicting impacts during natural exposures. In addition to histopathological lesions, gross lesions on the head and body of sockeye in Trial 3 following copepod development to the pre-adult and adult stages were similar to those reported from naturally infected adult sockeye salmon (Johnson et al 1996) and from farmed (Jóns-dóttir et al 1992) and laboratory infected Atlantic salmon (Jónsdóttir et al 1992, Grimnes & Jakobsen 1996, Jakob et al 2011. These lesions are induced by the feeding activities of the larger mobile parasites and may be exacerbated by secondary bacterial infections (Wootten et al 1982, Johnson et al 1996, Jakob et al 2011.…”

“…These authors reported a mean abundance of 41.1 lice fish −1 among adult sockeye collected by hook and line in coastal British Columbia with only minor skin damage. Similarly, the abundances of L. salmonis reported from relatively healthy adult sockeye collected by seine nets from the Alberni Inlet on 5 occasions over 2 yr ranged from 7.4 to 77.1 lice fish −1 (Johnson et al 1996). In the latter study, a mean abundance of 300 lice fish −1 was associated with death and morbidity of sockeye, suggesting that adult salmon have a greater tolerance to the pathological effects of infections at intensities capable of causing disease in the smaller salmon.…”

“…Providing a rare example of disease in a wild salmon population, Johnson et al (1996) reported heavy infections of Lepeophtheirus salmonis on adult sockeye salmon whose migration into freshwater was delayed. These infections were associated with skin lesions ranging from discolouration to large open ulcers with exposed musculature and with elevated morbidity and mortality.…”

Vector potential of the salmon louse Lepeophtheirus salmonis in the transmission of infectious haematopoietic necrosis virus (IHNV)

“…This is in accordance with previously reported effects of Lepeophtheirus salmonis infections on Atlantic and Pacific salmon (Jónsdóttir et al 1992, Grimnes & Jakobsen 1996, Johnson et al 1996. Severe lesions, as seen in Challenges 4.1 and 4.2, are known to cause osmotic stress and an increased susceptibility for secondary infections (Wootten et al 1982, Jónsdóttir et al 1992).…”

Ectoparasitic sea lice (Lepeophtheirus salmonis and Caligus elongatus) infestations of wild, adult, one sea-winter Atlantic salmon Salmo salar returning to Scotland

“…Johnson et al 1996, Jacobsen & Gaard 1997. For individual fish, higher abundance L. salmonis infestations correlated positively with a greater proportion of females (r = 0.166, n = 403, p = 0.001; annual mean min.…”