Early development of resistance to the salmon louse, Lepeophtheirus salmonis (Krøyer), in juvenile pink salmon, Oncorhynchus gorbuscha (Walbaum)

Abstract

This study examined the effect of fish weight on the susceptibility of post-emergent pink salmon to Lepeophtheirus salmonis (Krøyer). Three trials were conducted, each with two stocks of pink salmon, Oncorhynchus gorbuscha (Walbaum), at starting weights of c. 0.3, 0.7 and 2.4 g, respectively. In each trial, duplicate tanks of fish were exposed to 0, 25 (only in Trial 1), 50 or 100 copepodids per fish. Mortality in Trial 1 was c. 37%, regardless of stock following exposures to 50 or 100 copepodids. Mortalities occurred up to 26 days after exposure, and more than 80% of the lice on the dead fish were chalimus stages. Infections with adult or preadult lice were observed on c. 35% of fish surviving to 37 days after exposure. Mortality was 5% in Trial 2 and there was no mortality in Trial 3. The abundance of L. salmonis was lower in Trial 3 compared with Trials 1 or 2. Histological changes in the skin coincident with fish growth included a thickening of the epidermis, infiltration of the dermis with fibroblasts by the end of Trial 1 and the first evidence of scales by the end of Trial 2; scales were evident throughout Trial 3. These results showed that the previously reported innate resistance to L. salmonis displayed by pink salmon develops in fish heavier than 0.3 g and appears to be functional by 0.7 g. This resistance coincided with changes to the epidermis and dermis, including the formation of scales. The present results indicate that elevated risk associated with L. salmonis infection among migrating post-emergent pink salmon may occur during a relatively brief period before the fish reaches 0.7 g.

Physiological consequences of the salmon louse (Lepeophtheirus salmonis) on juvenile pink salmon (Oncorhynchus gorbuscha): implications for wild salmon ecology and management, and for salmon aquaculture

“…The level of shedding we observed is not unusual compared with previous studies where pink salmon were artificially infected (abundance was reduced within 5 to 12 DPI; [44]) or naturally infected pink salmon were held [66,67]. A high rate of louse shedding is also of importance in mathematical models, where the incorporation of realistic shedding rates [67] reduces the predicted mortality of pink salmon owing to salmon louse infection by 95% relative to an earlier model where shedding was not considered [10].…”

“…However, none of these sublethal effects is evident once pink salmon reach about 0.5 g, which would typically take about one to two months of normal growth. The 'no effect' threshold for sublethal disturbance defined by Sackville et al [12] of 0.5 g with one chalimus 4 is consistent with the developmental stage at which pink salmon develop scales and exhibit a heightened immunocompetence [44].…”

“…Laboratory studies have shown that juvenile pink salmon are less affected by the salmon louse once they reach 0.7 g and have developed scales [44]. This introduces the possibility that delayed SW entry may confer reduced sensitivity to louse parasitism if fish are larger when they first encounter salmon lice.…”

“…Shedding of salmon lice by juvenile salmon is well documented [44,62 -66] and is influenced by both biotic and abiotic factors, including parasite mortality, immune response by the host, water temperature and water salinity [32,44,63]. Thus, all controlled infection studies of Pacific and Atlantic salmon performed to date and designed to investigate the effect of fixed levels of infection have been confounded by the intensity of salmon louse infection decreasing with time.…”

“…This analysis revealed a threshold fish mass of 0.5 g beyond which one chalimus 4 louse no longer significantly elevated total body Na þ . This 0.5 g size threshold for maintaining ionic homeostasis while carrying a load of one salmon louse corresponds with the body size when pink salmon increase their hypoosmoregulatory capacity [23], and closely approximates the 0.7 g size threshold when louse resistance increases owing to epidermal and immune system development [44]. These findings show for the first time that louse infection does impair ionic homeostasis in small juvenile pink salmon, and suggest that fish are particularly sensitive to louse infection as homeostatic systems (ionic, physical barriers and immune defence) develop from precocial states at SW entry.…”