No abstract

This review surveys the available literature on the effieaey and eeotoxieology of chemotherapeutic agents for the control of sea lice (Copepoda: Caligidae) infestations in marine aquaculture. To date, treatment for the disease has relied almost exclusively on the use of the organophosphorus pesticides, trichlorfon and dichlorvos. While use of dichlorvos and trichlorfon appears to have little, if any, environmental impact, protracted use in some areas has led to reduced efficacy. Compounds proposed for sea lice control currently under field evaluation include azamethiphos (organophosphate), ivermectin (avermectin), pyrethrum (pyrethrin) and hydrogen peroxide. Although all of these eompounds are effi cacious, at present published information on marine environmental toxicology is scarce and is limited to in vitro toxieity testing. Other compounds that have been considered for sea lice control include formalin, malathion (organophosphate), earbaryl (earbamate), diflubenzuron (insect growth regulator) and natural remedies (i.e. onions and garlic). In general, formaldehyde (aldehyde), malathion and natural compounds show either poor efficacy or unsuitable therapeutic margins. Although earbaryl and diflubenzuron are both efficacious and exhibit suitable therapeutic margins, available data suggests that the compounds, due to undesirable environmental toxieologieal characteristics, would be unsuitable for sea lice control.

With 2 plates and 2 figures in the text)A study was undertaken of the settlement and attachment of larvae of Lepeophtheirus salmonis (Krsyer, 1837) on a salmonid host Salmo salar Linnaeus, 1758. Preferred settlement sites were the fins and other protected areas. Initial, reversible settlement was followed by attachment with the hooked second antennae. The filament used for subsequent attachment was found to comprise three distinct regions; basal plate, stem and external lamina. The filament was secured to the epithelial basement membrane by the basal plate which was formed by secretion down the axial duct from the posterior A-group of cells. The stem was fibrous and carried the axial duct along its length. A thin external lamina covered the stem and was apparently continuous with the chalimus cuticle. A number of filament-associated organs are also described, comprising cell groups A-C although only the function of A was characterized. Contents 0952-8369/91/006201+ 12 $03.00 20 1 0 1991 The Zoological Society of London J . nut. Hist, 8: 681-700. 6 9 373-390. marine animals: 83 -84. Newell, R. C. (Ed.). London: Butterworths. 1593. J . Fish. Res. Bd Canada 2 9 1015-1013. J . Fish. Res. Bd Canada 3 0 88 1-903. californiensis (Copepoda: Lernaeopodidae). J . Fish. Res. Bd Canada 28: 1143-1 151. PaciJ Sci. 17: 195-242. 1ahru.x (L.) in Bardawil lagoon. Annls Parasit. hum. Comp. 55(6): 687-706. Abstr. int. ( B ) 32(9): 5547-5548. 9 205-212.

The effects of temperature and salinity on the settlement, subsequent survival and development of the copepodids of Lepeophtheirus salmonis on Atlantic salmon were investigated experimentally. There was a significantly greater settlement and survival of copepodids at 10 days post‐infection (dpi) at 12 °C compared with at 7 °C at a constant salinity of 34‰. Development of L. salmonis was also more rapid at 12 °C. Settlement was significantly greater at a salinity of 34‰ than at 24‰. In one experiment, survival at 10 dpi was significantly greater at 34‰; however, a second experiment found that there was no significant difference between the two saline levels. This may have been because of a rise in water temperature for 2 dpi, which appears to have overridden the effect of low salinity. Development of L. salmonis was more rapid at 34‰. Copepodids settled on all of the external surfaces of the salmon, although the proportion on different surfaces varied between experiments. The gills, particularly at low temperatures, the body surface, and the pectoral and dorsal fins were especially favoured.

. Anecdotal reports from the Scottish fish farming industry suggested a reduction in the sensitivity of salmon lice, Lepeophtheirus salmonis, to the organophosphate dichlorvos. In this study, pro‐adult and adult lice were collected from marine salmon sites showing a variety of ‘sensitivities’ to dichlorvos and subjected to dichlorvos exposure at a range of doses for a 48‐h period in vitro. From the lice mortalities, 24 and 48 h LC50 values were calculated, and using a ‘sensitive’ source of lice as a reference, resistance ratios calculated. The data generated suggests that L. salmonis from different geographic areas shows differences in sensitivity to dichlorvos. The implications for the salmon farming industry are discussed.

A histopathological study was carried out on rainbow trout suffering from acute and chronic diplostomiasis. Clinical signs were few in acutely infected fish but in chronic cases fish were blind, exophthalmic and emaciated. Acute infections resulted in subcapsular cataract formation with varying capsular change. Chronically infected fish frequently showed lens dislocation, capsular rupture or duplication and the formation of Elschnig's pearls and Wedl cells. Phacogenic uveitis was seen in many fish with capsular rupture and retinal detachment frequently occurred as a sequal to vitreous detachment.

The efficacy of emamectin benzoate as an oral treatment of sea lice, Lepeophtheirus salmonis (KrÒyer), infestations in Atlantic salmon, Salmo salar L., was evaluated in a dose titration study and two dose confirmation studies. Replicated groups of salmon with induced infestations of sea lice were given emamectin benzoate on pelleted feed at doses of 0, 25, 50 and 100 μg kg−1 biomass day−1 for seven consecutive days. Sea lice were counted at 7, 14 and 21 days from the start of treatment, and comparisons made with control fish given the same diet without emamectin benzoate. Total numbers of sea lice were significantly reduced at all doses in all three studies when compared to control fish. There was no significant difference between doses of 50 and 100 μg kg−1, but the 50 μg kg−1 dose resulted in significantly fewer lice than the 25 μg kg−1 dose. Emamectin benzoate was highly effective in reducing numbers of preadult and adult lice and prevented the maturation of chalimus to motile stages. The optimum therapeutic dose was selected as 50 μg kg−1 day−1 for seven days. Treatment reduced the incidence of epidermal damage by sea lice and, in one study, survival of treated fish was 48% higher than in control groups. No fish mortalities or adverse effects were attributed to treatment with emamectin benzoate at any of the doses tested.

The duration of efficacy of emamectin benzoate in the oral treatment of sea lice, Lepeophtheirus salmonis, infesting Atlantic salmon, Salmo salar L., was evaluated in a tank study. One group of salmon was treated at a nominal dose of 50 μg kg−1 biomass day−1 for 7 consecutive days and a second group was untreated. Fish were then redistributed to 16 tanks, each holding 17 control and 17 treated fish. On days 34, 41, 48, 55, 62, 69, 76 and 83, two tanks were challenged with L. salmonis copepodites. Eight to 14 days after each challenge, fish were anaesthetized and numbers of lice recorded. Treatment with emamectin benzoate prevented development of copepodites for up to 62 days from the start of treatment, and chalimus numbers remained low for 69 days. Treated fish, challenged from days 34 to 69, had significantly (P<0.01) fewer lice than control fish. Treated fish challenged at days 76 and 83 still had fewer lice than control groups, although differences were not statistically significant for both replicates. When chalimus appeared on treated fish challenged at days 69–83, survival of chalimus to adult stages was lower than on control fish. Louse egg production on treated fish challenged at days 62–83 was not reduced compared to control groups.