Assessing the physiological impact of sea lice (Lepeophtheirus salmonis krØyer) on Atlantic salmon (Salmo salar L.)

• Main Author: Wagner, Glenn Nick
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/15145

The objective of this thesis was to investigate the physiological changes that sea lice (Lepeophtheirus salmonis) cause in Atlantic salmon (Salmo solar). Sea lice are parasitic copepods that naturally infect salmonids in low numbers across the Northern Hemisphere with infrequent epizootics. The increase in the period of these epizootics in wild salmonids has mirrored the rise of the aquaculture industry in coastal areas. Therefore, it is important to answer basic physiological questions about parasite-host interactions such as sub-lethal lice levels required to affect performance, and the behavioural implications of increased lice infection. Two preliminary experiments were performed using rainbow trout (Oncorhynchus mykiss) to refine the techniques used to measure fish physiological variables. The first was designed to determine which anaesthetic, between clove oil and MS-222, better minimised fish stress during surgery and recovery. Both clove oil and MS-222 reduced Cortisol levels compared to non-anaesthetised controls within 10 minutes of blood sampling. However, fish exposed to MS-222 had elevated Cortisol levels 1 hour after sampling. Based on this result, few other differences occurring between the two anaesthetics, and the fact that MS-222 is a human retinotoxicant, clove oil was used for all subsequent experiments. The second preliminary experiment was performed to determine proper recovery and interval times for critical swimming speed tests (Ucrit), using hypoxia to stress fish in place office. Four hours of recovery following minor invasive surgery did not hinder the swimming performance of fish. Both 10 and 20 minute time intervals were found to be valid for measuring Ucrit, blood and cardiac variables. During exercise, cardiac output (Q) and heart rate (ƒH ) of normoxic fish increased (70 % and 25 % respectively) while those of hypoxic fish did not. A trend of a compensatory increase in stroke volume (Vs) with decreased ƒH of hypoxic fish also was evident. Hypoxia reduced the Ucrjt of trout and lactate accumulation is not one of the physiological mechanisms. Blood variables were measured in both studies, while cardiac variables and Ucrit of fish also were measured in the latter. The succeeding two experiments involved the artificial infection of Atlantic salmon with sea lice. In the first, the physiological impact of two sub-lethal levels of sea lice was measured on Atlantic salmon to determine when the health of fish is compromised. Fish with infection levels of 0.13 lice g-1 suffered a significant decrease (19 %) in Ucrit and impaired osmoregulatory ability. Infection levels of 0.05 lice g-1 did not affect the swimming performance of salmon compared to controls. The second infection study was designed to determine if the observed behaviour of heavily infected wild salmonids in seawater (SW) returning early to native freshwater (FW) streams had a physiological basis. The Ucrit of infected fish in SW decreased significantly (22 %) compared to controls, similar to the preceding study. However, salmon exposed to FW for 4 hours did not differ from controls with respect to Ucrit. Evidence of the osmoregulatory ability of infected fish in SW being compromised did not occur. It is likely that energy use increased in the latter group to maintain osmotic balance. Blood and cardiac variables, and Ucrit were measured in both studies. The final study had an experimental and theoretical component. Lice with visibly ingested blood were first collected from infected fish, then their gut contents weighed. Juvenile trout then had repeated blood samples taken of two known quantities for one week and then had their Ucrit measured. Gut contents and decline in performance due to blood loss were used to predict the possibility of blood loss due to sea lice infection decreasing the performance of their host. It was predicted that 5-10 % of the tissues consumed by lice is blood. At an infection level known to decreases Ucrit (0.1 lice g-1) lice would consume only 1 % of host blood, while 8 % loss is required to decrease Ucrit- Significant reductions in Ucrit purely due to blood loss would not occur until lethal infection levels (0.75+ lice g-1) are reached. This series of studies has helped to answer some basic physiological questions about the interaction between sea lice and salmonids. It is of great concern that sea lice can significantly affect salmonid physiology at infection levels lower than previously found on wild fish in areas of intensive aquaculture (0.5-2.1 lice g-1). These findings support the implementation of continuous lice removal programs at all aquaculture sites to reduce infection levels in intensive aquaculture areas to those found in pristine areas (0.02 lice g-1) that do not impair Atlantic salmon performance.