Hatching times are temperature dependent, shortening as the temperature increases. Below 8-10°C, eggs of all three UK species do not appear to develop, possibly a mechanism to increase metanauplius survival by ensuring that hatching only occurs in the most favourable spring and summer conditions. Eggs that overwinter have very low hatching success, but can survive at low temperature for up to 2 years.
In the UK, argulid numbers on fish tend to be low in winter, with laboratory experiments suggesting that most lice die in cold conditions and after breeding periods. An increase in temperature in spring causes eggs to begin hatching. These early generations give rise to subsequent generations that cause population numbers to peak towards late summer/early autumn. As the temperature drops with the onset of winter, recruitment stops and the population declines again. Any adult lice that survive the winter become relatively inactive until the water temperature rises above 10°C, when they will leave their host to lay eggs.
Studies have shown that in trout fisheries, poor water clarity, low stock turnover rates (the rate at which the trout population in lakes is removed and replaced), and high temperatures show a significant correlation with high numbers of A. foliaceus in trout. Low water clarity was also associated with lower stock turnover, possibly because lower indian email address list clarity changes the distance over which trout are likely to react to prey, thereby reducing the likelihood of being caught.
These results suggest that high numbers of A. foliaceus alone may not affect catch rates unless worked into combination with other environmental factors. The mechanisms behind the effects of water clarity and stock turnover on parasite dynamics are currently unknown, although several hypotheses have been developed. Rapid stock losses are likely to reduce parasite numbers by removing infected fish before the parasite can leave its host to lay the eggs needed to produce subsequent generations. Low water clarity can influence parasite dynamics in several ways. Firstly, through its effect of slowing population turnover, secondly, through a reduction in predation on the parasite by trout (which readily eat it in the laboratory) and finally, lower water clarity may change host behaviour in a way that would increase infection success. Further research is being undertaken between Cefas and the Institute of Aquaculture (University of Stirling) to determine the mechanisms driving these relationships and to develop specific management strategies.
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