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Tom Baird

The Fish Louse

Evening all, Hope you are all safe and well. Looks like their is light at the end of the tunnel. In the mean time I thought I would put my knowledge to good use. I hope you enjoy…..

Stillwater fisheries contain a whole host of parasite and host interactions and one of the most common is Argulus foliaceus, or the ‘fish louse’.  Fish louse have a relatively simple life cycle and are known as ectoparasites, which require the external attachment to a fish host to feed and develop. Argulus deposit eggs within the lake/river and seek out hard substrate to attach. In 2000 it was observed that female argulids lay 2 to 4 egg clutches, then return to a host, before detaching again to lay another batch within 2 to 4 days. These eggs start as a pale yellow and as they develop the darker yellow they become. The egg strings can contain up to 400 eggs. Research carried out in 2007 suggests that these egg strings are more likely to be laid between the water surface and 1m, with lower levels of egg deposition occurring below this, suggesting shallow lakes may proliferate parasite numbers.

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 Development of eggs is dependent on temperature and eggs usually hatch above 10°C with the highest numbers seen during mid-summer. During these warmer months, hatching can occur 2 weeks after laying, (anecdotal evidence suggests, this could happen after only 8 days). The first mobile stage of Argulus life cycle is following hatching, where the free swimming metanauplius are well adapted for this life stage and are very mobile in the aquatic environment using developed thoracic legs to seek out a host.

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Within a well-balanced ecosystem, such as most river conditions, factors such as variations in depth, flow and relative low numbers of hosts, reduce the likelihood of Argulus finding a host. Should the metanauplius not find a host within 3-4 days, it will die and this results in minor problems due to Argulus in these conditions. However, within a fish farm or over stocked Stillwater, the factors mentioned previous may be more favourable to parasite/host interactions with a greater likelihood Argulus will succeed in attaching to a fish in greater numbers. An increase in fish activity linked to warmer water temperature is likely to improve the chances of interactions with Argulus.

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Once attached to a host, which has been seen to be fish, frogs and tadpoles. Argulus begin to develop suckers which enable them to move across the host to feed causing epithelial damage and increasing the potential for infection. Argulus then perform a number of moults, where the adult stage is eventually reached. The adults then mate and females detach from the host, where they will seek to find suitable substrate to lay eggs, leading to the next generation of the parasite. Adults will then seek out another suitable host, where they will feed, detach and lay further eggs. During spring, summer and autumn, it is understood up to 4 generations of Argulus can add to the population, cumulatively adding to greatest infection rates seen in August/September.

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As temperature drops towards the end of autumn this process slows down and once temperatures around 10°C are again reached feeding begins to stop. Throughout the winter most adults die and it is the eggs laid in late autumn which over winter and will produce the following season’s population. As such, warmer winter temperatures can increase the likelihood of Argulus overwintering at all life stages, giving rise to a greater outbreak when spring returns.

Possible solutions to reduce the impact of Argulus on fisheries

For example to understand any ecological impacts within a Stillwater trout fishery it is essential that sustained monitoring before, during and after an event is undertaken. In the case of Argulus infections it is important for the trout fishery owner to monitor levels of the parasite affecting fish. This will highlight at what time of the year significant pressure and stress is placed upon the stock. Monitoring of lake temperature should accompany these observations to determine at what point temperatures around 10°C are noted, providing evidence for suitable stock management.

Many trout fisheries choose to undertake single large stockings, which reduces transport costs and reflects well with their members seeing large numbers of fish entering the lake. This method gives the parasite the perfect opportunity to increase the likelihood of a host interaction and greater chance of survival/reproduction. The spring hatch of Argulus makes up the bulk of the lice population at this time.

Consequently stocking large numbers of trout in spring/early summer provides the perfect vector for a population explosion. Trickle stocking, whilst expensive, is a viable option to reduce Argulus population blooms. To reduce costs, it is a good idea to link up with other fisheries in the area to maximise the efficiency of any fish deliveries to an area. If transport costs can be shared, then a greater number of small scale stockings may be possible.

Other stock related solutions to combat high levels of Argulus infection include;

  • Stop stocking during summer – late summer/autumn additions may benefit from low levels of parasite numbers and reducing the period suitable for Argulus reproduction
  • Stop, catch and release – Whilst popular with some anglers, removal of stock through catches will help reduce stress, and parasite interaction potential
  • Maximise stock turn over – target an increase in membership which will aid revenue and further stocking
  • Reduce stock through the winter – Less effective, due to fishing effort.

Further reduction in parasite numbers can be achieved by the removal, temporary or permanent, of suitable egg laying substrate. Studies carried out by C.Williams pers comm demonstrated the addition of plastic pipe suspended vertically in the water column provided an excellent substrate for Argulus to lay their egg strings. These were removed every two weeks and the eggs were left to dry out and die. The pipes were then reinstated and the study continued. Over the course of a 3 month period approximately 12 million eggs were removed from one fishery, suggesting this method may be effective at reducing Argulus numbers. The study required a significant degree of effort which may be restrictive to some fisheries with lower angler numbers, so a programme targeting key spawning times in spring and summer may provide an agreeable alternative to year round collection.

Once again i hope you enjoyed this little read. Please remember this isn’t aimed at one particular fish species.

Tight lines………

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