Info

Lepeophtheirus salmonis

There are two species of salmon louse, Lepeophteirus salmonis and Caligus elongatus von Nordmann, which is found on salmon in the atlantic ocean. Of these two the Lepeophteirus salmonis has caused large losses among the salmon producers located in the north-eastern parts of the atlantic ocean. As Lepeophteirus salmonis is the one causing the large losses this salmon louse will be investigated. Any referral to salmon louse or lice in the thesis is therefore to Lepeophteirus salmonis, L.salmonis. The salmon louse (Lepeophteirus salmonis) is a sea louse, a parasite mainly living on salmonoids. It is a species of copepod in the genus Lepeophteirus. It has a direct life-cycle (i.e. a single host) with eight life stages and each life stage is separated by a moult. The Lepeophtheirus salmonis is on an average twice the size of most Caligus specieses. The body of an adult L.salmonis consists of four regions: abdomen, leg-bearing segment, cephalothorax and genital complex. The cephalothorax acts like a suction cup where it holds itself on the fish. It forms a broad shield that includes all body segments up to the third leg-bearing. It has a mouth part shaped as a siphon. In addition to the cephalothorax, second antennae and oral appendages are modified to assist the louse in holding the louse on the fish. The second pair of antenna is also utilized by adult males to grasp the adult female during copolation.

 

 

The salmon louse is not a parasite that multiplies on the host, which is essential in understanding the salmon louse problem. The salmon lice in one farm is a consequence of a generated copepodid pressure from salmon lice located elsewhere. Even for a large production site it is unlikely for one cage to infect another within the site. This is due to the earliest stages of the salmon louse where it drifts with current and is non-parasitic for minimum 2 days at temperatures aorund 15 $^{\circ}$C.

 

It has been believed that L.salmonis does not reproduce or grow during the winter. However, studies conducted at temperatures down to 2 $^{\circ}$C has proven that eggs develop. It took 45.1 days and a large portion of the nauplii developed into copepodid down to temperatures of 4 $^{\circ}$C \parencite{Boxaspen}. Which disproves this statement.

The salmon louse hatches directly into the water as a planktonic nauplii. These nauplii are non-feeding and not infectious until they molt into copepodid. Nauplii 1 is very salinity responsive, and newly hatced nauplii 1 does not survive at salinities of 15\% or less, and poor development at salinities between 20\% and 25\%. The time spent in nauplii stage 1 and 2 is estimated to be approximately 45 hours at water temperatures of 15 Celsius and 222 hours at 5 Celsius \parencite{Tully1993.}. Theoretically in the winter season the nauplii can be transported 279 km at sea currents of 0.35[m/s] before molting into the infectious copepodid stage. Both the nauplii and copepodid stages are positively phototactic and has a daily vertical migration, sinking at night and rising at day.

 

The nauplii is part of the plankton feed chain and is by that equally susceptible to predation. It is estimated that 70\% of all zoo-plankton is subject to predation, and there are no reasons why nauplii 1 \& 2 should not suffer same losses. It is also found that they are susceptible to bacterial infections, virus and parasites, their effects are little known, but they tend to increase with temperatures rising above 15 $^{\circ}$C

\subsubsection{Copepodid}

 

In this thesis copepodid refers to the infectious copepodid stage between Nauplius 2 and Chalimus 1. Other papers may refer to copepod for this stage, however copepod may be confused with the meaning of any very small crustaceans of the subclass copepoda. And therefore is being used, however literature widely uses copepodite as any of the five stages in the life-cycle of a copepod prior to the sexually mature adult. This should be in mind when searching for additional information regarding the subject.

 

The copepodid actively search for a host through sensing vibrations in the water, and their ability to find host is not light dependent. \textcite{Heuch1995} proved that copepodid swim upwards in response to pressure and a change in water flow or a mechanical vibration, which induced a burst in swimming speed. This is backed by \parencite{heuch1997} Heuch and Karlsen (1997) whom reported that copepodid are sensitive to low frequency water accelerations, such as for example produced by a swimming fish. When a host is found thecopepodid clasps the host tissue, then undergoes a molt to the first sessile stage in the life cycle if it decides to stay on the host. It then uses its claws to penetrate the salmons skin, and a form of glue is released into the salmon forming a more permanent attachment\parencite{ultrasound}. The salmon is now infected by salmon louse and the copepodid will molt further until it becomes mature able to reproduce. After initially attachment of copepodid, the process to final stage takes approximately 52 days for male and 57 days for females at water temperature of 10 $^{\circ}$C. The development of the copepodid is strongly correlated to water temperature as for the other stages as well. \textcite{Samsing} has stated a formula for estimated days from hatched to infectious copepodid and the estimated amount of days of infectiousness of salmon louse as a function of temperature. Which will mean after how many days does it take for a newly hatched nauplii 1 to develop to a copepodid, and how long does it stay in this stage before it eventually dies/finds a host. The functions has been plotted and can be seen in \ref{fig:Copdev} They also found that below 5 $^{\circ}$C there were not much infection at all.