It is quite common to use parameters like Prevalence, intensity or incidence in epidemiology. So kindly refer any concept book of Veterinary Epidemiology for your advance information and formula. However, Prevalence is a statistical concept referring to the number of cases of a particular disease that are present in a particular stock or population at a given time, whereas incidence refers to the number of new cases that develop in a given period of time  is a statistical concept referring to the number of cases of a disease that are present in a particular population at a given time, whereas incidence or occurrence refers to the number of new cases that develop in a given period of time 

You should use the software Quantitative Parasitology in this web:

http://www.zoologia.hu/qp/qp.html

Analysis of parasite prevalence is not simple. Parasites are not normally distributed, either in the fish or in the population. The negative binomial is often found to most closely describe the distribution of parasites so non-parametric statistics are essential.. Further, fish populations are also not randomly distributed, so the fish sample may not be representative of the population. I studied the parasites of orange roughy off New Zealand in the 1990's and found that the variation in parasite prevalence between tows was greater than between areas. Lester et al. (1988) got away with their paper on orange roughy parasites  because their sample size (number of tows within an area) was too small. 

Perhaps the best approach for your research topic would be to collect the data, and then develop a model of the parasite-host system, using what you know of the life cycles and the real data you have collected.  Its not that difficult to do. Google scholar "Anderson and May, parasites and model" as a start - much of the analysis can be done with excel :-) Another approach is Gibson and Jones 1993 "fed up with parasites" Marine Biology 117:495-500

The fishes were examined for the endoparasites by killing them by the usual method of a blow on the head. Fishes were dissected & body cavity was thoroughly examined for any parasite. Intestines were placed in Petri dish containing normal saline (0.75% Nacl) to allow adhering parasites to be released from the lumen. In case of acanthocephalans, if the anterior end was deeply bored in the mucosa of the intestine, a few crystals of the methanol were added to the normal saline, containing the parasites adhered to the intestinal wall. This led to immobilization of the parasites & loosening of the grip on the intestinal wall & facilitated the detachment of proboscis in case of acanthocephalans without causing any distortion in the arrangement of hooks. The regular record of the collection was maintained and the prevalence of Pomphorhynchus kashmirensis was carried out by the following formula:

Total number of hosts infected

Prevalence = X 100

Total number of hosts examined

Prevalence is the percentile representation of infected hosts divided by hosts examined multiplied by 100.

In Chile we are very affected in culture salmon, by the ectoparasite sea lice Caligus rogercresseyi

For this prevalence and intensity are measured:

1. Prevalence: Is the total number of cases of disease or pest in an animal population or certain vegetable cultivation in a space clearly defined time.

P = (N° parasitized fish / Number of sampled fish)

2. The intensity: Is the number of infectious agents from infected host within a sample, expressed as the average number, or qualitative category.

I = (Number of caligus by parasitized / number of parasitized fish fish)

Best regards