Introduction

A study of the external lesions of the American eel (Anguilla rostrata) captured by commercial fishermen in Quebec and Ontario, Canada, was conducted during the year of 1992. Approximately 12000 eels were examined at two processing plants located in the province of Quebec. A questionnaire survey was also sent at the end of the fishing season to 82 fishermen from five fishing areas studied during the processing plant survey, and to fishermen from the area of Quebec City. The use of both surveys proved to be a rapid and economic means to obtain information on eel health status. This paper emphasizes the advantages and limits of each survey and provides recommendations on their construction and elaboration.

Advantages and limitations

Processing plant surveys - When catches obtained by commercial fishermen are used, catches are sampled, and not the fish population in its biological sense. However, this can be a valuable first-hand source of data. Processing plant surveys can give access to a large number of fish, from extended geographical areas and at different times through one fishing season. Study expenses are cut considerably since there is no cost associated with the capture and the transportation of the fish. Travelling expenses are also reduced by concentrating the work of researchers in one or few locations instead of several fishing landing areas. The relative frequency of lesions among fish captured, as well as the length, the weight, and other variables can be determined. Temporal variation may be assessed. In certain circumstances, internal lesions can also be surveyed and organs collected.

On the other hand, the time available for the examination of fish may be limited. Information on the type of gear used to capture the fish, their selectivity, the fishing effort, fishermen movements, etc. , might be hard to obtain. The verification of bias may be difficult. Dead fish are generally found floating in the water or stranded on beaches, therefore, a processing plant survey is not a reliable method to estimate mortalities. Fishermen may also reject fish that look severely sick or injured. Fishing gear may induce lesions sometimes hard to differentiate from disease induced ones.

Questionnaire surveys - Most of this information can generally be obtained through questionnaires with commercial fishermen. These can be performed before the processing plant study in order to obtain material that will help its preparation. For example, first-hand data can be obtained on the temporal variation of diseases. During the eel survey, fishermen were able to specify at which times of the fishing season diseases were observed the most frequently. Questionnaires can be used alone if the information sought can only be obtained through this method, such as verifying if mortalities had occurred during the fishing season. Finally, questionnaires can be repeated after a processing plant survey to extend the geographical area of study, or to verify if fish rejection occurred.

Fishermen, however, can only approximate the number of sick fish observed during a certain period. Therefore, one cannot quantify and follow the prevalence of diseases during one fishing season, or through the years. Only strong variations are likely to be perceived. The accuracy of each fisherman's observations may vary with individual quality of observation, the size of catches, the fishing technique employed, other manipulations such as counting or cleaning the fish, the true prevalence of the disease, etc. Certain fishing techniques might increase the chances of observing external lesions (for example, long lines as compared to large fixed traps where fish are manipulated mechanically). These fishing techniques may be associated with a particular area or fishing site. If so, the frequencies of lesions reported by fishermen from these areas might appear higher, even if true prevalences are similar. Therefore, areas highlighted by a questionnaire as areas of higher frequency of observations, must be verified with further study. However, from the experience acquired with the survey on eels, it appears that areas where a disease occurs can be differentiated from another area where it does not.

Recommendations

Processing plant surveys - The best recommendation is surely to undertake a preliminary study before the primary study. This will allow the identification of most of the practical problems that might be encountered in each processing plant. It is also valuable to go fishing with different fishermen using different fishing gear. Many scars result from the fishing techniques employed and researchers should be able to identify these scars so they can be ignored during the survey. The fishing gear, fishermen movement, fishing calendar, etc., cannot be controlled. However, the origin of each fish lot inspected and the way they have been captured should be known. All possible information on fish shipment to the processing plant should be obtained. Are the fish screened before the shipment? Are they grouped in different containers according to their size? Is it possible to mark containers so they will be easily retrieved at the processing plant? Are the catches from more than one fisherman, or fishing area, pooled in the same container? Depending on the goal of the study, many questions must be addressed, and answers found, beforehand.

By working at the processing plant, one may find out that some manipulations must be modified, so it does not interfere with the regular operations of the plant. During the pre-study, it would be important to verify if the sampling technique limits sampling bias. Because the time available for manipulations is often limited in a processing plant, the lesions of interest should be chosen before the survey, and their number limited. Fins and skin hemorrhages are often induced by the capture technique or during the transportation and are therefore unreliable for such studies. Each lesion chosen should be fully described and precise classification criteria established. This should increase the degree of agreement between different inspectors, or help each individual maintain continuity during the whole period of the study. In order to obtain information on the sources of variance, the observations of each inspector after the examination of the same lot of fish should be compared. The repeatability of each inspector's observations should be assessed as well.

Questionnaire surveys - During the eel survey, a questionnaire of 12 pages and 52 questions, accompanied by pictures of external lesions studied, was mailed to every fisherman selected. The responses were collected during telephone interviews approximately 10 days later. Because the number of fishermen contacted was relatively small, this survey method appeared to be valuable, despite the additional cost of the telephone interviews. A large number of fishermen could be reached in a short period of time (approximately two months), including those experiencing reading problems who may not have answered a simple mail survey.

Most of the following information was sought through the questionnaire survey on eel. Some modifications are also proposed in order to improve the quality and reliability of the data.

 the fisherman's experience with this particular type of fish

 the fishing gear used (there may be more than one type)

 a good description of the period of the fishing season

 a description of the fishing area for each fishing gear type used. Include detailed maps of the area, divided in multiple squares, each numbered. The fisherman can describe his fishing area by specifying the appropriate squares. This is especially important and useful in areas where traps are not fixed

 the total size of the catch for the fishing season. In order to limit the apprehension sometimes encountered with this question, it is probably better to obtain this information through a closed question, which offers different exclusive categories of catch size

 the average weight of the fish caught with each fishing gear type. This information, with the total catch size of the fishing season, will allow estimation of the number of fish caught during the season, and further classification of the fishermen

 where does the fisherman sell his catch (processing plants, local markets, etc.)

 if questions on mortalities are included in the questionnaire, the maps used to describe the fishing territory should also be used to mark the location of fish carcasses observed

 and their number; - only external lesions easily identified from a picture can be successfully surveyed through a questionnaire. These might be, for example, lesions of lordosis, scoliosis, other malformations, tumors, scars, etc. Tests should be done with some fishermen in order to verify if the pictures are clear enough.

 as mentioned earlier, fishermen can only give an approximate number of the diseased fish they observed during a fishing season. During the pre-test undertaken with the questionnaire on eel external lesions, it appears that fishermen would rather answer with an absolute number than with predetermined frequency categories. Fishermen did not seem to be able to differentiate between categories of 1/1000 and 1/100, and 1/lOO and 1/10. When they were asked, through an open-ended question, how many eels were observed with a particular lesion during a year, the responses provided were generally no sick eel, 1 or 2, 10 to 20, or over a hundred sick eels. Therefore, we suggest to survey the frequency of diseases in absolute numbers.

 Information over many years might be hard to obtain, especially if many fishermen have stopped or started their fishing activities during the period of interest. Moreover, the information may not be recalled by the fishermen. If this information is important for the research, get the help from a researcher in cognitive psychology to construct the questionnaire.

Before the questionnaire is used for the final survey, test it several times with different fishermen, from different areas, and using different fishing gear. Test any questionnaire on which you made corrections. References on questionnaire construction, administration and analysis are numerous and many are of great quality. Finally, try to validate the answers obtained with some fishermen by the examination of their catches during the fishing season surveyed. If possible, choose fishermen from different areas, using different gear and whose catch sizes vary. Be also aware that fishermen who are more involved in the study (for example, those whose catches were inspected during the fishing season), may make better observations than those who are not. This may be attributed to an increase in a fisherman's efforts to make a "good" study.