Quantifying the risk of fouling of non-indigenous species and spread by recreational boats using fuzzy logic: a case study from Italy

In the Mediterranean Sea, recreational boating is an underestimated and unregulated vector of non-indigenous species (NIS) introduction and spread. Furthermore, travel and hull maintenance (e.g. cleaning) habits of recreational boaters are largely unknown. In order to quantify the risk of fouling by NIS and consequent spreading of caused by recreational vessels, a two-levels model based on the fuzzy logic approach was developed and tested on a sample of about 200 Italian boaters. The first level of the fuzzy system separately assesses the risk of hull fouling and the risk of spreading of NIS; the second level combines the two risks into an overall index, ranging from 0 (very-low risk) to 100 (very-high risk). The input variables of the model were defined after a thorough literature survey; they are: type of vessel, frequency of antifouling painting, hull cleaning and hauling out, and time of permanence at sea after an antifouling treatment (fouling risk model); journey length and journey frequency (spread model). Input data were collected by means of questionnaires submitted to boaters face-to-face and online; at the same time, the questionnaires allowed to gather data about the awareness of boat owners on NIS in marine environments. The results of this study showed that 53.9% of the respondents fell into the very-low and low risk categories, and 46.1% into the medium, high and very-high risk categories. Most of them knew about non-indigenous species along the Italian coast, but were not aware that recreational boats can play the role of vectors increasing their dispersal. In a context of prevention and management programs on NIS introduced by recreational vessels, the fuzzy analysis of data obtained by questionnaires proved to be a cost-effective and rapid method to estimate the magnitude of the risk posed by infected boats. Further testing and validation of the proposed model are needed before its routine use in implementing mitigation measures.