Using a multivariate approach to define Irish metiers in the Irish Sea

Abstract

There is an increasing need to take into account a “mixed fisheries” approach in management, assessment and sampling of fish stocks. To do this effectively it is necessary for groups of fishing trips with homogeneous fishing patterns or tactics to be defined into métiers. Presented here is the result of an Irish case study analysis in applying multivariate statistics to declared logbook landings to define Irish fleet métiers operating in the Irish Sea. Established multivariate statistical procedures, a combination of factorial and cluster analysis, were applied to five variables; landing profile, vessel length category, gear, mesh size range and month. The investigation has shown this methodology to be a suitable tool in identifying métiers without prior assumptions and enabled data to be described from a multivariate statistical perspective providing useful and informative results. In total twenty-two métiers were identified and defined from 2003 landings data, and a further 5 groups were formed to include trips which could not be assigned to a métier. Definitions were applied to 2003-2005 landings and effort data to examine their dynamics and stability. The dominant métier (in terms of effort and trips) contained high proportions of landed Nephrops taken by bottom otter trawlers using 70-89mm meshes. The effort in this métier was fairly stable over time. The greatest increases in effort and trip allocation occurred within métiers employing pots and traps for crustacean species. A small number of minor métiers (in terms of landings and effort) became obsolete over the three years examined. The effects of recent management measures are likely to have contributed to declines in the numbers of trips and effort in some métiers. For example, days-at-sea, limited mobile gears such as bottom otter trawls and beam trawls with ≥100mm mesh, have declined. Such changes within métiers over recent years have contributed to a deeper understanding of fleet dynamics in the Irish Sea. This analysis has identified and highlighted a segment of polyvalent Irish fishing vessels, which move between several métiers within a year. The definition of métiers can be used to enhance the Irish sampling programme in the Irish Sea by developing a finer scale, métier based, stratification of sampling. This in turn enables increased precision and robustness of national assessment data thus improving assessments and management advice. Defining métiers will prove advantageous in developing mixed fisheries assessments and advice.