Abstract
The state of marine systems subject to natural or anthropogenic impacts can be generally summarized by suites of ecological indicators carefully selected to avoid redundancy. Length-based indicators capture the status of fish community structure, fulfilling the Marine Strategy Framework Directive (MSFD) requirement for Descriptor 3 (status of commercial fish species). Although the MSFD recommends the development of regional indicators, a comparison among alternative length-based indicators is so far missing for the Mediterranean Sea. Using principal component analysis and dynamic factor analysis, we identified the most effective subset of length-based indicators, whether or not based on maximum length. Indicator trends and time series of fishing effort and environmental variables are also compared in order to highlight the individual and combined capability of indicators to track system changes across geographical sub-areas. Two indicators, typical length and mean maximum length, constitute the smallest set of non-redundant indicators, capturing together 87.45% of variability. Only in combination can these indicators disentangle changes in the fish community composition from modifications of size structure. Our study supports the inclusion of typical length among the regional MSFD Descriptor 3 indicators for the Mediterranean Sea. Finally, we show dissimilarity between the western and eastern-central Mediterranean, suggesting that there are sub-regional differences in stressors and community responses.
Highlights
Operational ecological indicators are generally used for summarizing the status of marine communities and ecosystems in a comprehensive and accessible way (Cury and Christensen 2005, Shin and Shannon 2010)
This paper aims to explore six indicators representing the size structure of the fish community: typical length (TyL), mean maximum length (MML), the large fish indicator (LFI), the large species indicator (LSI), mean weight and evenness
For all geographical sub-areas (GSAs) the Kaiser criterion suggested the use of two factors, except for GSAs 1, 5, 15, 19 and 25, for which the use of only one factor was suggested
Summary
Operational ecological indicators are generally used for summarizing the status of marine communities and ecosystems in a comprehensive and accessible way (Cury and Christensen 2005, Shin and Shannon 2010). Indicators of fish community size structure can reveal the fishing effects caused by removing certain sizes and altering the abundance of different-sized species Given their ability to summarize such complex dynamics, indicators of fish community size structure are increasingly used by national and transnational legislation, such as the European Marine Strategy Framework Directive (MSFD), which seeks to achieve Good Environmental Status (GES) by 2020 for all European seas (European Commission 2010). Because no single indicator can capture the diversity of dynamics and processes within a system, suites of indicators are required (Greenstreet et al 2012a), though the selection of indicators to retain should be carefully considered: indicators included should be sensitive to specific impacts (Houle et al 2012) or be unambiguous to monitor single species, communities or ecosystems (Rochet and Trenkel 2003, Shin and Shannon 2010). The selected suite should avoid redundancy (Methratta and Link 2006, Greenstreet et al 2012a) and ensure applicability across different geographical areas (Shin et al 2018)
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