Fishing and environment drive spatial heterogeneity in Celtic Sea fish community size structure

The Large Fish Indicator (LFI) is a univariate size-based indicator of fish community state that has been selected to support the OSPAR fish community Ecological Quality Objective (EcoQO). To operate this EcoQO, a survey-based LFI for each OSPAR region needs to be developed. However, fish communities in these regions are spatially heterogeneous, and there is evidence of within-region spatial variation in the LFI that could confound an overall indicator series. For Celtic Sea trawl-survey sites, spline correlograms indicate positive spatial autocorrelation at a similar range (∼40 km) for the LFI and for fishing effort (h year ^-1) from vessel monitoring systems. Statistical models reveal a strong negative effect on annual LFI by site of fishing effort within a radius of 40 km. There was a weak effect of fishing within 20 km and no effect at 10 km. LFI also varied significantly with substratum and with local fish community composition identified by a resemblance matrix derived from the survey data. Finally, there was a weak effect of survey year on LFI. Spatial stratification of LFI calculations may be necessary when developing size-based indicators for OSPAR or Marine Strategy Framework Directive regions.