Can guild- or site-specific contrasts in trends or phenology explain the changed role of the Dutch Wadden Sea for fish?

Abstract The Wadden Sea bordering the Dutch, German and Danish coast, is traditionally a region with important functions for many fish species: as a nursery area for juveniles (marine juveniles), as a feeding area, as a transit to and from fresh water, and resident species complete their whole life cycle there. Because of indications that the importance of the Dutch Wadden Sea has changed drastically for many species during the past decades, we analysed and classified trends of 24 common fish species in the last 45 years, which were assigned to 5 different ecological guilds. Trends were examined for three Wadden Sea regions and compared to trends in the adjacent two North Sea coastal regions. For these analyses we made a combined use of two longterm time series: an annual beamtrawl survey, the Demeral Fish Survey (DFS) with a high spatial but poor seasonal resolution and a fyke series with a high seasonal but poor spatial resolution. We investigated for which species the DFS survey was appropriate for trend analysis, and we evaluated whether a change in timing may contribute to patterns in DFS time trends. Total fish biomass showed a similar pattern in all tidal basins with an increase from 1970 to 1980, a peak in the mid-1980s and a strong decline from 1980 to 2000, with a subsequent stable trend. The pattern in the coastal region deviated especially in the past 10 years, with a further decline along the Dutch Wadden coast and an increase along the mainland coast. Most dramatic declines throughout the Wadden Sea occurred in species belonging to the marine juvenile guild, notably plaice, sole and dab. A declining trend in marine juveniles is on-going in the western part, while it recently stabilised or even increased in the central and eastern part and in the coastal regions. Resident species showed more variable trends in the Wadden Sea with less pronounced directions: both increases and decreases occurred. In the coastal regions, several resident species have increased considerably in the last 15 years, a pattern not observed in the Wadden Sea. Also the size structure of the fish community changed in all regions, with generally the strongest declines in the largest size classes. The combined use of the two surveys showed that for some species the DFS was not timed in the period of peak occurrence. Although the phenology of several species has changed, the DFS survey period still encompasses the peak period of most species.