ICES CM 2013/K:05

Marine under – impacts on coastal and

Martin Snickars, Benjamin Weigel, Erik Bonsdorff Åbo Akademi University, Dept. Biosciences, Åbo, Finland. Presenter contact details: [email protected], Phone +358 2 215 4604

Summary Coastal benthic habitats are productive and central for the benthos-fish link as a majority of feed on benthos in these habitats during at least part of their life-cycle. Environmental change, i.e. combined and climate effects, may alter this link, as many habitats are sensitive to these changes. In this study we examplify how fish and benthos change over time, and how the link in photic and aphotic habitats may be affected by climate change and eutrophication. We analyzed the response of benthos and benthic feeding (BF) fishes to temperature and Secchi depth over time, and across depths. During the 1980s, BF increased in shallow (<5 m) as Secchi depth decreased. Sine late 90s the main response had changed to be negative to temperature. However, in deep water (>6 m), the temperature response was positive, suggesting a spatiotemporal shift in the response of BF, following a change to cooler, yet warming, aphotic habitats. However, here the benthos biomass declined during the 2000s indicating potential food shortage that may be amplified with future environmental change. Whilst models predict increasing pelagic decreasing photic depths and levels, trophic links between different habitats may be affected.

Introduction During the last decades, environmental change has altered many . For example, in the Baltic , eutrophication and climate effects constitute major factors. Eutrophication has increased pelagic primary production, risk of low oxygen levels in deep water and decreased photic depth, while increased temperature and lower have contributed to these changes (Kautsky et al. 1986; Olsson et al. 2012; Rousi et al. 2013), i.e. affecting fish and benthos in vegeated and unvegetaded habitats. The combined effect of the factors may be even more important, as temperature has an positive effects on pelagic primary production, strengthening the eutrophication effects (Meier et al. 2012). The coastal of the northern Baltic Sea comprise of a mosaic of hard and soft substrates. Vegetated habitats are common in photic areas, with macroalgae such as Fucus vesiculosus dominating on hard substrate, and a diverse dominating on soft substrates. In aphotic areas soft bottom benthos is dominating. The fish community consists of both marine and warm-water coastal species, a majoity of the species feed on benthos.

Materials and Methods The study area (c. 30 km2) in the Åland Islands, N Baltic Sea comprises of small islands and has a maximum depth of c. 40 m.We used two data sets to analyse the long-term changes in Benthic feeding fish (BF) community1), and vertical distribution 2). The first data set used coastal survey nets (1983- 2007). Two linked gillnets (five mesh sizes, 17–50 mm) were set (2–5 m depth) at 2-8 stations, which were fished in August 4-6 times/year. The second set used Nordic gillnets (12 mesh sizes, 10–55 mm) in 2002-2012. 45 stations (depths: <3, 3-6, 6-10, 10-20 m) where fished once in August with one gillnet. Abundance was given as CPUE (abundance of BF per year for the first data set, and per depth and year for the second data set). The BF was defined as species eating benthos during at least part of the life-cycle. The two methods can not be compared, but each provides trends in the BF community. Bottom temperature and Secchi depth were measured at each station during (Government of the Åland Islands). Benthos was sampled in May (1990-2012) at five stations (depths 8-42 m) using three Van Veen graps sieved through a 1 mm sieve. Species were identified to nearest possible taxon (g/m2) by the Swedish Board of . A 7-year moving window linear regression was used to analyse the the change in response of benthos and fish to temperature and Secchi depth over time. Results and Discussion The BF community included 15 species, dominated by perch (Perca fluviatilis). In the first fish data set, the BF community was positively related to the decreasing Secchi depth until early 90s, suggesting that the community abundance increased due to eutrophication. The relationship between temperature and BF community was weakly negative during most of the study period, but since late 90s it was strongly negative although temperature and BF increased over time. Since mid 90s temperature and Secchi have been negatively related, with decreasing photic depth while water temperature has increased, suggesting that temperature may facilitate pelagic primary production. The negative response of BF to temperature is likely explained with re-distribution and changed habitat preference. Cold summers,

many of the warm-water fishes have a narrower Log vertical distribution as they favour shallow, warm habitats. During warm summers, feeding habitats may be expanded to deeper, aphotic waters, reducing food in shallow water. This pattern is strenghtened by the second data set showing a positive response to temperature during period 2002-12 (Adj. R 0.37, p=0.03) in deep water (>6 m) but not in shallow water, indicating a temperature-mediated, vertical shift in distribution 2 with warming deep water. However, paralell to this increase, benthos showed a declining trend from early 2000s, indicating that BF may affect the Adjusted R benthos biomass (Adj. R 0.47, p=0.01), but also eventually may face a shortage in food. Models predict increased temperature and primary production, decreasing photic depth and oxygen levels Although, this study shows that BF may Figure 1. Top: Secchi depth (black line), bottom water expand feeding habitats with warming deep water, temperature (dark grey) and CPUE of BF during 1983.2007. Log values. Below: 7-years moving linear regression combined climate and eutrophication scenarios between abundance of benthic feeding fish (BF) and imply responses to both vegetated and Secchi depth (black bars) years 1983-2007, BF and unvegetated habitats (Meier et al. 2012; Bergström temperature (white bars) and temperature and Secchi et al. 2013) that may affect trophic links as suitable depth (grey bars). + and – at Y-axis denote the sign of the slope. Year denotes the period’s median year. habitats for benthos and fish may decrease.

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