Northern inland fishery will be challenged by climate change case Lake Säkylän Pyhäjärvi (SW Finland)

Transcription

1 Northern inland fishery will be challenged by climate change case Lake Säkylän Pyhäjärvi (SW Finland) Anne-Mari Ventelä, Marjo Tarvainen, Teija Kirkkala, Pyhäjärvi Institute, Eura, Finland Jouko Sarvala, University of Turku 1

2 Säkylän Pyhäjärvi Surface area 15,400 ha Mean depth 5.4 m Maximum depth 26 m Coastline 80 km(47.71 mile) Water residence time 3-5 y The lake is located in the boreal temperate zone (cool climate type). The winter time mean air temperature in the area is -2.1 ºC and the lake is normally ice-covered for 141 days on average. Drainage basin area Total 61,500 ha River Yläneenjoki 23,400 ha River Pyhäjoki 7,750 ha Remaining area (small sub-basins) 14,950 ha 2

3 25 Total phosphorus, summer mean Long term management challenge Long term Pyhäjärvi Restoration Program since Catchment measures Education, information services Biomanipulation Research and monitoring Chlorophyll a, summer mean

4 Finnish sauna and summer cottage culture intensive recreational use of lakes and rivers 4

5 Fish community The fish community consists of 18 species, dominated by perch, ruffe, roach and vendace The lake has a vital commercial fishery, with 22 professional fishermen The most important commercial catch species is vendace (Coregonus albula), which is the main planktivore in Pyhäjärvi (Sarvala et al., 1999). 5

6 Data on fish community composition were obtained from: extensive test fishing with gillnets in 1984 (non-standard series) and in 2000, 2004, 2006 and 2009 (Nordic gillnets; CEN 2005) catch samples and surveys (Sarvala et al. 1998, Ventelä et al. 2011, and unpublished). The winter seine catch of all species was monitored in , but samples for vendace and whitefish were collected since Sampling was done in the fishing harbour after the fishermen returned from the lake with their daily catch in special containers. At the first stage, samples of some kilograms consisting of a variable number of fish were taken from every container, sorted by species and weighed. Subsamples of each species were saved for individual measurements and age determinations; a minimum of thirty 0+ vendace individuals were measured each time. 6

7 Daily seine catch records for vendace (age 0+ and older fish separately) obtained for winters from each seine crew and/or from the most important wholesale fish agent. For most years, the year-class size of age 0+ vendace in the autumn could then be calculated from the decrease in the catch per unit effort during winter (Helminen et al. 1993). The water chemistry and hydrology of the lake have been monitored since the 1960s, and in 1980 monitoring was extended to cover biotic components such as phyto- and zooplankton (Ventelä et al., 2007, 2011), chl a and nutrients. Ice data for were recorded by local observers and the Finnish Environment Institute s Oiva data service ( which also provided water temperature and water quality data. 7

8 Biomanipulation The annual harvest rate approaches the total production of vendace, which means that the fishery has for decades acted as intensive long term biomanipulation. In addition to this self-supporting biomanipulation, a restoration project has also subsidized the harvest of commercially unwanted fish since The fishing was especially intensive in and apparently resulted in water quality improvement (Ventelä et al., 2007). 8

9 Long-term biomanipulation in Pyhäjärvi = Commercial fishery of vendace (Coregonus albula) and white fish (Coregonus lavaretus) + Fishing of commercially unwanted fish species: Ruffe Roach Smelt 9

10 1. Via food chain Strong fish stocks, strong predation pressure on zooplankton especially number of Daphnia decreaces Phytoplankton not controlled by zooplankton, becomes more abundant, Sarvala et al

11 23 professional fishermen working in the lake = top predators of the system! 11

12 2. Via nutrient removal 26 % of the annual phosphorus load is removed from the lake with the fish catch! to Eurajoki - 18 % from the air +10 % Ventelä et al. 2007, 2011 from ditches in the nearest drainage area + 24 % remain in the lake 56 % from Pyhäjoki + 12 % the catch of fish 26 % from Yläneenjoki + 54 % 12

13 Fishing methods The majority of the annual fish catch is taken in winter by seining through holes in the ice Four seine groups Very efficient method, daily catches can be kg/ one group/day Fishery during the open water period has not developed and is not cost effecient Legislation, need for cooling Quality problems in summer Storms, open windy lake, need for big boats Old fishermen, no willingness to change traditions Fyke nets, < 10 % the total catch 13

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18 Biomanipulation, total catch coregonids unwanted

19 Climate related challenges 19

20 mean Ice-out has shifted to an earlier date and the duration of icecover is decreasing, quite dramatically so in the 2000s. 20

21 160 Julian day of ice-out NAO winter index Ventelä et al

22 bm catch ice days

23 The maximum summer water temperature has increased highly significantly during (0.9 C per decade; adjusted R 2 = 0.41, F 1,47 = 34.75, P<0.001). May (spring) temperature records show no significant change over years (temperature vs. year in : P>0.18). Jeppesen et al

24 For vendace, there was a significant decline in yearclass size during the period (adjusted R 2 = 0.20, F 1,38 = 10.57, P<0.003), was related to increasing annual maximum temperatures (linear regression of log 10 -transformed variables: adjusted R 2 = 0.18, F 1,38 = 9.82, P<0.004), but not to summer mean temperature (P>0.51). In multiple linear regressions including either TP or chl a of May September, the temperature effect on vendace year-class size remained significant, none of the trophic state variables were significant (period , log 10 -transformed variables, temperature and phosphorus (R 2 = 0.21, F 2,28 =3.77, P<0.04) or chl a (R 2 = 0.22, F 2,28 = 3.86, P<0.04). Jeppesen et al

25 Jeppesen et al

26 The timing of ice break and the following temperature development are key factors affecting the year class variation in vendace (Helminen & Sarvala, 1994). Ice-out triggers the hatching of vendace larvae Highly vulnerable to predation from 8 to about 15 mm total length. A longer early larval period results in higher larval mortality (Helminen et al., 1997) Changes in the length of the ice cover 26

27 Conclusions Decreasing length of the ice cover period and increasing maximum water temperature will challenge/change The fish community in the lake The professional fishery Long term biomanipulation Lake and fishery management Other effects? 27

28 Thank you! 28