Long-term impacts of different fishing methods on the ecosystems in the Kattegat and Öresund

Transcription

1 Long-term impacts of different fishing methods on the ecosystems in the Kattegat and Öresund Henrik Svedäng Swedish Institute for the Marine Environment

2 Background of the study THE DEPLETION OF MARINE FISH STOCKS IS A WORLDWIDE DILEMMA THAT THREATENS GLOBAL FOOD SECURITY ALTHOUGH THERE ARE ENCOURAGING SIGNS OF DECREASING FISHING MORTALITY FOR SOME STOCKS, EVALUATION OF FISHERIES MANAGEMENT IS NEEDED

3 Aim of this study To analyse and compare the Kattegat and Öresund with different fishing regulations and observed fish abundance To provide evidence of the potential effects of using different technical regulations on similar ecosystems

4 The study area Skagerrak Kattegat Sweden Denmark Sound Baltic Sea 100 km

5 The study area Skagerrak Kattegat Sweden Denmark Sound Baltic Sea Trawling ban since 1932! 100 km

6 Description of the fisheries The fisheries in the Öresund and Kattegat go back to the Middle Ages Degradation of the demersal fish stocks begun in the middle of the 19 th century Trawling was adopted at the start of the 20 th century Since 1932, there has been an almost total ban on towed fishing gears in the Öresund but no such limitations have ever existed in the Kattegat At present, the fishery in the Kattegat and Öresund is managed both by TAC and effort regulation

7 Assessment / management areas: Kattegat -SD21, Western Baltic- SD22-24, Eastern Baltic- SD25-32 Kattegat cod Western Baltic cod Eastern Baltic cod

8 Ecosystem decription BOTH AREAS SHOW STRONG VERTICAL STRATIFICATION AND TYPICAL SEASONALITY IN TERMS OF THERMAL REGIME. THE PRODUCTIVITY OF THE ECOSYSTEM IS FAIRLY HIGH THE FISH COMMUNITY IS SIMILAR TO THE NORTH SEA MOST COMMERCIAL DEMERSAL FISH STOCKS ARE RECOGNISED AS COMMERCIALLY EXTINCT IN THE KATTEGAT (E.G. HADDOCK, POLLACK, WHITING, LING, HALIBUT, SKATES) EXCEPT FOR SOLE AND PLAICE.

9 Swedish landings of haddock from the Kattegat and Skagerrak Landings of haddock (tonnes)

10 Size distributions have changed: Truncation of the size distribution

11 Cod spawning areas: Baltic Sea

12 Present cod spawning sites in the Kattegat: Swedish cod fishery in the first quarter of the year in a 5x5 nautic mile grid (left panel), mean number of spawning cod in monotoring fishery (middle), and probably spawning sites (right panel): (Vitale et al. 2008).

13 Quarterly fishing activity in the western part of the Baltic Sea: at Kullen aggregation only occurs during the spawning period From: DTU Aqua, Denmark

14 WATER CIRCULATION IN THE AREA: black arrows surface currents purple counter circulation

15 STOCK SEPARATION ISSUES Cod spawn at several locations in the Kattegat and the Öresund Juvenile cod in the Kattegat are an assortment of North Sea and Kattegat stock components, and the proportion of the two stocks in the area varies between years Kattegat and Öresund cod are well-separated from each another

16 Taggings indicate different subpopulations in the area: Cod in the Skagerrak migrate towards the North Sea, whereas cod in the Sound and Kattegat are resident. Some cod switch between the Kattegat and the Sound at spawning Sound Kattegat Skagerrak

17 Euclidean distance from release to recapture point (km) Euclidean distance from release to recapture point (km) Low degree of dispersal over time indicate stationary behaviour both in the Kattegat and Sound cod Kattegat Days at liberty Öresund Days at liberty

18 An otolith chemistry study Three putative subpopulations in the Kattegat-Sound were depicted according to their migratory history during the spawning period Noteworthy: Some fish migrated towards Kullen during spawning period from the north and the south Svedäng et al. 2010b

19 Tonnes THE EVOLUTION OF THE COMMERCIAL FISH STOCKS IN THE KATTEGAT AND ÖRESUND Cod landings in the Kat tegat and Öresund Öresund Kattegat

20 CPUE ( cod kg per hour) Cod in the Kattegat (black bars) between 1981 and 2010, and for cod in the Öresund (red bars) between 1991 and 2000, and between 2004 and

21 Marked differences in abundance and size structure between the Sound and Kattegat: implication of stock separation (Number at age in the first quarter) Svedäng et al. 2010a

22 Environmental status of the Kattegat and Öresund Temperatures are now as high or higher than at any time since ICES began measurements in Oxygen deficiencies may occur in both the Öresund and Kattegat in late summer at low wind stress. The problems are aggravated by eutrophication.

23 Conclusions It is natural to believe that the much better performance of the cod stock in the Öresund is related to the absence of trawling It is unlikely that the difference is caused by more severe environmental problems in the Kattegat A prerequisite for these effects of the differences in technical regulations is the existence of separate, rather stationary fish stocks in the two areas It is suggested that fisheries management of the Kattegat has been a fiasco, and the better management of the Öresund is just incidental The findings presented in this briefing paper call for much more restrictive management actions

24 I knew it! Technology would save us.

25 Publications on the Kattegat and Öresund Cardinale, M. and Svedäng, H Modeling recruitment and abundance of Atlantic cod, Gadus morhua, in the eastern Skagerrak- Kattegat (North Sea): evidence of severe depletion due to a prolonged period of high fishing pressure. Fisheries Research, 69: Hagberg, J Utökad analys av historiska data för att säkerställa referensvärden för fisk (Improved analysis of historical data for ensuring fish reference points). Swedish Board of Fisheries (in Swedish) 22 p. Knutsen, H., André, C., Jorde, P.E., Skogen, M.D., Thuróczy, E. and Stenseth, N.C Transport of North Sea cod larvae into the Skagerrak coastal populations. Proc R Soc Lond B 271: Munk, P., Larsson, P.-O., Danielssen, D.S., and Moksness, E Variability in frontal zone formation and distribution of gadoid fish larvae at the shelf break in the northeastern North Sea. Marine Ecology Progress Series, 177: Pihl, L. and Ulmestrand, M Kusttorskundersökningar på Svenska Västkusten (Studies on coastal cod on the Swedish west coast). Länsstyrelsen i Göteborg och Bohus län 1988 (in Swedish). 61 pp. Svedäng, H. and Bardon, G Spatial and temporal aspects of the decline in cod (Gadus morhua L.) abundance in the Kattegat and eastern Skagerrak. ICES Journal of Marine Science, 60: Svedäng, H., Hagberg, J., Börjesson, P., Svensson, A. and Vitale, F Bottenfisk i Västerhavet. Fyra studier av beståndens status, utveckling och lekområden vid den svenska västkusten (Demersal fish the Kattegat-Skagerrak. Four studies on stock status, development and spawning areas along the Swedish west coast). Finfo 2004:6. 42 p. (in Swedish). Svedäng, H. and Svenson, A Cod (Gadus morhua L.) populations as behavioural units: inference from time series on juvenile cod abundance in the Skagerrak. Journal of Fish Biology 69 (Supplement C): Svedäng, H., Righton, D. and Jonsson, P Migratory behaviour of Atlantic cod Gadus morhua: natal homing is the prime stockseparating mechanism. Marine Ecology Progress Series 345:1-12. Svedäng, H., Stål, J., Sterner, T. and Cardinale, M Subpopulation structure in cod (Gadus morhua) puts strain on the management toolbox. Reviews in Fisheries Science 18: Svedäng, H., Jonsson, P., Elfman, M. and Limburg, K. 2010b. Migratory behaviour and otolith chemistry suggest fine-scale subpopulation structure within a genetically homogenous Atlantic cod population. Environmental Biology of Fishes (minor revision). Vitale F., Cardinale, M. and Svedäng H Evaluation of the temporal development of the ovaries in cod (Gadus morhua) from the Kattegat and Sound. Journal of Fish Biology, 67: Vitale, F., Svedäng, H. and Cardinale M Histological analysis invalidates macroscopically determined maturity ogives of the Kattegat cod (Gadus morhua) and suggests new proxies for estimating maturity status of individual fish. ICES Journal of Marine Science, 63: < Vitale, F., Börjesson, P., Svedäng, H. and Casini, M The spatial distribution of cod (Gadus morhua L.) spawning grounds in the Kattegat, eastern North Sea. Fisheries Research, 90,