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Evidence for impacts by jellyfish on herring recruitment in the North Sea

Evidence for impacts by jellyfish on herring recruitment in the North Sea. Chris Lynam 1 , Andrew Brierley 1 , Mike Heath 2 & Steve Hay 2 1 Gatty Marine Lab. University of St Andrews 2 Marlab, Aberdeen. Fishing down the food web.

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Evidence for impacts by jellyfish on herring recruitment in the North Sea

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  1. Evidence for impacts by jellyfish on herring recruitment in the North Sea Chris Lynam1, Andrew Brierley1, Mike Heath2 & Steve Hay2 1 Gatty Marine Lab. University of St Andrews 2 Marlab, Aberdeen

  2. Fishing down the food web From Pauly and MacLean In a perfect ocean. Island Press 2003

  3. Benguela Sardine Sardinops sagax(also known as pilchard) High fish landings Fishery collapse & 1st reports of jellyfish Rise of the jellyfish No jelly? Boyer et al. SA J Mar Sci 2001

  4. Anecdotal evidence from the Benguela • Many tonnes of • jellyfish but only • 1 sardine caught • in 66 trawls. • ~1/2 of which • contained >80% • by mass jellyfish • 12m diameter • circular mouth • opening. • Mesh size • reducing from • 400 to 36 mm • Towed for ~5 min • at 3 knots

  5. Interactions between fish and jellyfish • Top down, medusae prey on fish eggs/larvae • Bottom up effect, medusae prey on zooplankton which regulate fish survival • Dietary overlap (Purcell & Sturdevant 2001) 73% 50% Pacific herring (Clupea pallasi) Aurelia labiata Cyanea capillata

  6. North Sea Jellyfish Cyanea capillata Cyanea lamarckii Aurelia aurita Fish larvae trapped in C. capillata tentacles C. capillata feeding on A. aurita C. capillata feeding on C. lamarckii

  7. Predatory impact by jellyfish on herring larvae • Aurelia aurita is predatory on Atlantic herring larvae in the Baltic Sea (Möller Science 1984). • Aequorea victoria is predatory upon Pacific herring larvae in British Columbia • (Purcell & Grover MEPS 1990)

  8. Our data • Jellyfish by-catch from IYGPT nets, during ICES International 0-Group Gadoid surveys of the North Sea. • 15 years (1971-86, not 1984) • >430000 medusae from 2030 IYGPT trawls • ICES Autumn-spawning herring data, for North Sea, Eastern English Channel, Skagerrak and Kattegat combined Survey area

  9. Herring recruitment, SSB, and jellyfish abundance

  10. Herring residual survival • The residual survival, i.e. the difference between the observed and the expected (Ricker-modelled) survival = ln(R) – ln(SSB), is a measure of external impacts on larval survival. • Residual survival = observed survival -expected survival • Residual survival = ln(Robserved) - ln(Rmodelled)

  11. r = -0.61, p = 0.02 Jellyfish do impact on herring survival to age-1

  12. The North Atlantic Oscillation A climatic phenomenon that contributes greatly to the variability in the weather system over the North Sea and Europe. 1971-86 + NAO leading to strong westerlies, high turbulence, and warmer SST in the North Sea. From Hurrell 2003 The NAO impacts on the sea level pressure, turbulence, sea surface temperature, ocean heat transport and deep ocean convection and has the potential to influence profoundly the pelagic environment.

  13. Jellyfish: links to the NAO • The abundance of A.aurita in the North Sea is negatively correlated with the winter North Atlantic Oscillation Index • (Lynam et al. Limnol. Oceanogr. 2004)

  14. Herring: links to the NAO • Climate variation governs alternating periods of herring and sardine (Sardina pilchardus) dominance in the English Channel(Alheit & Hagen Fish Oceanogr. 1997). • low NAO, herring dominate over sardines in English Channel • The NAO is linked to herring year-class-strength in the Baltic Sea (Axenrot & Hansson Limnol. Oceanogr. 2003)(r2 = 0.35 p < 0.03 n = 10) • low NAO, weak year class of herring in the Baltic Sea

  15. Jellyfish-herring-climate

  16. Regulation of A. aurita abundance by C. capillata? Predation/competition impact on herring?

  17. Jellyfish-herring-climate

  18. Summary • A. aurita are detrimental to North Sea herring recruitment and survival to age-1, via • Predation on herring larvae by medusae, and • Competition between larvae and medusae for zooplankton food. • This impact is mediated by the climate, as quantified by the NAO, through • Increased predation/competition between herring and C. capillata in low NAO years. • > reduced herring recruitment • Regulation of A. aurita by C. capillata in high NAO years. • > enhanced herring recruitment

  19. Acknowledgements Many plates from Aquascope 2000, Strömstad, Sweden and the Sir Alister Hardy Foundation for Ocean Science image library

  20. Removal of long-term trend due to SSB from recruitment • The Ricker model estimates recruitment from the Spawning Stock Biomass • RRicker = (a.S).exp(-b.S) • where, RRicker = numbers (millions) age-1 for year class y, • S = SSB (tonnes) in year y. The constants a = 1.16 x 10-1, b = 8.53 x 10-7 • were fitted by non-linear least-squares estimation using the Gauss-Newton algorithm.

  21. Herring survival index • The survival of herring to age-1 was calculated from the difference between the natural logarithms of the SSB and recruitment level (R): • Survival = ln(R) – ln(SSB)

  22. Herring residual survival • The difference between the observed and the expected (Ricker-modelled) survival, the residual survival, is a measure of external impacts on larval survival. • Survival residuals = observed survival -expected survival

  23. Orkney/Shetland Buchan Central Spawning grounds • In each spawning ground analysed the abundance of jellyfish was positively correlated with the herring larval abundance. • Aurelia aurita • Buchan • (r2 = 0.44 p = 0.04 n = 10) • Central • (r2 = 0.61 p = 0.02 n = 8) • C. capillata • Orkney/Shetland region • (r2 = 0.48 p < 0.01 n = 14)

  24. Jellyfish and fish Fish larvae trapped in C. capillata tentacles Whiting eating A. aurita Whiting secure among C. capillata tentacles

  25. Currents Possible North Atlantic Oscillation (NAO) driven changes in Atlantic currents influencing the North Sea showing the four regions of the North Sea where jellyfish data were analysed. NAC = North Atlantic Current, GS = Gulf Stream, CSJ = Continental Shelf Jet, LSIW = Labrador Sea Intermediate Water, NADW = North Atlantic Deep Water, and + signifies a positive effect on jellyfish abundance, adapted from Reid et al. (1998).

  26. Regime Shift 1983 linked to Atlantic Inflow Long-term series of a combined index of Candacia armata and Metridia lucens in the NW North Sea. Reproduced from Corten 1999. The abundance of zooplankton Atlantic water indicator species Candacia armata and Metridia lucens EoS

  27. Concomitant changes in North Sea gadoids and plankton Jellyfish survey period From Beaugrand et al. Nature 2003

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