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Effect of Predators on Pacific Herring in the Gulf of Alaska Ron Heintz Alaska Fisheries Science Center Auke Bay Laboratories June 17, 2015 Why Is It Important to Understand Predation Effects on Herring • Predation rates often exceed levels of natural mortality used in stock assessments • Predation can be a force in structuring population dynamics • Incorporation of predation rates in assessments alters the biological reference points used for management • Accounting for predation is key to developing ecosystem based approaches to fishery management 2 How To Tell If Predators are Important to Herring Identify predators Predator abundance Diet composition Daily consumption Abundance of prey, mortality rates, production Do predators select herring? 3 Who Eats Herring in the GOA 39 bird species 5 cartilaginous fish 33 bony fish 32 inverts 14 mammals 123 predators 4 Herring Stocks in Alaska and Northern BC Based on 2009 Assessments Prince William Sound Lynn Canal Closed to fishing > decade Sitka Gulf of Alaska Sporadically open/closed Open to fishing 5 Consumers of Spawn and Larvae Photo by Frans Lanting Spawn loss rates up to 95% Most of predation due to birds, invertebrates Larvae vulnerable to invertebrates – Larval vulnerability to fish increases with size Photo by Sandy McRuer 6 Who Eats Juveniles? 27 birds species 35 fish species 1 baleen whale 3 pinnipeds dogfish 7 Do Pink Salmon Eat Herring in PWS? Pink salmon consumption of herring in PWS Adults 0% volume 20091 70% volume 20101 3% volume 20111 Freq. of Occurrence in 22001 stomachs (2010-2011) 0 % larvae 0.7 % 25-150 mm <0.1 % > 150 mm Juveniles DeRiso et al. 2008 1. Sturdevant et al. 2013, 2. Willette et al. 1997, 3. Boldt and Haldorson 2002, 4. Cross et al. 2005, 5. Sturdevant et al. 1996 unclear record 30% mass = larvae2 1.1% mass = fish3 0.6-40% vol = fish4 0% mass = fish5 8 Birds Eat Substantial Numbers of Juveniles in PWS During Winter Recruitment 2 years after consumption Winter Consumption of Juvenile Herring by Birds Juvenile herring consumed (millions) 6000 All birds 5000 recruits 4000 3000 2000 1000 0 1990 1991 1993 1994 1996 1998 2000 2004 2005 2007 Bishop et al. 2015 9 Groundfish Consume Juvenile Herring in Winter Longline surveys conducted over 3 winters in age-0 habitats 16% of 1930 stomachs contained herring 50% of Pac. Cod 27% of Pollock 5% Great sculpin 16 18 Herring in pollock stomachs (November 2010, 2011) Frequency 14 12 10 8 6 12 10 Herring in cod stomachs (March 2011, 2012) 8 6 4 4 2 2 0 14 Frequency 16 40 80 1 20 1 60 Herring length (mm) 200 0 Bishop et al. 2013 90 1 20 1 50 1 80 Herring length (mm) 21 0 10 Who Eats Adults? 11 Estimated Consumption of West Coast Vancouver Island (WCVI) Herring Adults Do consumption rates track abundance? Humpback Whales Estimates are for WCVI including La Perouse Bank N. Fur Seal P. Cod Dogfish Cumulative consumption is ~54% of WCVI and SOG spawning biomass. Similar to earlier estimates for Hake, sablefish, cod ~41% Lingcod Hake Schweigert et al. 2010 Ware and McFarlane 1995 12 Ecopath Estimates of Consumption in GOA 1990-1994 % of Adult Herring Biomass Consumed in GOA • Cumulative = 68.5% % of Adult Biomass 40 35 30 25 20 • Summer only • 50+ m 15 10 5 • Bottom trawl survey 0 • SEAK not included Aydin et al. 2007 13 Arrowtooth Removals Depend on Herring Abundance 160000 PWS Pre-fishery Biomass from ASA Tonnes 120000 80000 40000 0 1970 1980 1990 2000 2010 2020 14 Are Whales Important Predators? 25000 Whale Abundance – N. Pacific 20000 15000 10000 5000 Number of Whales 20,000 whales ~7.5 million people 0 1970 1980 1990 2000 2010 ( Darling, Nicklin, Barnes 2009) Similar rate of increased obs’d • BC • PWS • SEAK 15 Zooplankton (may include euphausiids) unknown Whales Can Eat Lots of Herring Euphausiids Pollock Herring Forage fish (may include herring) 16 Whale Diets Viewed Through Isotopes and Fatty Acids 15 N enrichment PWS whale fatty acids more similar to herring than krill Whale, Herring , Krill Fatty Acids Euphausiid 1 .0 SEAK PWS Krill 0.5 PWS SEAK 0.0 Pacific herring PWS SEAK Thysanoessa raschii -0.5 Herring PWS whale trophic position is highest among sampled whales Herring -1 .0 -2 Eupausiid -1 0 1 2 Whale 17 Whale Consumption Rates Depend on Location Tonnes consumed 18 PWS Ly nn Canal Sitka Sound Winter 07-08 Location & Winter ∑ Tonnes Consumed PWS 07-08 2639 PWS 08-09 4388 1-Mar-08 Sitka 07-08 1018 PWS Sitka Sound Ly nn Canal Sitka 08-09 788 Lynn Canal 07-08 732 Lynn Canal 08-09 501 12 6 0 1-Sep-07 Tonnes consumed 30 1-Nov -07 1-Jan-08 Winter 08-09 20 10 0 1-Sep-08 1-Nov -08 1-Jan-09 1-Mar-09 18 Whales in PWS Consume a Significant Biomass 24000 20000 2008 Stock biomass 16000 12000 2007 Stock biomass 8000 Range of winter mortality 4000 ‘98 Harvest level 0 Low Best High 2007-2008 Low Best High 2008-2009 19 Proportion Consumed Number Consumed Paradoxically…. Schooling May Attract Predators Number of Prey 20 Summary The importance of predators depends Life stage Population abundance Ecosystem conditions Habitat herring occupy Location in the GOA Herring schooling behavior increases their susceptibility to predation at low stock sizes Effects of predators on post-metamorphic juveniles is a significant data gap 21