Thomas - Thorne New Approach to Understand PWS Ecosystem draft

The SEA program and a subsequent decade of quantitative monitoring built around acoustic assessment techniques has documented three major biomasses in PWS: two pelagic fishes Pacific her

A New Approach to Understand the Prince William Sound Ecosystem

(This is a historic research draft document authored by Gary L Thomas and Richard E Thorne)

Traditional concepts for biological forcing in food webs are top-down and bottom-up processes (Dyer and Letourneau 2003; Sinclair et al 2003) However, a more

contemporary concept that has been advanced by Cury et al (2003) is for wasp-waist

or middle-out forcing Arctic and subarctic food webs that are dominated by one or two planktivorous fishes have been classified as wasp-waist ecosystems with middle-out forcing (Rice 1995)

In 1991-93, GLOBEC planning documents were used to design an ecosystem program

in Prince William Sound (PWS), Alaska (Cullen 1989; GLOBEC 1991a,b,c,d,e) This effort evolved into several programs, including the Sound Ecosystem Assessment (SEA) program (Thomas et al 1997; Cooney et al 2001) The SEA program attempted to balance top-down and bottom-up approaches by recognizing that both food limitation and predation/prey sheltering were important concepts in ecosystem function (Cooney

et al 2001; Willette et al 2001) This approach led to recognition, even prior to Cury et

al (2003) that there were a select few species that tended to dominate the ecosystem through sheer numbers and biomass

The SEA program and a subsequent decade of quantitative monitoring built around acoustic assessment techniques has documented three major biomasses in PWS: two pelagic fishes (Pacific herring, Clupea palasi and walleye pollock, Theragra

chalcogramma) and large-bodied copepods of the genus Neocalanus that dominate the spring zooplankton biomass (Thomas et al 1997; Thomas and Thorne 2003; Thorne and Thomas 2008; Thorne 2008) In addition, three other species likely have important impacts that are less well documented: euphausids (Thorne 2005), pterapods

(Armstrong et al in press) and pink salmon (Thorne and Thomas in prep)

One result of the SEA program was the documentation that both food supply

(Neocalansus) and predation (pollock and herring) were critical in pink salmon survival Thorne (2008) verified that pollock and herring switch from feeding on Neocalanus to predation on pink salmon fry when Neocalanus is in low abundance During the late 1980s, herring was the dominant biomass in Prince William Sound, reaching over 100,000 metric tons (Thomas and Thorne 2003; Hulson et al 2008; Thorne and

Thomas 2008) Subsequent to the Exxon Valdez Oil Spill, the herring abundance collapsed to less than 20,000 metric tons The pollock abundance was first measured in

1995 (Thorne 2008) Pollock abundance peaked in 1998 at 43,000 mt, then declined to about 22,000 mt in 2003, the last year that it was surveyed

The relative abundance of pollock and herring has major impacts on the structure of the PWS ecosystem From November through March, adult pollock are distributed primarily

in deeper basins adjacent to the Gulf of Alaska and at depths between 175 and 300 m (Figs 1-2) In contrast, herring are distributed in protected bays and inlets at depths of 10-40 m (Thomas and Thorne 2001, 2003) Many marine mammals and seabirds

depend on herring for winter-period forage (Irons et al 2000; Thomas and Thorne 2003) For example, Thorne (2008) showed that Steller sea lion numbers were

positively correlated (99%) with herring abundance (r = 0.88, n = 18, t = 7.41) from synoptic aerial and acoustic surveys In contrast, only one Steller sea lion was ever detected above pollock concentrations in PWS throughout 5 years of extensive, winter-period aerial and vessel surveys Pollock are largely inaccessible to the surface

oriented animals However, pollock is a major predator on other fishes, as well as zooplankton During spring, some pollock clearly migrate into surface waters to feed on Neocalanus, and move further into inshore waters to feed on small fishes, including juvenile pink salmon and herring, when Neocalanus is in low abundance (Fig 3) Less documented, but clearly present, is movement of pollock into shallow bays and inlets during winter, where they remain near the bottom but prey on age 0 herring

We hypothesize that pollock, the current dominant biomass in PWS, has major impacts

as a predator on other fishes, including herring and pink salmon, but that the predation

is moderated by the abundance of zooplankton Euphausid abundance would tend to keep pollock in the deep central basins Pollock that move into surface waters in spring feed on Neocalanus unless Neocalanus is in low supply It is likely that pteropods, which become abundance in late spring/summer, also serve to moderate pollock

predation on juvenile fishes We know that pink salmon survival is a function of pollock predation It is likely that herring recruitment is similarly dependent What we don’t fully understand is how prey sheltering by dominant zooplankton groups impacts pink

salmon and herring survival, or how the circumstances of prey sheltering might create loopholes (Bakun and Broad 2003) that result in high survival and subsequent

recruitment

Literature Cited

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comparative pattern recognition with focus on El Nino effects in the Pacific Fish Oceanogr 12: 4/5, 458-473

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Milton, J Olsen, V Patrick, A.J Paul, D Salmon, D Scheel, G.L Thomas, S.L

Vaughan and T M Willette 2001 Ecosystems controls of juvenile pink salmon (Onchorynchus gorbuscha) and Pacific herring (Clupea pallasi) populations in Prince William Sound, Alaska,” Fish Ocean 10 (Suppl 1):1-13

Cullen 1989;

Cury P., L Shannon and Y Shin 2003 The functioning of marine ecosystems, In Responsible

Fisheries in the Marine Ecosystem, M Sinclair and G Valdimarson, Eds Cambridge, MS: CABI Publishing

Dyer and Letourneau 2003

GLOBEC 1991a,b,c,d,e

D Irons, S Kendall, W Erickson, L McDonald and B Lance “Nine years after the EXXON

VALDEZ oil spill: effects on marine bird populations in Prince William Sound, Alaska,” The Condor vol 102, pp 723-737, 2000.

Rice, J 1995 Food web theory, marine food webs and what climate change may do to

northern marine fish populations, in Climate Change and Northern Fish

Populations, R Beamish Ed Can Spec Publ Fish Aquat Sci vol 121, pp 561-568

Sinclair et al 2003

Thomas, G.L., E.V Patrick, J Kirsch and J.R Allen 1997 Development of a

multi-species model for managing the fisheries resources of Prince William Sound”,

In D.A Hancock, D.C Smith, A Grant and J.P Beumer Developing and

Sustaining World Fisheries Resources - the State of Science and Management,

2nd World Fisheries Congress CSIRO Publishing, Collingwood, Australia Pp 606-613

Thomas, G.L and R.E Thorne 2001 Night-time Predation by Steller Sea Lions

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and their predator assemblage in Prince William Sound, Alaska Aquatic Living

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Thorne, R.E 2008 Walleye pollock as predator and prey in the Prince William Sound

ecosystem In: G.H Kruse, K Drinkwater, J.N Ianelli, J.S Link, D.L Stram, V

Wespestad and D Woodby (eds), Resiliency of gadid stocks to fishing and climate change Alaska Sea Grant, University of Alaska, Fairbanks (16 p.)

Thorne, R.E and G.L Thomas 2008 Herring and the “Exxon Valdez” oil spill: an investigation

into historical data conflicts ICES Journal of Marine Science 65(1):44-50.

Willette, T.M., R.T Cooney, V Patrick, D M Mason, G.L Thomas and D Scheel 2001

Ecological processes influencing mortality of juvenile pink salmon

(Onchorynchus gorbuscha) in Prince William Sound, Alaska, Fish Ocean 10 (Suppl 1):14-41

Fig 1 Typical winter-period vertical distribution of adult walleye pollock in PWS

Fig 2 Typical horizontal distribution of adult walleye pollock in Prince William Sound during winter

Fig 3 Progressive shoreward movement of herring and pollock during spring 2001, a year of low macrozooplankton abundance