The influence of climate on cod, capelin and herring in the Barents Sea Dag Ø. Hjermann (CEES, Oslo) Nils Chr. Stenseth (CEES, Oslo & IMR, Bergen) Geir Ottersen (IMR, Bergen; linked to CEES) Willem Barents 1598

Roles of cod, capelin and herring young herring cod main zooplankton feeder in Norwegian Sea lives in Barents Sea as young (age 1-2) main predator in Barents Sea largest remaining Atlantic cod stock zooplankton phytoplankton capelin main zooplankton feeder in Barents Sea key food for many birds and mammals main predator in Barents Sea largest remaining Atlantic cod stock main zooplankton feeder in Barents Sea key food for many birds and mammals

Variation in cod and herring recruitment: depends on (correlated with) sea temperature Cod (1946-) Ellertsen et al Herring (1921-) Annual average sea temperature Recruitment Warm conditions are necessary – but not sufficient - for cod and herring recruitment

Variation in capelin recruitment: Depends on presence of age 1-2 herring – predator of larvae capelin herring

How does the interplay between climate and trophic interactions work? young herring cod capelin WARM CONDITIONS reproduction + _ _ indirect, delayed climate effects growth/survival - Harp seal Zooplankton ? Sea birds

Model based on statistical analysis Survey data ( ) Models for abundance / length of cod, capelin (herring) Inference about causal relationships Inference about probable effects of climate change / changes in management

The exploited Barents Sea ecosystem - simplified capelin young herring cod zooplankton phytoplankton benthic invertebrates other pelagic prey harp seal minke whale CLIMATE INDIRECT CLIMATE man CONSTANT

Statistical model log(N age+1, year+1 ) = a + b∙ log(N age, year ) + c∙log(X year ) + … +  year where X are covariates such as Sea temperature Abundance of other species

Data Cod: bottom trawl sampling (1981-present) Capelin & herring: acoustic survey (1973-present)

Modeling, phase 1: Cod only Capelin Young herring Cod Climate Model: cod demography External factors (model input) Internal factors (feedback) Accepted, Can. J. Fish. Aq. Sci.

Modelling, phase 2: Capelin only Capelin Young herring Cod Climate Model: capelin demography External factors (model input) Internal factors (feedback) Manuscript, in review

Modelling, phase 3: Cod-capelin(-herring) Climate Dynamics/interactions cod-capelin, herring given by climate Climate Capelin Young herring Cod External factors (model input) Internal factors (feedback)

Variation in the abundance of young herring (age 1-2) in the Barents Sea (1980s and 1990s) Herring age 1-2 biomass Sea temperature (1-2 years before)

Some main effects in final model (indicated by statistical analysis) Herring has negative effect on capelin reproduction (to age 1) Cod has negative effect on capelin survival age yr old cod eat 1-3 yr old cod (when few capelin) Warm sea has positive influence on cod and herring (and capelin) reproduction Capelin has positive effect on cod growth (i.e., lowers cod maturation age) - Capelin has negative effect on capelin growth (i.e., increases maturation age) Not shown in figure: Survival dependent on cohort abundance Survival dependent on harvest

Simulations of the Barents Sea ecosystem model (input: climate, fishery) Red: observations

Simulations of the Barents Sea ecosystem model (input: climate, fishery) Red: observations, green: model simulation (input: climate, fishery)

How does climate influence capelin biomass? Importance - and instability - of indirect pathways

Preliminary results: The effects on the Barents Sea cod of climatic fluctuations Red lines: simulated with a warmer climate (an increase of appx. 0.6 C° / 1.1 F°) 12 % increase

Conclusions Most effects are documented in the literature, but are now put together in one single, (relatively) simple model Indirect effects of temperature via trophic changes: large, delayed effects The indirect effects of a given change in temperature depend on ecosystem state (i.e., to which degree populations are able to respond to a favourable climate) The ratio capelin/cod is a good predictor for both the survival of young cod (cannibalism) and for the survival of capelin... which leads to unstable dynamics (positive feedback)

Thank you for your attention

Summary of cod sub-models Climate affects reproduction, survival of small fish, length growth Negative density-dependence: survival age 1-2, 2-3, 3-4, length growth Negative effect of age 6 cod on survival of small fish (cannibalism?) Capelin abundance affects growth (but not survival) Fishing mortality affects survival age 5 and up

Summary of capelin sub-models Reprod.SurvivalLength growth Age→1→ → Abundance ---0/- Abund. 1 yr older - Body length +-0/- Temperature winter + Winter NAO index +/-+0/+ NAO previous yr -/+ Herring biomass -0/- + Cod biomass (rel.) -----/-- Autumn catch (rel.) --- Additivity test (P)0.55<.001 < R 2 (w/ interactions) Everything is affected by density-dependent effects Everything is (negatively) affected by cod and/or capelin More effects of NAO than of temperature; non-linear effects

The effects on the Barents Sea cod of reducing the herring population Red lines: Simulations with herring biomass decreased by 67% 5 % increase

The effects on the Barents Sea cod of reducing the capelin harvest Red lines: Simulations with capelin harvest effort reduced by 75% 7 % increase

The effects on the Barents Sea cod of both increasing the capelin population and reducing the herring population Red lines: Simulations with herring population reduced by 67% and capelin harvest effort reduced by 75% 15 % increase (i.e., larger than 5% + 7%)

Predictions : effect of warmer climate ( increased NAO + temperature) Observations ( ), predicted (current levels), predicted (climate variables increased by 1.5 s.d.) Capelin (log age 1) Capelin (biomass) Capelin (age 1-2) Cod (log age 1) Cod (log age 7) Cod (biomass age 5-10) Mean increase: t (27%) (but variability increased)

Climatic influence on capelin - decomposed climate manipulation Climate effect via cod Climate effect via herring Direct climate effect Cod*herring interaction

Capelin model, version II: without length (implicit part of maturation function); linear; no model selection BM age (biomass) is a function of N age Proportion of matures(age) is a function of N age Coefficients (a, b, c, d) estimated using ordinary linear regression for each age Density dependence Harvest Cod Herring

Capelin model, simulations The 1980s All reduced models perform less well - cod. herring and harvest are all important

Capelin model, simulations The 1990s Models without herring perform less well - herring is important