Evidence Synthesis
Stage 1: Spawning Output, Egg Production, And Early Larval Supply
Working hypothesis: spawning biomass and spawning location set initial propagule supply, but most cohort variation is filtered later.
Smart et al. found that eggs, yolksac larvae, and preflexion larvae were associated with spawning stock biomass, while late larvae and juveniles were not (Smart et al. 2012). That supports a causal path from adult spawners to early-stage abundance, but also implies substantial mortality or redistribution after early larval stages. Duffy-Anderson et al. review the broader first-year evidence and emphasize that first-year survival, rather than egg production alone, is central to year-class strength (Duffy-Anderson et al. 2016).
DAG implication: include spawning stock biomass -> egg abundance, but avoid a direct dominant path from egg abundance to age-3 recruitment without intervening survival nodes.
Stage 2: Temperature, Spawning Distribution, And Transport
Working hypothesis: climate affects recruitment partly by changing where and when adults spawn, and by changing transport pathways and spatial overlap with prey and predators.
Petrik et al. used an individual-based biophysical model and concluded that advection alone and spawning timing shifts alone did not reproduce warm-cold distribution differences. Changes in spawning areas, especially contracted spawning in cold years, best matched observations (Petrik et al. 2015). Mueter et al. found support for larval transport, with survival increasing under stronger northward and weaker eastward transport, but noted that transport and temperature were difficult to separate statistically (Mueter et al. 2006). Smart et al. found that winds affected early-stage transport depth and abundance patterns but did not support a simple encounter-turbulence effect for all feeding stages (Smart et al. 2012).
DAG implication: represent temperature -> spawning time/location and wind/circulation -> transport separately from temperature -> prey and metabolic demand.
Stage 3: Larval Feeding And Early Growth
Working hypothesis: local prey concentration and temperature influence feeding success, development rate, and vulnerability to size-dependent mortality.
Feeding stages were positively associated with local copepod concentrations in Smart et al., while broad-scale zooplankton biomass was less informative (Smart et al. 2012). Brodeur et al. showed spatial differences in age-0 condition around frontal habitat near the Pribilof Islands, with offshore fish in better condition than inshore fish (Brodeur et al. 2000). This supports using local prey and habitat metrics rather than only basin-scale prey biomass.
DAG implication: use local prey availability and distributional match as proximate parents of larval growth.
Stage 4: Age-0 Summer And Fall Energy Storage
Working hypothesis: age-0 fish need both size and energy stores before winter; lipid-rich prey availability in late summer and fall is one of the strongest mechanistic links to survival.
Heintz et al. found that age-0 pollock diets had about threefold higher lipid content in cold years than warm years; fish energy density was 33% higher in cold years; and total energy in fall was positively correlated with age-1 recruits per spawner (Heintz et al. 2013). Siddon et al. showed a shift in energy allocation: larvae prioritize somatic growth, while post-metamorphic age-0 juveniles allocate more to lipid storage. The period after larval development and before winter is a short critical period for storage (Siddon et al. 2013).
Moss et al. found age-0 energy density was greater in cool years, with euphausiids important in cool-year diets and intra-cohort cannibalism higher in warm years (Moss et al. 2009). Sigler et al. synthesize the mechanism as timing plus location: late ice retreat can support copepods/euphausiids and a spatial match with age-0 pollock, allowing lipid-rich diets (Sigler et al. 2016). Gann et al. explain the poor 2007 year class as a cold-year exception: strong stratification and few summer wind-mixing events limited nutrient flux, primary production, and age-0 growth (Gann et al. 2016).
Strasburger et al. found that pollock and Pacific cod partitioned prey in cold 2008, but warned that warm years could increase competition for suboptimal prey and predation (Strasburger et al. 2014). Buckley et al. show that copepods and euphausiids remain key prey groups, with climate-mediated zooplankton changes likely affecting pollock differently by size class (Buckley et al. 2016).
DAG implication: age-0 total energy should be treated as a central mediator between environment/prey and recruitment. Cold years can be beneficial, but only when summer nutrient flux and prey production are adequate.
Stage 5: Winter Survival From Age 0 To Age 1
Working hypothesis: overwinter survival depends on body size, lipid reserves, winter duration, temperature, and prey availability.
Sogard and Olla experimentally showed that large size, high initial condition, and cold temperatures increased starvation survival. Bering Sea age-0 fish sampled in September were smaller and lower in lipid than lab fish that survived starvation, implying field vulnerability (Sogard and Olla 2000). Heintz et al. connected fall total energy to age-1 recruits per spawner (Heintz et al. 2013). Siddon et al. and Sigler et al. both frame the late-summer/fall period as preparation for winter rather than merely a growth period (Siddon et al. 2013; Sigler et al. 2016).
DAG implication: age0_size and age0_energy_density should be parents of winter_survival_to_age1.
Stage 6: Age-1 And Age-2 Survival To Age 3
Working hypothesis: after age 1, recruitment to age 3 is filtered mainly by predation, spatial overlap with predators, growth, and distribution relative to the cold pool and prey.
Mueter et al. found the strongest environmental effects on survival at the early juvenile stage, with little evidence for environmental effects at the late juvenile age-2 stage. Age-1 predation accounted for a large share of variability in stock-recruit residuals, and juvenile-adult spatial overlap was a strong predictor (Mueter et al. 2006). The earlier PCCRC report reached a similar conclusion, finding the strongest environmental effects at age 1 and emphasizing cannibalism or juvenile-adult overlap (Mueter et al. 2004).
Spencer et al. modeled arrowtooth flounder predation on pollock ages 1-3. Arrowtooth avoid the cold pool, juvenile pollock are less temperature-sensitive, and warm-year predator movement into the northwest middle shelf could increase age-1 predation mortality (Spencer et al. 2016).
DAG implication: include both adult_pollock_biomass -> cannibalism and arrowtooth_biomass/cold_pool -> predator_overlap -> age1_age2_survival.
Stage 7: Climate Regime And Switching Control
Working hypothesis: bottom-up and top-down controls alternate or interact depending on ice timing, temperature, prey production, adult biomass, and predator biomass.
Mueter et al. concluded that bottom-up and top-down processes are both important. Ice effects were modified by adult pollock biomass, consistent with oscillating control (Mueter et al. 2006). Sigler et al. argue that late-ice years can improve copepod, euphausiid, and age-0 pollock production; if such years occur successively, top-down control may increase (Sigler et al. 2016). Mueter et al. projected lower future recruitment under warmer late-summer SST, especially when predation is included (Mueter et al. 2011). Spencer et al. found projected biomass declines were driven mainly by the negative SST-recruitment relationship, with predation overlap adding risk under some predator-distribution scenarios (Spencer et al. 2016).
DAG implication: use time indexing to keep the graph acyclic, e.g., cohort_strength_t-1 -> predator/adult_biomass_t -> predation_on_cohort_t.
Candidate Indicators
| Process | Candidate indicators |
|---|---|
| Spawning and early-stage supply | SSB, egg abundance, maturity or spawning timing, observed or modeled spawning location |
| Physical forcing | Ice retreat date, cold pool area, spring transition or bloom date, late-summer SST, summer wind-mixing index, stratification, surface silicic acid |
| Prey field | Local copepod concentrations for larvae, Calanus/euphausiid biomass, prey lipid content, spatial match indices for age-0 fish |
| Age-0 state | Length, weight, energy density, percent lipid, total energy, diet lipid, stomach fullness |
| Top-down risk | Adult pollock biomass, arrowtooth biomass, Pacific cod abundance, juvenile-adult overlap, juvenile-arrowtooth overlap, age-1 predation mortality |
| Recruitment response | Survival to age 1, survival to age 2, age-3 recruitment residuals, year-class strength from assessment |