Fish

teh barents sea ecosystem

  • the barents sea is a highly productive arctic-subarctic shelf sea with strong coupling behaviour between:

    • climate (ice, temperature)

    • lower trophic levels (zooplankton)

    • fish populations

    • fisheries

  • the key species interactions we are focusing on:

    • capelin -cod - haddock

capelin

  • life history

    • small pelagic fish

    • max length 22 cm

    • max age 7 years

    • matures at 14 cm (age 3-5, growth dependent)

  • distribution and feeding

    • distribution closely linked to ice retreat

    • main prey:

      • copepods (especially for small capelin)

      • krill (increasingly important with size)

      • amphipod

  • ecosystem role

    • capelin grazing has a strong negative effect on zooplankton biomass

    • this can cause apparent competition among zooplankton groups

    • capelin is a key forage fish, trasnferring energy from zooplankton to higher trophic levels

  • stock collapses

    • four historical collaspes with ecosystem wide effects

    • likely cause:

      • predation on larvae by young herring

      • fishing pressure

      • multiple interacting drivers

  • predators

    • cod (major predator)

    • other fish

    • marine mammals (harp seals, minke whales)

    • seabirds

haddock

  • life history

    • can reach 110 cm and 14 kg

    • matures at 4-7 years

  • feeding

    • primarily benthic feeder:

      • echinoderms

      • molluscs

      • polycahetes

      • crustaceans

    • larger haddock also consume fish

    • during spawning migration:

      • feed heavily on capelin and capelin eggs

  • population dynamics

    • strong variability in recruitment (pulsed recruitment)

    • recent increase in abundance

    • influencing factors:

      • temperature

      • age diversity

      • stock size

      • predation by cod (negative effect)

    • evidence for density-dependent growth

atlantic cod

  • stock status

    • currently the largest cod stock in the world

    • biomass can exceed 4 million tonnes

    • two main ecotypes:

      • NEA cod (skrei) - highly migratory

      • coastal cod - limited migration

  • life history

    • can live >30 years

    • can exceed 1.2 m in length

    • mature at age 6-9

    • extremely high fecundity (millions of eggs)

cod distribution, spawning and early life

  • spawning:

    • occurs along the norwegian coast

    • main spawning area: lofoten

    • eggs and larvae are pelagic

    • drift northward into the barents sea with currents

  • feeding by life stage

    • larvae: mainly calanus finmarchicus

    • juveniles (0-group): diverse diet, euphasiids very important

    • adults: very generalist diet (>200 prey species)

      • capelin is the main prey

      • cannibalism and haddock also important

growth age and demography

  • age and growth studies using otoliths

  • rapid early growth:

    • 6-7 months: 10.6 cm

    • age 1: 21 cm

  • ages 2-9 8-10 cm growth per year

  • mean age of the population has declined historically

  • changes in age strucutre can reduce reproductive output

migration and tradeoff

  • migration triangle

    • feeding areas

    • spawning grounds

    • nursury areas

  • migration involves trade-offs

    • energy cost

    • time cost

    • predation risk

    • offspring survival

    • growth conditions

climate change as a driver

  • observed patterns

    • climate warming causes redistribution of cod

    • spawning location have shifted northward

    • temperature influences:

      • recruitment

      • drift routes

      • growth of larvae

  • key paradox

    • spawning stock biomass (SSB) is falling

    • recruitment remains moderate/low despite

      • hogh SSB

      • high temperaautres

      • moderate fishing pressure

spawning location and juvenile size

  • drift model results

    • northern spawning → larvae experience colder temperatures

    • results in smaller juveniles

    • mean size reduction: 20%

  • why size matters

    • a 20% reduction in juvenile sixe can reduce first- winter survivial by a factor of 6

  • this links migration decisions directly to population dynamics

conclusions

  • cod, haddock, and capelin are tightly linked through predation and food availability

  • capelin is a critical energy bottleneck species

  • lsimate change currently appears more importnat than demography for cod spawning location

  • long migration costs may be offset by imporved offspring survival consequences

  • population dynamics cannot be understood withotu considering:

    • climate migration

    • size strucutre

    • species interactions