fisheries

habitat destruction

  • bottom trawling and dredging is one of the biggest human impacts on the seafloor ecosystems

  • effects of bottom trawling and dredging:

    • it scrapes the seafloor → removes structure forming organisms (sponges, corals)

    • crushes and kills benthic animals (e.g., Arctica islandica clams)

    • resuspends sediments → decreases light, clogs filter feeders

    • increases turbidity, affects primary production

  • direct vs indirect effects:

    • direct : mortality of benthic species, mechanical destruction

    • indirect: sediment resuspension, smothering, nutrient release

  • natural vs fishing disturbance:

    • fishing disturbance overlaps with natural disturbance (storms), but unlike storms, fishing happens frequently, year round, and over huge areas

  • other damaging practices:

    • explosive fishing, drive fishing

    • ghost gear: lost nets continue fishing for years → kills turtles, fish, mammals

overfishing (and the theory behind it)

comback to the maths section later; slides 11 to 20

overfishing consequences

  • exploitation curve rate:

    • total catch → rises then falls (overfishing collapses yield)

    • total biomass → declines steadily as exploitation increases

    • mean size (Lmax) → declines (fisheries induced evolution)

    • number of collapsed species → increases

    • the MMSY point marks multispecies MSY - beyond that, things go bad

  • evidence of global overfishing

    • reported catch peaked in 1996 and declines

    • but reconstructed total catch is much higher → because illegal, unreported, small scale catches are huge

  • northern cod collapse

    • 1960s-1980s heavy industrial fishing

    • 1992 → collapse → moratorium

    • no recovery after 30+ years

    • this collapse became a global warming example

  • why rebuilding is hard

    • predator prey reversal

    • cod decline → prey (herring, seals) increase

    • prey then eat cod eggs/juveniles → making cod recovery extremely difficult

examples of rebuilding (or failure)

  • norwegian spring spawning herring

    • collapsed in the 1970s

    • rebuilt due to strong recruitment, strict quotas, and international cooperation

    • this is now a major success story

  • food web changes

    • shows how fishing removes top predators → ecosystem shifts → kelp forests collapse

  • are we overfishing or rebuilding?

    • some areas appear to be rebuilding

    • others remain overfished

governance and jurisdiction 

  • the EEZ map illustrates how much ocean is now controlled by coastal states

  • slide 21 to 22

tools for rebuilding fisheries

  • gear restrictions, capacity reduction, total allowable catch, efforts control, closed areas, catch shares, certification, community co-management (critical in small-scale fisheries)

  • different regions use different tool combinations

modern issues: are global fisheries recovering?

  • some methods suggest recovery

  • some do not

  • uncertainty is very high

  • the global average is near the MSY line → risky zone

fisheries induced evolution

heavy fishing of large individuals selects for earlier maturation at smaller sizes

bycatch

  • definition → incidental take of juveniles of target species, non-target species → can be releases, injured, or killed

  • bycatch + targeted fishing = severe decline

the papers - worm (2009), Britten (2021), Hilborn (2004)

  • overfishing is widespread, but stock rebuilding is possible - however, recovery is uneven, uncertain, and depends strongly on management quality, fishing pressure, and ecosystem context. Marine protected areas (MPAs) help only in certain situations

paper 1- worm et al. 2009

  • this paper was a major turning point in fisheries science.

  • before 2009, Worm et al. (2006) prediceted that all global stocks could collapse by 2048. This paper caused a huge controversy

  • in this paper, the authors combined their perspectives and concluded that some fisheries are indeed collapsing, but others are rebuilding - and good management works

  • the main findings are as such:

    • the global fisheries are not universally collapsing. some regions show clear recovery

    • overfishing is still widespread, but:

      • where catch limits, monitoring, and enforcement are strong

      • stocks show rebuilding trajectories

    • recovery takes decades, not years

    • key management tools that work are catch shares/ITQs, strong TACs, gear restrictions, closed areas, community co-management, and science based stock assessments

    • fisheries management quality determines stock health more than global trends do

paper 2 - Britten et al. 2021

  • this paper revisits worm et al. 2009 conclusion with more data and a more cautious tone

  • recoveries exists - but are much more uncertain than previously thought

  • assesment methods matter a lot. small changes in model structure, natural mortality assumptions, catchability, and harvest control rules can completely change whether a stocl appears rebuilt or still overfished

  • global pre- and post-2000 trends differ

    • in the early 1900s/early 2000s, there has been clear rebuilding in some regions

    • after 2010, the signals weaken, and uncertainty grows

  • many assessments show paper recoveries, meaning management models say a stock is rebuilt but actual biomass estimates remain low or flat

  • rebuilding is easier in simple ecosystems (e.g. some pelagic species), but hard in complex food wbes (e.g. cod in northwest atlantic - prey increase prevents recovery

paper 3 - hilborn et al. 2004

  • this paper focuses on marine protected areas/no-take zones and asks “do MPAs help rebuild fisheries?”

  • main findings:

    • MPAs do not universally rebuild stocks. They only help under specific conditions

    • they work best when the species is sedentary, a portion of the population is protected from harvest, overfishing is the main problem, and teh spillover from MPAs benefits fished areas

    • MPAs work poorly when fosh move long distances, overfishing happens outside the MPA, migration links populations, and effort shifts (displacing fish) cancels the benefit

    • quota based systems can be better than MPAs

  • MPAs are not a magic solution - context matters

how do these papers fit together

  • overfishing is real and a widespread problem

  • rebuilding is possible. but not guaranteed

    • recovery is possible, but extremely dependent on high quality management and ecosystem context

  • ecosystems matter

    • cod collapses fail to recover because ecosystem feedbacks supress recovery

    • MPAs help only if species and ecosystems are suitable

    • ecology determines recovery success

  • the best fisheries management = combination of tools

    • TACs

    • Quota systems

    • Monitoring

    • Enforcement

    • Effort limits

    • seasonal closures

    • gear restrictions

    • MPAs when appropriate

    • rights based systems (ITQs)