PYTR Class

Learning Objectives

1. Explain the variation of productivity in terms of stratification and nutrients
2. Evaluate the assessment of fish stocks and monitoring of harvest rates
3. Explain the risk of harvesting at MSY
4. Describe different perspectives influencing the recovery of overexploited species

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Part 1: Productivity, Thermal Stratification, Nutrient Mixing and Nutrient Loading

Ocean Productivity and Photosynthesis

• Productivity is higher in:
  • Shallow areas (sunlight reaches seabed).
  • Nutrient-rich zones (e.g., estuaries, upwelling currents).
  • Coastal areas (nutrients from rivers and land runoff).
  • Some lake systems.
• Ocean productivity = Organic matter production by phytoplankton.
  • Phytoplankton are photoautotrophs using sunlight to create food energy.

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Light and Photosynthesis in the Ocean

• Sunlight is essential for life, including in the deep ocean.
• In the ocean, light is absorbed and scattered:
  • Very little penetrates beyond ~80 m.
  • Penetration depth ranges:
    • Up to 150 m in low-productivity subtropical areas.
    • As shallow as 10 m in highly productive coastal areas.
• Photosynthesis mainly occurs in the photic zone (upper sunlit layer of the ocean).

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Ocean Layers and Mixing

• In low- and mid-latitude oceans:
  • Sunlight warms surface water, making it less dense and more buoyant.
  • Warm water stays above denser cold deep water.
  • The boundary between them = thermocline.
  • Winds can mix water across the thermocline, moving nutrients from deep to surface.
  • Light + seawater density interaction is crucial for phytoplankton success.

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Nutrients and Productivity

• Deep Chlorophyll Maximum (DCM):
  • Zone with enough light for photosynthesis and nutrient supply from below.
• Thin buoyant surface layer limits productivity:
  • Organic matter sinks, taking nutrients with it.
  • Nutrients accumulate in deep, dark waters.
  • Due to density differences, nutrients rise slowly or only via upwelling.
  • Surface waters limit productivity by moving nutrients away from light.

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Geographic and Seasonal Variations in Productivity

• Satellites track ocean surface color to monitor chlorophyll (photosynthesis indicator).
• Higher productivity found:
  • Near the Equator and coastlines.
  • In eastern margins (wind pushes warm water aside, allowing deeper nutrient-rich water to rise).
  • In high latitude oceans (frequent upwelling and vertical mixing).
• Low/mid-latitude oceans:
  • Warm surface water stays separate from cold, nutrient-rich deeper water due to density differences.
  • Limited vertical mixing = reduced nutrient supply = lower productivity.
• High latitude oceans:
  • Cold surface water sinks, enhancing vertical mixing.
  • Nutrient supply exceeds phytoplankton demand due to good light and mixing.
• Seasonality:
  • Strongest at high latitudes.
  • Summer has more sunlight (intensity + duration) → increased photosynthesis.

Part 2: Assessment of Fish Stocks and Monitoring of Harvest Rates

Assessment of Fish Stocks and Monitoring of Harvest Rates

• Purpose:
  • Ensures conservation and sustainable use of fish stocks.
  • Helps estimate the size and health of exploited fish populations.

Methods for Estimating Fish Population Size and Health:

• Catch Data:
  • Includes age, size, weight, and species caught by commercial/recreational fishers.
  • Used to estimate fish population size and distribution.
• Scientific Surveys:
  • Conducted from research vessels.
• Tag and Recapture Surveys:
  • Tracks movement and population data over time.

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Monitoring Harvest Rates:

• Done through analysis of logbooks:
  • Records size of catch, species, equipment, and location.
  • UK vessels must submit data within 48 hours of landing.
  • Allows long-term monitoring but does not detect illegal fishing.
• FAO Stock Status Plot (1996):
• Analyzed trends in ~400 well-studied fisheries.
• Stocks categorized as:
  • Developing: Catch levels increasing.
  • Senescent: Catch levels declining.
• By the mid-1990s, 20% of stocks exploited in the 1950s had collapsed.

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Challenges in Predicting Fish Stock Sizes:

• Many influencing factors:
  • Policy changes, energy costs, market shifts, disasters, climate change, acidification, demand changes.
• Catch data remains a vital indicator.
• Limitations of Stock Assessments:
  • Costly and difficult, especially when using research vessels.
  • National governments may use catch data to evaluate industry viability.
  • A 2013 study (Nature) estimated under-reporting of catches:
    • 100–500% in LICs/MICs.
    • 30–50% in HICs.

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Catch Numbers vs. Actual Fish Populations:

• Number of fish caught ≠ number of fish in the ocean.
• By mid-2000s, 14% of 166 stocks had collapsed.
• Fishing restrictions often led to stock recovery (e.g., Rougheye rockfish).
• Example: East USA – haddock & redfish abundance rose over 5× between 1995–2007 due to regulations.

Catch Size Doesn’t Always Indicate Stock Health

• Smaller catches can result from:
  • Market changes.
  • New fishing regulations.
• E.g., 34 USA West Coast stocks seemed collapsed due to low catches, but only 3 had truly collapsed.
• Shark catch declines may reflect reclassification, not actual depletion.

Additional Influences on Catch Data:

• Changes in exclusive economic zones.
• Disasters (e.g., oil spills).
• Fuel cost increases and price drops.
• Most nations only monitor their most economically important fisheries.

Part 3: Risks of Fishing at the MSY

Risks of Fishing at the Maximum Sustainable Yield (MSY)

• MSY = Maximum Sustainable Yield:
  • Largest long-term annual harvest that can be taken from a stock without depleting it.
  • Used to estimate how much fish can be sustainably caught.
• Risks of MSY:
  • MSY is only an estimate and hard to calculate accurately.
  • Harvesting at or above MSY can:
    • Reduce fish reproductive potential.
    • Cause rapid decline in fish stocks.
    • Trigger positive feedback loops leading to stock collapse.
  • MSY is affected by multiple environmental and biological factors.

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MSY and Biomass:

• Biomass = Total weight of a stock; influenced by:
  • Growth, reproduction, and mortality rates.
• Fish species vary in:
  • Growth speed (e.g., cod vs. herring).
  • Reproductive output (e.g., cod vs. sharks).
  • Lifespan and stability of stock size (e.g., sand eels vs. cod).
• Fishing impacts biomass by:
  • Removing large numbers of fish → reduces reproductive capacity.
  • Long-term → population decreases.
• Factors Influencing MSY Calculations:
  • Growth rate of fish.
  • Reproduction timing.
  • Fish mortality rate.
  • Abiotic factors (e.g., temperature, salinity, oxygen, acidification).
  • Biotic factors (e.g., predator-prey relationships, parasites).
  • Fishing methods used.

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Criticism from Peter Larkin (1977) – An Epitaph for the Concept of MSY:

• MSY risks population health because it:
  • Ignores spatial productivity variation.
  • Ignores impact on non-target species.
  • Only considers benefits, not costs of fishing.
  • Is vulnerable to political manipulation.
• MSY also:
  • Assumes all individuals in a population are the same.
  • Assumes harvest levels are the same each year—ignores variability.

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Scientific Concerns:

• Some scientists see MSY as a maximum average yield, not a fixed number.
• Natural and human-induced changes cause harvest fluctuation.
• Real-World Consequences of Overfishing at MSY:
  • Overfishing has devastated large fish stocks in:
    • Northwest Atlantic, North Sea, Peru’s anchovy industry, and whale fisheries.
  • Leads to depletion of large, high-value species (e.g., cod, tuna).
  • Fishing fleets move down the food chain to catch smaller, less valuable species.

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MSY Challenges in Practice:

• Often based on unreliable or insufficient data.
• Accurate estimates require data on:
  • Population size.
  • Growth rates.
• Makes MSY application difficult and sometimes impractical.

Part 4: Recovery of Overexploited Fish Species

• Stakeholders in the fishing industry include:
  • Governments
  • Consumers
  • Non-governmental organizations (NGOs)
  • Retailers (e.g., fish markets and supermarkets)

Stakeholder	Interests / Goals
Government	– Maintain employment in the fishing industry – Generate tax revenue – Ensure a healthy and sizable fish population
Fishing Industry	– Maximize profits – Increase catch sizes – Shift to other fishing grounds if local stocks decline
Consumers	– Affordable fish in markets – Continuous availability – Support for large-scale fishing and aquaculture
NGOs	– Protect fish stocks – Promote sustainable fishing practices – Advocate for bans on harmful practices (e.g., fishing young or endangered fish)
Retailers	– Secure and sufficient fish supply – Affordable prices for consumers

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• Managing Differences:
  • Typically resolved via discussion and negotiation
  • Restrictions may apply to:
    • Certain fish species
    • Specific times or areas for fishing
  • Penalties may be imposed on law-breaking vessels
• Recovery of Overfished Species:
  • Requires cooperation among stakeholders (governments, industry, NGOs, markets, etc.)
  • Recovery measures include:
    • Temporary fishing bans
    • Fishing license limits
    • By-catch prevention
    • Public education on sustainable fish choices
• Global Decline of Fish Stocks:
  • Overexploitation is widespread
  • Agencies must create rebuilding plans when stocks become overfished
• Rebuilding Fish Stocks involves:
  • A reduction in fishing to let populations reach sustainable levels (MSY – Maximum Sustainable Yield)
  • Once the fish reaches MSY, it’s removed from the overfished list
  • It remains on the rebuilding list until it hits the target population
• 2021 USA Example:
  • 45 fish stocks had rebuilding plans
  • Only 6 were no longer classified as overfished