Fish and Pollutants

This lecture examines:

• Persistent bioaccumulative toxic pollutants (PBTs)

• Their effects on fish (general and mechanistic)

• Why fish larvae are uniquely vulnerable

• How sublethal pollutant exposure during early life stages can alter survival, development, and year‑class strength

The lecture focuses specifically on fish larvae, because:

• The larval phase is the most sensitive stage of the life cycle

• Small increases in larval mortality can cause orders‑of‑magnitude declines in recruitment

• Sublethal pollutant levels that do not kill adults can still severely impair larvae

2. Persistent Bioaccumulative Toxicants (PBTs)

2.1 What are PBTs?

PBTs are:

• Synthetic industrial chemicals

• Highly resistant to degradation (abiotic + biotic)

• Highly mobile (ocean currents + atmospheric transport)

• Long‑lived (long half‑lives)

• Bioaccumulative (build up in tissues faster than eliminated)

• Biomagnifying (increase in concentration up the food web)

• Toxic via:

  • Endocrine disruption

  • Neurotoxicity

  • Carcinogenicity

  • Developmental deformities

  • Energetic/metabolic costs

They are found globally, from tropics to poles.

3. Major PBT Groups Covered

The lecture reviews five major contaminant groups:

1. PCBs (polychlorinated biphenyls)

2. PBDEs (polybrominated diphenyl ethers)

3. Organochlorines (DDT, chlordane)

4. Mercury (Hg) and methylmercury (MeHg)

5. Other legacy pesticides (briefly)

4. PCBs

4.1 Characteristics

• Legacy industrial compounds (no longer produced but still leaking into environment)

• Used as coolants/insulators in electrical equipment

• Half‑life: 8–15 years (depending on chlorination)

4.2 Biological effects

• Endocrine disruption → impaired reproductive development

• Neurotoxicity → impaired locomotion

• Carcinogenicity

• Body deformities

• Energetic costs → reduced growth/metabolism

5. PBDEs

5.1 Characteristics

• Flame retardants in plastics, foams, textiles

• Widespread use over last 30 years

• Half‑life: months to years (depending on bromination)

5.2 Biological effects

Similar to PCBs:

• Endocrine disruption

• Neurotoxicity

• Impaired reproduction

• Altered swimming behaviour

• Growth/metabolic impacts

6. Organochlorines (DDT, Chlordane)

6.1 Characteristics

• DDT = insecticide

• Chlordane = herbicide

• Long environmental persistence

  • DDT half‑life: >10 years

  • Chlordane: several years

6.2 Biological effects

• Endocrine disruption

• Neurotoxicity

• Reproductive impairment

• Behavioural changes

• Developmental deformities

7. Mercury & Methylmercury

7.1 Characteristics

• Naturally occurring, but most environmental Hg from fossil fuel combustion

• ~70% present as methylmercury (microbial conversion)

• Strong biomagnification

7.2 Biological effects

• Endocrine disruption

• Neurotoxicity

• Impaired locomotion

• Growth/metabolic impacts

8. Global Meta‑Analysis of PBTs in Marine Fish

Benito et al. meta‑analysis (1969–2012):

• 2662 measurements

• 842 species

• Global coverage (but biased toward Northern Hemisphere)

• Tissues sampled: muscle, liver, gonad, etc.

• Concentrations standardised for body size

8.1 Geographic patterns

• Highest concentrations generally in:

  • Atlantic Ocean

  • Pacific Ocean

• Significant regional differences only for:

  • Chlordane

  • PCBs

8.2 Habitat patterns

Expected: benthic > demersal > pelagic
Observed: no consistent differences, except mercury (opposite pattern: pelagic highest).

8.3 Trophic level patterns

Expected: biomagnification (higher trophic level = higher concentration)
Observed: no consistent biomagnification, except mercury (which did increase with trophic level).

8.4 Temporal trends

• Overall decline of 15–30% per decade

• Significant downward trends for all contaminants

• BUT:

  • Very high variability

  • Data mostly from juveniles + adults

  • Almost no data for larvae

9. Why Focus on Fish Larvae?

9.1 Larval stage = critical phase

• Morphologically and ecologically distinct

• All major organs develop during larval ontogeny

• Larvae are:

  • Small

  • Poor swimmers

  • Highly vulnerable to predation

  • Highly sensitive to environmental stress

  • Dependent on rapid growth to escape planktonic predators

9.2 Ontogenetic priorities

9.3 Larval mortality

• Natural mortality extremely high

• Mortality strongly size‑dependent

• Small larvae may experience 40–70% mortality per day

• Larger larvae: 5–10% per day

• Adults: ~0.2% per day

9.4 Consequences

Small increases in larval mortality → massive declines in year‑class strength.

10. Survivorship Curves & Mortality Models

10.1 Survivorship curves

• Show abundance vs age

• Slope = instantaneous natural mortality (M)

• Early life stages have extremely steep slopes

10.2 Example model

• Start with 10¹⁴ eggs

• Larval mortality ~11% per day

• Juvenile mortality ~5% per day

• Adult mortality ~0.2% per day

• Result: 6–7 orders of magnitude decline within first year

10.3 Effect of small mortality increases

Doubling larval mortality (e.g., from 11% → 22% per day):

• Leads to 1000‑fold reduction in juvenile abundance

• Even if juvenile mortality stays the same

• Shows how sublethal pollutant effects can devastate recruitment

11. Why Larval Mortality Is Invisible

• Adult fish kills are obvious (float, wash ashore)

• Larvae:

  • Are microscopic

  • Break apart quickly after death

  • Disperse in currents

  • Are not detected unless specifically surveyed

Therefore, larval mortality from pollutants is almost always undetected, even when ecologically catastrophic.

12. Why PBTs Are Especially Dangerous for Larvae

• Endocrine disruption affects organogenesis

• Neurotoxicity affects swimming → feeding → predator avoidance

• Growth impairment delays escape from planktonic predators

• Metabolic costs reduce energy available for development

• Sublethal concentrations (safe for adults) can be lethal for larvae

Thus, even low environmental concentrations can:

• Reduce feeding success

• Increase predation

• Delay metamorphosis

• Reduce survival

• Collapse year‑class strength

13. Summary

• PBTs are globally distributed, persistent, and toxic.

• They bioaccumulate and biomagnify (especially mercury).

• Adult fish show declining contaminant levels globally.

• BUT larvae are far more sensitive than adults.

• Small increases in larval mortality → huge declines in recruitment.

• Larval mortality is invisible, making impacts easy to overlook.

• Sublethal pollutant exposure during early life stages may be a major, under‑recognised driver of fish population decline.