Fish population dynamics P2

2. Today’s Focus: Rocky Reefs of California & Mexico

These temperate rocky reefs share key features with coral reefs:

  • High habitat complexity

  • High biomass and species diversity

  • Strong environmental forcing (especially upwelling)

Today’s lecture examines:

  1. Larval production

  2. Settlement

  3. Post‑settlement processes (predation, competition)

  4. How oceanography shapes recruitment

3. Reproductive Modes and Larval Duration

3.1 Pelagic vs Non‑Pelagic Larvae

  • Previous lecture assumed pelagic larvae transported by currents.

  • Many rocky‑reef fishes lay benthic eggs that hatch near the substrate.

  • These larvae:

    • Still have a larval phase

    • But are not fully pelagic

    • Are less influenced by currents

    • Show higher local retention

3.2 Larval Duration as a Driver of Dispersal

  • Larval duration = time larvae spend in the water column.

  • Longer duration → greater potential dispersal distance.

  • Otolith microstructure used to estimate larval age:

    • Daily growth rings (day/night feeding cycles)

    • Allows reconstruction of larval duration for each species.

3.3 Empirical Findings

  • Huge variation across species:

    • Garibaldi: ~20 days

    • Pacific rockfish: ~270 days

  • Implication:

    • Species differ dramatically in dispersal potential.

    • Oceanographic models treat larvae as particles to simulate dispersal distances.

3.4 General Patterns Across Taxa

  • Seaweeds: dispersal <5 km

  • Invertebrates: extremely variable

  • Fishes: typically 20–200 km, depending on larval duration

4. Two Functional Groups on Rocky Reefs

4.1 Mid‑Water Schooling Species

  • Live in kelp canopy/mid‑water

  • Larval duration: 3–4 months

  • Examples: olive rockfish, yellowtail rockfish, black rockfish

4.2 Benthic Solitary Species

  • Live near substrate, under kelp

  • Larval duration: 1–2 months

  • Examples: various benthic rockfish species

5. Upwelling Systems and Recruitment

Upwelling = deep, cold, nutrient‑rich water rising to the surface.

5.1 California Upwelling System

  • Strong but not as extreme as Peru/Chile

  • Drives:

    • Productivity

    • Larval transport

    • Recruitment success

5.2 Counterintuitive Larval Depth Patterns

  • Mid‑water adults → larvae found deep (90–100 m)

  • Benthic adults → larvae found shallow

  • Why?
    Larval depth is an evolved strategy to exploit upwelling cycles.

6. ENSO (El Niño / La Niña) Effects on Recruitment

6.1 El Niño

  • Weak upwelling

  • Weak offshore winds

  • Weak offshore currents

  • Benthic species recruit better

    • Their shallow larvae are pushed onshore into correct habitat.

6.2 La Niña

  • Strong upwelling

  • Strong offshore currents

  • Mid‑water species recruit better

    • Deep larvae are brought upward and onshore by upwelling.

6.3 Neutral Years

  • Both groups recruit moderately well.

7. Within‑Year Variability

Upwelling is not constant even within a single ENSO phase.

7.1 Temperature as a Proxy

  • Upwelling pulses → sharp drops in temperature

  • Temperature record shows:

    • Highly stochastic fluctuations

    • No smooth seasonal pattern

7.2 Recruitment Tracking Temperature

  • Mid‑water species:

    • Recruit during cold pulses (upwelling events)

  • Benthic species:

    • Recruit during warm periods (weaker upwelling)

Even over 4 months, recruitment switches back and forth depending on oceanographic conditions.

8. Case Study: Sheephead Wrasse Recruitment

8.1 Geography

  • California + Baja California

  • Strong ENSO‑driven current reversals:

    • La Niña: north → south flow

    • El Niño: onshore → north flow

8.2 Findings

  • Northern sites show:

    • Low recruitment in normal/La Niña years

    • Huge recruitment spike during El Niño

  • Mechanism:

    • El Niño currents push larvae northward, enhancing recruitment at northern sites.

9. Habitat Effects on Settlement: Giant Kelp (Macrocystis) Forests

Macrocystis forests:

  • 10–15 m tall

  • Extremely complex habitat

  • Support diverse fish assemblages

9.1 Classic Experimental Studies

Multiple studies manipulated kelp density to test:

  • Settlement preferences

  • Habitat dependence

  • Recruitment success

9.2 Key Findings Across Studies

  • Some species prefer dense kelp (mid‑water species)

  • Others prefer open rock (benthic species)

  • Kelp structure strongly influences settlement patterns

10. Detailed Example: Kelp Bass Recruitment

10.1 Experimental Manipulation

  • Increased kelp density → order‑of‑magnitude increase in kelp bass recruitment

  • No kelp → almost no recruitment

10.2 What Part of the Kelp Matters?

Two metrics tested:

  1. Stipe density (kelp “trunks”)

  2. Blade biomass (actual leaf area)

Findings:

  • Recruitment increases with stipe density up to ~50 stipes

  • But blade biomass explains recruitment far better

  • Relationship is asymptotic:

    • More kelp ≠ infinite recruitment

    • Plateau due to predation and habitat saturation

10.3 Spatial vs Temporal Variation

  • Spatial: denser forests ≠ always more fish

  • Temporal: year‑to‑year kelp density changes → recruitment changes

  • Blade biomass is the key driver, not stipe count

11. Post‑Settlement Processes

Once larvae settle, several processes shape survival:

11.1 Predation

Evidence

  • Higher recruit density → higher per capita mortality

  • Why?

    • More prey → more predators attracted

    • Predation scales non‑linearly with prey density

Experimental Predator Removal

  • With predators present:

    • Mortality increases with recruit density

  • With predators removed:

    • Mortality becomes flat (density‑independent)

Habitat Complexity

  • Complex rocky substrate → lower mortality

  • Flat substrate → high mortality

  • Mechanism: more hiding spaces reduce predation risk

11.2 Interspecific Competition

Case study: Striped vs Black Surfperch

Diet Preferences

  • Striped surfperch:

    • Dominant competitor

    • Prefers Gelidium robustum (high‑quality algae)

  • Black surfperch:

    • Subordinate competitor

    • Forced to eat turf algae when striped surfperch present

Experimental Removal

  • Remove striped surfperch:

    • Black surfperch expands diet to include Gelidium

  • Remove black surfperch:

    • Striped surfperch unaffected (dominant)

Depth Distribution

  • Striped surfperch dominate shallow areas (more algae)

  • Black surfperch pushed to deeper areas

  • Competition drives zonal habitat partitioning

Community‑Level Effects

  • Similar patterns seen across multiple rockfish species

  • Competition structures:

    • Vertical zonation

    • Habitat partitioning

    • Species coexistence

12. Synthesis and Conclusions

Temperate rocky reefs show that:

Recruitment is shaped by:

  • Larval behaviour (depth, duration)

  • Oceanography (upwelling, ENSO)

  • Habitat structure (kelp density, substrate complexity)

Post‑settlement survival is shaped by:

  • Predation

  • Competition

  • Habitat availability

Population dynamics emerge from the interaction of all these processes.

  • No single factor explains adult abundance

  • Multiple processes act simultaneously

  • Understanding these interactions is essential for:

    • Fisheries management

    • Marine protected area design

    • Predicting climate‑driven changes