Fisheries: Dynamics, Overfishing & Food-Web Consequences

Fishery
- Business/enterprise centered on harvesting one specific aquatic species (can be a fin-fish, shellfish, squid, etc.).
- Unit of analysis = single species; all curves/metrics in this lecture refer to that one species.
Fishing Industry
- Broader umbrella term covering all commercially harvested species.
Catch (or Landings)
- Primary success metric.
- Expressed as either
- Number of individuals caught, or
- Biomass (pounds, tons).
- Common formal metric: \text{CPUE}=\frac{\text{Catch}}{\text{Unit\;Time}}, usually annual.

Formal, science-based management practices only became widespread in the last ~50 years.
Aim: prevent species extinction, preserve ecosystem integrity, and sustain economic vitality.

X-axis = time since exploitation began (could be a few years to multiple decades).
Y-axis = catch (number or biomass).
Typical pattern observed for swordfish, cod, tuna, striped bass, clams, shrimp, squid, etc.:
1. Low initial catch – anglers are still learning effective gear, locations, seasons.
2. Rapid growth phase – technological & tactical improvements; focus on largest fish; catch rises steeply.
3. Plateau – maximum sustainable catch approached; resource limits start to appear.
4. Decline & Crash – catch drops sharply; effort no longer profitable ⇒ fishery closes/shifts.

Y-axis = probability of death.
X-axis = age of individuals.
General pattern (applies to salmon, trout, striped bass, humans!):
- High juvenile mortality (larvae/egg stage): small, immobile, weak defenses.
- Lower mortality during prime adult years: increased size, better feeding & defense skills.
- Rising mortality in senescence: old, slow, disease-prone.
Reproductive age occurs in the mid-section of the curve; beyond that lies expected lifespan.

X-axis = body size (closely tied to age).
Y-axis = % of population at that size.
Shaped by the mortality curve:
- Few very small fish (most die young).
- Majority in intermediate “prime” sizes (low mortality, high survival).
- Few very large, old individuals (die of age, predation, or fishing).

Stage 1 – Targeting Giants
- Harvesters select the largest/oldest fish for maximum profit per effort.
- Quickly erodes the extreme right tail of the size distribution.
Stage 2 – Targeting Prime Adults
- After giants become scarce, effort shifts to middle-aged, prime-size fish (still lucrative).
- Population keeps shrinking; size structure compresses.
Stage 3 – Targeting Sub-Adults / Pre-Reproductive Fish
- Fishers now capture individuals before they reach reproductive age.
- Removes future spawners ⇒ recruitment failure ⇒ fishery collapse.

Visual link back to curves:
- Stage 1 overlaps high age, low population fraction.
- Stage 2 overlaps peak of size-distribution.
- Stage 3 intrudes into the left side of reproductive window, dooming population recovery.

Fishers rarely abandon the profession; instead they switch target species.
At the industry level this produces a series of overlapping fisheries curves:
- e.g., Swordfish → Tuna → Cod → Striped Bass.
Consequence = Fishing Down the Food Web
- Sequentially removing higher-trophic-level predators, then mid-level species, effectively shortening food chains.
- Alters ecosystem structure & function; to be explored in subsequent lectures.

Economic: jobs, food security, export revenues depend on sustainable yields.
Ecological: trophic cascades, biodiversity loss, habitat alteration.
Ethical: stewardship duty, intergenerational equity, preventing extinctions.

Understanding species-specific physiology & population dynamics is prerequisite to setting quotas, size limits, closed seasons, gear restrictions.
Real-time monitoring of \text{CPUE}, size structure, and reproductive metrics can signal approach to Stage 3 and avert collapse.
Multi-species perspective essential; single-species management may still drive “fishing down” unless whole food-web effects are incorporated.