AP Syllabus focus:
‘Overfishing can make some fish species extremely scarce, which can reduce biodiversity in aquatic ecosystems.’
Overfishing occurs when fish are removed from a population faster than they can reproduce. This shifts population size, age structure, and genetic diversity, sometimes causing long-term declines or sudden collapses.
Core idea: harvest rate vs. population growth
Fish populations can grow when reproduction and survival replace losses, but fishing adds an extra source of mortality. When total deaths exceed births, populations decline.
Overfishing: harvesting fish at a rate that exceeds the population’s ability to replace individuals through reproduction and growth.
A key APES lens is comparing population growth capacity to harvest pressure, especially for species that mature late or produce few offspring.
How overfishing drives fish scarcity
Direct reduction in population size
High fishing mortality removes individuals faster than natural processes can compensate.
Repeated heavy harvest can push a stock below a level where it can recover quickly, creating extremely scarce populations.
Loss of breeding adults (spawning stock)
Many fisheries preferentially catch larger fish, which are often the most fertile.
Removing large, mature individuals reduces spawning biomass (total mass of reproductive adults).
Fewer eggs/larvae are produced, lowering recruitment (young fish entering the breeding population).
Time-series data for North Sea cod showing recruitment (bars) alongside spawning stock biomass (line) across multiple decades. The figure illustrates the core fisheries concept that the reproductive adult population (spawning stock) is closely tied to the system’s capacity to generate new individuals (recruitment), even though recruitment can fluctuate strongly year to year. Source
Truncated age and size structure
Overfishing commonly shifts populations toward younger, smaller fish.
Fewer older age classes means fewer consistent spawners across variable years.
Populations become less resilient to environmental swings (e.g., temperature changes affecting larval survival).
Slower recovery for “slow life-history” species
Species with these traits are especially vulnerable:
Late maturity
Long lifespan
Low fecundity (few offspring)
Top-predator roles Heavy harvest can cause prolonged scarcity because replacement rates are inherently low.
Population dynamics: why declines can accelerate
Fish population growth is often approximated by logistic growth: growth is fastest at intermediate population sizes and slows near carrying capacity, but also becomes limited when populations get too small.
A logistic (S-shaped) population growth curve showing how population size increases rapidly at intermediate sizes but levels off as it approaches carrying capacity (). This diagram supports interpreting the logistic model as density-dependent growth rather than unlimited exponential increase. Source
= population size (individuals)
= intrinsic growth rate (per time)
= carrying capacity (individuals)
When overfishing reduces , growth may not “bounce back” if:
the remaining breeding population is too small,
individuals have difficulty finding mates (a low-density limitation),
or age structure has been depleted.
Selective harvest and evolutionary change
Because fishing often targets specific traits (like large body size), it can act as artificial selection:
Earlier maturation at smaller sizes can become more common.
Average size may decline over generations. This can reduce long-term yield potential and alter population productivity even if fishing pressure later decreases.
Biodiversity impacts through population loss
The specification emphasises that scarcity can reduce biodiversity in aquatic ecosystems. Overfishing contributes by:
Driving local extirpations (loss of a species from an area).
Increasing the risk of regional or global extinction for highly targeted or slow-reproducing species.
Reducing genetic diversity within a species as population size shrinks, lowering adaptability to disease or environmental change.
Common signs that a fish population is being overfished
Catch sizes decline despite similar effort.
Average fish size decreases over time.
More juveniles appear in catches due to scarcity of adults.
Formerly common species become rare in landings and surveys.
FAQ
They combine long-term catch records with fishery-independent surveys (e.g., trawl or acoustic surveys).
Indicators often include declining spawning biomass and consistently weak recruitment across multiple years.
Possible reasons include very low breeding density, loss of older spawners, altered genetic traits (earlier/smaller maturation), or environmental conditions that no longer favour successful recruitment.
Recruitment is the number of young that survive to join the fishable or breeding population.
It varies with currents, temperature, food availability for larvae, and predation, so heavy fishing can be especially risky in poor recruitment years.
No. Species with fast growth and early maturity can rebound more quickly, while sharks, many deep-sea fish, and other slow-growing species can become scarce with relatively low fishing mortality.
Stable catch can mask depletion if effort rises or fishers shift to smaller/younger individuals.
A declining mean size often indicates fewer older age classes and reduced spawning potential, increasing the chance of future scarcity.
Practice Questions
Describe two ways overfishing can cause a fish species to become extremely scarce. (2 marks)
Explains that harvest exceeds reproduction/replacement, causing population decline (1).
Explains loss of breeding adults/age structure leading to reduced recruitment and faster decline (1).
Explain how overfishing can reduce biodiversity in an aquatic ecosystem by altering fish populations. Include effects on population size, breeding capacity, and genetic diversity. (6 marks)
States that overfishing reduces population size and can cause severe scarcity (1).
Links removal of mature adults to reduced spawning biomass and fewer offspring (1).
Describes truncated age/size structure and reduced resilience/recovery (1).
Explains local extirpation or increased extinction risk lowering species richness (1).
Explains reduced genetic diversity in small populations (1).
Links reduced genetic diversity to lower adaptability and higher vulnerability to change (1).
