Impacts of Overfishing
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AP Environmental Science › Impacts of Overfishing
A marine ecosystem loses many large predatory fish; scientists note reduced genetic diversity in remaining populations. What is a likely driver?
Population bottlenecks from overharvest reduce effective population size, increasing inbreeding and loss of alleles through genetic drift.
Reduced predation increases population size, which necessarily decreases genetic diversity because more individuals share fewer alleles.
Higher mutation rates caused by fishing gear introduce harmful alleles, which increases genetic diversity and stabilizes populations.
Increased gene flow from distant populations always decreases genetic diversity by homogenizing alleles across the entire species range.
Explanation
Overfishing creates population bottlenecks, reducing effective population size and genetic diversity through drift and inbreeding. This makes species less adaptable to changes like disease or climate. Loss of alleles can impair long-term survival. Monitoring genetic health is key in management. Protecting populations prevents these genetic impacts. This highlights overfishing's effects beyond abundance.
A coastal community switches from longlines to gillnets; seabird mortality increases. Which management action best reduces this bycatch?
Remove all catch limits because higher fish abundance will distract seabirds from nets and lower entanglement rates.
Require acoustic pingers and net visibility modifications, and restrict fishing during peak seabird foraging times to reduce entanglement.
Increase mesh size so more seabirds can pass through, which also increases catch efficiency for target fish.
Add fertilizer to coastal waters to increase plankton blooms, moving seabirds offshore and away from fishing activity.
Explanation
Gillnets can entangle seabirds as they dive for fish, increasing mortality in foraging areas. Management actions like acoustic pingers emit sounds to deter birds, while net modifications improve visibility. Restricting fishing during peak foraging times further reduces risks. These measures help minimize bycatch without halting fisheries entirely. They demonstrate adaptive strategies to address overfishing's non-target effects. Protecting seabirds supports overall marine biodiversity and ecosystem health.
A fishery targets the largest groupers first; within a decade, average size and age at maturity decline. What explains this change?
Carrying capacity rises after fishing, so density-dependent competition increases and forces individuals to delay reproduction until larger.
Genetic drift increases body size because population bottlenecks always favor larger individuals that can store more energy for migration.
Artificial selection favors earlier maturation at smaller sizes because large, late-maturing individuals are removed before reproducing as often.
Mutualism with cleaner fish is disrupted, directly causing groupers to mature earlier through reduced parasite removal rates.
Explanation
Overfishing that selectively targets the largest individuals in a population, such as groupers, can drive evolutionary changes through artificial selection. Larger, later-maturing fish are removed before they can reproduce multiple times, leaving smaller, earlier-maturing individuals to pass on their genes. Over generations, this shifts the population toward smaller average sizes and younger ages at maturity. This change reduces the overall productivity and resilience of the fish stock, as smaller fish produce fewer offspring. Such fishing-induced evolution highlights the long-term genetic impacts of overfishing beyond just population decline. Effective management, like size limits, can help mitigate these effects and preserve natural traits.
A coastal fishery sets a minimum size limit for lobster harvest. Which rationale best supports this rule?
Minimum size limits prevent habitat destruction by stopping traps from contacting the seafloor and crushing benthic organisms.
Minimum size limits increase biodiversity by ensuring predators only eat small lobsters, preventing trophic cascades in kelp forests.
Minimum size limits reduce ocean acidification because larger lobsters store more carbon in shells, raising seawater pH.
Allowing individuals to reproduce at least once before harvest increases recruitment and helps maintain a sustainable breeding population.
Explanation
Minimum size limits ensure fish reach reproductive maturity before harvest, allowing at least one spawning cycle. This boosts recruitment and sustains populations. It counters size-selective fishing pressures. Such rules are vital for species with high juvenile mortality. They promote larger, more fecund adults in the stock. This strategy mitigates overfishing by preserving breeding potential.
A coastal ecosystem loses large predatory fish; scientists observe smaller average prey size and earlier prey reproduction. Which pressure likely drove this?
Increased prey density and competition after predator release favors earlier reproduction and smaller size, a density-dependent life history shift.
Overfishing increases mutation rate in prey, guaranteeing smaller size and earlier reproduction regardless of ecological conditions.
Predator removal increases salinity, which directly shortens prey lifespans and forces earlier reproduction in all marine species.
Reduced sunlight after predator removal decreases photosynthesis, forcing prey fish to mature earlier due to limited oxygen production.
Explanation
Removing predators increases prey density, triggering density-dependent shifts toward smaller size and earlier reproduction to cope with competition. This is an evolutionary response to overfishing pressures. It can reduce overall productivity. Protecting predators prevents such changes. Ecosystems adapt but at a cost to resilience. Overfishing alters life histories profoundly.
A fishery uses cyanide to stun reef fish for the aquarium trade; coral and fish communities decline. What is the primary environmental concern?
Cyanide increases dissolved oxygen, causing oxidative stress only in predators and leaving reef communities otherwise unaffected.
Cyanide acts as a fertilizer, increasing algal growth and thereby increasing reef fish abundance and ecosystem stability.
Cyanide reduces ocean temperature locally, leading to coral bleaching because corals require warmer water to survive.
Chemical damage kills non-target organisms and corals, degrading habitat and reducing biodiversity beyond the targeted fish removal.
Explanation
Using cyanide in fishing stuns target fish but causes widespread chemical damage, killing non-target organisms and degrading habitats like corals. This reduces biodiversity and disrupts reef ecosystems beyond just the removed fish. Corals, essential for reef structure, suffer mortality, leading to habitat loss for many species. The practice highlights the collateral damage of destructive fishing methods. Sustainable alternatives are needed to protect marine life. Overfishing via such methods accelerates ecosystem decline.
A coastal ecosystem shows reduced carbon storage in seagrass meadows after overfishing predatory fish. Which pathway best explains this change?
Overfishing increases carbonate precipitation, converting organic carbon into limestone and permanently increasing seagrass carbon storage.
Seagrass declines because fishing reduces atmospheric carbon dioxide, limiting photosynthesis and causing global seagrass loss.
Trophic cascade increases grazers that damage seagrass, reducing biomass and sediment carbon burial, lowering long‑term blue carbon storage.
Predator removal increases seagrass photosynthesis by reducing shade from fish schools, increasing carbon storage in sediments.
Explanation
Overfishing predators triggers trophic cascades, increasing grazer populations that damage seagrass, reducing biomass and carbon burial in sediments. This lowers blue carbon storage, affecting climate regulation. Seagrasses are vital for coastal carbon sequestration. Protecting predators maintains balance. Restoration efforts can reverse damage. Overfishing thus impacts carbon cycles indirectly.
A government bans fishing during peak spawning months for a depleted salmon run. Which principle supports this regulation?
Protecting reproductive individuals increases recruitment and future population size by allowing more adults to spawn successfully.
Spawning bans reduce river flow, preventing eggs from drifting and increasing survival by keeping them stationary on gravel.
Closed seasons eliminate predators like bears and seals, which depend on salmon and therefore must decline without access.
Closed seasons increase genetic mutations, which quickly creates more salmon and offsets fishing mortality within one generation.
Explanation
Spawning seasons are critical for fish reproduction, and banning fishing then protects breeding adults. This increases egg production and recruitment, aiding population recovery. It prevents disruption of spawning behaviors and aggregations. Such temporal closures are a common tool in fishery management. They help maintain genetic diversity and stock structure. This approach mitigates overfishing by focusing on reproductive success.
A fishery experiences high bycatch of dolphins in purse seines; a new rule requires setting nets only when dolphins are absent. What is the goal?
Eliminate the need for quotas because protecting dolphins guarantees tuna stocks cannot be overfished under any conditions.
Increase ocean productivity by forcing dolphins to migrate, which increases nutrient upwelling and raises fish biomass.
Increase dolphin harvest to reduce competition with fishers for tuna, thereby increasing tuna abundance and catches.
Reduce incidental mortality of non-target species to protect populations and maintain ecosystem integrity while continuing target harvest.
Explanation
Bycatch of non-target species like dolphins in fisheries increases mortality and threatens populations, disrupting ecosystem integrity. Rules requiring nets to be set only when dolphins are absent aim to reduce incidental deaths while allowing target harvest. This protects biodiversity without halting fishing. Effective bycatch mitigation supports sustainability. Overfishing exacerbates bycatch issues through intensified effort. Such measures highlight the need for selective practices.
A fishery’s target species is a slow-growing, late-maturing shark. Compared with sardines, it is more vulnerable to overfishing because it has
a producer trophic role, so harvesting directly reduces primary productivity and collapses the entire ecosystem immediately.
a dependence on freshwater, so any harvest in marine waters causes instant mortality from osmotic shock.
high fecundity and short lifespan, so individuals reproduce rapidly and are removed before contributing to the gene pool.
low reproductive rate and long generation time, so populations recover slowly after harvest reduces breeding adults.
Explanation
Species with low reproductive rates and long generation times, like sharks, recover slowly from overfishing. They produce few offspring, making populations vulnerable to harvest. In contrast, fast-reproducing species like sardines rebound quicker. Life history traits determine susceptibility. Management must account for these differences. Protecting slow-growers prevents extinction risks.