Sharks have evolved a reproductive strategy vastly different from most other fish species. Unlike many fish that produce numerous offspring, sharks employ a strategy of late maturity and low reproductive output. This means they reach sexual maturity late in life and produce relatively few offspring compared to other marine species. The combination of late maturity and low offspring production poses significant challenges to shark conservation efforts. Unlike species with rapid reproduction rates, sharks cannot quickly replenish their populations in the face of threats such as overfishing, habitat degradation, and climate change.
Sharks have been roaming the oceans for over 400 million years. They first appeared in the fossil record during the late Ordovician period, around 450 million years ago, and have since evolved into a diverse array of species occupying various ecological niches. Throughout their long history, sharks have adapted to changing environmental conditions, survived mass extinctions, and persisted as one of the ocean's most successful predators. Sharks' evolutionary strategy worked perfectly for millions of years, allowing them to thrive in diverse marine environments and maintain their status as apex predators. Their strategy remained largely unchanged for millions of years, a testament to its effectiveness in the evolutionary arms race of the oceans. However, now sharks face a new set of challenges unlike any they've encountered before.
Take, for example, the white shark (Carcharodon carcharias), one of the most iconic apex predators of the ocean. Female great whites reach sexual maturity at around 30 years old, after which they only give birth every two to three years to 2-15 pups. This late maturity ensures that sharks have enough time to grow and develop before investing energy into reproduction. In what aspects is this evolutionary adaptation favorable to sharks' survival and on contrary, why is it dangerous for them - especially in current conditions, characterized by challenges like habitat destruction and overfishing?
Advantages of Sharks' Reproductive Strategy
Sharks have evolved their unique reproductive strategy of late maturity and low reproductive output for several reasons, each with its own set of pros and cons. Let's first look into the advantages of the reproductive strategy employed by sharks. By reaching sexual maturity later in life, sharks have the opportunity to grow to substantial sizes and develop strong survival skills before investing energy into reproduction. This longevity allows them to occupy their ecological niche effectively and maintain stability within marine ecosystems. Sharks also invest significant energy into each offspring, resulting in higher-quality individuals. Their gestation periods last from 5 months to 3 years, and the avarage is between 9-12 months. Also, unlike species that produce numerous offspring with lower chances of survival, sharks produce fewer offspring but invest more resources into their development. This increases the likelihood of offspring survival and contributes to the overall health of shark populations. The slow reproductive rate of sharks helps maintain stable population sizes over time. While fluctuations in environmental conditions or prey availability may impact population numbers, sharks are less susceptible to rapid population declines compared to species with faster reproductive rates. By prioritizing individual growth and survival over rapid reproduction, sharks have adapted to thrive in diverse marine environments and maintain their status as apex predators for millions of years.
Challenges of Sharks' Reproductive Strategy
This evolutionary adaptation, while advantageous in certain aspects, makes sharks highly vulnerable to current challenges and it also prolongs their populations' recovery times. The low reproductive output of sharks makes them especially vulnerable to overfishing. Unlike species that can quickly replenish their populations, sharks cannot sustain high levels of exploitation without facing the risk of population collapse. Humans simply cannot keep fishing sharks in the same quantities and same speed without bringing sharks to the edge of extinction. Due to their slow reproductive rates, shark populations require longer periods to recover from declines compared to species with higher fecundity.
Even with effective conservation measures in place, it may take decades for shark populations to rebound to sustainable levels. This prolonged recovery time increases the risk of extinction for many shark species. In addition, factors such as habitat destruction, pollution, and climate change also lead to population declines. Understanding these challenges is essential for devising effective conservation measures to ensure the survival of these iconic marine predators.
The Role of Fisheries Management in Shark Conservation
As explained above, sharks are extremely vulnerable to overfishing, therefore effective fisheries management is critical for the conservation of shark populations worldwide. By implementing sustainable, selective fishing practices and protecting critical habitats, fisheries management measures can help mitigate the threats facing sharks and promote their long-term survival. Some fish is just more resilient than others and sharks are unfortunately one of the least resilient species when it comes to overfishing.
A good indicator of the resilience to fishing pressure is a population doubling time. What is that? It tells us the projected amount of time that it will take for a given population to double if all fishing ends - depending on the factors discussed in this article, such as number of pups, frequency of pregnancies, etc. For example, minimum population doubling time for thresher sharks, bull sharks, white sharks or blue sharks is more than 14 years. For scalloped hammerhead sharks or silky sharks, it is between 4,5 - 14 years. For comparison, the minimum population for other large predatory fish is much lower: for wahoos, mackerels or snappers it's 1,5 years, for mahi-mahi it's even less.
But effective fisheries management is not only about the choice of targeted species, other factors play a role too. One of the most important ones is the fishing technique - the more selective, the better. Selective fishing techniques create less impact on the ecosystems and produce less bycatch. For example, all of the above-mentioned predatory fish can be targeted by the same fishing technique, baited longlines, which is a non-selective technique. If longlines are used in fisheries, they can result in high levels of bycatch, including unintended catches of sharks, sea turtles, seabirds, and other non-target species. This indiscriminate fishing method not only harms shark populations but also disrupts marine ecosystems and threatens the survival of vulnerable species. Additionally, implementing time and area closures during peak breeding seasons can help protect critical habitats and reduce the risk of overfishing.
No-fishing zones as places of future hope
Especially for the over-exploited shark populations, closures and quotas may not provide adequate protection for vulnerable shark populations, and additional conservation measures such as establishing no-fishing zones (also known as marine protected areas or MPAs) become necessary. No-fishing zones are designated areas where all fishing activities are prohibited or highly restricted, allowing marine ecosystems to recover and thrive without human interference. No-fishing zones safeguard critical habitats, including breeding and nursery grounds, feeding areas, and migration routes, which are essential for the survival and reproduction of sharks and other marine species. By preserving these habitats, no-fishing zones help maintain healthy ecosystems and support the resilience of shark populations. The MPAs can serve as the place of refuge for depleted shark populations to recover and rebuild their numbers. By reducing fishing pressure within these areas, populations can grow and expand, benefiting adjacent areas through natural dispersal and replenishment. Effective MPA, as described by Edgar er al 2014, should have 5 key features: well enforced, no take, isolated by deep water or sand, older than 10 years, and larger than 100 square kilometers. An effective MPA has twice as many large fish, five times more large fish biomass, and fourteen times more shark biomass than in unprotected areas.
Moving forward, it is imperative to prioritize conservation efforts that address the unique reproductive biology of sharks. By implementing science-based management strategies, protecting essential habitats, and establishing well-enforced marine protected areas, we can safeguard the future of these iconic marine predators and maintain the health of our oceans for generations to come.
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