Slow reproduction

All life on earth has evolved for survival. Whether a living organism is in the form of a bacterium or a tree, a mushroom or a shark, the way it lives has been gradually tuned and adapted to suit conditions at the time. When conditions change, the organism changes with them.

The way that sharks and rays live has been very successful. For 420 million years, sharks of some sort have been on earth. Their survival strategy is to live a long time, grow slowly, get big, mature late in life and have fewer offspring – but they invest a lot of time and energy in the pups they do produce, which means that each individual stands a good chance of surviving.

Sharks in general reproduce slowly, but even within this group there is variation. The dogfish shark is one of the slower species, with a pregnancy that can last for up to two years, whereas the blue shark has on average 25–50 pups a year (although a litter size of 134 pups has been recorded).

This strategy has worked well for sharks, but it is starkly different to the lifestyle of animals that have evolved to live fast and die young. Sardines, for example, live for a short time, grow quickly, stay small, mature early and have many offspring in which they invest little. This has worked well for them too.

However, when a species is put under pressure from threats such as fishing, habitat loss, climate change and persecution, those that live fast and die young, like sardines, are more likely to survive than species that live slow and die old, like sharks. Why is this? Sharks have always been high on the food chain, which means that they are not ‘designed’ to handle high levels of predation – by other marine species or by humans. When their numbers decline, they are slow to recover. If the threat continues and there is no time for recovery, the numbers are likely to continue to dwindle.

Left alone, sharks have been able to persist in a changing world for millions of years. But now they are being pushed to their limit. Their biological characteristics are such that they can’t support losses from large-scale fishing – or from other causes, particularly when those causes are combined, as they are now. If we want to ensure that sharks are not pushed to their biological limit, it is important to understand their lifestyle characteristics – and that is why the Save Our Seas Foundation funds research into shark and ray reproduction. Sharks need help from us to ensure that they aren’t pushed to their biological limit. Research, policy development, management measures, effective enforcement and awareness all contribute to achieving this goal.