UNDERSTANDING THE DELICATE BALANCE OF PREDATOR-PREY RELATIONSHIPS WHEN CONFRONTED WITH OVERFISHING
When I think about predator-prey relationships I often think of the natural balance that exists in any given ecosystem. Predators and prey use a wide spectrum of adaptive strategies to cope with their interactions, such as morphological and behavioral adaptations. Extermination of a species in a naturally balanced ecosystem would not occur as a result of predator-prey interactions. However, high-level predators such as sharks and groupers are being depleted in the oceans worldwide. Reports have shown that the more people there are in a nation, the fewer large fish exist along their reefs. The demand for seafood will increase with an increase in human population. The higher the demand, the higher the fishing effort. Fishermen will automatically go after the biggest fish first but shift to smaller individuals once the bigger ones become depleted. Given that about half the world’s populations live near coastlines, and that the world population is still on the increase, demands for ocean-derived protein continues to increase. This brings us to the conclusion that overfishing is the most likely reason for the disappearance of large predatory fish. Overfishing particularly of predatory species, exacerbated by destructive fishing practices, such as the use of longlines and gillnets, complicated by pollution and global climate has led to changes in structure and function of marine food webs over time.
Perhaps if you have lived in a coastal community, some of the tales you heard in the company of old fishermen is big fish stories. With the disappearance of large predatory fish, they are now nothing but just old tales. More and more smaller fish are being captured instead and a typical overfished ecosystem will lack apex predators. A case in point, are the reefs in East Africa, sharks have become extremely rare in these reefs! A study by Coastal Oceans Research and Development- Indian Ocean- (CORDIO) East Africa underwater surveys on reefs in Tanzania, Comoros, Madagascar and Mozambique to assess population densities of large reef associated fish recorded no sharks except for one site in Cabo Delgado, northern Mozambique. In Kenya, one of the major changes in the state of the coral reefs is the dramatic decline in the number and individual size of finfish and very rare shark sightings. Fishing activities have reduced fish populations in studied reefs causing a severe decline in the species richness of the fished lagoons.
Why should we care about the disappearance of large predatory fish?
There are several reasons why. Removal of high-level predators’ results in a degraded marine ecosystem. Large predatory fish such as sharks and groupers are vitally important in marine food webs as they keep numbers of their prey in check by directly limiting the populations. To explain this in a much simpler way, let’s look at it this way. Sharks are the large predatory fish in this case and they are with no doubt big, they are the apex predators, sitting at the very top of the food web. This is because they naturally have very few natural predators. As apex predators, sharks will feed on big fish that occur below them such as groupers or rays, and their prey will feed on the smaller fish in relation to their size, such as parrot fish. The predator-prey relationship trickles down the food web in a similar manner. In turn this limits the populations of the prey species of those animals and so on. It’s a whole web! It is also important to note that the diet of most apex predators is varied, hence sharks will not depend on a single prey, but they will have a number of prey species they can switch to when populations of another prey are low. To top it all, apex predators remove the sick and weak individuals from their prey populations.
Imagine an ocean without sharks
So let’s take a minute to imagine an ocean without sharks. All sharks have been fished from the ocean, we would automatically think groupers will increase in numbers, right? Of which we might think that’s still better for the ecosystem, right, but that’s wrong. As one scientist clearly puts it ‘You can’t replace a 10-foot shark with a one-foot grouper and expect there to be no effect on reef communities’. By taking sharks out of the coral reef ecosystem, the larger predatory fish, such as groupers, will increase in abundance. A population explosion of groupers would dramatically reduce the groupers prey populations such as herbivores. With less herbivores, macroalgae will overgrow and coral recruitment is inhibited. This shifts the ecosystem to one of algae dominance, affecting the survival of the reef system. In the absence of sharks, even coral and butterflyfish populations will suffer after some time. But then again, this is not always the case. Groupers too are overfished. Groupers are among the highest valued fish in fisheries globally and their population is on the decline. Their life history characteristics of forming aggregations during spawning makes them highly vulnerable to heavy fishing pressure. At the Marine Education Centre, we have recorded spawning aggregations of Epinephelus fuscoguttatus (brown marbled grouper) in Diani Kenya during turtle surveys. However, decline of the brown marbled grouper has been reported on a global-level by at least 30% or more since the 1970s.
A shift in abundance of species eventually occurs over time. From the disappearance of big predators to reduction in medium sized predators, which can also result in an increased number of prey. With more prey than predators in the ecosystem, there is risk for an increased competition. A commonly used example to explain this scenario is the case of overfishing large sharks along the coast of the US, which has led to an increase in rays who no longer have predators to keep their populations in check. The rays prey on scallops and other bivalves. Greater predation on scallops and other bivalves by rays has in return destroyed the scallop fishing industry. In addition to shifts in species abundance and decreased diversity there is degradation of habitats. Still using an increase in ray populations due to a lack of sharks as an example, there will be increased competition within the ray populations. Hungry rays ferociously grazing while hunting for food has the potential of uprooting seagrass at higher rates. Healthy seagrass serves as nursery grounds for fish hence a degradation of seagrass habitat from this behavior can lead to poorer quality nursery grounds for fish. Additionally, disappearance of bivalves can lead to poor water quality. Other than a good source of food for rays, bivalves also act as a filtration system for the ocean. Bivalves feed on phytoplankton that they filter from the water, which helps maintain a high level of water quality. With the decline in scallops, clams and other bivalves, the water quality decreases, which could lead to uncontrolled algal blooms and dead zones, further damaging the ocean ecosystems.
Predators can increase diversity in communities. By preying on competitive dominant species, predators prevent the species from monopolizing a limited resource. A good example is when there is an outburst of sea urchins due to a decrease in their predator population, the result of which is overgrazing by the urchins in a seagrass, algae, or kelp habitat. Kelp forests are home to numerous species of fish and invertebrates and when completely destroyed by sea urchins the result is barren areas completely devoid of life. Urchins however are readily consumed by sea otters in the Northern Pacific Ocean and California, and if urchin numbers are kept low, otters assure that the kelp forest community remains healthy.
As the numbers of large shark decline, the oceans continue to suffer and to some extent these consequences are far more unpredictable and devastating than any of us can fathom because of the complexity of predator-prey interactions! As Thorsten, founder of MLP, scarily puts it, “the most dramatic, and not completely impossible end to the extinction of bigger predatory fish will be a complete collapse of the whole reef ecosystem with only a few surviving species and many other reef fish becoming locally extinct”. Fishing needs to be managed sustainably to ensure that the ocean’s stocks are not depleted. This is why at the Marine Education Centre in Diani Kenya, we are currently developing a sustainable fishing awareness programme. To learn more about this project, check out a blog written by Marine Education Centre marine biologists on “a guide to source sustainable fish and seafood” at https://ceskenya.org/sustainable-fish-and-seafood
Fatma Manyenze, based in Kenya, joined the Marine Life Protectors team in July, providing online content of marine conservation in Kenya. Her primary work is with the Conservation Education Society, where she is involved in marine research and conservation Education. To check out their work, please visit www.ceskenya.org.
For more simple illustrative graphics to better understand predator-prey relationships, check out the Marine Life Protectors Facebook Page.