Threat 2: Predator loss

Overview

Predator loss releases prey populations from both the pressure and risk of predation. In both marine and terrestrial ecosystems, predator removal can cause a potentially irreversible cascade of complex knock-on effects that destabilise food-webs and the marine environment as a whole.

Predators, such as sharks, tuna and billfish, have substantial influence on the structure and function of ecological systems, both directly by regulating prey populations and indirectly through the interactions between their prey and other members of the ecosystem. These indirect effects that occur further down the food-web are referred to as trophic cascades.

Marine predators are experiencing cataclysmic declines worldwide: it is estimated that in excess of 90% of all marine predators have already been lost from the oceans, including tuna, billfish, swordfish and sharks.^{1 2} This is almost entirely due to overfishing, but other factors also contribute. For instance it is estimated that 96.1% of all threats posed to shark populations stem from fishing (57.9% by-catch, 31.7% directed commercial fishing, 5.8% artisanal and 0.7% recreational), with habitat destruction and pollution comprising 2.9% and 0.4% of threats respectively.³ Soaring demand for sharks in Asian markets is accelerating these declines in shark populations, and it remains largely uncertain just how severe the knock on effects will be.⁴

“Soaring demand for sharks in Asian markets is accelerating these declines in shark populations.

Sharks

Sharks are found in nearly all ocean habitats⁵ and form many connections within food-webs due to their high mobility and varied diets.⁶ Something that makes sharks of potentially greater influence on food-web dynamics than other marine predators is their ability to consume larger prey than bony fish of similar size (since most sharks can extend their jaws and ‘saw’ with their teeth), and large megafauna (eg. turtles, marine mammals and other elasmobranchs) often have sharks as their major or only predators.⁷

There is a considerable lack of accurate fisheries data, particularly for non-commercial species landed as by-catch, and so much of our understanding of the top-down impacts of marine predator loss remains limited to the speculations of mathematical modelling. The cascading influence of predators in terrestrial ecosystems is better understood; for example the removal of wolves from Yellowstone National Park, USA, caused elk populations to boom and overgraze the land, devastating the habitats and food source of many other species (fortunately this is now reversing, after wolves have been reintroduced).⁸ However, marine ecosystems are by comparison far more difficult to access and study in detail, and so clear documentation of marine trophic cascades remains sparse.

“A severe decline in sharks off the coast of North Carolina, USA, has led to the complete collapse of a century-old bay scalloped fishery.

But there are a few examples of note. For instance severe declines in sharks – blacktips, in particular – off the coast of North Carolina, USA, has led to the complete collapse of a century-old bay scalloped fishery that supported the local community.¹ The removal of their sharks caused their prey, the cownose ray, to soar in numbers and expand into areas that were previously too risky to forage in. The rays decimated the scallop populations in the area to a point beyond which, combined with ongoing fishing pressure, they may not be able to recover; perhaps permanently altering the ecosystem and severely affecting local livelihoods.

Further evidence of cascades induced by predator loss comes from the Kwa-Zulu Natal shark netting programme in South Africa. Here, whilst the catch rates of large sharks declined over 1956-1976, fishing tournaments revealed growing numbers of smaller sharks and a subsequent decline in bony fish.⁹ It is estimated that during this period, due to the decline of large sharks, between 419,000 and 2.8 million small sharks escaped predation.^{9 10} Beyond 1977, the smaller sharks started to decline from increased targeting and at the same time catches of rays and bony fish started to increase, presumably due to the reduced predation and competition from the sharks.¹¹

Even if predators only consume particular types of prey infrequently, the risk for the prey can be sufficient to significantly change their behaviour. For instance tiger sharks only seasonally present in Shark Bay, Australia, cause their prey (turtles, dugongs, dolphins) to give up foraging opportunities to enhance safety.¹² Similarly the presence of sleeper sharks in Alaska alters the diving behaviour of seals, despite the seals making up only a minor portion of the sharks’ diet.¹³

Interestingly, large predators in the open ocean are thought to be less pivotal in governing particular trophic cascades due to their ranging behaviour and varied diet, preventing sustained predation on any particular species in any particular location,¹⁴ and if other large predators are present, they may be able to substitute the role of another with minimal disruption of food-web dynamics.¹⁵ Essentially, the existence of predator diversity may dampen cascade effects. Consequently it is thought that the influence of predator loss can be more pronounced in coastal and seabed fisheries, and in particular where megafauna are targeted as prey.

Nonetheless the full extent of potential and already occurring trophic cascades triggered by predator loss remains unknown, warranting further study amid growing concerns that cascades could be widespread and exacerbating declines in both commercial and non-commercial fish species.

Remedies

Here the solution is intrinsically linked, and directly comparable to, those proposed for overfishing:

• Sustainable fisheries management is required, with enforced, scientifically-informed quotas.
• More selective fishing gear, as much of the problem is by-catch.
• Consumers exercising their purchasing power to favour sustainable fish stocks, and avoiding shark-fin soup and other shark-derived products.

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