The argument that ‘whales eat fish', and thus threaten commercial fisheries and/or ‘food security', stems from a simplistic correlation between slowly increasing whale numbers (since the IWC's moratorium on commercial whaling came into effect in 1985-86) and declining fish catches. The argument is not borne out by science.
Over-fishing causing declining fish catches
The current worldwide decline in fish stocks is a direct result of overfishing. The UN Food and Agricultural Organisation has found that 35% of 200 major fish resources are over-fished and 25% are being fished to their limit. Studies show that fishing effort may need to be halved to make world fisheries sustainable. The FAO recommends achieving increases in world fish stocks by ensuring fishing levels are sustainable, increasing the age at first capture, increasing net mesh sizes and restricting access to areas important to juvenile fish.
Little overlap between whale diets and fishery target species
The prey species of the baleen whales are often of little commercial interest, and in many cases are unsuitable for human consumption. In the Pacific Ocean, more than 65% of the food consumed by baleen whales, sperm whales and other marine mammals consists of deep-sea squids and deepwater fishes not harvested by humans.
Baleen whales are just one of many higher order species in complex food webs. Research has found that other predatory fish (eg tunas and billfish) are generally much more significant predators of fish than are mammals or birds. That study found that marine mammals feed on 1.1 million tons of pollock annually, while predation by other fish was estimated to be 2.7 million tons, and cannibalism accounted for 7.4 million tons annually.
No conclusive evidence that whales negatively affect fishery yields
While there is undisputed evidence of human overfishing, there is no conclusive evidence that whales negatively affect fishery yields. Most populations of great whales are today a small fraction of their pre-exploitation levels, due to unregulated whaling over the past 200 years. The argument that these comparatively small whale populations could impact on today’s fish resources cannot be substantiated.
No evidence in favour of culling whales
There is no scientific evidence to demonstrate that the culling of whales or other higher order predators improves fishery yields. It is extremely difficult to determine the influence of a particular predator on controlling the available production of a commercially desirable prey. In those ecosystems where detailed modelling studies have been undertaken, research clearly rejects the assumption that reduction of populations of top order marine species improves fisheries yield. In some cases a cull may in fact lead to a lower fishery yield overall because there are a large number of interacting species in marine ecosystems that may also exert a positive or negative predatory influence on a commercial species. Actions such as culling amount to experimental ‘ecosystem engineering’ and are considered potentially fraught with difficulty because of unforeseen long-term ecological consequences.
Any benefit resulting from the reduction of marine mammal predators is far from certain. Marine ecosystems are highly dynamic and complex systems such that the response to a marine mammal cull would likely be highly diffused through the ecosystem and involve many other species. Simple equations — for example, ‘less of predator A equals more of target species B’ — are misleading. There is a great deal of uncertainty regarding interactions in marine ecosystems. They consist of numerous trophic levels with species that interact in multiple ways comprising a complex food web.
Any increase in the number of fish that might result from culling of whales is likely to be eaten by other predators. Others will never be encountered by the fishery and are part of the natural mortality. Therefore any potential benefit is likely to be only a tiny fraction of that originally calculated on the basis of overly-simplistic food-web models using the total food consumed by marine mammals.
Australian Antarctic Magazine article: Why whales don’t threaten fisheries in the South Pacific.