Marine mammal and fisheries interactions

Report: Net Loss: Are Whales a Threat to Fisheries?

Report contents:

  • Introduction
  • Fishing & fishery management
  • By catch, ghost fishing & illegal fishing
  • Pollution, global warming & habitat destruction
  • The interaction between whales and fisheries
  • Summary & conclusions
  • References

Download a printable version of the ‘Net Loss: Are Whales a Threat to Fisheries?’ report.


The Food & Agriculture Organisation of the United Nations (FAO, 2000) calculates that 75% of the world’s fishery resources are fully or over exploited. Many fisheries have collapsed through poor management and overexploitation. However, consumer demand for seafood is constantly increasing and fishery management committees are desperately searching for new methods to increase fishery output from our oceans. Whales and other marine mammals have been blamed for reducing fish stocks by direct consumption of target species or their prey. However, there is no scientific evidence that an increase in whales has ever decreased fishery production (IFAW, 2001). In spite of this, some governments have proposed culling marine mammals to increase fishery output.

Fishing & fishery management

When people first started to catch fish they were an abundant source of food harvested using simple inefficient catching methods. That situation has changed rapidly in the last two centuries. Abundant Humans now use highly advanced technology to rip out vast quantities of fish from scarce and dwindling stocks (Kemf et al., 1996). The increase in demand and the improvements in technology have not been matched by improvements in methods for estimating fishery abundance or managing the harvest. The result has been massive overfishing and the collapse of some fisheries. Further increases in output to match increasing demand are no longer possible using the present system.

Traditional fishery assessment involves taking small samples of a species and analysing them to extrapolate information from the whole population. Sample sizes are minute compared to whole populations and would be considered statistically invalid in a land-based experiment under controlled conditions. In addition, the models that are used to estimate population size take into account very few of the factors affecting the abundance of fish stocks, most of which we simply do not know and are very unpredictable. It is as hit and miss as predicting our daily weather and the computers used are very much smaller!

Population size and estimated sustainable yield are calculated by extrapolation from these dubious figures. Fishery Managers and organisations such as the International Council for the Exploitation of the Sea (ICES) use the figures to suggest sustainable quotas and Total Allowable Catches adding margins for error in the calculations. Unfortunately, politicians adjust the figures upwards in order to keep their voters happy and take into account the social and immediate financial impact of reduced catch allowances on fishing communities as well as international treaty obligations. The result is that catches are invariably too high from a biological point of view and fisheries decline. Stocks that are managed by a number of different countries are particularly at risk.

By catch, ghost fishing & illegal fishing

Initial high catches are further exacerbated by other factors, which are simply ignored in most population abundance estimates. Whenever a fish species is targeted, there are invariably other non-target species caught in the net as fishing is based partly on luck. These other species may have a value and be retained and sold.

However, 5-12 million tonnes annually of unsalable fish are thrown back into the sea dead as by catch (Bricklemeyer et al., 1989). These fish include prey items for other species as well as fish that are highly valuable but simply over the quota allowance for that vessel. In addition, no allowance is made for the destruction caused by discarded nets or nets that have broken away and continue, ‘ghost fishing’ as they drift around the oceans. It is estimated that around 30 million tonnes of fishery product are lost each year through by catch and ghost fishing (Kaschner & Pauly, 2004). A further factor in reducing fish stocks is the massive trade in illegal fish products caught by international poachers ”pirate” fishing under flag’s of convenience and selling through unscrupulous markets.

Pollution, global warming & habitat destruction

Pesticides, excess nutrients and just about everything discarded on land ends up in the sea through run off into our rivers. In addition, a good proportion of the particles and chemicals in the air will end up in the vast expanse of the oceans. The oceans have also been a traditional dumping ground for unwanted substances that we assume will be diluted in their supposedly infinite mass. This pollution together with alterations to the marine environment through global warming put further pressure on fish stocks. Loss of habitat, particularly estuarine and coastal habitats that are the nursery areas for many fish species, is a further factor driving fish stocks down. The affects of these factors are mainly on the prey organisms particularly algae that drive the whole biological production process, but they also affect both fish and whale stocks through decreased abundance of prey, increased susceptibility to disease, impaired reproductive capacity and cancers (Perry et al., 2001).

The interaction between whales and fisheries

The complex interactions between predators and their prey are known as food webs and they change with seasons and weather patterns and are impossible to model accurately with present technology due to the vast number of interconnected factors. Whales and other marine mammals are at the top of the food chain and are therefore highly vulnerable to changes at all the lower levels. For example a small reduction in algal biomass will have a greater impact on each higher level in the chain due to losses at each stage. It is also likely that the impact of one predator may limit the impact of another. Therefore, the removal of one predator may result in the increase of another. Species exploited by Man are often the main prey species of other predators such as seabirds and larger fish. Scientists have been unable to predict the results of a marine mammal cull in any particular fishery despite trying for over twenty years and one study actually concluded that a seal cull might reduce fishery output (IFAW, 2001).

The prey of whales is mostly made up of species that Humans do not exploit in areas that are not fished e.g. squid in the deep sea and plankton in Arctic and Antarctic waters (Kaschner & Pauly, 2004). There are relatively few small areas where whales interact directly with exploited fisheries and in these areas it is highly likely that overexploitation of the fishery will impact the survival of the whales rather than vice versa. Removing whales from the food web would not necessarily result in an increase in their prey or of an exploitable alternative predator and would not necessarily affect areas that are commercially exploited.

In fact, whales and fish have coexisted in balance for millennia. Before whales were over-exploited by man, they were present in far higher numbers than they are today and yet fish numbers were also far higher. Today, many whale populations remain far below their pre-exploitation abundance (possibly as low as 99% of pre-exploitation biomass) and it is very hard to see how they can be having any impact at all on fish stocks at their present levels (Kaschner & Pauly, 2004).

Summary & conclusions

The mismanagement of global fisheries has led to the collapse of many fish stocks, which are now unable to supply increasing demand for seafood. Most fisheries are fully exploited or overexploited as a result of a long history of mismanagement due to the inability of scientists to accurately assess populations together with other negative impacts such as illegal fishing global warming and habitat destruction. It has been suggested that marine mammals are contributing to the pressure on fish stocks and that they should be culled. However, it is likely that overfishing is contributing to the decline in whale populations rather than the other way around. If global fishery output is to be increased then new methods of fish production must be developed in tandem with better wild fishery assessment and management techniques.


Bricklemeyer, E., Ludicello, S. and Hartman, H, 1989. Discarded catch in the US commercial fisheries. In: W.J. Chandler (ed), Audobon Wildlife Report, 1989/90, Academic Press, San Diego, USA.
Food & Agriculture Organisaton of the United Nations, 2000. The State of World Fisheries and Aquaculture 2000, Rome Italy.

International Fund for Animal Welfare, 2001. Whales and Fisheries. Yarmouth Port, Maryland, USA.
Kaschner, K. and Pauly, D., 2004. Competition between marine mammals and fisheries: Food for thought.
Kemf, E., Sutton, M. and Wilson, A., 1996. Marine fishes in the wild. WWF Species Status report, Gland, Switzerland.

Perry, C., Clark, E. and von Post, C., 2001. Swimming against the tide: Environmental threats to the world’s whales and dolphins. Environmental Investigation Agency, London, UK

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