Population control

Fishery yields are both an economic and humanitarian concern, especially since human population growth has given rise to food supply issues. This is why marine mammals that prey on fish are sometimes perceived as competitors to be controlled in order to increase our share of the pie. Fishermen worry about the potential impacts of seals and whales on target fish species, while whaling countries use this argument to underscore the necessity for their harvesting activities. But what role do marine mammals truly play in the reduction of fish stocks? And does controlling their populations really allow for an increase in fishing yields?

Are marine mammals at fault?

Let’s take a look at the case of the collapse of cod stocks in the Gulf of St. Lawrence. Although the government and fishermen disagree on the primary cause of this situation, they do agree that the abundance of seals in the Gulf of St. Lawrence may undermine the recovery of cod stocks. Undebatable scientific truth or scapegoat syndrome? Would reducing harp and gray seal populations really help us reverse the cod situation and see this fish thrive once again? The question is still open to debate, as demonstrated by the Fisheries Resource Council of Canada (FRCC) September 2011 report entitled “Towards Recovered and Sustainable Groundfish Fisheries in Eastern Canada”, which advocates the culling of 140,000 gray seals over a 5-year span, in order to verify their impact on the recovery of cod in the southern Gulf. Science? Or politics?

Caution advised

In the case of the Atlantic salmon, it has also been pondered whether or not seals were to blame for its decline. Studies show that salmon are seldom consumed by seals. For example, of the 700 stomachs of gray seals harvested at Anticosti Island, a high density sector for Atlantic salmon, only one contained salmon. Likewise, examination of 9,000 stomachs of harp seals over the past 30 years has revealed the presence of but one salmon. The mystery of the salmon decline is a complex one, but seals do not seem to play an important role.

History recap: the case of St. Lawrence belugas

In the 1920s, St. Lawrence belugas were blamed for the scarcity of cod and salmon. The government distributed rifles and cartridges and offered bonuses to fishermen to kill as many belugas as possible. After eight years of this scheme, Dr. Vladykov undertook a study on beluga diets. His work proved that belugas feed essentially on species of no commercial value such as sand lance, capelin, nereid worms, and various molluscs and crustaceans.


Fewer marine mammals, more fish?

Whatever the case, if we wanted to guarantee a more substantial share of fish stocks, wouldn’t controlling marine mammal populations be an effective method? The argument in favour of this method appears to be quite straightforward: simply reduce the size of a predator population to increase the population of a species (Species 1), and thereby boost the catch. However, if we add to this reasoning another fish species (Species 2) that is both a prey of the predator and a predator of Species 1, the equation becomes more complex. Indeed, a reduction in the predator’s numbers will favour the population of Species 1, but by the same token will also favour that of Species 2. An increase in the population of Species 2 might then translate into an even greater predation pressure on Species 1! It is quite difficult to predict whether the population of the desired fish species (Species 1) will increase or decrease in response to a decline of either of these predators. And this model is still oversimplified compared to what really occurs in the natural environment: often times there are not two but thousands of “paths” between the predator targeted and the prey sought by the fishing industry. To make such a model more realistic, the notion of time must be accounted for: as certain “paths” are longer than others, the desired effect might be achieved in the short term, only later to be followed by the opposite effect.

of the Institut des sciences de la mer à Rimouski (ISMER) examined the impact of great cetaceans on the tropical ecosystems in northwestern Africa and the Caribbean in response to the controversial Japanese theory, i.e. that baleen whales are the main culprits in the global collapse of fish stocks, and whaling is an effective method to counter this decline. Certain countries of coastal regions support this model and speak out in favour of whaling for their economic security, which often depends on fisheries. The results of the study, published in the journal Science, demonstrate that even a complete elimination of baleen whales would not lead to any significant increase in economically important fish stocks in these regions. Not only is the impact of whales on these fish practically non-existent, but it is also one hundred times less than that of fishing. 

Despite the uncertainty that prevails, considerable pressure remains to shift the blame to marine mammals. Are they held accountable because they are more visible than other predators? Might it be because they are already considered pests by fishermen due to the problem of incidental catches in their fishing gear?

Less fish… fewer marine mammals?

What if we asked the question the other way around? And we asked what effects fisheries can have on marine mammals? Are there cases in which Man has entered into competition with them and depleted their food resources? What would happen if the moratorium were lifted on forage species, in other words those at the bottom of the food chain? And if a fishing industry targeting krill – a small crustacean that a multitude of predators rely on – were to take off, what would be the effects on bird, marine mammal and fish populations?

Humans, unlike natural predators, have access to powerful technologies and an abundance of alternative resources. Fisheries can have a significant impact on predator populations as well as on the entire ecosystem. Better managing what we harvest from nature and curbing excessive exploitation of the marine environment seem to be the best solutions to consider, as much to ensure fishery yields as to protect marine ecosystems.