Balanced Harvesting
Ensuring fisheries are sustainable is no easy task. We have a growing human population that needs food. We have different legislative and management systems operating around the world. We still have a lot to learn about fish populations and marine ecosystems, and on top of that, there is a certain vagueness about terms like ‘sustainability’. Nevertheless, fisheries managers, fishers and researchers around the globe are working towards minimising our impact on the ocean environment and the very populations on which we depend.
Generally, it is felt that we need to protect juvenile fish from extraction, allowing them to reach maturity and breed before being caught. The ‘minimum landing size’ tries to do exactly that (the age of a fish is proportional to its size). Recently, however, there have been controversial calls for something different – ‘balanced harvesting’ – an unselective type of fishery that catches predominantly small, immature fish.
Nis Jacobsen, Henrik Gislason and Ken Andersen from the Technical University of Denmark set out to figure out which type of fishery would produce the greatest conservation and fishery objectives.¹ Using a ‘trait-based size-spectrum model’, Nis and the team compared the impacts of four different types of exploitation scenarios on the structure of their model fish community (through changes of biomass) as well as on fishing catches:
Selective unbalanced fishing, a pattern that is a good approximation to the current exploitation of the North Sea fishes, where juveniles are protected, and adults are fished with approximately equal mortality across groups.
Unselective unbalanced fishing, where all individuals are exploited with the same fishing mortality irrespective of whether they are juveniles or adults.
Selective balanced fishing, where juveniles are protected from fishing and adult fishing mortality scales with productivity at the population level.
Unselective balanced fishing, which is the ‘ideal’ balanced pattern where each individual is exploited relative to its productivity.
The research team found that different exploitation patterns do indeed alter the community structure slightly differently. This is because as populations are exploited, their growth and reproduction can alter, which can have knock-on effects on whatever it eats or gets eaten by (a trophic cascade). For example, a decrease in the population of one fish may mean less of its prey is being eaten, resulting in a population explosion of that prey species.
These trophic cascades will happen no matter which type of exploitation scenario is chosen – and in fact would happen without any fishing whatsoever as populations can undergo ‘boom’ and ‘bust’ cycles for a whole host of reasons. But it seems that selective fishing – where juvenile fish are deliberately avoided in the fishing process, focusing just on the big fish – caused the most pronounced cascades.
Furthermore, selective fishing seemed to reduce the ‘maximum sustainable yield’ – the maximum catch that can be removed year after year – available from the community. It seems that balanced-unselective fishing may have the smallest impact on fish communities because the biomass was not significantly altered by focusing on the largest fish species and individual fish available.
As a bonus, it also increased the level at which the maximum sustainable yield is reached, which means (theoretically) more fish can be caught. I say theoretically because the use of maximum sustainable yield for determining fishery catches is somewhat controversial too, which is another story.²
The Proceedings paper isn’t the only one to suggest that unselective-balanced fishing may be the way to go. Andrea Belgrano and Charles Fowler in Science suggest that balanced fishing may help reduce the impact of commercial fishing on the genetics of fished stocks.³
Sounds good. So if it turns out that the best thing is to change our usual fishing practices and move to exploiting individuals in proportion to their productivity, what’s stopping us?
Well, there are a couple of problems – namely with the size of the fish being caught. Balanced-unselective fishing is likely to catch more smaller-bodied fish, which in a Western market is not very profitable. It’s big fish we love to tuck in to, and that’s where the money is. And it’s not just about making as much money as possible – fishing isn’t a cheap business. Low income from small fish could cause many a fisher problems.
But the problems aren’t just economic. Many small fish aren’t just small because they are juveniles … some are small because they are, well, small. And many of these small fish are what are known as forage fish – like sardines and anchovies. If you are a forage fish, you have a lot of predators.
And if your population size is kept low for sustained periods of time, the knock-on effect in the wider community through trophic cascades could cause significant alterations in community structure in the long run – and ultimately on our fishery catches.
This article originally appeared in Marine Scientist in May 2014.
