Ensuring our 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 of course there is a certain vagueness about terms like ‘sustainability’.  Never the less fisheries managers, fishers, and researchers around the globe are working towards minimizing our impact on the ocean environment and the very populations on which we depend on.  Generally, it is felt that we need to protect juvenile fish from extraction, allowing them to reach maturity and breed before being caught.  Makes sense right?  You may have heard of the minimum landing size, which tries to do exactly that (the age of a fish is proportional to its size).  Recently however, there have been calls for something different - ‘balanced harvesting’ –catching predominantly small, immature fish.

Controversial?  Yes.  Which is why Nis Jacobsen, Henrik Gislason, and Ken Andersen from the Technical University of Denmark set out to figure which type of fishery would produce the greatest conservation and fishery objectives.

Using something called 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 – e.g. – and a little simply put - is most of the weight in a few large fish or many small fish), as well as on fishing catches.  The researchers explain these scenarios pretty well in their open-access paper, so I’ll just paste in what they wrote…

i) 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;

ii) unselective unbalanced fishing, where all individuals are exploited with the same fishing mortality irrespective of whether they are juveniles or adults;

iii) selective balanced fishing, where juveniles are protected from fishing and adult fishing mortality scales with productivity at the population level;

iv) unselective balanced fishing, which is the ‘ideal’ balanced pattern where each individual is exploited relative to its productivity

The research team found (rather expectedly) that different exploitation patters 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 to whatever it eats or gets eaten by (a trophic cascade).  For example, a decrease in 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 we choose – and in fact would happen without any fishing what-so-ever as populations can undergo ‘boom’ and ‘bust’ cycles for a whole host of reasons.  But it seems that selective fishing – where we deliberately avoid juvenile fish in the fishing process and just focus on the big fish – caused the most pronounced cascades.  Furthermore, selective fishing seemed to reduce what is known as 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 of 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 in a nutshell means you can (theoretically) catch more fish.  I say theoretically because the use of maximum sustainable yield for determining fishery catches is somewhat controversial too.  That’s a story for another post*.

This paper isn’t the only one to suggest that unselective-balanced fishing may be the way to go.  Another (pay-walled) paper published in the journal Science by Andrea Belgrano and Charles Fowler suggests that balanced fishing may help reduce the impact of commercial fishing on the genetics of fished stocks.  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.

* If you fancy reading up on the issues of maximum sustainable yield, try this open access paper I mentioned in a previous post, this Southern Fried Science blog entry by Amy Freitag.