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Sustainable yield definition is first used in forestry practices in the 1700s to describe the strategy of cutting trees that should not exceed the forest’s capacity to regenerate.
The term has since been extensively used in the fisheries sector and other wildlife management regimes. Sustainable yield defines the stock that can be harvested without compromising the ecosystem’s carrying capacity to reproduce the same species’ stocks.
But in this day and age, when the finite resource is fast dwindling – that we could even hardly reach the minimum stock – does the term maximum sustainable yield (MSY) still relevant?
What is Maximum Sustainable Yield
Maximum Sustainable Yield Definition
In the Encyclopedia of Ecology, Maunder (2008) defined the maximum sustainable yield as the maximum catch (in numbers or mass) that can be removed from a population over an indefinite period.
Other operational definitions of MSY from notable institutions such as Food and Agriculture Organizations (FAO), World Wide Fund (WWF), Atlanta State Marine Fisheries Commission (ASMFC), and European Commission (EC) include the’ highest’ or ‘largest’ catch taken continuously or sustained over time without significantly affecting the capacity of the remaining stock to reproduce or replenish.
Think of MSY as a never-ending game of tug-o-war between land and human populations. We want to take as much as we can without losing balance or destroying populations forever.
The MSY principle is also being used in other renewable resources in forestry and agriculture.
The concept of MSY became popular in the 1950s by introducing the ‘surplus production models’ in fisheries management. The said model thrives on the assumption that when an unfished area has reached its full potential or so-called carrying capacity, the growth rate and the reproduction rate of species slow down due to an increase of competition on the existing resource.
But what happens when factors such as mortality, migration, or fishing step in? Of course, this would reduce the fish population. Is this good or bad? I can say it is nothing but a silver lining.
When the reduction of fish stocks occurs, the remaining fish stock gains some space and opportunity to grow and reproduce until it bounces back to reach its full population capacity. If the reproduction rate is faster than the factors that reduce the population size, we called a ‘surplus.’ This is where the surplus production model was derived.
The principle is also true in forest management. If left alone undisturbed, a stand of trees will create some fierce competition on light, nutrients, and space among the population of trees, thus slowing their growth rates. A natural disaster such as typhoons and the natural death of trees due to decay or old age creates some space for other understory species to grow until they reach the peak of saturation or carrying capacity (given no take or harvest has ever occurred).
The surplus is the biomass of trees that can be taken or harvested from the ecosystem without compromising its ability to regenerate and remain stable.
Why is maximum sustainable yield important?
I love eating fish, and so do many of the remaining 7.5 billion people on our planet.
But do we know the population status of our favorite fish? You’ll be surprised to know that some of them are already on the brink of extinction.
Stock assessment of the National Marine Fisheries Service (NMFS) showed that the West Coast Pacific sardine population has dramatically decreased by 95% from its 2006 biomass level. This means from 1.8 million metric tons of biomass in 2005, it was down to only 86,000 metric tons in 2017. And in 2019, this is further reduced to 27,000 metric tons.
Further research of NMFS showed that sardines’ replenishment rate between 2010-2014 is so low that it could no longer cope with the sardine industry’s intensive fishing. As a result, no U.S. commercial sardine fishery was allowed to operate from 2015 to 2019.
The National Oceanic and Atmospheric Administration (NOAA) proposes an annual catch limit of only 4,288 metric tons of sardines from 2020-2021. This makes parameters such as maximum sustainable yield (MSY) still highly relevant.
The fate of your favorite fish chips, Atlantic cod, or Gadus morhua is also in threat of depletion due to overfishing and predation. Because it lost its resilience to rebound back to its average population level, it could no longer cope with the natural selection process such as predation.
The grim reality of overfishing beyond sustainable levels has already affected hundreds of fish stocks around the globe. Of the 600 marine fish stocks monitored by FAO:
- 52% are fully exploited, or fishing operation is already close to an optimal yield level with no more room for further expansion;
- 17% are overexploited and are in the higher risk of stock depletion/collapse; and
- 7% are depleted or have catches that are well below historical levels, irrespective of the amount offishing effort exerted.
Sadly only 1% or around 6 species of fish are recovering from depletion or catches that are again increasing after depletion.
As a fish consumer, it pays to look at UN FAO’s “General situation of world fish stocks” so we may know which fish sold in the market that is overexploited. Somehow, we do not want to take part in the cause of their overfishing.
How is maximum sustained yield measured?
MSY follows the mathematical model of the Sigmoid curve, representing that the biomass of species grows until they reach their carrying capacity over time. Carrying capacity is at the highest point of the curve where the growth rate is already slow due to over-competition of the resource.
MSY is at the inflection point located halfway to the carrying capacity. It is assumed that when you take or harvest at this inflection point or MSY, the population of species will still recover. The ability of the species to rebound to its original stock population is called species resilience.
Harvesting near the carrying capacity might be detrimental to the ecosystem because the probability of taking almost ALL mature and breeding individuals from the population is most likely high. If this happens, it will take very long before the species bounce back to its stable state.
So how then do we calculate for the MSY? First, we have to know the carrying capacity of the stocks.
Using the Schaefer model on surplus production, a fish stock grows towards its environment’s carrying capacity (symbol as K) at a maximum rate (denoted as ‘r’). The formula for this is expressed as:
Nt+1 = Nt + rNt (1 – Nt/K)
Nt = population size
r=growth rate of the population (which is equal to the difference between the birth rate and the death rate); was originally defined in terms of the continuous-time exponential growth equation
dN/dt = r N.
The maximum sustainable yield will be around half of the estimated carrying capacity. This is expressed as BMSY.
Challenges to the MSY model
New management paradigms suggest that MSY is a good sustainability indicator but should be not be used as a target – but rather a reference point. The reference point is defined in FAO’s framework guide to stock assessment as indicators that provide specific values to aim (target) or avoid (limit).
Reference points are also scientific benchmarks used to indicate the status of fish stock.
Using MSY as a target reference point (TRP) is dangerous because whenever there is an overestimate of the MSY values, there is also an overestimate of the surplus production, thereby leading to overfishing. In the long term, this will significantly reduce the population of the fish stock.
Therefore, FAO suggests that the target reference point and any precautionary reference points for the fishing mortality rate should always be below the MSY point. The general rule is that the fishing mortality rate will not exceed 2/3 of the MSY value.
This approach is a significant shift from historical fisheries management practice where MSY has most often been treated as a target, rather than as a limit, and in most times often exceeded the target.
Other challenges to the MSY mathematical model are dynamic factors such as:
1) breeding seasons – suppose the harvest was made during the breeding season. It accidentally took/harvested most mature and breeding individuals; this could jeopardize the population’s ability to replace itself. In fisheries, this is known as recruitment overfishing.
2) frequency of the harvest/take – if the harvesting frequency is way faster than the species’ growth rate, you might end up harvesting under-mature or juvenile species. This phenomenon is termed in fisheries as growth overfishing.
3) natural patterns of stock fluctuations – long term records of unexploited fish populations, for instance, show that different species of fish exhibit different natural patterns of stock fluctuations. These are as follows:
Steady-state stocks – population of species that demonstrate minimal changes in their population;
Cyclical stocks – population of species that fluctuates over a predictable series of time;
Irregular stocks – population of species that fluctuates significantly with no regular pattern;
Spasmodic stocks – population of species with short alternating periods of plenty of stores, and more extended periods of great scarcity.
Each of these would require its own management goal and strategy.
Despite the loopholes and the over-simplicity that MSY theory offers, MSY is still widely recognized by institutions such as FAO and other national governments to be indicators for sustainable resource harvest. MSY still expresses ideal resource extraction levels to sustain future harvest, mainly if used appropriately
Before the 1950s, considerable damage to some species’ population occurred due to the absence of sustainable parameters to be wary of. MSY may not be perfect, but having some specific parameters to contemplate somehow placed red flags on resource extractive industries to be more cautious of their operations on the environment.
Let biologists and fish scientists reconcile their maths. For now, we should take a personal stand, absorb all the information and knowledge we could get, and support advocacies that aim to save depleting species – because if we do not do it now, no one in this generation ever would.
I just hope our children and grandchildren would not wake up one-day consuming fish in broth cubes instead of the real ones.