Quick notes on Sustainable Yield

Sustainable Yield (SY) refers to the increase in natural capital. It is the natural income that can be exploited each year without depleting the original stock or affecting its potential for replenishment.

If you see it as a business, you could consider sustainable yield as the 'retained profit' of a company. It is the amount that one has left over and can use to invest in other aspects of the business. That money can be used to expand and grow or to improve the business in any other way. It is the same with Sustainable Yield, that increase in natural capital is what is available for use and would not affect the environment. It is like the money a company can use to make itself better without going into debt.

* MSY means the maximum sustainable yield, and it is the one that is of interest commercially speaking.

Some important aspects to consider when calculating sustainable yield are:

• carrying capacity
• population size
• total biomass or energy at a given time
• Rates of change of population, biomass, and energy.

However, there is a convenient formula for calculating Sustainable Yield:

SY= Annual Growth and Recruitment - Annual Death and Emigration

Basically, what this calculates is how many organisms are there at a given point in time. It considers new individuals that came in, and individuals that died or left.

Sustainable Yield can also be calculated by

SY= (Total Biomass or energy at a Time T)+1 - (Total Biomass or energy at a Time T)

This is useful when calculating the changes in SY over a period of time. It would be used when comparing biomass in 2009 and biomass in 2010.


Here is an example on how sustainable yield is important for economic systems. It is a report on the commercial value of estuaries in Australia.
http://www.ozcoasts.org.au/indicators/econ_value_commercial_fisheries.jsp

Also, here is an ecological assessment of rivers and estuaries (also in Australia). This is the type of information that is useful when evaluating an ecosystem and its natural resources.
http://www.anra.gov.au/topics/coasts/pubs/estuary_assessment/est_ass_int_wpdd.html

Measuring Biomass

Biomass refers to the total number of living organisms in a certain area. It is the amount of energy that is available for the next trophic level. It is important to measure biomass because it gives an idea of the state of the ecosystem. By knowing the amount of energy available, one can know how many species it can support.

One way to measure biomass is to obtain the dry weight of an organism (since it is a rough approximation to the amount of biomass) and multiply it by the number of those organisms in a given area. The units are grams per meter squared (or cubed if it is an aquatic ecosystem). This is a commonly used method. However, the difficulty here is knowing with accuracy the total number of individuals, particularly if one wants to measure biomass in a wide area.

The way to get the 'most accurate' biomass measurement would require counting absolutely every organism. However, there are ways to get an estimate. One method is by making a transect, which means tracing an imaginary line across the selected ecosystem, and counting the organisms that are in the quadrants following the transect. That data can then be extrapolated to the rest of the ecosystem, or several transects can be made, to get a more accurate estimate.

Quadrats are another method in which a rectangular area is selected and biomass is counted in that specific place only. Random quadrats can be selected, and as with the transect, the more measurements (quadrats) you select, the better the estimate.

There is also a remote sensing technique that surveys the earth's surface from the air or using satellites. Images generated are analyzed to determine total biomass productivity. However, this is only used with producers, and works mostly on dense woodlands.

In the future, more technological tools will be developed and improved in order to measure biomass easily. This could include better imaging to determine biomass, as well as a technological tool that will help get an accurate number of the individuals of a species in an area. The main benefit is that the measurement will be non-destructive, harming no ecosystem.

After biomass is collected at different trophic levels, a biomass pyramid can be made. Here's an example of an estuary, more specifically, Chesapeake Bay. Taking the following food chain:

phytoplankton -> clams -> blue crabs -> sandbar sharks

Firstly, the units will be measured in kg per cubic kilometer, since it is a aquatic ecosystem. We can calculate the amount of each species in the food chain, and we get the following information:

There are 31,333,333 units of phytoplankton in 2320 cubic km of water in Chesapeake Bay.
There are 9,200,000 clams in that same area.
There are 278,666 blue crabs.
There is one sandbar shark.

Then, considering the weight of each specie and the area, we can calculate biomass, which then can be shown in what is known as a biomass pyramid. Results are shown below.

Bibliography:
Rosillo Callé, Francisco. The biomass assessment handbook. Illustrated ed. Earthscan, 2007. Print.

Kimball, John W. "Food Chains." Biology Pages. N.p., 23 Apr 2010. Web. 27 Aug 2010. .

Tackenberg, Oliver. "A New Method for Non-destructive Measurement of Biomass, Growth Rates, Vertical Biomass Distribution and Dry Matter Content Based on Digital Image Analysis." Oxford Journals (2007): n. pag. Web. 27 Aug 2010. .

Chesapeake Bay

Chesapeake Bay is an estuary located in the west coast of the United States. It receives water from over 100,000 streams and rivers and empties in the Atlantic Ocean. It is home to around 350 species of fish, dozens of species of shellfish and crabs, 16 species of underwater grasses. Waterfowl, ospreys, and shorebirds also inhabit nearby areas.

All of these species interact in this ecosystem, and form a very complex food web (made up of many food chains). As explained previously, the food chain begins with a producer, which in this case is the phytoplankton, which is then eaten by a number of first consumers, which are then eaten by other predators.

The main purpose of the food chain is to see how energy is transferred from one organism to another. However, pyramids are ways to obtain more data. They represent graphically how energy, matter, and population are distributed in a system. Two of them are the pyramid of biomass and the pyramid of productivity.

The pyramid of biomass shows the amount of biomass at each trophic level, which is calculated by multiplying the mass of an organism times the amount of organisms. The dry weight of an individual is roughly the same as the energy it contains.

The pyramid of productivity shows the flow of energy over time. Since energy is lost as it moves from 1 trophic level to another, the upper levels are always smaller than the lower ones. Only 10% of the energy in one trophic level is passed on to the next one.