1 Fish Farm Wastes in the Ecosystem
39
state and which do not? In the absence of adequate knowledge, the WFD Annex V
assessment strategy serves to provide an empirical appraisal of change away from
a reference state which is by definition healthy. If a sufficiently large proportion of
points fall outside the reference envelope, then the PCI can be used to indicate a
change in quality from high or good to moderate or worse. This is the second use,
but even it needs agreement about critical values of the PCI – at the good/moderate
boundary, above all.
1.14 Assimilative Capacity
Given regulation according to the WFD and the need to maintain ecosystem health
and sustainable human use, how many finfish or shellfish can be farmed within a
water body? The size of a sustainable aquaculture is said to be the carrying capacity
of the water body for the stock concerned; I approach it here from the alternative
perspective of the assimilative capacity of the water body for the wastes of – or,
more generally, the pressures generated by – fish-farming and other human activities. What, for example, is a water body’s ability to absorb anthropogenic DAIN
without significant adverse effect on the health of the ecosystem?
Figure 1.8 shows some of the principles involved in the estimation of assimilative capacity. The horizontal axis represents increasing pressure from anthropogenic activity. This pressure could be quantified as the number of fish farms in a
water body or the number of humans who would produce waste equal to the total
input to the water body from all sources, but it is better to relate the waste input to
the receiving system. Thus, suitable indicators of pressure would be the annual rate of
organic matter arriving on each square metre of seabed in the AZE below a farm,
or the daily total of nutrients input to a zone B water body, divided by the volume of
the water that is replaced each day from the adjacent sea.
The vertical axis is something that measures impact on the ecosystem – that is,
the change in state from a reference condition as defined for the WFD or a decrease
in the health components organization and vigour. Examples of such benthic indicators include the AZTI Marine Biotic Index (AMBI) (Borja et al. 2003) and the
Infaunal Trophic Index (ITI) (Word 1990). These assess the balance of the several
kinds of large benthic animal needed to maintain a healthy ecosystem in the mud.
Examples for the water column include the excess of chlorophyll concentration
over that in a reference condition, and the PCI described above.
There is a scale issue: the pressure variable on the x-axis and the impact variable
on the y-axis must relate to the same scale: A, B or C as defined previously. Given
that, the next part of the task is to find a relationship between the two axes, as
shown by the diagonal line in the diagram. A simple relationship might be that of
linear regression, so that y = a + b.x, where a and b are constants. As suggested by
the curve in Fig. 1.6, the true relationship in Fig. 1.8 is unlikely to be simple, but
this is not a problem so long as it can be expressed by a mathematical equation, or
by a table in which values of impact, y, can be looked up for values of pressure, x.
39
state and which do not? In the absence of adequate knowledge, the WFD Annex V
assessment strategy serves to provide an empirical appraisal of change away from
a reference state which is by definition healthy. If a sufficiently large proportion of
points fall outside the reference envelope, then the PCI can be used to indicate a
change in quality from high or good to moderate or worse. This is the second use,
but even it needs agreement about critical values of the PCI – at the good/moderate
boundary, above all.
1.14 Assimilative Capacity
Given regulation according to the WFD and the need to maintain ecosystem health
and sustainable human use, how many finfish or shellfish can be farmed within a
water body? The size of a sustainable aquaculture is said to be the carrying capacity
of the water body for the stock concerned; I approach it here from the alternative
perspective of the assimilative capacity of the water body for the wastes of – or,
more generally, the pressures generated by – fish-farming and other human activities. What, for example, is a water body’s ability to absorb anthropogenic DAIN
without significant adverse effect on the health of the ecosystem?
Figure 1.8 shows some of the principles involved in the estimation of assimilative capacity. The horizontal axis represents increasing pressure from anthropogenic activity. This pressure could be quantified as the number of fish farms in a
water body or the number of humans who would produce waste equal to the total
input to the water body from all sources, but it is better to relate the waste input to
the receiving system. Thus, suitable indicators of pressure would be the annual rate of
organic matter arriving on each square metre of seabed in the AZE below a farm,
or the daily total of nutrients input to a zone B water body, divided by the volume of
the water that is replaced each day from the adjacent sea.
The vertical axis is something that measures impact on the ecosystem – that is,
the change in state from a reference condition as defined for the WFD or a decrease
in the health components organization and vigour. Examples of such benthic indicators include the AZTI Marine Biotic Index (AMBI) (Borja et al. 2003) and the
Infaunal Trophic Index (ITI) (Word 1990). These assess the balance of the several
kinds of large benthic animal needed to maintain a healthy ecosystem in the mud.
Examples for the water column include the excess of chlorophyll concentration
over that in a reference condition, and the PCI described above.
There is a scale issue: the pressure variable on the x-axis and the impact variable
on the y-axis must relate to the same scale: A, B or C as defined previously. Given
that, the next part of the task is to find a relationship between the two axes, as
shown by the diagonal line in the diagram. A simple relationship might be that of
linear regression, so that y = a + b.x, where a and b are constants. As suggested by
the curve in Fig. 1.6, the true relationship in Fig. 1.8 is unlikely to be simple, but
this is not a problem so long as it can be expressed by a mathematical equation, or
by a table in which values of impact, y, can be looked up for values of pressure, x.