50 Priyantha I. Hathurusingha and Kenneth R. Davey
overlooked the opportunity of comparing the predictive capabilities of other
models (Howgate, 2004).
A significant concern is that bio-magnification models are based on
steady-state assumptions. It has been reported that steady-state is not readily
reached in RAS and therefore bio-magnification factors are difficult to
measure. Moreover Sijm et al. (1992) reported that steady-state conditions do
not necessarily imply ‘equilibrium’. For example, experiments carried out with
guppies (Poecilia reticulata) and rainbow trout showed that chemical
accumulation continues if fish stop growing (Sijm et al., 1992). A similar
experiment conducted with growing- and non-growing fish under field and
laboratory conditions showed that the use of steady-state assumption is not
valid (Sijm et al., 1992).
Moreover, it has been reported that steady-state conditions and kinetics of
uptake/depuration are unrelated variables (independent); for example, fish
growth impacts on the steady-state conditions whereas gender influences the
kinetics of elimination (Sijm et al., 1992).
Finding reliable empirical data for model validation is demanding and also
an expensive challenge to measure the biological variables. For example,
uptake and elimination rate constants for large-scale are difficult to calculate
accurately from laboratory-scale and also to replicate in the natural
environment. In the kinetic models, net changes of chemicals across the fish
body and environment are assumed to be negligible, or zero, under steady-
state conditions. In reality, it is difficult to measure or establish when steady-
state is attained (Hawker and Connell, 1988). Additionally it is questionable
whether steady-state assumptions can be reasonably made for practical RAS
systems.
It is apparent there is no empirical evidence that the net chemical
exchange rate is zero between the RAS water and fish-flesh phases. Therefore
an adequate model to predict GSM and MIB in RAS farmed fish needs to be
developed considering these inadequacies.
FACTORS TO BE CONSIDERED IN DEVELOPING AN
ADEQUATE MODEL FOR RAS FISH
Although RAS is a complex system, biological and physical variables can
be determined and a consumer acceptable threshold concentration can be
established experimentally from harvested fish-flesh.