Modelling of Taste-Taint in Fish ... 47dependent on the uptake and elimination efficiencies of the chemicals concern.
A low elimination rate is known to be the governing factor for higher bio-
accumulation (Neely et al., 1974).
A number of models have evolved. Notably, there are some that predict
the level of chemical concentrations in fish, but few have been published (S.
Poole, Department of Agriculture Fisheries and Forestry, Australia (DAFF),
pers. comm.). A simple model can be based on the chemical equilibrium
partitioning between two phases. Generally, a living organism is treated as
one-compartment by applying a mass balance. In this case, it is assumed that
chemicals are immediately distributed in the fish-flesh once ingested. In more
complex models such as the Pharmacokinetic models (PBPK), the organism is
considered as a combination of individual organs or tissues (Wen et al., 1999).
In these types of models, chemicals are thought to be distributed among
various organisms within the fish with different kinetic rates (Howgate, 2004).
All possible exposure routes of the taste-taint chemicals as well as
eliminating routes have to be taken into account when developing models.
There are three (3) uptake routes: food, respiration and dermal diffusion, and
six (6) meaningful elimination paths: respiration, dermal diffusion, egestion,
metabolic conversion, reproductive losses and growth dilution.
It is important to understand the appropriate processes.
MODELS FOR PREDICTING GSM AND MIB IN FISH-FLESH
Howgate (2004) derived simplified kinetic equations for the uptake and
elimination of GSM and MIB based on a bio-concentration factor.
What was modelled were the uptake kinetics under constant and varying
concentration of GSM in water and, the depuration kinetics in terms of
constant concentration with time (day). Howgate (2004) reported a greater
accumulation under-steady concentration than that under-increasing
concentration, starting from an initial zero concentration. He concluded that
taste-taint concentration in fish-flesh is related to the concentration in water of
the GSM and MIB chemicals. For depuration kinetics, he developed an
exponential model for reduction in GSM and MIB over time, with half-lives of
2.3 day for GSM and 1.3 day for MIB.
However, this work cannot be seen as a ‘true’ model because important
elements such as the growth of the fish and taste-taint dilution due to fish
growth and metabolic elimination, have not been considered with regard to
accumulation of GSM and MIB.