Factors affecting
anesthesia
Many factors affect the efficacy of
anesthetics in fish. These can be
divided into biological and environmental
factors.
Often, the rate at which anesthetic
drugs become effective is related to
the gill area to body weight ratio,
which can vary considerably
among fish species. Aquatic species
also have different metabolic rates
that affect the rate at which chemicals
are absorbed and anesthesia is
induced. For example, cold-water
species seem to respond to lower
concentrations of anesthetic than
warm-water species.
There are also factors that can
affect anesthesia within a particular
species. Larger individuals
generally require a greater concentration
of anesthetic than smaller
individuals. In contrast, it has also
been reported that the larger,
more active fish in a group are
anesthetized faster than smaller
ones. Many drugs such as MS-222
and benzocaine are fat-soluble;
therefore, in larger fish or gravid
females, anesthesia may last
longer and recovery may be slower
as the drug is removed from
the lipid reserves. Also, diseased
or weakened animals are much
more susceptible to anesthetic
treatment.
Environmental factors can also
profoundly affect the efficacy of
certain anesthetics. Aquatic invertebrates
and fish are ectotherms;
their body temperature closely follows
that of their environment. As
a result, physicochemical passage
of the drug into the fish is also
temperature related. At lower
water temperatures, higher doses
or longer exposure times are
required with MS-222, benzocaine
and 2-phenoxyethanol, presumably
because the absorption rate
decreases at lower temperatures.
The pH of an anesthetic solution
also can influence its efficacy, possibly
by affecting the ratio of
charged to uncharged molecules.
This is most pronounced with
quinaldine, which loses its efficacy
in solutions with low pH.