Fish Biology: Respiration in Marine Fish
by J. Charles Delbeek, M.Sc.

Fish need energy to move, to find and digest food, to grow, and to
reproduce, in addition to maintaining the body and internal
environment (Moyle and Chech, 1982). This energy must be obtained
from the environment and then assimilated. In animals this
assimilation depends on the use of oxygen to metabolize their food.
Therefore, the collection of oxygen is probably the most important
function an animal undertakes. In terrestrial animals this is
accomplished by the use of lungs, which extract oxygen from the air
through the bidirectional movement of air in their lungs. Fish use a
slightly different mechanism, utilizing the unidirectional passive
movement of water through external gills. However, fish have one
problem, water contains only 1/30 as much oxygen per volume as air
(Moyle and Check, 1982). There are a number of mechanisms that fish
can utilize to extract oxygen from the water but the most common one
involves the gills and this article will concentrate on them.

Due to the low concentration of oxygen in water, the gills must be as
efficient as possible in order to extract oxygen. The gills consist
of bony or cartilaginous arches which hold pairs of gill filaments. 
Each gill filament consists of an upper and lower surface
covered with minute ridges known as lamellae. These lamellae are made
of extremely thin membranes (1 cell thick) and are the primary sites
of gas exchange. Water flows across the gill filaments and oxygen is
removed and passes into the blood by diffusion. To increase the
efficiency of oxygen uptake a countercurrent method is used (the same
principle as used in force air furnaces); blood flows through the
lamellae in a direction opposite to the water flow through the gill
filaments. Countercurrent flow insures a steady oxygen
tension gradient along the entire diffusion surface (Moyle and Chech,
1982). The actual uptake of oxygen depends on a number of factors,
the surface area of the lamellae, the thickness of the gill epithelia
across which the oxygen moves and the oxygen tension across the
membranes. As a result fast moving, highly active fish tend to have
larger gill surface areas and very thin gill epithelia. Since the
rate of oxygen diffusion depends on an appreciable gradient existing,
any decrease in oxygen tension that occurs will affect the rate of
oxygen uptake. As the blood absorbs more oxygen from the water in the
gill chamber, the oxygen content of the water decreases and the rate
of diffusion decreases as well, therefore, fresh, oxygen rich water
just be introduced. Renewing this water is called ventilation of the
gills (Moyle and Check, 1982).

Gill ventilation consists of an expansion of the area behind the
mouth (buccal cavity) causing water to enter through the mouth. The
water is then accelerated over the gills by a simultaneous
contraction of the buccal cavity and expansion of the gill chamber
(branchial cavity) aided by the opening of the plate covering the
gills (operculum). When the branchial cavity finally contracts,
expelling the last of the water, the cycle repeats itself. This is
exactly what we see as a pumping action of the mouth and opercula in
our fish. Interruptions of this cycle produce brief periods of
reversals of flow or "coughs" which fish use to clear their gills of
excess foreign matter and mucus. Drummond et al (1973)
found that the frequency of coughs in brook trout could be used as a 
reliable indicator of sublethal concentrations of copper.

There are a number of behavioral actions in fish which can be
observed in response to the oxygen concentration in the aquarium. The
most common of these is an increased respiration rate. By increasing
the rate of water flow over the gills the oxygen tension is maximized
by the rapid displacement of water next to the lamellar surfaces. In
some fish species, the ventilatory cycle ceases all together and the
fish ventilates its gills by swimming at a "critical" velocity.

So what does all this mean to the average hobbyist? Well, for one
thing, by keeping an eye on the ventilation rate of your fish you can
get a pretty accurate reading on a) their health and/or b) the oxygen
concentration of your water. also, the frequency of the coughing
behaviour can be an indicator of the presence of copper in the water
or excess mucus production. It is for this reason that I am a little
leery of these so called "stress coat" solutions that protect your
fish against disease and injury through the production of body slime.
Although no one has shown that these solutions inhibit respiration in
fish, I believe the possibility exists. Furthermore, the use of
copper sulfate to treat parasitic diseases is a common practice,
however, its prolonged use can affect the long term health of your
fish because copper increases the production of body slime. This
slime will also coat the gills of a fish, interfering with the
exchange of oxygen. The fish can literally suffocate in its own slime
if too much copper is used and/or used over an extended period.

References
1. Drummond, R.A., Spoor, W.A., and Olson, G.F. 1973. Some short-term
indicators of sublethal effects of copper on brook trout, Salvelinus
fontinalis. J. Fish. Res. Bd. Canada 30:698-701.

2. Moyle, P.B. and J. J. Chech, Jr. 1982.  Fishes: An Introduction
to Ichthyology. Prentice-Hall, Inc., New Jersey.