THE OXYGEN CONSUMPTION OF FISH

Introduction

Fish obtain their oxygen from the water pumped across the gills. A fish in a sealed container of water will deplete the oxygen; if the partial pressure is measured before and after a known interval, the rate of oxygen consumption (MO2) can be calculated. Water flows past the gills in one direction, due to the double pump system of mouth and opercular cavities. The rate of water flow depends on the need for oxygen (i.e. the activity level of the fish) and on the oxygen level in the water. A fish in a sealed container should thus increase its ventilation rate as it depletes its oxygen supply. In this practical, you will measure the partial pressure of oxygen in aerated water before and after a fish has been sealed in it for a known interval. You will also measure the ventilation rate of the fish, and of a second fish in an open vessel acting as a control for behavioural stress.

Each pair will have two fish (the common goldfish, Carassius auratus) of similar size, one for oxygen consumption measurement and one control. You will also need to collect class data from the board to examine the effect of body size on oxygen consumption. By discussion with the demonstrator and others in class ensure that a "control jar" without fish has been included, to compensate for any bacterial oxygen consumption.

Procedure

Determination of MO2

  1. Take one of the glass sample jars which will be used as a respirometer and determine the volume it holds using a measuring cylinder.

  2. Select two goldfish of similar size. Place one in an open dish of water, to be the control for ventilatory movements. Practise measuring the ventilation rate of this fish with a stopwatch; time the duration of 10 respiratory cycles (opercular or mouth movements) and express this as a rate (min-1).

  3. Determine the volume of the other fish by the volume of water it displaces in a measuring cylinder. Note the volume.

  4. Place the experimental fish in the jar and tighten the lid ensuring no air bubbles are contained within. Note the starting time; this should be timetabled at 5 min intervals to avoid congestion at the meter. Measure the ventilation rates of both fish at 5 min intervals through the experiment.

  5. After approximately 45 min measure the partial pressure of oxygen (PO2) in the respirometer. The calibration and use of the meter and electrode will be demonstrated. Remove a sample of water using the syringe and take this to the oxygen meter.

  6. Weigh the fish. Measure the PO2 of the aerated water in the container from which the experimental water was taken since this represents the starting value, and record the temperature of the water.

Calculation of oxygen consumption


Writing your report

Draw a graph of ventilation rate against time for both experimental and control fish. Does the ventilation rate of the experimental fish increase, relative to the control, as its oxygen is used up? Can you think of another mechanism that could explain this pattern?

Your report will use data from the entire class so all data must be written up on a board for all students to copy. This must include the mass of each fish and the O2 uptake rate. Draw a graph of the rate of oxygen consumption MO2 against log mass for the class data. Does the rate of oxygen consumption per gram of fish increase or decrease with body size? Write a paragraph (i.e. about half a page) to accompany your results, explaining how and why size influences metabolic rate. (You will need sources other than "Raven & Johnson" to answer this.)