Feature

Underwater Sampling Pioneered

Taking blood samples can be a problem at the best of times, but it's even harder when the subject is under water.

By Cathryn Crane

How do you tell what is happening inside a fish when it's swimming around in a normal environment? Many studies of the inner workings of fish involve raising laboratory stock or removing the fish from the water while taking measurements. Naturally this tends to place stress on the fish, and questions hang over the validity of the results.

In a new approach, scientists at Auckland University's Leigh Marine Laboratory are taking the laboratory to the fish. Trained divers are learning how to take blood samples from fish populations at the Leigh Marine Reserve.

It's the first time such a technique has been developed successfully, and it's the brainchild of Dr Ned Pankhurst. He wanted to know how the hormone levels in fish change when they are under stress or in various reproductive stages. No-one knew very much about this and the results obtained from laboratory stock were questionable. Most fish sampled in the wild have been near-surface species, such as rainbow trout, with the sampling conducted out of the water, with equally unsatisfactory results.

Pankhurst's team of divers practiced the blood sampling technique on laboratory fish, learning the best way of taking samples. As fish have small blood volumes, it was important to determine how much blood was required for a sample and when to stop. When it came to working underwater, further problems were encountered.

"[The] potential difficulties included the basic act of performing coordinated tasks underwater," Pankhurst notes. Syringes were prepared and labelled prior to the dives and attached to a weighted board. A plastic writing slate and pencil were used to record details, and a mesh prevented the syringes floating away after the sample was taken.

Diver-friendly fish at the reserve were caught, netted and a blood sample taken. The divers had to be careful to prevent contamination of the sample from the surrounding sea water, so the sample was not taken until the needle was firmly in contact with the fish's vertebrae. The sampling procedure took 30 to 60 seconds to complete and the sexual condition of the fish noted by assessing their sperm state.

In tests involving blue cod, divers moved across the coastline to minimise the chances of sampling the same fish too often. Fin clips were used in other tests to tag fish that had been sampled. The sampling didn't seem to worry the fish any -- demoiselles returned to their nest sites 10-15 minutes after handling.

The results of the various studies are likely to be of considerable interest to fish farmers. There are suggestions that farmed fish or fish under stress from transportation may undergo significant hormonal changes, particularly with regard to reproductive function. In some cases, the reproductive phase is severely inhibited.

By establishing a baseline of normal hormone levels in wild fish, Pankhurst's team are able to compare this with stock fish and assess the effects of confinement and handling. They have also been able to clarify hormonal changes and the different reproductive stages of the fish. Pankhurst's work has been recognised by the Department of Conservation with the awarding of the first Marine Reserves Science Prize.