Herring Larval Surveys on Explorer by John Dunn

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Herring larval surveys on Explorer

One of the jobs which the Explorer did regularly throughout her service career were surveys in the North Sea and up and down the West Coast trying to determine the potential density of herring. It is almost impossible to overstate how important the herring was and indeed still is to the Scottish fishing community and economy. This seasonal fish shoaled in massive numbers off the English and Scottish coasts within easy reach of coastal communities who in the early years would sail or even row out and set their drift nets to catch this rich bounty. The fish move up and down in the water column chasing their food , which are copedods, in fact it is the oil in the copepods which make herring and mackerel oily. The copepods move up in the water column at night to feed on phytoplankton, and then back down in daylight to avoid being eaten by predators. This is what made the herring relatively easy to catch, the drift nets were just like long curtains of net hanging from the surface and down to about six to eight feet. The fish chasing their food simply blundered into the net and got caught by the gills as they tried to back out.

In the early days of the larval surveys they used to use one meter ring nets which were made of silk, however these were extremely vulnerable to damage , and the ship had to tow them very slowly to avoid splitting them.

In the early days of the larval surveys they used to use one meter ring nets which were made of silk, however these were extremely vulnerable to damage , and the ship had to tow them very slowly to avoid splitting them.

Later developments included the Bridger Gulf three which had a monel metal mesh cone inside it , and was less prone to damage, but the biggest advantage to the ship was that it could be towed at five knots which meant a huge saving in time and meant that larger areas could be covered than before.

A Bridger Gulf three on the after deck of Scotia two.

Subsequent developments were the Aberdeen version of the Dutch Gulf three which was made of Aluminium and had a tightly stretched polyester net inside it on a frame which could easily be taken out to allow it to be washed down and cleaned. These high speed samplers were nicknamed the bomb by the crew, who never knew them as Gulf three high speed samplers.

Aberdeen Design Dutch Gulf Three Being Launched off the Stern of ExplorerAn Aberdeen version of a Dutch Gulf three in Aluminium being deployed over the stern of Explorer.

The sample once collected up aft was taken down to the plankton lab, basically the sink in the dry lab. It was then washed into a jar glass originally and then plastic in the later years, the scientific label was written out in pencil, recording all the details of where and when the sample was taken, this was then popped inside the jar, ensuring that it would not be lost or rubbed off. A dilution of formalin was added to the sample to preserve it, and then it was set aside in a wooden box below the bench. In between sample stations sample jars which had sat for at least forty eight hours were opened and the contents poured out into shallow glass dishes with a black plastic board below them. This was to allow scientific staff to pick out the herring larvae which now preserved had turned white. They were just like white threads with little black eyes, eggs which had also turned white were also picked out. This was a tedious and smelly job, as despite the preserved sample having been washed out using a fine mesh bag over the sink the sample still retained formalin and as an angle poise lamp was often used to illuminate the dish this also caused the fumes to rise off the dish. However it did ensure that a reasonably accurate estimation of the potential stock abundance was obtained fairly quickly.

Pitfalls of this type of sampling were that you had to use a set of tables which had been worked out to give the winch man instruction as to how much wire to pay out. This of course if everything was perfect mean that you could get quite close to the bottom and back again without hitting the bottom. However if the ship’s speed was slower than that expected the net sank faster, with disastrous consequences. If the winch man had not reset the mechanical warp counter on the winch or if indeed the counter wheel was not turning properly , could mean you had way more wire out than you thought again with serious consequences.

When high speed samplers were introduced all of these sampling problems were made even worse as the ship was now moving more quickly and therefore everything happened faster and with even more alarming consequences.

This meant that close collaboration between the scientists , bridge officers and the winch man were essential to ensure that the survey was carried out without serious damage or loss of equipment.

Despite the same internationally agreed survey lines, stations and areas being used year after year, it could be very tricky to obtain good samples. The bridge officer had to watch the echo sounders , and the ships speed ,also ensure that he towed the nets or samplers on a course he knew would avoid underwater peaks obstructions etc. This was not always possible as generally we towed into the tide and wind, as the ship could be more easily kept to a constant speed. The scientists had to be aware of where they were in the survey and prepare for emergency action to be taken if a peak suddenly appeared on the bottom. Ironically trying to pull the net or sampler in quickly had the opposite effect as the depressor or weight would bite into the water and actually take the net or sampler straight into the bottom, so stopping paying out wire and then slowly starting to recover it was the best way to avoid hitting the bottom, but required experience and a certain degree of bravado.

Variants of these high speed samplers are still used to this day in larval surveys, however quite sophisticated electronic devices are now used which can identify and count individual fish in a shoal as well as estimate the number in the entire shoal.

An early ARIES System Being Launched off Scotia 3
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