Tackle and Tactics.
THINK LIKE A FISH Part 24 Cod shoaling and feeding.
Eat or be eaten! Whether on land or in the sea this is a fact of nature. We have all watched TV pictures of hunting predators on the plains of Africa, the lion, the cheetah or the hyena singling out the young or weak specimen of its prey before catching and eating it. Things are no different for the fish around our shores. There is a constant struggle between hunters and hunted, large and small, weak and strong. Most of the fish that we like to catch are designed to prey on smaller animals and many of them eat other fish. Of course, in its young stages even a fierce predatory species like the cod has to "watch its back" or risk being devoured by something bigger, even including its own uncles, aunts or grandparents.
A couple of studies show just how tricky it is to be a cod. Fishery scientists like to study what they call "habitat" - which simply means the sort of places that fish like to live in. It is well known that young/small fish often live in or around cover (hiding places) to give them a chance of dodging predators. To test this idea some Canadian researchers studied how baby cod managed to avoid their older brother and sisters. The experiment was carried out in tanks and the baby cod were given the choice of sand, gravel, big stones or a clump of kelp to hide in. Bigger, three year old, cod were introduced to the tanks as predators.
If there was no big cod in the tank or if the big cod was in there but had decided to take a rest (no they aren't always on the feed) then the little cod preferred to swim about over the sandy sea bed and avoided the kelp (at the same time keeping well clear of the predator). If, however, the big cod decided it was hungry and began to forage about the tank, every thing changed and the little cod, sensing the danger, dived for cover. Surprisingly, given the choice, the baby fish hid among the big stones and the kelp was only regarded as a second best refuge. When they were able to tuck themselves away in either stones or weed there was much less chance of being eaten.
Presumably hiding among and under stones gives these baby fish better protection than lurking among waving weeds and little cod are not the only ones to realise this. I have often seen young mullet and bass, which normally swim around in shoals, disappear like magic when they were disturbed. By searching about with my hands and turning over stones the refugees were often discovered neatly tucked away in crevices.
Some time ago there was a television programme in which several species of tropical fish were shown turning over large rocks to get at the animals hiding beneath while other opportunist fish looked on. It would surprise me if some of our own predators, perhaps large cod or wrasse, do not use exactly the same tactics. In fact cod are well known stone turners.
From a very different point of view another Canadian study was interested in just how much big cod move about when they are in the sea and also how they shoal together. In this case sensitive echo sounders were used to follow the yearly shoreward migration of cod shoals over the Newfoundland shelf, which occurs every spring. It was found that both the tightness of the shoals and the rate at which the cod swam were closely associated with the presence of pink shrimps in the water. Pink shrimps are those big pink "prawns" which you can buy at any fishmongers. These juicy crustaceans, even when dead and cooked, are a superb salmon bait and when they are alive and kicking they are an important food for cod and other fish.
The shrimp shoals show up on echo sounding traces as background "scatter" so it was possible to tell where their numbers were thickest. The migrating cod which were usually swimming towards the coast at a speed of about 20 kilometres per day (roughly 14 miles per day) dropped their speed to only about 5 kilometres per day when they encountered large numbers of shrimp. Presumably the wealth of food was just too much for them to resist and the fish got stuck in when they had the chance. Even the urge to migrate was overcome by the chance of a square meal.
A second interesting observation was that the feeding cod shoals spread out from the sea bed to match exactly the depth range of the shrimp hordes. This upward and outward spread meant that the cod were swimming in open water anything up to 85 metres off the bottom. The cod would not rise any further from the bottom than this even if there were lots of shrimps at shallower depths. When the cod were feeding on these big concentrations of shrimps the shoals actually dispersed to do so and at times the individual fish were as much as eight body lengths apart. This means that in a shoal of big fish (say 1 metre long) the feeding individuals could be as much as eight metres from one another. The scientists interpret these changes in swimming speed and shoal structure as ways of making the most of the rich food supply.
Why do cod shoal in this way? Well, for any fish, being in a shoal probably gives them a number of advantages when it comes to avoiding predators. Of course you might think that a fish swimming 'in a crowd' would have more competition for food (less shrimps to go round) but at the same time it can feed more freely because there are more pairs of eyes on the look out for danger. All in all these snippets of information are simply a couple more pieces in the great puzzle of what makes our favourite fried fish supper the great success which it is.
If you have any comments or questions about fish, methods, tactics or 'what have you.' get in touch with me by sending an E-MAIL to - firstname.lastname@example.org
Think like a fish.
Plan your fishing.
Spinning for cod in Greenland.