MARINE BIOLOGY: USING SCIENCE TO MAKE PREDATORS PRAY!
United Kingdom, August 29, 2003 By Daniel Bagur [About the Author: Daniel Bagur is an English marine biologist, currently living in the UK. He gained his degree in Marine and Freshwater Biology at Aberystwyth University in Wales. At present he is finishing a book on the behaviour of animal life along the rocky shores of Great Britain. His works in wildlife photography and scientific research have been published both in the UK and in Australia. At present he is focused on research into fish behaviour. As a child, Daniel spent most of his time diving in the beautiful freshwater lakes of Europe and along the coastal waters of the Mediterranean Sea. He became a qualified scuba diver and has since pursued a passion for wildlife photography and fish behaviour]
The Fish and Me
Fish as a group are the number one prey item for a huge number of predators, even some invertebrates prey on fish. Predation is so important in the lives of fish that it even affects their sex lives, certain fish are known to reduce the intensity of their sexual displays or court for shorter periods of time when predators are in the area. They are under attack from all angles with danger lurking in the air (herons, gannets and other piscivorous birds) on the land (cats and man amongst others) and in the water (otters, mink and larger fish). Although certain areas are less dangerous than others, fish are not really safe anywhere. A group of small freshwater fish swimming in open water for example could be chased into the weeds by a shoal of perch. Once in the safety of the weeds a pike may chase them into the shallowest regions of the river or lake only for them to be eaten by a heron. How do they deal with these constant threats and how can their behaviour be used to help us catch predatory fish?
Krause and Godin carried out research into the influence that prey behaviour had on the predators’ choice of victim. They found that a predator was more likely to attack fish that were unaware of its presence than those that had become wary upon spotting it. The fishes heightened awareness made them more difficult to catch. By making a plug look ill or weak with a jerky, seemingly uncontrolled retrieve, a fisherman can give predators the impression that their lure is unaware of its surroundings and will make an easier meal than its shoal mates encouraging the predatory fish to make a lunge for it.
Magurran, Oulton and Pitcher found that when a model of a pike was pulled towards two different sized shoals of foraging European minnows, the fish in the larger of two shoals stopped feeding earlier than fish in the smaller shoal. A larger shoal means more eyes and more eyes means that any predator will be seen sooner. The more fish present in a shoal the less chance each individual has of being eaten. Some scientists have suggested that shoaling reduces encounters between predators and prey as the predators must travel through a larger amount of fishless water between sightings of prey than if the prey fish were spread out. Another reason that predators dislike shoals is that they become confused during the chase, changing their focus from one fish to the next in the chaos. This is called the confusion effect and it has been shown to lower the success rate of predators like the pike. For this reason piscivorous fish often prefer stragglers and fish of a different size or colour to the rest. They target these individuals making the chase a little easier. This is called the oddity effect. In this continuous, evolutionary arms race the prey species have fought back. Allan and Pitcher found that fish like to shoal with similar fish in terms of size and colour. Fish seem to know how big they are and seek out others of the same size. This is especially the case when predators get close. Perhaps a lure that was slightly bigger or looked different to the other prey fish present in the water would encourage a predator to attack.
Fish are, quite understandably very wary and when faced with an unfamiliar fish they begin edging their way over towards it, pausing at regular intervals, to evaluate things from each slightly closer viewpoint. This behaviour is called predator approach and is thought to be a method of assessing the level of threat the newcomer will pose. The fish inspecting the newcomer are able to judge the relative safety of certain areas probably based on the position of the eyes and seem to know that the most dangerous place to be is in the area directly in front of the predator. A lure passing in front of a predator is much more likely to be attacked than one approaching its tail. Studies suggest that fish are able to recognize the facial features of predators. Those traits intimidating prey species the most were large eyes and a large mouth. It is interesting to note here that the angler fish’s eyes are greatly reduced in size and the shape of its head and mouth are distorted by protuberances. The angler fish relies on not being detected during close scrutiny from its prey. Perhaps plugs or lures with large eyes and a large mouth would be more likely to scare away other prey fish, making the now lone plug seem lost and vulnerable and therefore more attractive to the predator.
A study by Murphy and Pitcher looked at the predator approach behaviour of the European minnow when faced with a pike. The minnows would approach cautiously and then retreat back to the safety of the shoal. They showed their nervousness in the usual ways, moving jerkily and spending less time feeding. Interestingly, the minnow that had approached the pike just prior to its attack appeared to be more stressed than the others. This research suggests that the minnows are able to read the pikes body language and are aware of imminent attacks.
Brönmark and Miner found that carp exposed to predators grew to become shorter but fatter than those grown in the absence of predators. Fatter fish are harder to swallow and it seems that the presence of a predator has the effect of changing the way certain fish grow. By making our lures thinner they will be more appealing to hungry fish.
When looking for predators, finding the prey species is always a good start. Oceanic species such as wahoo and marlin for example are known to congregate in the presence of prey. This is also true for freshwater predators. Helfman noted that prey fishes liked to aggregate beneath floating objects. These provided protection from aerial predators and the suns glare. Mahi mahi are often found beneath floating weeds and debris in search of sheltering prey fish. Aggregations of piscivorous birds can also indicate the presence of a shoal pushed up towards the surface by predatory fish.
In his book ‘The Captive Sea’ Craig Phillips described an occasion when he placed 200 topminnows into a tank of piranhas whilst working at a seaquarium. Within an hour, half of the minnows had been chased down and eaten. The next day only 12 of the topminnows were left. These 12 fish remained in the tank untouched. Phillips suggested that this was the result of familiarity. The piranhas no longer associated these lucky individuals with food. The really interesting part came when other topminnows were added to the tank. These newcomers were rapidly chased and eaten but the locals were left alone. It seems that prey familiarity detracts from feeding incentive. Perhaps changing lures often when fishing is a good thing, fish may recognize a lure and be less interested after only a few casts.
Research has shown that in green water, red, and in blue water, yellow are the most conspicuous colours to predatory fish. It would make sense to include flecks of such colours on our lures to attract the attention of predators from as far away as possible. Most predators are known to hit their prey side on at speed. It makes sense therefore for these flecks of colour to be placed along the fishes flanks. Fishing lures are often designed to pull fishermen towards the till as opposed to pulling the fish out of the water! It is important not to use too much colour on a lure. This is especially the case when fishing over reefs where fishes protected by toxins, strong spines and tough skin often advertise their inedibility with bright colours. It would be a mistake to advertise the inedibility of your lure. It seems the ideal solution is to use a lure that imitates the predator’s natural prey in size and colour, adding these red or yellow flecks to increase their visibility. In the case of the wahoo, species like the mullet, hardtails and bonita are good examples of prey species to replicate. For marlin squid, tuna and mackerel look-alikes may be more effective.
During the twilight hours predators have been found to be most active and most successful. Hobson found that 10 – 15 minutes after sunset the water column above coral reefs remained empty for 15 – 20 minutes. This period was known as the ‘quiet period’ and occurred between the hiding of the daytime reef fishes just after sunset and the arrival of the nocturnal fish. The absence of fish was due to the fact that predatory fish such as sharks, groupers, jacks and snappers made the most of the light conditions at this time. The predators kept low and were difficult to detect against the darkening reef by any fish above them. The prey species on the other hand were easily visible to the predators as they were silhouetted against the evening sky. By attacking from below the predators were very successful. When the light faded the predator’s success rate dropped dramatically and the nocturnal reef fish took to the open water. The colour of the lure wouldn’t matter during this period as it would appear as a silhouette to all predatory fish. As a side point, all of the open water, predatory teleosts (bony fishes) tested have been found to possess colour vision. Fish dwelling in caves or the deep sea however, are likely to lack this ability. Hobson noted that the ‘quiet period’ ended about 30 minutes after sunset. The optimum fishing time for predators and those aiming to catch them would therefore be the 20 minutes between 10 minutes and 30 minutes after sunset.
Most nocturnal fish do not take the same precautions against predation as their day time counterparts (shoaling for example). To some extent they rely on the lack of light for protection. Nocturnal reef fish lack the vivid colours of those present during the day. Day time species use their colouring as a means of recognizing one another. At night fish cannot clearly see so colourful body patterns become useless. In fact, it is thought that bold, contrasting marks increase s fish’s conspicuousness at night and therefore attract predators. For this reason a lure fished at night could benefit from the presence of sharp contrasting colours.
If we can learn from research into their behaviour which are the best ways a fish can behave and look in order to avoid being detected and eaten then we can make our plugs and lures look and behave in exactly opposite ways encouraging predatory fish to make the most of an irresistibly easy meal.
[Daniel Bagur's E-mail: email@example.com]
Reported by: Sol Jose Vanzi
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