There is a huge difference in survival rates between male and female pike. Small females survive much longer than small males. This could be because the males may have a behavior which increases the likelihood to be eaten, but the cause could also be caused by behavior differences in the spawning period. It is then that the males have to fight for access to the females.
Thrond Haugen, pike scientist and post-doctor of the Centre of Ecological and Evolutionary Synthesis (CEES) at Biologic Institute suggests that even though pike eat very little during the spawning period, if they catch a rival in their mouths, they would likely eat it.
He has completed detailed and timed research about pike in England”s largest lake, the mile wide and 6 mile long Windermere in the Northwest of England. The numbers are unique because British scientists had total control over all net catches in the lake for more than 50 years. From 1949 to 2000, 9000 were pikes tagged and put back again.
The British scientists, however, did not do a deeper number analysis. Instead, the data was given to professor Nils Christian Stenseth at CEES.
This is the first time in history scientists got access to so detailed an analysis of fish population in a natural lake. Previous studies usually came from artificial environments such as vessels or ponds and were often significantly shorter without any information on a individual level.
After three large computer calculations and statistic analysis, Thrond Haugen found results which could both interest pike scientists as well as the fishing industry. He not only calculated the likelihood for the a pike to end up in the net as well as its natural mortality rate, but he also managed to calculate that the potential for a pike to end up in a net during the winter is dependent of how big the fish is during the spring and how the summer turns.
Currently, he is now working with generalizing the model to make it transferable to other fish species, such as cod, and then one can predict the catch rates nearly a year in advance.
The scientists of CEES are also planning to make a mathematical model so the administration management can use it to optimize the harvest of predator fish.
The English research lake is separated into two pools with a shallow area in between. It is easy, however, for the pike to move between the pools. Thrond Haugen has shown that at any time, the pike choose the pool which optimizes the life time production of offspring.
A pike”s whole purpose is to survive and reproduce as much as it can before it dies.
Thrond Haugen states, “We found the mathematical formula which says something about when a pike should change pool and compared this with historical data on the pike distribution between the pools. The results shows that the pike behaved optimal. This is never been presented earlier in a natural setting.” Haugen”s calculations conclude that is not wise to harvest too many cannibal fish as it can actually influence the growth of small pike.
Haugen summarizes that even though a greater density of cannibalistic fish increases the chance of being eaten, it will also create less competitors for the survivors. The survivors will then have much better growth conditions meaning that a pike which survives though a cannibal regime has good chance for a long and good life.
It appears that both large and small females have a similar high survival rate but is the opposite with males. When males are 20 – 24 inches long (50 to 60 cm), the mortality rate decreases drastically because they are often too large for other fish to eat them. It is only the size of one”s mouth which sets the limits.
It is not unusual that pike eat their own siblings. Because pike make for high quality food, they will grow quite large, and will keep doing this for their whole life.
Thrond Haugen can however not quantify how much pike mortality is caused by cannibalism. Female pike can be significantly larger than the males since twenty percent of their body mass is used for egg production. To lay most eggs possible, it pays off to be as large. The world record is 63 inches (1.6 meters) long and 77,16 lbs (35 kg) in weight.
Lethal Sex Fight
During the mating season, females are a limited resource so the males use a lot of energy fighting. Because pike have a whole mouth full of razor sharp teeth, the mortality is significant after being bitten.
Sex tactics ate important. With some fish species some males stay too small and have to sneak for sex, but with pikes there are no sneakers. They will be eaten by their sex competitors.
The bigger the males become, the greater chance they have to mate with a female. Several males can access the same female after each other, but they have to be close to each other to have a successful mating. The sperm has very low survival in water. The fertilization must therefore happen incredibly fast and abdomen against abdomen.
Thrond Haugen has found a clear connection between temperature and survival. While the likelihood to survive in a cold environment is small with many competitors and little food, it changes dramatically in a warmer environment. Even then the survival rate is even lower with a lot of competition and a lot of food than with much competition and less food.
Haugen comments that the results are not intuitively understandable because there could have been number errors or confusion effects. When there is so much competition for food, pike cannot choose what to eat. Instead, they use their energy to keep their competitors away. These tactics are extra precious because pike have a higher metabolism and therefore need more food in a warmer climate.
These findings have been seen in earlier experimental studies but no one has been able to prove this in the field. Thrond Haugen, in cooperation with Norwegian Institute of Water Research, is also doing analysis on the population of large trout in Mjoesa, Norway.
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