Different fish may have different life strategies in terms of growth, maturity and spawning.
When we investigate life history traits of a population, we mean:
- are the fish very fast growing, reproduce at a young age and have a short life span? or
- are they slow growing, reproduce relatively late in life and have a long life span?
Roach demonstrate contrasting life history traits between fisheries. In some lakes, they grow very fast (growth index values of >150 per cent) but rarely live above five years old. Their age of first spawning may be as young as two or three years old. In some lakes and rivers, roach growth can be very slow (growth index values <80 per cent) and live to over 15 years old. Their age of first spawning may not be until five or six years old.
What is the mortality rate? Why should we be interested?
The mortality rate is the rate at which fish die in a population. It is usually expressed as an average, annual value. It can be converted into a survival rate to demonstrate how many fish survive in a given year.
The mortality and survival rate is crucial to our understanding of a fish population. Following a successful spawning year of chub, we may expect some of those fish to be present in anglers' catches for the next 15 to 20 years, as chub generally have a very long life span.
However, if our life history traits indicate that the chub are very fast growing and the mortality rate is high (and so survival low), then these chub may only be present in angler catches for five or six seasons. Such information is crucial in our understanding of how and why angler catch rates change in both the short and long-term.