Understanding Fish Population Dynamics and Sustainable Fishing
Population Definition
A population is a set of individuals of a species that lives in a particular habitat, exhibits the same biological behavior, and is kept in balance with the environment. Within a population, there is competition between individuals for food and favorable reproduction ratios. An area typically hosts multiple populations of different species that are interrelated, often forming part of a food web. If we consider the volume of each population, we can analyze the proportion of individuals incorporated each year due to adult reproduction (recruitment) and the individuals lost due to natural deaths (mortality).
Biological Factors Influencing Population Size
Biological factors responsible for population size can be categorized as follows:
- Factors that increase biomass:
- Recruitment: New individuals incorporated into the population each year.
- Growth: The annual weight increase of individuals of all ages.
- Factors that decrease biomass:
- Mortality: Natural death of individuals.
Populations tend to remain in balance without large variations in biomass, represented by the equation: (Recruitment + Growth = Mortality). These terms are neither constant nor independent, as each depends on the number of individuals in the population.
Impact of Increased Mortality
If, for any reason, the number of deaths is higher than normal, the population will have fewer individuals. This can create more favorable conditions for new recruits, allowing a greater proportion to be incorporated due to reduced competition. This decrease in population biomass is offset by an increase in recruitment and growth, eventually returning the population to its original equilibrium level.
The Influence of Fishing on Population Balance
Another factor influencing population decline is human activity, specifically fishing, which adds to natural mortality. This disrupts the natural balance, leading to a new equation: Biomass = Recruitment + Growth – (Mortality + Fishing).
Maximum Sustainable Yield (MSY)
The amount of fish taken by fishing should not exceed a certain value. If it does, the proportion of adults will be significantly reduced, affecting recruitment and decreasing the population. From a fishing standpoint, the goal is to extract the largest possible amount of fish, not just in the short term, but continuously. To maintain this amount over time, the population balance must be maintained: Recruitment + Growth = Mortality + Fishing.
Maximum Sustainable Yield (MSY) is the determination of the maximum catch that can be obtained from fish stocks continuously.
Rational Exploitation of a Fishery
Efforts and Yields: Yields refer to the catch in metric tons (TM). Efforts represent the activity involved in fishing or the means used for it. This includes the number of boats, their power, and the type of nets used. Assuming that the efforts of all the ships in the same fishery are equal, the total effort is proportional to the number of boats. There comes a time when catches decrease despite increased effort. The point on the curve representing optimal fishing, or a sustainable equilibrium, is where fishing is neither excessive nor insufficient.
Overfishing and its Consequences
Overfishing occurs when the catch decreases with increasing effort. This decline in yield is due to:
- Growth overfishing: Smaller, younger individuals are caught. Although more individuals are caught, the total tonnage may be equal to or less than before.
- Recruitment overfishing: Many individuals are captured before reaching sexual maturity, leaving an insufficient number to reproduce and replenish the species.
These two types of overfishing are related, as sexually mature individuals are typically the largest.
Relationship Between Recruitment and Parental Stock
Recruitment refers to new individuals (offspring), while parental stock refers to individuals capable of sexual maturity and reproduction. In certain species, a decrease in parental stock leads to a decrease in recruits. However, in other species, reducing parental stock can increase recruitment due to greater food availability, as the recruits face less competition and can grow better.