Determining Population Size

Measuring population density How do we measure how many individuals in a population? number of individuals in an area mark & recapture methods Difficult to count a moving target sampling populations

Random Sampling A random sample is when every object (people or things) has an equal chance of being chosen every time something is chosen

Quadrant Analysis Another technique uses random sampling based on the quadrat method. A quadrat is a known area of space (ex: an acre of land) that is subdivided into a grid using quadrats Larger areas and larger organisms require larger quadrats and smaller areas and organisms require smaller quadrats.

Quadrant Analysis To calculate the # of plants in an given area -count the number of plants in a set number of quadrats -average -multiply by the total number of grids This will give you an idea about the total # of that organism in the area.

Transect Used in areas that show variation (slope or seashore) Used in conjunction with quadrant analysis Keep a measurement of distance from water or change in elevation

Mark-and-Recapture To calculate population size -animals are captured -marked -released -recaptured. By comparing the number of animals originally captured (n1) with those recaptured (n3), an estimate of population size can be made. The equation used to make this estimate is called the Lincoln index.

Mark-and-Recapture

Estimating Commercial Fish Stock Population The ICES (International Council for the Exploration of the Sea) surveys fishermen to collect information about the numbers and kinds of fish caught, average size, logbook records to look for changes in the stocks/effort to gain stock over the years.

Estimating Commercial Fish Stock Population Research vessels are also employed to: 1. Cast nets (trawling) to randomly sample fish populations 2. Use echo readers to track schools of fish 3. Calculate the age of fish by reading the rings of their scales; this indicates if the species is reaching maturity to replenish removed stock. 4. Tagging fish to track movements and behavior

Estimating Commercial Fish Stock Population As fish grow, new material is deposited in the ear bones -too few young fish = lack of spawning -too few old fish = over- fishing is taking place

Estimating Commercial Fish Stock Population Maximum Sustainable Yield: Highest proportion of fish that can be removed from the total population without jeopardizing the maximum yield in the future … must be sustainable All of the models are then analyzed mathematically to produce “Maximum Sustainable Yield” predictions. These then help set fishing seasons, limits, and sizes.

Measures to Control Over-Fishing Ban drift nets and bottom trawling (like strip mining the ocean floor). Eliminate by-catch and establish programs for fishermen to access the resources to prevent this (trap doors in tuna nets). Establish marine reserves and no catch zones Enact national and international fish quotas based on scientific models and MSY. Enforce the laws and punish offenders.