AFRF: Fisheries Assessment Primer

FISHERIES ASSESSMENT PRIMER:

Information below was excerpted from "Fisheries Management for Fishermen: A manual for helping fishermen understand the Federal management process" by Richard K. Wallace, William Hosking, and Stephen T. Szedlmayer, originally published in 1994 by the Auburn University Marine Extension & Research Center (Publication #MASGP-94-012; now out of print)

Re-created for the Internet in 2000 by the American Fishermen's Research Foundation.

TABLE OF CONTENTS:

What Makes Fish and Shellfish a Renewable Resource?
Survival · Surplus Production · How Many Fish Can We Catch? · More on Surplus Production · Carrying Capacity · Habitat Loss · Ever-changing carrying capacity · Summary

Stock Assessment:
Some Basics · Stock Assessment Based on the Fishery (Catch & Effort) · Assessment Based on a Little Biology (Age at First Stock) · Information for a More Complete Assessment · Best Available Data

Age, Growth, and Death:
Aging Fish · More Information From Age Structure · Mortality and Spawning Potential Ratio (SPR) · Determining Mortality From Age Structure · Spawning Potential Ratio

Virtual Population Analysis (VPA):
At times, fishery biologists have more information available than is provided by the snapshot of the age structure. Sometimes the number of fish caught from a single year class is known for each year that the year class is fished. Year class refers to the group of fish born in the same year. Using the number caught each year from a year class and the mortality rate, the size of the year class can be reconstructed. For example, if the fish born in 1988 (1988 year class) were first harvested in 1990 and 1,000 fish from the year class were caught during the first year, 900 fish the second year, 800 fish the third year, 700 fish the fourth year, and 600 fish the fifth year (1994), then there had to be at least 4,000 fish alive (1,000 + 900 + 800 + 700 + 600) in the year class when fishing started in 1990.

If the natural and fishing mortality rates are known or can be estimated, then the number of fish in the year class that should have been alive to produce the catch of fish can be calculated. If 600 fish were caught in 1994, there had to be more than 600 fish alive at the end of 1993, because some would have died of natural causes during 1994 and it is unlikely that fishermen would catch all the fish in that year class (fishing mortality of 100%). For the purpose of illustration, assume that natural mortality equaled 20% and fishing mortality also equaled 20% (remember that these should be converted into instantaneous rates to be mathematically correct). Since a 20% fishing mortality removed 600 fish from the stock, then a 20% natural mortality would remove an equal number of fish (600) from the stock. This means at least 1200 fish were alive at the end of 1993. However, only some of the fish that were alive were caught or died, so there must have been more than 1200 fish alive. Dividing 1200 fish alive by the total mortality rate (20% + 20% = 40%) (1200/0.4) gives 3,000 fish alive at the end of 1993. This process can be continued backward until the total number of fish in the 1988 year class is estimated. The reconstructed year class can then be tested with different rates of fishing mortality to see what the affects might be, or the information can be used in other calculations such as determining the spawning stock biomass.

A FEW DEFINITIONS:
Species - a group of similar organisms that can freely interbreed.
Population - A group of individuals of the same species living in a certain area.
Stock - A harvested or managed unit of fish.

Fish stocks:
Ideally the various populations of a species would be the units that are managed; however, this is rarely practical and fishery biologists often refer to stocks rather than populations.

For example, Spanish mackerel occur from Maine to the Yucatan Peninsula in Mexico. For purposes of management in the US, Spanish mackerel are divided into two stocks. Fish from one stock migrate from Florida northward along the each coast of the United States and the others migrate from Florida into the Gulf of Mexico. The two stocks may represent one or several populations that make up the species. However, current knowledge about harvesting patterns and migration patterns dictates that they be managed as two stocks.

Sometimes more than one species is included in a stock because they are harvested together as though they were one species. In other cases, different species may be managed together for convenience.

Summary:
A stock of fish is the practical unit of a population that is selected for management or harvesting purposes. In some cases a managed stock may include more than one species.