AFRF: Fisheries Assessment Primer

FISHERIES ASSESSMENT PRIMER:

What Makes Fish and Shellfish a Renewable Resource?

Renewable resources like finfish and shellfish are living things that replenish themselves naturally and can be harvested, within limits, on a continuing basis without eliminating them. The scientific principles behind this renewability are well-known and provide the basis for fish and game management.

Survival:
All animals produce more offspring than survive to adulthood. This is a kind of biological insurance against the natural calamities all animals face. Actually, for a species to maintain itself, each pair of fish only has to produce two offspring that survive to reproduce. Most individual fish and shellfish produce tens of thousands to millions of eggs. Most of their eggs do not survive to become juveniles and even fewer live to become adults.

Surplus Production:
This extra production together with the effects of harvesting fish can result in surplus or sustainable production.

The theory of surplus production goes something like this. In an unfished population, the biomass (total weight) of fish in a habitat will approach carrying capacity (maximum amount that can live in an area) of the habitat. Furthermore this population will have a lot of older, larger fish compared to a fished population. These fish dominate the habitat and their presence prevents all but a small percentage of the young fish produced each year from surviving to become old fish. When fishing begins, many large older fish are removed. Removal of these older fish and other fish reduces the biomass below the carrying capacity and increased the chances of survival for smaller, younger fish.

The unfished population can be viewed as a relatively stable population with moderate production. The fished population, on the other hand, is a dynamic population with a higher rate of turnover of individual fish as the older fish are replaced by younger, faster growing fish. Some of this new production must be allowed to survive and reproduce to maintain the population. The remaining or surplus production is available for harvest.

How many fish can we catch?
The basic goal of fishery biology is to estimate the amount of fish that can be safely removed (total allowable catch - TAC) while keeping the fish population healthy. These estimates may be modified by political, economic, and social considerations. Overly conservative management can result in wasted fisheries production due to under-harvesting, while too liberal or no management may result in over-harvesting and severely reduced populations.

More on surplus production:
As you may have guessed, surplus production is a complex biological process that is influenced by several factors. These factors merit further discussion.

Carrying capacity:
One factor is that of carrying capacity. Carrying capacity can be thought of as the amount of fish an area of habitat will support. Habitat that historically supported 100 million pounds of red drum is unlikely to support a lot more or a lot less red drum unless conditions change. For example, if the amount or quality of habitat is reduced, carrying capacity will likewise be reduced.

There is no question that human activity has altered, and in some cases, reduced fish habitat. Water pollution, loss of coastal wetlands and seagrasses, destruction of spawning areas, and changes in freshwater flows are some habitat alterations that have led to habitat reduction. Unfortunately, fishery managers and fishermen have had little say in habitat alterations. Fishery managers are saddled with managing the fish populations that the habitat can support today, not the fish populations that past habitat conditions supported.

Ever-changing carrying capacity:
Another aspect of carrying capacity is that it changes as environmental conditions change from year to year. The most obvious example of this is found in the brown shrimp fishery of the Gulf of Mexico. From 1980 to 1992 landings were as high as 193 million pounds (1986) and as low as 125 million pounds (in 1983). Most of this variation can be attributed to salinity conditions in the marsh habitat used by very small shrimp. When conditions were good (high salinity), there were more suitable habitat and more young shrimp survived. When conditions were poor (low salinity), there was less suitable habitat and fewer young shrimp survived. The biological principles that cause surplus production are the natural methods that a species uses to increase the population when environmental conditions are favorable.

Summary:
Harvesting fish lowers the population below the carrying capacity of the environment. Continued harvest depends on the ability of the population to produce enough offspring to move toward the maximum carrying capacity. Variations in natural conditions can alter the carrying capacity, resulting in good years and bad years for survival of young.