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Size Limits: Management Tools of the Fisheries Biologist

Crappie_iStock_000002119643Small_400k-of-500k-used.jpgFisheries biologists utilize regulations to manage angler harvest habits to protect and enhance fish populations. Size limits are intended to increase the number of total or larger fish and are among the most widely used tools available to resource managers for enhancing angling opportunities. There are basically three types of size limits used by the Division of Wildlife and Freshwater Fisheries: minimum length limits, maximum length limits, and slot limits. A better understanding of when and why size limits are implemented will ultimately lead to the success of these types of regulations to improve a fishery.

The most commonly used fish size restrictions are minimum length limits, (e.g., 15 size limit). The intent of minimum length limits is to protect smaller fish from angler harvest, or fishing mortality. This type of size limit is suitable when a fish population exhibits good growth and low survival of young fish to a harvestable or adult size each year (i.e., recruitment). Allowing fish to spawn before they may be legally harvested will benefit a fishery that suffers from low recruitment. Ultimately a minimum length limit will improve the number of large fish in the population provided there are acceptable levels of angler harvest.

General Guideline for Size Limits

Growth Rate

Low Recruitment

High Recruitment

Fast Growing Fish

Length Limit

No Size Limit

Slow Growing Fish

No Size Limit

Slot Limit

On the other side of the spectrum are maximum length limits (e.g., 34 maximum limit) which reduce or ban the harvest of larger fish. Though less common, this type of size limit is imposed on a fishery to protect larger fish from excessive harvest in order to maintain greater numbers of these fish in a population. Growth rates of fish to reach a large size may be slow and these fish represent a smaller proportion of the fishery warranting further regulation.

Slot limits prohibit anglers from harvesting fish within a designated size range (e.g., 13–16 slot limit) while allowing fish under and over the slot limit to be harvested. Typically, a fishery with slow growth and high recruitment will benefit from a slot limit. The goal is to allow anglers to remove a significant number of overabundant small fish below the slot. This will improve or sustain the growth of fish protected within the slot and ultimately increase the abundance of fish above the protected range.

Fisheries biologists are aware that there are tradeoffs when size limits are used. A high minimum length limit may increase the abundance of large fish, but the number of fish an angler could harvest or weigh-in at a fishing tournament would decline. Also, this may impact local economies because of anglers or tournaments going to other less restrictive lakes and reservoirs to fish. Determining when a size limit is appropriate for the fishery is an important decision, especially when multiple groups may be affected by the regulation.

Since the 1970’s, the harvest of bass by anglers has significantly declined because anglers have become conditioned to release all bass in order to promote conservation. However, the effectiveness of a size limit at restructuring a fish population declines with high amounts of voluntary catch-and-release by anglers.

This imposes several problems for fisheries biologists attempting to maximize the potential of a bass fishery. Size limits are dependent upon angler harvest to effectively restructure fish populations. For example, at low levels of fishing mortality the benefits of a slot limit are lost due to lack of harvest below a slot limit. If small fish are not removed from the population, growth rates will not improve. While the catch-and-release ethic by bass anglers has provided benefits to some bass populations, this is not the case for all bass fisheries.

Other angling groups, such as crappie and catfish anglers, have a greater tendency to catch and harvest fish. Since crappie recruitment can be variable from year to year, minimum length limits may protect smaller fish from harvest by anglers, particularly during years of low recruitment. Most catfish populations have good growth and adequate recruitment, and do not warrant size limits. However, trophy-size catfish may live more than 20 years and protection of these fish may be needed to maintain their abundance.

Size limits are a useful tool for biologists to manage fish populations, but for size limits to be successful anglers must accept that keeping some fish to eat is not a bad thing. These regulations depend on angler cooperation and law enforcement to ensure success. Anglers who are educated regarding the purpose of length limits and practice selective harvest will be the active conservationists who help biologists manage, protect, and enhance our valuable fisheries resources.



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