Fishery Management biology

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Fishery Biology

Finfish and shellfish are a renewable resource. They replenish themselves so they can be harvested within limits and still survive. Animals produce more offspring than survive. In theory if 2 fish produce 2 offspring that live the species will continue to survive. Fish lay thousands to millions of eggs. Very few of these eggs survive to be juvenile Fish. Fewer survive to be adults.

Surplus production

In theory the total weight (biomass) of fish in a habitat will approach the carrying capacity of the habitat (the maximum amount that can live in that area). If there is no fishing the population will be dominated by older larger fish. They prevent the survival of many of the younger fish. When fishing begins older fish are removed from the population and smaller fish survive because the biomass is below the carrying capacity. The fished population becomes a dynamic population with high turnover of individual fish. Some of the young fish must be allowed to survive to reproduce in order to maintain the population. The remaining surplus population is available for harvest.. You must always leave enough young and spawners in the population to continue to reproduce.

The basic goal of fishery biology is to determine the amount of fish that can be removed from the fishery and still have a healthy population. Political, economic and social considerations will influence the final optimum yield figure. The fishery is wasted by measure that are too conservative and decimated if decisions are too liberal.

Carrying Capacity

Habitat that supports a certain amount of fish historically is unlikely to support much more or less than that unless the habitat changes.
When habitat is lost then fewer fish can be supported. Habitat can be lost by

• Pollution
• loss of wetlands
• loss of seagrasses
• destruction of spawning areas
• changes in freshwater flow- causes change in salinity

Fishery managers can’t change habitat, so they concentrate on how many fish the existing habitat can support. The carrying capacity can change over time as conditions in the habitat change.

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Stock assessment is all of the activities fishery biologists do to describe the status of a stock. magnuson requires that all fished stocks be assessed on a regular basis to prevent overfishing.

Stock assessment has 2 basic activities
1 to learn all they can about the biology of the sepcies
2 to learn all they can about the fishing activities for the species

Stock assesssment based on the fishery- Catch and Effort

Here little has to be known about biology. It is based on the history of landings for the stock and the effort expended to catch that amount.

CPUE - catch per unit of effort
Divide the yearly landings by the total effort ie: 3million pounds of shrimp divided by 6000 vessel days of effort = 500 pounds per vessel day CPUE

Vessel days are used in trawler type fishing
Hook hours are used for longliners = the numer of hooks multiplied by the hours they were in the water.

A decrease in the CPUE means less fish are in the fishery.

Assessment on catch and effort isn't used often because

• Insufficient landing data
• Insufficient effort data
• New technology makes direct comparisons with previous data hard

Over capitalization of the fishery is when fishing effort is too high for the fishery. Either there are too many boats or the technology has improved enough to increase the fishing pressure.

Assessment Bases on a little Biology

Age at first spawning.
What age do the fish first spawn ?
What proportion of total catch are of each age.

Recruitment overfishing -
If some of the species spawn at 2 years and all spawn by 3 years, and most of the fish caught are 2 years or less , the the fishery can be in danger. This is recruitment overfishing.
Protection usually comes in the form of size limits or gear restrictions that favor larger fish.

Overfishing can still occur because larger fish usually produce more eggs than smaller fish.

A closed season during spawning doesn't always work, because the fish are still removed prior to the next spawn which can still lead to a stock reduction. This technique does work however when a species spawns in a very limited area and they are vulnerable to being caught in huge numbers at that time- Protected areas and marine reserves are often used for this purpose.

A full assessment of a fishery would have to include

• the kinds of fishermen in the fishery- rod and reel, longliners, netters etc
• Pounds of fish caught by each type of fishermen
• Fishing effort by each type of fishermen
• the ages of fish caught by each type of fisherman
• ratio of males to females in the c atch
• prefewrred size for marketing
• the value of the fish to each group
• the time and area of the best catches

Biological info needed

• age structure of the stock
• age at first spawning
• number of eggs each fish can produce (fecundity)
• ratio of males to females in the stock
• natural mortality
• fishing mortality
• groth rate of the fish
• time and place of spawing
• habitats of larval fish, juveniles and adults
• migratory habits
• food habits for all ages of fish
• estimate of the total number or weight of the fishery

Fishery dependent data - when the above data is collected by examining landings

Fishery independent data - when biologists collect their own data by sampling

Best available data. Assessments must proceed on the best available data. When fishermen do not supply good data the the data avaiolable must be used. Often it seems that supplying the data leads to more regulation, but use of incorrect or incomplete dats can lead to mismanagement of the fishery.