FISHERIES ASSESSMENT PRIMER
American Fishermen’s Research Foundation
Stock assessment is all the activities that fishery biologists do to describe the conditions or status of a stock. The result of a stock assessment is a report on the health of a stock and recommendations that would maintain at sustainable harvest levels.
For Pacific albacore tuna stocks the stock assessment is carried out for the two stocks, South Pacific and North Pacific by the two regional fishery management organizations, the Inter American Tropical Tuna Commission (IATTC) and the West Central Pacific Fishery Commission (WCPFC). A scientific working group for each stock performs a stock assessment periodically. Scientists from the nations harvesting albacore compile the available relevant data on catch, fishing effort, and length- weight information to carry out a stock status analysis. Additionally information from tagging studies, age-growth studies and ecological information are also evaluated for the status of stocks evaluation. The assessment-working groups analyze the fisheries and other available research data for suitability for inclusion in stock assessment analysis and models to determine stock condition and estimates of near term abundance.
For North Pacific Albacore the reported catch and effort is the primary input to the assessment model. Length compositions collected from fish landed are also utilized along with estimates for age at length derived from studies of growth to convert the length data to age. These data, catch, effort and number of fish-at-age in the harvest, are the primary data in the albacore analysis. Estimates of the natural mortality rate (non-fishing deaths) and the portion of fish vulnerable to fishing gear are also important parameters of the analysis important in the determination of the rate of fishing on the stock.
All of the data are input to models that estimate the removals and forecast the trend in the fishable stock in each age group. The current stock assessments are conducted with mathematical models that incorporate all of the available data in a manner that statistical reliability and data uncertainties can be addressed within the model and estimates of stock abundance can be estimated along with the uncertainty level of the estimates. This builds on basic methods of assessment presented below that forms the framework of the current stock assessment methodology.
A stock assessment based on the fishery (catch and effort):
One of the simplest stock assessment methods requires almost no knowledge about the biology of the stock. However, good information about the fishery is required. In this assessment, the manager only needs to look at the history of landings for the stock and the effort expended to catch the stock. The key word here is effort. Landings data (the amount of fish caught and landed per year) alone are not very useful. Landings can fluctuate up and down for a variety of reasons. A trend of decreased landings may be a cause for concern, but the amount of effort made by fishermen to catch the stock tells the real story.
In order to account for effort, fishery biologists divide the yearly landings by the fishery effort to calculate the catch-per-unit effort (CPUE). For example, three million pounds of shrimp caught by 6,000 vessel-days of effort gives a catch-per-unit effort of 500 pounds per vessel-day. (Fishery biologists often express effort in ways that are foreign to fishermen. For example, “vessel-days” is an attempt to estimate the total days all shrimpers trawled. In a longline fishery, the effort might be called hook-hours where the number of hooks multiplied by the amount of time the hooks were in the water can be used to estimate effort.) The catch-per-unit effort is directly related to the amount of fish in the stock.
A number of fisheries have followed a pattern in relation to the catch-per-unit effort. At the beginning of a new fishery, the catch-per-unit effort is high and the effort is low. As interest in the fishery grows, the effort increases, the catch increases and the catch-per-unit effort usually levels off or declines. Finally, as more effort is applied, the catch declines and the catch-per-unit effort declines even more. When both the catch and the catch-per-unit effort decline, it is an indication that the stock is probably overfished. This means too much effort is being applied for the stock to maintain itself. Landings decline despite increasing effort. The obvious solution is to reduce the amount of fishing until the catch-per-unit effort returns to the earlier stages of the fishery.
This seems simple enough. But why isn’t this assessment used more often? The reasons include:
- Insufficient landings data
- Insufficient effort data
- Fishermen using new technology that make it hard to compare the effort today with the effort of several years ago
Adequate landings data are often available, but the effort data is usually missing, incomplete or unusable. The other problem is that by the time there is a clear decline in catch-per-unit effort, stocks may be well overfished, even to the point of collapse.
If fishing effort is too high, it usually means that there are too many boats in the fishery. Fishery managers call this over-capitalization. This means more money (capital) has been invested in boats than the fishery can support. Over-capitalization can also refer to the ability of fishermen to increase effort without increasing the number of boats. If no new boats are added to a fishery, but each boat doubles its fishing power by carrying twice as much longline or using new technology (sonar, GPS, etc.) The new effort can have the same effect as doubling the number of boats.
Summary of catch and effort:
Landings data are often used to suggest that there are problems in a fishery. Declines in landings or increases in landings are signals that something has changed in the fishery. In either case, the effort by fishermen to catch the stock must be considered. The catch-per-unit effort is the appropriate way to evaluate changes in catch because CPUE is an indicator of stock abundance. Problems arise in measuring effort over time in a fishery that may have changed from sailboats pulling one net to diesel- powered vessels with sophisticated electronics pulling multiple nets.
Assessment bases on a little biology (age at first spawning):
When little is known about the biology of a fish stock, one of the first questions asked is, “At what age does the fish spawn?” The second question is, “What proportion of the fish caught are one-year, two-years, and three-years old?” If some of the fish spawn when they are two-years old, and all spawn at age three, and most of the fish caught are two-years old, then there is a danger that too many fish may be caught before they can spawn and replace themselves. This is called recruitment overfishing.
Harvesting some fish before they spawn does not automatically doom the stock, but it is something that needs to be evaluated. Declining landings, greater effort to catch the same or smaller amounts of fish, or declines in average size of fish are all signs of possible problems. Determining the age of spawning and the age of fish caught is one step toward management.
When fishermen appear to be catching fish before they have a chance to spawn and there are other signs of trouble in the fishery, the usual management response is to protect small fish. Protection most often comes in the form of length limits or gear restrictions that favor the catch of larger fish. Minimum mesh size limits for gill nets is a gear restriction that allows smaller fish to escape.
Unfortunately, protecting small fish does not necessarily get at the larger problem of overfishing. Remember, recruitment overfishing occurs when more fish are being removed than can replace themselves. Overfishing can still occur on the remaining fish in a stock even when the small fish are protected because small fish produce fewer eggs than large fish.
Fishermen sometimes suggest a closed fishing season during the period when a stock is spawning. This would seem logical but the idea is usually rejected by biologists. A fish caught before, during, or after the spawning season is still not available to spawn the next year. As a result, the focus is more on protecting fish until they are old enough to spawn and then determining how many fish can be safely removed without harming the stock. Exceptions to this approach are cases where spawners gather in certain locations and are very vulnerable to being caught in unusually large numbers.
Summary of age at first spawning:
Knowing the age of first spawning and the age of fish being caught is an important aspect of fishery assessment. Size limits and gear restrictions can be put in place to protect fish until they have a chance to spawn at least once. Protecting small fish, however, still does not guarantee against overfishing.
Information for a more complete assessment:
Few fish stocks, if any, have been fully assessed. Fishery biologists and managers always wish they knew more about the fish and the fishermen. Full assessment would include some of the following information about the fishery:
1. The kinds of fishermen in the fishery (longline, rod and reel, netters, recreational, etc.)
2. Pounds of fish caught by each kind of fisherman over many years
3. Fishing effort expended by each kind of fisherman over many years
4. The age structure of the fish caught by each group of fishermen
5. The ratio of males to females in the catch
6. How the fish are marketed (preferred size, etc.)
7. The value of fish to the different groups of fishermen
8. The time and geographic area of best catches
The biological information would include:
1. The age structure of the stock
2. The age at first spawning
3. Fecundity (average number of eggs each age fish can produce)
4. Ratio of males to females in the stock
5. Natural mortality (the rate at which fish die from natural causes)
6. Fishing mortality (the rate at which fish die from being harvested)
7. Growth rate of the fish
8. Spawning behavior (time and place)
9. Habitats of recently hatched fish (larvae), of juveniles and of adults
10. Migratory habits
11. Food habits for all ages of fish in the stock
When the above information is collected by examining the landings of fishermen, it is called fishery- dependent data. When the information is collected by biologists through their own sampling program, it is called fishery-independent data. Both methods contribute valuable information to the stock assessment.
Best available data:
Even in the best stock assessments it is rare that everything that should be known about a stock is known. Assessments proceed with the assumption that the best available information (data) will be used. Fishermen often disagree with this assumption when they are adversely affected. Fishery managers respond that they are obligated to protect the stocks, and in the case of federal fishery management, are mandated by law to use the best available data.
The best available data principle sometimes creates a conflict for fishermen. In the past, when managers have asked for more and better data from fishermen, the result has usually been more regulations. The data appear to have been “used against the fishermen.” From the managers’ point of view the data were used to ensure that the fishery could continue. When fishermen don’t provide good data then the fishery will be managed on the data available, which may be incomplete. This can result in overly restrictive management, which is wasteful or can result in continued overfishing and declining catches. In either case, fishermen are the losers. It is in the long-term interest of fishermen to provide the best data possible.
Some information 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)