Improving Fish Stock Assessments (1998) / Chapter Skim
Currently Skimming:
4 Harvest Strategies
4 Harvest Strategies
Pages 37-58
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 37... ...
Management of fish populations to sustain catches and abundance levels can be based on several alternative means of strategic catch regulation. Fixed exploitation rate strategies are commonly employed in marine fish stocks under such names as FMSY, FO I, FmaX, and F40% (defined later in this chapter)
|
From page 38... ...
, and indicators of how the uncertainty in key population parameters is likely to change in the future depending on current management actions (uncertainty indicators)
|
From page 39... ...
For stocks that are managed using age-structured assessments, spawning stock biomass (SSB, the weight of reproductively mature fish) is an important indicator of reproductive capacity.
|
From page 40... ...
Variability in annual landings is generally considered undesirable compared to maintaining the average yield at a constant level because variable landings create uncertainty in fishing communities and lead to the inefficient and intermittent use of capital by fishers. In fisheries with threshold harvest policies, an important yield indicator is the number of years in which the stock falls below the threshold, closing the fishery (Hall et al., 1988; Quinn et al., l990~.
|
From page 41... ...
provides guidelines for applying precautionary reference points for managing populations of highly migratory fish species and those that straddle national boundaries (UN, 1995~. Fishing Mortality Reference Points The natural mortality rate (M)
|
From page 42... ...
and YPR can be used to define FMSY' the fishing mortality for maximum sustainable yield (MSY; Figure 4.5~. Spawning Biomass per Recruit A common SPR-based reference level, FX%, is defined as the F that would reduce the spawning stock biomass per recruit to X% of the level that would exist with no fishing.
|
From page 43... ...
A dynamic pool model (a and d) describes the effect of fishing mortality rate (F)
|
From page 44... ...
to generate reference fishing mortality rates. An example is Fme`3, the fishing mortality rate that results in an SPR equal to the inverse of the median survival ratio (number of recruits produced per unit of spawning biomass [R/SSB]
|
From page 45... ...
CLOSED-LOOP MANAGEMENT STRATEGIES Harvest strategies based on fishing mortality rate or biomass targets modify harvest tactics depending on the stock assessment results and thus are "closed-loop" strategies in the terminology of control systems theory. In this closed loop, there are tight linkages: regulatory performance depends on assessment performance and assessment performance in turn depends on the regulatory pattern.
|
From page 46... ...
This has led to the recommendation that annual quota should vary with stock size and that quota shares should be set as proportions of the annual TAC. Implementation Problems with Constant Exploitation Rate Strategies Even where quota management has not proceeded to the individual fishing quota approach recommended by many economists, there has been wide acceptance of the simple strategy rule TAC = F x EB, where F is the target fishing mortality rate (e.g., FmaX, Fo i, Fx%)
|
From page 47... ...
Yield losses could still be substantial for CV > 0.5 in some trials, especially when estimation errors were
|
From page 48... ...
Biomass Estimates C.V. Biomass Estimates "Herring," r = 0.3 "Herring," r = 0.8 "Cod," r = 0.3 "Cod," r = 0.8 FIGURE 4.6 Effect of stochastic errors in annual biomass estimation on performance of escapement and fixed exploitation rate strategies.
|
From page 49... ...
When evaluating such regulatory schemes, the costs of placing a large number of fishers in competition in a substantially reduced time period and geographic area must be weighed against the costs of following more standard harvest tactics under realistic assessments of risks. The results of simulation experiments such as those cited above are particularly worrisome because they indicate that yield losses due to biomass estimation error in systems where TACK= FBt can exceed the gains in economic performance expected from individual quota management (from reduced competition and more efficient deployment of fishing activity)
|
From page 50... ...
Biomass Estimates "Cod," r = 0.8 FIGURE 4.7 Effect of stochastic errors in annual biomass estimation on the probability that stock biomass falls below 20% of average carrying capacity at least once during the first 20 years of simulation. Each simulation starts with stock biomass at the optimal equilibrium size.
|
From page 51... ...
Standard assessment methods do not work well unless fishery removals are significant because catches are generally the only quantities measured in absolute units that provide the scale of biomass estimates. Fishery-independent surveys constitute the only way to guarantee that actual abundance trends will be detected by the assessment system, although having such surveys does not guarantee that trends will be assessed correctly, as some of the results of simulations reported in Chapter 5 indicate.
|
From page 52... ...
often is a deliberately oversimplified model that is not too badly biased (in a safe direction) , and gives less variable estimates than would be achieved with a more realistic model.
|
From page 53... ...
Biologists have not been too concerned in the past about overfishing in recreational fisheries, believing that recreational methods are generally relatively inefficient and that any depletion of stock should result in decreasing effort and cause a feedback reduction in harvest rate. This lack of concern about modern recreational fisheries is unwise; recreational methods are becoming much more efficient, recreational fishers are becoming more numerous, and they often target a mixture of species so that a decline in particular species is no guarantee that total effort and fishing mortality risk will decrease in response.
|
From page 54... ...
, and the move to responsible fishing practices may require major changes in fishing gear toward more selective methods to reduce bycatch. From a stock assessment perspective, movement to better gear types will add uncertainty about changes in age-size selectivity patterns, natural mortality, and pre-recruit mortality rates.
|
From page 55... ...
One such simplification is to abandon attempts to mimic the complex sampling schemes followed in practice and instead simulate data using standard sampling probability distributions, but with sample sizes much smaller than those used in real assessments, as an alternative means to achieve realistic sampling variances. The second, more drastic, simplification commonly used in policy evaluation is to replace testing of the actual assessment method on simulated data by a much simpler simulation of the estimation errors that would affect the management parameters used in the decision rule.
|
From page 56... ...
Decision tables have the advantage that scientists and managers must explicitly examine alternative hypotheses about stock condition or population dynamics, the probability of these conditions, alternative actions, and their consequences. Decision tables are limited in the number of different states of nature that can be considered, however; in models with several uncertain parameters, all of the alternative conditions cannot be shown together, yet they must be integrated across stock conditions to produce summary statistics such as expected value and distribution of outcomes for each management action.
|
From page 57... ...
Expectation 750 950 1150 1350 1550 1750 1046 Probability 0.099 0.465 0.317 0.096 0.020 0.003 TACa (thousand tons) 100 0.51 0.63 0.70 0.75 0.78 0.81 0.66 150 0.26 0.45 0.56 0.63 0.69 0.72 0.49 200 0.22 0.26 0.42 0.52 0.59 0.64 0.34 aShown in the interior cells of the table as ratio of stock size at end of management period to unexploited biomass.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.