Improving Fish Stock Assessments (1998) / Chapter Skim
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Pages 13-26
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From page 13... ...
BASICS OF SAMPLE SURVEY DESIGN Survey sampling techniques almost always employ a probability- or design-based sampling scheme, the basic ingredients of which are as follows (Cochran, 1977; Thompson, 1992~: · A finite population of unique and identifiable sample units (e.g., fishing vessels, area of bottom swept by 13
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From page 15... ...
Methods that rely on spatiotemporal frames are called direct methods for commercial data collection and on-site methods for recreational data collection because interviews of anglers or vessel captains are conducted as fish are landed and the landed fish can be observed directly, measured, and weighed. Time-tested approaches based on the sampling design selected are used to sample from both types of sampling frames.
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From page 16... ...
Degradation in data quality also results when fishers do not trust stock assessments or other fishery management because they believe that such activities are responsible for increasingly restricted quotas, shortened seasons, and diminished opportunities. When stocks are overexploited, catch and effort quotas are mandated through fishery management plans to allow the stocks to increase.
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From page 17... ...
The two methods used in on-site sampling of commercial fisheries are port sampling (Burns et al., 1983) and observers on-board fishing vessels (Murawski et al., 1994~.
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From page 18... ...
Spatial and temporal frames should be the same as for commercial fisheries so that data can be combined for assessments of stocks that are used by both commercial and recreational fishers. Although catch-at-age and surplus production models have been the mainstay of commercial fishery management, they are only beginning to be used in recreational fisheries (e.g., Quinn and Szarzi, 1993~.
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From page 19... ...
In either case, these survey methods are relatively expensive; only trained interviewers can be used and their travel can be costly. On-site methods rely on spatiotemporal sampling frames; hence, the sampling frame is delineated by the geographic extent of the fishery and the timing and length of the fishing season.
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From page 20... ...
Limitations Aside from biases inherent in recreational data mentioned previously, violation of the assumptions underlying the relationship between catch and population size can compromise the accuracy of model predictions. Many stock assessment models assume that C= qpV (2.1)
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From page 21... ...
For stock assessments in such mixeduse fisheries, imprecision in recreational catch estimates add little to the overall uncertainty in total catch. However, in a few fisheries recreational catch is a considerable or predominant component of total catch, for example, for bluefish and striped bass.
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From page 22... ...
numbers and weight of fish caught; length and age compositions; and biological information such as gender, maturity, fecundity, and condition. Most trawl surveys conducted for stock assessment purposes also collect abundance and demographic information for other species captured and therefore offer an opportunity for ecosystem-based assessments over broad temporal and spatial scales.
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From page 23... ...
The robustness of the models and associated estimators with respect to likely violations of their basic assumptions have to be assessed. Most assessment models assume that survey catchability and the relative vulnerability and availability of different age classes stay constant over time, so that survey catch rates (either by age or overall)
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From page 24... ...
Responding to decreasing survival or shrinking geographic range caused by changes in climate and other environmental factors could require different assessment and management strategies than those now used to detect changes in fish abundance presumed to be caused by overfishing. The accuracy of fish stock assessments can be affected by environmental factors.
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From page 25... ...
3. Can these predicted effects be incorporated into fish stock assessments and fishery management strategies?
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