Appendix 4: 

 Stock Assessment 

 Principles and Terms 



Much ot the information in this report comes 

 from the scientific anah'sis of fisheries data to de- 

 velop stock assessments. Stock assessment in- 

 cludes an estimation of the amount or abundance 

 ot the resource, an estimation of the rate at which 

 it is being removed due to harvesting and other 

 causes, and one or more reference levels nt har- 

 vesting rate and/or abundance at which the stock 

 can maintain itself in the long term. Such assess- 

 ments often contain short-term (1-S years, typi- 

 cally) projections or prognoses for the stock un- 

 der a number of different scenarios. This infor- 

 mation on resource status is used by managers to 

 determine what actions are needed to promote the 

 best use of our living marine resources. 



Stock assessment analyses rely on various 

 sources of information to estimate resource abun- 

 dance and population trends. The principal in- 

 formation comes from the commercial and recre- 

 ational fisheries. For example, the quantity of fish 

 caught, the individual sizes of the fish and their 

 biological characteristics (e.g. age, maturity, and 

 sex), and the ratio of fish caught to the time spent 

 fishing (catch per unit of effort) are basic data for 

 stock assessments. In addition, the National Ma- 

 rine Fisheries Service (NMFS) conducts resource 

 surveys with specialized fishery research vessels or 

 chartered fishing vessels. These surveys, often con- 

 ducted in cooperation with state marine resource 

 agencies, universities, the fishing industry, inter- 

 national scientific organizations, and fisheries 

 agencies of other nations, produce estimates of re- 

 source abundance. 



Resource surveys are conducted differently 

 from commercial fishing. Commercial operations 

 seek out the greatest aggregations of fish and tar- 

 get on them to obtain the largest or most valuable 

 catch. Fishery research vessels operate in a stan- 

 dardized manner, over a wide range of locations 

 and within waters inhabited by the stocks, to pro- 

 vide unbiased population abundance and distri- 

 bution indices year after year. The survey results 

 are then used with commercial and recreational 

 catch data to assess the resource base. The final 

 critical data comes from studies on the basic biol- 

 ogy of the animals themselves. Understanding the 

 natural histor\' of the harvested species and the 

 other species with which thev interact is crucial to 

 understanding the population dynamics ot living 

 marine resources. 



Fish abundance or population size can be ex- 

 pressed as either the number of fish or the total 

 fish weight (or biomass). Increases in the amount 

 ot fish are determined by body growth of indi- 

 vidual fish in the population, and the addition or 

 recruitment of new generations ot young fish (i.e. 

 recruits, recruits from the same year are said to 

 comprise a year class (or cohort). Those gains must 

 then be balanced against the proporrion ot the 

 population removed by harvesting (called fishing 

 mortality, F) and other losses due, for example, to 

 predation, starvation, or disease (called natural 

 mortality, M). In stock assessment work, removals 

 ot fish from the population are commonly ex- 

 pressed in terms ot rates within a time period. The 

 fishing mortality rate is a function ot fishing ef- 



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