VETTER NATURAL MORTALITY IN FISH STOCKS 



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NATURAL MORTALITY ESTIMATES: FROM PAULY (1980) 



(FISH STOCKS) 



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 0.5 1.0 1.6 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 



ESTIMATED NATURAL MORTALITY (M) 



Figure 1. — Frequency of estimated instantaneous annual rates of natural mortality (M) in 175 different fish stocks, populations, or 

 species. Estimates include both freshwater and marine species. Data from Pauly 1980. 



concerns variability of M within groupings that 

 would be used commonly to estimate M , such as 

 stocks of single species, rather than general pat- 

 terns across species. If M truly varies relatively 

 little during these ages (so that the log of catch- 

 at-age decreases linearly with age), if the age 

 structure has been relatively constant histori- 

 cally (so that catch curves are actually linear, 

 rather than curvilinear as seen in stocks with 

 inconstant age structure, e.g., chapter 2, Ricker 

 1975), and if catch curves actually reflect rela- 

 tively accurately this constancy and low variabil- 

 ity, then most estimates of M derived from analy- 

 sis of appropriately processed catch curves cannot 

 help but be relatively close to the true rate. Model 

 predictions, although in theory sensitive, would 

 in practice be fairly robust to any particular value 

 chosen from the true range of values for M . 



Despite the potential problems with accuracy 

 or precision of existing estimates from single 

 groups of fish, I list in Table 3 most of the esti- 

 mates available for unexploited populations, and 



some of the few existing estimates from exploited 

 populations. My purpose is to identify the appar- 

 ent range of variability in M within single stocks. 

 The estimates are drawn from references cited by 

 Pauly (1980) and other sources. Only references 

 that reported multiple estimates for M are in- 

 cluded, thus excluding most of the references re- 

 viewed. Because these estimates are derived from 

 catch data, the stated ranges are "apparent", 

 rather than demonstrably the "true" values. 



Estimated rates of natural mortality are not 

 particularly constant for either unexploited or ex- 

 ploited groups, and are only slightly less variable 

 within stocks than they are within species. Al- 

 though the range of rates within groups may ap- 

 pear relatively small compared to the total range 

 of rates reported for all fish species (e.g., 0.36 to 

 0.56 for sauger from Lake Nipigon [Table 3] vs. 

 approximately 0.1 to 3.0 for most species listed 

 by Pauly 1980), the maximum and minimum 

 rates reported for single groups differed by at 

 least 50% in 20 of the 22 comparison listed in 



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