Kleiber and Perrin: Reappraisal of catch and stock status in US North Pacific albacore fishery 



385 



It can furthermore be shown that as a increases above 

 1, E[CPE old ] and E[CPE new ] rise monotonically and 

 approach d h as a gets large (Fig. 4). Thus, old and new 

 CPE are both unbiased when there is no favoritism; 

 the bias increases at different rates for the two as 

 favoritism increases; and they both approach the same 

 bias as favoritism becomes large. 



Relating the model results back to the U.S. North 

 Pacific albacore fishery, we presume that an increas- 

 ing ability to locate high-abundance strata is equivalent 

 to a rising value of favoritism (a) in the model. The 

 divergence of the old and new time series (Fig. 2) in- 

 dicates that the effective a has been increasing toward 

 some intermediate level. Changes in the fishery in the 

 past decade— increasing availability and use of fishing 

 advisories and severe decline in effort with the possi- 

 bility of differential loss of less-able fishermen— are 

 consistent with such a change in a. 



In searching for corroborative evidence that the 

 fishery has been increasingly concentrating on high- 

 abundance strata, we found that the favoritism index 

 given by Equation 3 has been tending upwards, par- 

 ticularly over the last decade (Fig. 5). This coincides 

 well with the divergence in the old and new time series 

 of CPE starting around 1979 (Fig. 2). Given this 

 evidence of increasing ability to locate aggregations, 

 the recovery in the old CPE time series in the past 

 decade reflects a change in fishing operations and not 

 a recovery of the albacore stocks. In fact, if the albacore 

 population had just been holding its own over that time, 

 the new time series should also have been rising 

 because it is also subject to increasing positive bias. The 

 fact that it has been declining indicates that the popula- 

 tion must have been declining even more rapidly. 



Though we have not invented a new population in- 

 dex, free of variable bias, the different effect of bias 

 on our new CPE time series and the original CPE time 

 series has helped reveal a change in fishing operations. 

 This change has markedly affected our interpretation 

 of CPE in the United States North Pacific albacore 

 fishery. To reveal population trends with an unbiased 

 time series requires that we deal with unvisited strata. 

 The difficulties of doing so are great with the type of 

 spatio-temporal variability that we have in the North 

 Pacific albacore fisheries. Attempts are being made 

 (R. Mendelssohn, NMFS Southwest Fish. Sci. Cent., 

 Monterey, CA, pers. commun.) with the inclusion of 

 fishery-independent data (environmental data in this 

 case) to infer CPE in unvisited strata. 



Conclusion 



We have reexamined the details and justification for 

 a routine procedure for processing catch-and-effort 

 data from the U.S. Pacific albacore fleet. We have 

 found that the use of estimates of fishing power to 

 account for variation in catch rates due to vessel size 

 is of negligible value. Standardizing effort for vessel 

 size has had little effect on the observed time series 

 of effort and CPE in this fishery. 



On the other hand, the patchy distribution of albacore 

 is of great importance. In the routine procedure for 

 reporting overall fleet effort, the concentration of 

 effort on areas of high abundance has been dealt with 

 inappropriately, so that CPE trends are unrelated to 

 trends in the population available to the fishery. The 

 new CPE, which is an incomplete correction for con- 

 centration of effort, reverses what has been seen as 

 a rising trend in abundance over the past decade. 



We have shown that the divergence of the old and 

 new time series is consistent with a fishery for a 

 patchily distributed resource and a growing ability 

 to locate high-density patches. Such a scenario is con- 

 firmed by detailed examination of changes in the 

 distribution of catch and effort in the fishery. This 

 change is probably due to increasing use of advisories 

 aimed at locating dense albacore patches, but other 

 possible contributing factors include the differential 

 dropout of less-able fishermen from the fishery or a 

 decreasing representation of less-able fishermen in the 

 sampled landings. Whatever the cause, we have shown 

 that in such a scenario, the original time series would 

 have an increasing positive bias as an index of popula- 

 tion, and the new series would also have an increasing 

 positive, but reduced, bias. The nature of the fishery 

 is such that we cannot calculate a reliably unbiased 

 index of population based solely on the fishery data. 

 However, the implication of our results is that the 

 actual trend in the population should have been a more 

 severe decline than is indicated by the new time series. 



Citations 



Bartoo, N., and Y. Watanabe 



1989 Report of the eleventh North Pacific albacore workshop. 

 Admin. Rep., LJ-89-24, Southwest Fish. Sci. Cent., Natl. Mar. 

 Fish. Serv., NOAA, La Jolla, CA, 17 p. 

 Berude, C.L., and N.J. Abramson 



1972 Relative fishing power, CDC 6600, FORTRAN IV. 

 Trans. Am. Fish. Soc. 101:133. 

 Beverton, R.J.H., and S.J. Holt 



1957 On the dynamics of exploited fish populations. U.K. Min. 

 Agric. Fish., Fish. Invest. (Ser. 2) 19, 533 p. 

 Deriso, R.B., and A.M. Parma 



1988 Dynamics of age and size for a stochastic population 

 model. Can. J. Fish. Aquat. Sci. 45(6): 1054. 



