Jacobson et al.: A biomass-based assessment model for Engraulis mordax 



721 



mortality rates and recruitment 

 levels for 1985 to 1991 were ad- 

 justed. For simulations, fishing 

 mortality rates during the 1985 to 

 1991 fishing seasons were either 

 low (best-fit estimates from actual 

 data, 0.005 to 0.12 yr" 1 , Table 4) 

 or high (F=1.0 yr -1 ). Recruitment 

 levels were either low (one third 

 of best-fit estimates in Table 4), 

 equal to best-fit levels, high (three 

 times best-fit levels), or alternat- 

 ing (three times best fit for 1985, 

 one third of best fit for 1986, and 

 so on). There were eight scenarios 

 in total (two fishing mortality pat- 

 terns combined with four recruit- 

 ment patterns) and results for 

 each scenario are averages based 

 on fifty simulations. Each scenario 

 used the same series of random 

 numbers to facilitate comparisons. 

 Results indicate that the SMPAR 

 model is able to track trends in bio- 

 mass (Fig. 4) under a wide variety 

 of conditions. Uncorrected esti- 

 mates underestimated year to 

 year variability but this problem 

 would be reduced after bias correc- 

 tions were applied. 



Age composition in the Mexican fishery 



Catch curves (Ricker, 1975) for a segment of the 

 Mexican fishery based in Ensenada, Baja California, 

 indicate that age at full recruitment to the Mexican 

 fishery decreased from age 2 to age during 1982 to 

 1988. 4 Prior to 1983, most year classes reached full 

 recruitment at age 2 but the 1985 and 1986 year 

 classes were fully recruited at about age 1. The trend 

 toward younger recruitment continued with the 1987 

 and 1988 year classes, which were fully recruited at 

 age 0. 



The stock synthesis model interpreted the Mexi- 

 can age-composition data as evidence for increased 

 recruitment and biomass of northern anchovy. In 

 retrospect, this interpretation seems incorrect be- 

 cause biomass estimates for anchovy declined 

 steadily after the 1985 fishing season (Fig. 2). De- 

 clines in biomass were not evident at the time, how- 



4 Arenas, P., T. Barnes, and L. D. Jacobson. 1994. Fishery and 

 biological data for northern anchovy taken in Mexican waters, 

 1978-1989. U.S. Dep. Commer., NOAA, Nat. Mar. Fish. Serv., 

 Southwest Fish. Sci. Cent., P.O. Box 271, La Jolla, CA 92038. 

 Admin. Rep. LJ-94-03, 24 p. 



ever, because only one index of relative abundance 

 for northern anchovy (the modified historical egg 

 production index or MHEP, Lo and Methot, 1989) was 

 available, and because recent biomass estimates are 

 relatively uncertain. 



Status of the stock 



Northern anchovy biomass (Fig. 2) and recruitment 

 (Fig. 3) declined after the 1985 fishing season to about 

 the same levels as during the 1963 to 1971 fishing 

 seasons. Northern anchovy have been too scarce off 

 Baja California, Mexico, to support a fishery since 

 the 1990 season (Table 1). Declines in biomass dur- 

 ing recent years were due to low recruitment rather 

 than to high fishing mortality rates because fishing 

 mortality rates were moderate after the 1986 fish- 

 ing season (<0.14 yr -1 ) and low (<0.03 yr -1 ) during 

 the 1990 to 1991 fishing seasons (Table 4). The re- 

 cent period of low northern anchovy biomass occurred 

 as Pacific sardine, Sardinops sagax, biomass levels 

 began to increase in the early 1980's and water tem- 

 peratures began to warm (Barnes et al., 1992). We 



