Monthly abundance estimation for common sardine Strangomera bentincki and anchovy Engraulis ringens in the central-southern area off Chile (34-40°S)

Abundance, biomass, recruitment and fishing mortality of common sardine, Strangomera bentincki, and anchovy, Engraulis ringens, are estimated for stocks distributed in the central-southern area (34-40degreesS) off Chile. Monthly length-frequency data. from July 1990 to November 1999, were used to identify cohorts, to analyze growth, and to determine the monthly age composition of the catches. The abundance of cohorts was estimated using cohort analysis on a monthly time-scale that is sufficiently small for capturing the rapid seasonal changes that characterize both resources. An ADAPT approach was used to estimate the unknown parameters by using catch per unit effort (CPUE) data as auxiliary information. Growth was a regular process for different cohorts of both species, and a seasonal oscillating growth curve was fitted. The parameters of the seasonal von Bertalanffy growth function were: L-infinity = 17.7 cm, K = 0.79 yr(-1), t(0) = -0.347 yr, C = 0.996, and winter point (WP) = 0.373 (r(2) = 0.930, n = 207, P < 0.05) for common sardine; L-infinity = 20.8 cm, K = 0.444 yr(-1), t(0) = -0.392 yr, C = 0.849, and WP = 0.310 (r(2) = 0.906, n = 203, P < 0.05) for anchovy. The growth rates were slowest between April and May, before the spawning season. Recruitment of common sardine occurred in November, while recruitment of anchovy tended to occur in January. The yearly pulse of recruitment dominates the population dynamics of both species, and it was observed that the recruitment of the two species varies inversely. The 1997 cohort of common sardine was the lowest in the 1990s, but recruitment of anchovy was higher, probably as a consequence of the 1997-1998 El Nino event. The biomass of the common sardine stock declined from 1997 to 1999, while fishing effort was growing. On the contrary, the biomass of anchovy was higher in 1998 and 1999 due to the stronger cohorts of 1996 and 1997. Evidence for density-dependent catchability was found for sardine, but not for anchovy, At present, both species can be considered as overfished because a growing trend in fishing effort is not compensated by the productivity of the stocks. (C) 2002 Elsevier Science B.V. All rights reserved.