Abstract

The procedures used to estimate the daily spawning fraction of northern Chile anchovy (Engraulis ringens) have been revised based on information from 17 Daily Egg Production Method (DEPM) survey estimates of the anchovy spawning biomass. The most commonly used spawning marker is postovulatory follicles (POFs). POFs are transitory remnants of the ovulated follicles that remain within the ovary after spawning, until they are fully resorbed. As the process of Postovulatory Follicles (POF) degeneration is continuous, the assignment of POF stages to daily cohorts of spawners can be imprecise. Therefore, the main objective of this paper is to review the criteria used to assign females to daily spawning cohorts in anchovy (Engraulis ringens) located off northern Chile. For our first step, we review the peak spawning time using recently fertilized eggs. To study the POF duration, we divided the POF degenerative process into seven stages and then fitted a gamma probability distribution to the hourly POF stage fractions in a daily cycle. This approach is a quantitative, objective way to define the range of time during which the POF stages are present in the samples, as well as the overlap between daily cycles. The hourly distribution of recently fertilized eggs (Stage 1) indicated a strong daily spawning time synchrony that peaked at 4:00 GMT (00:00 local time). We found that the class Day(1) cohort (females that spawn on the night before their capture) belongs primarily to POF Stages 1 to 3, with some proportions belonging to POF Stages 4 and 5 according to the capture time. The migratory nucleus (MN) and hydrated (H) phases correspond to the cohort that spawns on the night of capture. The daily spawning fraction estimates the based on the POF or MN and H phases did not show significant differences. Our results indicated an average spawning frequency of four days (S = 0.27 day(-1)). The prevalence of POFs according to female size follow an asymptotic model of the von Bertalanffy type, indicating an asymptote around 0.27 day(-1). The POF presence was higher during cold periods, probably due to a longer period of decay. A general linear model analysis showed that body weight, time of catch and mean temperature significantly influenced the spawning fraction.