Inter-annual variability in forage fish population size structure: Comparison of selectivity of traditional vs. non-traditional sampling devices

Abstract

Despite the crucial role of forage fish in ecosystem functions globally, data on population demographic structure are often not readily available for many forage fish stocks. Data to characterize variability in population size structure can be highly informative for population monitoring and management, as such data can be used to estimate cohort strength and recruitment, variation in growth rates, and for body-size dependent broadcast spawners such as anchovy, variation in reproductive capacity. In this study, Anchovy Length Indices (ALI) were developed for the central California portion of the central stock of Northern anchovy using predator- and trawl-derived samples. Five different data sources were examined: seabird, sea lion and salmon diet, and midwater and acoustic-trawl surveys. Seabird and midwater trawl surveys had the longest time-series of data. Seabird and midwater trawl methods differed somewhat in their relative selectivity patterns, yet both sampled all age classes of anchovy and reflected similar patterns of inter-annual variability in length frequency. The combined seabird-trawl ALI (1998–2017) was used to characterize anchovy size structure for years when the fishery was not sampled (< 2014). The overall bimodal distribution of anchovy size was divided at 94 mm, suggesting that separation of age 0 and age 1+ anchovy in central California in summer (June) during this timespan was smaller than historic means for the region. The predator-based ALI provided further information prior to the initiation of anchovy length sampling during midwater trawl surveys (1993–1997). The ALIs encompassed periods of extremely high and low anchovy biomass, as well as extreme warm and cold ocean climate events; small mean anchovy lengths appeared to be related to warm conditions (ENSO).