A summary report of life history information on Pacific Mackerel (Scomber japonicus) for the 2023 benchmark stock assessment

The Pacific mackerel (Scomber japonicus) is one of five coastal pelagic species listed in the Pacific Fisheries Management Council (PFMC)’s Coastal Pelagic Species Fishery Management Plan, and has regularly scheduled stock assessments. This report provides a detailed summary of the life history information collected, analyzed, and used in the 2023 benchmark stock assessment for Pacific mackerel. In particular, the goals of this report are to: (1) provide updated age and length data; (2) compute ageing error matrices for use in the stock assessment model; (3) provide updated estimates of the length and age at maturity; and (4) discuss priorities for future life history research to address existing data gaps and assessment needs. To produce the data included in this report, biological samples from Pacific mackerel were collected during trawl surveys conducted in spring 2010-2021 and summer 2012-2022 by the NOAA Southwest Fisheries Science Center (ovaries and otoliths) and from fishery samples collected monthly by the Port Sampling Program of the California Department of Fish and Wildlife (otoliths only) in 2008- 2022. Sagittal otoliths from individual fish were evaluated whole, and absolute ages were assigned based on a July 1 birthdate and the date of capture. Ovaries from female fish were preserved and histologically processed to permit classification as either immature juveniles or mature adults using established methods and terminology. Maturity ogives were estimated for fork length and age using an analytical method based on logistic, non- linear regression. Pacific mackerel ages ranged from 0 to 7 years with 0, 1, 2, and 3 year old fish comprising 46%, 29%, 16%, and 6% of sampled fish, respectively. Fish aged 4-7 years made up only 2.3% of otoliths. Ageing agreement for fish 0 to 3 years old were estimated with high precision, whereas fish aged 4 years and older had lower agreement. Difficulty in estimation of age increased in older fish due to reduction in the spacing between annuli and interpretation of a check from an annulus. Length and age at maturity estimates were generated from females that ranged from 174 to 402 mm fork length and 0 to 7 years of age. The estimated average length (L50) and age (A50) at maturity was 274 ± 1.26 mm FL and 1.01 ± 0.06 years, respectively.