Our knowledge of the herring biomass comes from the fish that spawn each year. Some of the younger fish may not spawn though. The stock assessment model used by the Alaska Department of Fish and Game (ADF&G) includes the age at which a herring first spawns (or the age at maturation) as a parameter. The maturity function is then used to account for fish not observed during spawn. Sometimes a herring will not spawn if conditions aren’t right or if they don’t have enough energy stored up. These fish are known as skip spawners. The objectives of this study were to determine if using scales can be used to assess age at first spawn or maturation and skip spawning of older ages. Herring lay down annual growth rings on their scales in proportion to their body growth. Reproduction takes a lot of energy from the herring resulting in reduced growth of spawning fish relative to nonspawners. Therefore, growth in scales provide a spawning history for individual fish across their lifetime.
Using a captive population of wild adult herring, researchers from the National Oceanic and Atmospheric Administration (NOAA) conducted a lab study to determine if they could identify whether individual herring are immature (have never spawned), primiparous (have spawned only once), or repeat spawners. Researchers also tested to see if the age at first spawn could be determined by analyzing the history of egg development in the ovaries (histology), or by using growth increments on herring scales. They looked at over 1,700 measurements of scale growth from herring ages 4-6 collected from PWS spawning populations between 1986-2013.
Researchers confirmed that herring scales can be used to distinguish spawners from nonspawners. Comparison of maturation from histology to scale growth showed that 3 year old herring preparing to spawn for the first time had significantly smaller growth rates relative to non-spawning 3 year olds. Scale growth of herring ages 1 and 2 indicated that all fish of these ages were immature and do not spawn while ages 3-6 showed difference in growth patterns and that equated to spawning activity. Results indicated a difference in skip spawn probabilities between male and female herring. Females had a higher overall probability of skip spawn (13-50%) and males (10-15%).