Using repeat injury assessments in adult sockeye salmon (Oncorhynchus nerka) to predict spawning success and describe severity of migration conditions

Abstract

Pacific salmon (Oncorhynchus spp.) are semelparous and anadromous, and during their up-river migration to spawn can experience injuries and mortality from fisheries interactions and adverse environmental conditions. There is the potential to improve models used to predict those losses by including information on surficial fish condition, but for this information to be useful, we need to understand how injuries change over time, and how injury type influences this change. To do this, we used repeated individual assessments to examine whether injuries accrued during migration on Fraser River sockeye salmon (O. nerka), assessed in the last leg of their migration or on arrival at the spawning grounds, could be used to predict their final injury score at death after spawning. We found that injury scores increased over the spawning period, but this was not driven by any specific type of migration-related injury, and fish initially scored as ‘uninjured’ had the largest increase in injury score, indicating a ceiling effect to our scoring method. Females with higher migration-related injury scores were more likely to experience pre-spawn mortality. Initial injury score accrued during migration was positively correlated with final injury score after spawning but only when initially assessed 45 km from the spawning grounds, and females that spawned accrued more injuries than females that did not spawn. Thus our method was able to use pre-spawn injury scores to predict spawning success but was limited in its ability to use post-spawn injury scores to describe severity of migration conditions. This highlights the need to better understand injury and disease dynamics and further refine injury assessment and scoring methods at the individual level if information on surficial fish condition is to be used to either describe or predict migration severity or pre-spawn mortality at the population level.