Selawik River Inconnu Spawning Population Age Structure Evaluation and Spawner Recruitment Response to a 2004 Permafrost Thaw Slump, Selawik National Wildlife Refuge, Alaska

Abstract

In 2004 a large permafrost thaw slump (slump) began emitting substantial volumes of fine sediment into the Selawik River upstream from the Inconnu Stenodus leucichthys spawning area. Following this event, the once clear-flowing river ran turbid for about nine summers before the slump stabilized and temporarily ceased emitting large amounts of sediment. To determine if sediment input had a negative effect on Inconnu spawner recruitment age cohorts, we investigated: 1) age and length data from male Inconnu from the Selawik River spawning population in 2023 and 2024; and for comparison, we obtained similar age structure data in 2024 from Inconnu preparing to spawn in the Kobuk River, a neighboring drainage without a slump; and 2) the effect of the slump on spawner recruitment based on the results of age structure tests. In the Selawik River in 2023 and 2024, we collected otoliths (age samples), fork length, weight, and identified sex from 202 and 210 spawning Inconnu, respectively. In the Kobuk River, 174 Inconnu samples were collected in 2024, but samples were unavailable in 2023. The proportion of females in sampled Inconnu from the Selawik River, 2023–2024 was 0.40 and 0.28 respectively. In 2024, mean fork lengths of males in the Selawik and Kobuk River samples were not significantly different (t283 = -0.83, P = 0.407). The median ages of Selawik (age-18) and Kobuk River (age-17) Inconnu were significantly different (W = 42822.0, P = 0.004). Age histograms for both populations indicated similar recruitment patterns. Ages 16–20 were more strongly represented in the Selawik and Kobuk rivers in 2024. All post-slump age cohorts were present in the Selawik River age samples from 2023 and 2024, thus allowing us to conclude that the slump had negligible effect on Selawik River Inconnu spawner recruitment. Selawik River Inconnu have consistently returned on an annual basis to spawn from 2004 to present. The negligible response of the Selawik River Inconnu spawning population to the slump’s chronic and prolonged sedimentation events may be because of: 1) the reduced attenuation of sediment carried from the slump over the ~50 river kilometers to the spawning habitat and resulting sediment dilution; 2) the inherent spawning habitat’s hydrological qualities, such as potential upwelling groundwater and interstitial flow, that promote successful reproduction under marginal water quality conditions. Subsequent age collections are recommended in approximate 5-year intervals as a management tool that may identify disruptions from anthropogenic and natural environmental stressors to Inconnu spawning populations in the Selawik and Kobuk rivers.