Assessing developmental errors in branchiostegal rays as indicators of chronic stress in two species of Pacific salmon

Abstract

Stress during ontogeny reduces homeostasis, increasing the formation of developmental errors. Fused and partial branchiostegal rays were assessed as indicators of stress throughout embryogenesis in coho salmon (Oncorhy nchus kisutch Walbaum) and chinook salmon (Oncorhynchus tshawytscha Walbaum). Error frequencies and the proportion of fish possessing them should increase when development is stressed. Coho parr were examined from reciprocal crosses between two hatchery stocks experiencing artificially fluctuated (between 7 and 12 °C) or ambient (natural) temperatures throughout embryogenesis. Temperature means and ranges were equivalent between treatments, allowing patterns of thermal variance to be compared. Chinook presmolts, having high versus low levels of infection with bacterial kidney disease owing to similar levels of parental infection, also were examined. Sampled fish were cleared and stained and the errors analyzed using categorical linear models and observations of positional distributions. Although partial rays in coho were genetically influenced, fusion frequency increased significantly under fluctuating (551) versus ambient temperatures (163), as did the number of fish with fusions (207 versus 104, respectively). No significant difference between bacterial kidney disease groups was observed, indicating the disease had little influence over error development. Positional distributions of fused rays were bimodal in both species, showing anterior and posterior zones of weaker developmental control. Partial rays occurred at initial and terminal positions in the series, suggesting evolutionary vestigialization in both species. Branchiostegal ray fusions are shown to be useful traits for further investigation of thermal stress during embryogenesis in salmon.