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Assortative mating based on ecologically divergent traits is a major driver of speciation among three-spined sticklebacks, however, little is known about reproductive isolation and variations in courtship behaviour among nine-spined sticklebacks. Here we demonstrate assortative mating and divergent courtship behaviours between two cryptic species of nine-spined sticklebacks using no-choice mate trials and kinematic analyses. Strong assortative mating was demonstrated in our tank experiments, highlighting the importance of prezygotic reproductive isolation in these species. Kinematic analyses showed that the freshwater type exhibited aggressive courtship behaviour with frequent ‘Attacking’ and spent more time exhibiting displacement activities. In contrast, the brackish-water type demonstrated a higher frequency of the ‘Zigzag-dance’ and ‘Nest-showing’. Our study highlights the value of nine-spined sticklebacks as a comparative system for the study of speciation and paves the way for future studies on the role of courtship behaviours and assortative mating in the evolution of sticklebacks.

The Chukchi Sea environment changes considerably in physical and biological conditions, driven by the expanding volume of warm Pacific summer water. These environmental changes can affect the migration timing of baleen whales in the southern Chukchi Sea. However, few studies have been conducted in this area to determine the migration timing of bowhead whales (Balaena mysticetus), the only baleen whale species endemic to the Arctic region. In this study, we conducted a fixed passive acoustic monitoring of bowhead whales in the southern Chukchi Sea from July 2012 to October 2015 and compared the occurrence patterns of vocalizations to physical and biological environmental factors. Bowhead whale calls were detected in fall/winter and spring during the ice-freezing and retreating periods, respectively. The fall migration timing of bowhead whales through the southern Chukchi Sea was delayed in the years when the timing of sea ice formation was late, and it formed increasingly later in the years 2013, 2014, and 2012, in that sequence. Moreover, the sea surface temperature decreased before freeze-up, which affected the timing of fall migration of bowhead whales. There was no clear relationship between the occurrence of bowhead whale calls and the abundance of prey, especially in spring, suggesting that most bowhead whales use the southern Chukchi Sea as a corridor during their spring northward migrations. However, the occurrence of bowhead whales and high abundance of zooplankton in October-November present the possibility that bowhead whales expand their feeding area in the southern Chukchi Sea.

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