Pink Salmon - Oncorhynchus gorbuscha

Having been a rarity in Scotland for the preceding 50 years, in the last 7 years Pink Salmon (fig. 1) have seen a significant rise in observations around the country. As such, information regarding their presence is quite limited, although trends seem to be indicating that the species may become significantly prevalent in Scotland within just a few years. 

Native Range and Distribution:
As shown in figure 2, pink salmon are native to the Pacific Ocean (NNSS, 2019), with spawning grounds in North America and Asia (Skóra et a, 2023). Also visible in figure 2 is the non-native range of the species (NNSS, 2019). The spread of pink salmon from the Pacific to the Atlantic was the result of the intentional introduction of the species into Western Russian rivers. This allowed the species access to the North Sea, first reaching Norway, and then making the journey to the UK. (Armstrong, Bean and Wells, 2018).                  As it stands, the countries in which pink salmon has the potential to become invasive includes Belgium, Estonia, Finland, France, Germany, Iceland, Ireland, Latvia, Lithuania, Norway, Poland, Sweden, The Netherlands, and the UK (NNSS, 2019). In the UK, the species is most prevalent in Scotland (NNSS 2019), and within Scotland the species is most prevalent in South Aberdeenshire, Angus, Caithnesshire and the Scottish Borders region (fig. 3). An individual pink salmon was first observed in the UK in 1960, and the first example of spawning was observed 50 years later in 2017, in the Aberdeenshire River Dee (Armstrong, Bean and Wells, 2018).       

Habitat and Characteristics: 
Pink Salmon live in both fresh water and salt water, known as being ‘anadromous’ (NOAA Fisheries, 2023). Their life cycle (Fig. 4) begins in freshwater, where females lay eggs in nests called ‘redds’, to be fertilised by males (Forth Rivers Trust, 2018). Once hatched, they develop into ‘smolts’, the stage at which they begin to migrate toward the ocean (Armstrong, Bean and Wells, 2018). In the oceans they spend 18 months developing into their adult stage, taking on oval spots along their backs. (NOAA Fisheries, 2022). Finally, they return into freshwater rivers to lay eggs, becoming darker in colour and developing a large back hump (NOAA Fisheries, 2022). They will die soon after spawning (NOAA Fisheries, 2023). 
Notably, the species operate on a two-year reproductive cycle, with both odd and even year broods that do not breed together (Armstrong, Bean and Wells, 2018). The broods present in Scotland are those of the odd years (e.g. 2017, 2021).        

Current and Future Trends: 
Figure 5 demonstrates the trends in Pink Salmon numbers since 1960. In 2017, record numbers of the species appeared in Scottish waters. This spike was not repeated in the next breeding cycle (2019), but was observed in the 2021 cycle, reaching the highest ever recorded number at 171 individuals (Fisheries Management Scotland, 2024). 
It is still unclear whether the species could (or has already) become truly established in Scotland. This would require the survival of fish spawned in Scottish rivers to breeding age, wherein they would return to Scotland to lay their own brood. Recent research by Skóra et al (2023) found that Pink Salmon can complete the first freshwater component of their lives in Scotland, with smolt stage fish identified in the Thurso and Oykel rivers. 
However, survival to reproductive age may be the hurdle. Armstrong, Bean and Wells (2018) explain that Scottish rivers are warmer than may be optimal for the species. They suggest this warmth may lead to the species spawning in the autumn season, and thus moving into the oceans during winter when food is limited. As such, survival to reproductive age (when the fish would return to Scottish rivers) is unlikely, although further research is required to know for sure.