The Importance of Estuarine Environments for Pacific Salmon

Fish monitoring at Hollyhock flats will be starting this summer! We've summarized a scientific article explaining what kinds of habitat are important to salmonids.

The article Habitat use by juvenile salmon, other migratory fish, and resident fish species underscores the importance of estuarine habitat mosaics contends that limited and degraded estuarine environments pose a large obstacle to salmon survival. As they leave rivers, salmon enter and stay in estuaries before migrating to the ocean. All Pacific salmon species use these habitats during their downstream migrations, with some residing there for days to months.  Therefore, the abundance and quality of the habitat in estuarine environments greatly impacts salmon survival rates.

Determining the significance of estuarine habitats is complex. An estuary is subject to daily tidal variations and fluctuations from seasonal changes in temperature and hydrology. To compound the issue, each species using an estuary has a unique life-cycle pattern.

The Fraser River, where the study was conducted, is a primary source of freshwater and significantly impacts the marine ecosystem surrounding it. It supplies essential terrestrial nutrients that support the fish communities and influences the migration routes of emigrating salmon and it governs the nutrient cycling processes of the surrounding Salish Sea. The estuary’s lower reaches serve as a crucial habitat for over 50 fish species, including those found in freshwater, estuarine, and marine environments. 

Over 70% of the Fraser River Estuary has been permanantly transformed by industrial, agricultural, and urban development leading to reduced connectivity between habitat patches. Examples of this transformation include habitat degradation and shoreline modification, alteration of hydrology, nutrient and waste runoff, and noise pollution.

Within this estuarine environment, there are three habitats used by salmonoids and other migrating fish: seagrass, marsh, and sand flats, which together form a ‘seascape’. The article found that in terms of estuarine fish species richness and abundance, eelgrass made the largest contribution and exhibited the greatest seasonal variation. While marsh habitat supported the fewest species, it supported more distinct species as compared to sand flat and was particularly preferred by juveniles of the five commercially significant Pacific salmon species. In the Fraser River estuary, marsh was found to be the most used habitat by migrating salmon.

Concerning Pacific salmon species in the Fraser River Estuary, the authors of the study concluded that concentrating exclusively on eelgrass habitat remediation could exacerbate the reduction in salmon populations by depriving them of the remaining marsh. This highlights the idea that interconnected seascapes comprising various habitat types support higher biodiversity and productivity.

Chalifour L, Scott DC, MacDuffee M, Iacarella JC, Martin TG, Baum JK (2019) Habitat use by juvenile salmon, other migratory fish, and resident fish species underscores the importance of estuarine habitat mosaics. Mar Ecol Prog Ser 625:145-162.