Quantifying spatiotemporal variation of nearshore forage fish schools with aerial surveys in Prince William Sound, Alaska

Objective

Changes in abundance and distribution of schooling forage fish, such as the Pacific Sand Lance Ammodytes hexapterus and Pacific Herring Clupea pallasii, can be difficult to document using traditional boat-based methods, especially in the shallow, nearshore habitats frequented by these species. In contrast, nearshore fish schools are easily observed and quantified from aircraft when light and sea conditions are favorable. We used aerial shoreline surveys to assess interannual variability in the distribution and abundance of schooling forage fish in Prince William Sound, Alaska, during the summers of 2010 and 2012–2022.

Methods

During the surveys, aerial observers classified fish schools by their size, species, and (in some cases) age-class. All observations were georeferenced along the flight path, converted to estimated surface area (m²) based on school diameter, and standardized by effort (shoreline kilometers surveyed).

Result

Pacific Herring were widely distributed, and school densities varied annually; there were several spikes in school density of up to 54.38 m²/km interspersed among years of lower average densities (7.73–25.57 m²/km). In contrast, Pacific Sand Lance were usually limited in their distribution to a few predictable locations. School density in these consistent areas varied across years, from a high of 50.98 m²/km in 2010 to a low of 0.15 m²/km in 2017. We validated 88 schools during aerial surveys conducted in 2014–2016 and 2019–2022, of which 76 (86%) were correctly identified to species.

Conclusion

Here, we provide indices of Pacific Herring and Pacific Sand Lance school density over time in shallow nearshore coastal areas of Prince William Sound, Alaska. These indices were generated from aerial surveys, which offer an effective alternative to boat-based surveys for tracking forage fish schools when they occur in shallow and nearshore coastal habitats.