Enhancing in-house assessment of pink salmon returns at the Solomon Gulch Hatchery.

In the summer of 2007, PWSSC conducted a study with the Valdez Fisheries Development Association (VFDA) to develop a tool that would allow the VFDA to enhance their evaluation of fish abundance with their commercial-grade echo sounders. The goal was to optimize escapement for brood stock at the Solomon Gulch Hatchery while maximizing the opportunity for harvesting the surplus for cost recovery and the open fishery. A secondary objective was to explore the use of three-dimensional modeling for studying fish distribution and behavior in Prince William Sound by combining traditional fisheries acoustics techniques with the novel use Geographic Information System (GIS) modeling and graphical visualization. We jointly surveyed near the hatchery simultaneously with commercial-grade sounders and our BioSonics DTX 120 kHz scientific-grade echo sounder. This allowed hatchery personnel the opportunity to assess first hand the differences between the two technologies and gave us the insight needed to find the most user-friendly product that would be a useful tool.

We conducted the field survey at low tide when fish were concentrated near shore at the edge of extensive mud flats. The main body of fish (about 180,000) was near the diffuse outflow of Solomon Creek. A two-dimensional contour plot of fish density distribution revealed the shape of the school near the freshwater stream. However, three-dimensional volume visualization revealed that the densest aggregations of fish were restricted to the area where Steller sea lions were preying on salmon. A model of the distribution of the density polygons suggested that these dense aggregations were anomalous, perhaps the results of fish behavior in response to the presence of these predators in their midst. Elsewhere, there was a gradation in fish density distribution—gradually increasing from low to high—suggesting that the gradual change could be displayed in the color scheme of a properly adjusted commercial-grade sounder.

The results indicated that in the absence of stress factors that alter salmon schooling behavior, it should be possible to construct a set of templates that would help VFDA conduct semi-quantitative assessments of the number of hatchery returns. The templates would be based on photographs of echograms displayed on VFDA commercial-grade echosounders that were referenced to simultaneous research-grade quantitative determinations of fish density, affirmed with 2D and 3D visualization techniques. This new technique forms the basis for semi-quantitative calibration of a high-quality commercial grade echosounder.