Introduction & Objectives

Technical Approach

The survey area was divided into six strata of varying geometry to facilitate allocation of survey effort and improve sampling efficiency. Each complete survey of the Canal was randomly created using custom‐made functions and libraries developed for the open‐source software R. Systematic zig‐zag tracklines were created with a random starting point. A slightly higher effort per unit of area was allocated to Strata 1‐4 where the highest concentrations of harbor porpoises were expected to occur based on previous work, as well as to provide more coverage to areas that the U.S. Navy uses for testing and training. Strata 2‐4 and the northern half of Stratum 5 contain the areas of naval operations, Dabob Bay Range Complex and Bangor Study Area. 

In summary, surveys were designed to meet the following objectives: 

• Obtain an approximately uniform sampling coverage of the study area;
• Sample the entire Hood Canal, previously surveyed in 2016, for possible comparisons between these surveys and for future estimation of trends in abundance;
• Optimize survey effort within the allocated survey period. 

Progress & Results

Vessel surveys were carried out during 37 days in all seasons from 09 February 2022 to 08 February 2023. Observers located on two independent platforms (an upper and lower deck of the same vessel) searched for species of interest during good weather conditions. Acoustic recordings were also made when feasible. A total of 2175.8 kilometers of on‐effort trackline representing 691 lines were surveyed and 809 harbor porpoise groups (1,385 individuals) and four groups of transient killer whales were observed. In addition, 1,147 sightings of harbor seals (Phoca vitulina, 1,249 individuals), 4 sightings of Steller sea lions (Eumetopias jubatus, 4 individuals) and 23 sightings of California sea lions (Zalophus californianus, 57 individuals) were documented. Estimates of abundance showed a clear seasonal pattern in the occurrence of harbor porpoises in Hood Canal. The greatest abundance occurred in the fall totaling 1,336 individuals (CV=0.25) with the lowest, 308 porpoises (CV=0.25), occurring in the winter. These results indicate that harbor porpoises have increased significantly in abundance from previous surveys (2013‐2015), are most abundant in the summer/fall, leave the Canal in the winter, and begin returning in the spring. Density of porpoises was higher in the central region of Hood Canal, north of Hood Point, an area that coincides with DBRC.

Publications:

Distribution, abundance, and density of harbor porpoises in Hood Canal, Washington.
Rone, B.K., A.N. Zerbini, E.A. Falcone, E.L. Keene, and G.S. Schorr. 2024. Journal of Wildlife Management 88(3):e22543.