Distribution and demographics of mysids (Crustacea: Mysida) as prey for gray whales (Eschrichtius robustus) in northwest Washington state

Abstract

Background: The movement and distribution of gray whales (Eschrichtius robustus) during the feeding season is likely dependent on the quality of foraging opportunities and the distribution of prey species. These dynamics are especially important to understand for the Pacific Coast Feeding Group (PCFG) of gray whales which spend the feeding season along the coast from northern California, USA through northern British Columbia, Canada. In Washington state, no previous work has been done to describe available gray whale prey. The main goal of this research was to initiate studies on an important gray whale prey item in northwest Washington, mysid shrimp (Mysida), by establishing a baseline understanding of mysid swarm demographics in the area and investigating patterns in gray whale and mysid presence.

Methods: Prey samples were collected during June through November 2019 and June through September 2020 using a vertically-towed plankton net at seven sites in the Strait of Juan de Fuca and seven sites in the Pacific Ocean in areas where gray whales were known to feed. Mysids collected in the samples were counted and the sex, length, species, maturity, and gravidity were documented. Patterns in gray whale and mysid co-occurrence were explored through data visualization.

Results: Seven species of mysids were observed in the survey area. In 2019, the number of mysids per tow increased steadily through the season, the most abundant species of mysids were Holmesimysis sculpta and Neomysis rayii, and sampled mysids averaged 4.7 mm in length. In 2020, mysids were abundant in tow samples in June and July but were not abundant in the remaining months of the sampling season. The average length of mysids in 2020 was 13.3 mm, and a large portion were sexually mature males and brooded females identified as H. sculpta. Throughout the survey area, the majority of whale sightings occurred later in the season in 2019 and earlier in the season in 2020, coinciding with the trends of sampled mysids.

Discussion: This study provides the first description of mysid swarm composition and temporal variation in northwest Washington. Tows were dominated by a similar assemblage of mysid species as what is observed in other areas of the PCFG range. The differences in sampled mysid assemblages between years, and the presence of whales in the survey area in times and at sites where samples with high mysid counts were collected, suggest evidence for interesting predator-prey dynamics that warrant further investigation.

Keywords: Eschrichtius robustus; Foraging ecology; Mysida; Pacific gray whale; Predator-prey dynamics.

Figure 1. A map of the area where plankton tows and whale surveys were conducted along the northwest coast of Washington state during the 2019 and 2020 summer-fall gray whale (Eschrichtius robustus) feeding season.

Points represent plankton tow sampling sites. Gray lines show the area within which rates of gray whale observations per km were calculated for the Strait and Ocean regions. Map tiles by Stamen Design, under CC BY 4.0. Data by OpenStreetMap, under ODbL.

Figure 2. Bar chart of the average number of mysid shrimp (Mysida) collected per sample along the northwest coast of Washington state during June–November of 2019 and June–September of 2020.

Stacked bars show the species composition of the mysids sampled in each month. (*) Mysids per sample in June 2019 (0.4) and August 2020 (0.3) were low but not zero.

Figure 3. A map showing the total number of mysid shrimp (Mysida) and gray whale (Eschrichtius robustus) sightings along the northwest coast of Washington state during the summer-fall of 2019 and 2020.

Points in mysid plots (left column) represent individual samples that contained mysids, with the size of the point corresponding to the total mysid abundance. Whale icons in gray whale plots (right column) represent individual whale sightings, which indicates that one or more whale was observed. In all plots, the month of the sample or sighting is represented through the color fill of the point or icon. Inset maps show better detail of the Seal and Sail/Sail River/Bullman Beach area, which is where the majority of mysids were collected during this study. Map tiles by Stamen Design, under CC BY 4.0. Data by OpenStreetMap, under ODbL.

Figure 4. A box and whisker plot showing mysid shrimp (Mysida) length in mm for each month of sample collections along the northwest coast of Washington state during June–November of 2019 and June–September of 2020.

Figure 5. Mysid shrimp (Mysida) biomass and gray whale (Eschrichtius robustus) observations from surveys conducted along the northwest coast of Washington state during the summer-fall of 2019 and 2020.

Average mysid biomass is expressed in grams, and whales are represented as the average number of uniquely identified gray whales observed per km for each month.