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

Study area

Surveys were conducted in northwest Washington State on a 6.7 m rigid-hull inflatable in the Strait of Juan de Fuca from Sekiu Point (48°16.10′N, 124°17.73′W) in the east to Cape Flattery (48°23.22′N, 124°43.70′W) in the west and in the Pacific Ocean from Cape Flattery in the north to Sea Lion Rock (47°59.58′N, 124°43.45′W) in the south as described in Scordino et al. (2017). Plankton tows were conducted at discrete sites along the established survey route (Fig. 1).

Mysid sampling

Seven sites in the Strait of Juan de Fuca and seven sites in the Pacific Ocean were surveyed to collect information about the prey base available to gray whales and the demographics of mysid swarms in this region (Fig. 1). All sample sites were close to shore in depths of less than 15.3 m and were selected based on areas where gray whales have previously been documented to feed (Scordino et al., 2017) and a desire to stratify sampling sites throughout the study region. Exact sample locations moved closer or further from the shore throughout the season to consistently sample in habitats along the edge of the expanding and contracting kelp line, as prey tows could not be conducted through the dense kelp forests. The southernmost site was adjusted between years to better reflect recent gray whale distribution patterns; in 2019 the southernmost site sampled was the South of Bodelteh Islands site and in 2020 the sampling site was moved to Ozette Island (Fig. 1). We aimed to collect one sample per site each month of the study. Sampling occurred during the gray whale feeding season during June through November in 2019 and during June through September in 2020.

Samples were collected using plankton nets similar to those used by other regional research groups (Dunham & Duffus, 2002; Burnham, 2015; Hildebrand et al., 2022). In 2019, samples were collected using a plankton net with a single 30 cm diameter hoop with a 3:1 hoop diameter to length net and 500 µm mesh. In 2020, this net was replaced with a Bongo-style net manufactured by Sea-Gear Corporation (Melbourne, FL, USA). The Bongo-style net had twin hoops measuring 30 cm in diameter with a 3:1 mouth to length ratio and a 500 µm mesh. Due to the differences in total volume sampled by these two types of net, we refrain from directly comparing sampled mysid abundance between years of the study and instead focus on month-to-month patterns and catch composition. At each site, the net was lowered over the side of the research vessel by hand until we felt the line go slack when the net hit the bottom (later confirmed through the collection of benthic species and substrate sediments), then was towed near the bottom at approximately 0.25 m s⁻¹ through the water, accounting for current speed and direction for approximately 30 s and retrieved as fast as could be achieved by hand (approximately 0.6 m s⁻¹). This procedure was repeated for two tows at each site, and contents of the cod end from each tow were poured into a 500 mL container comprising a single sample. If the sample did not contain mysids, or other known gray whale prey items (Nerini, 1984; Kvitek & Oliver, 1986; Duffus, 1996), then the sampling effort and absence of prey was recorded but the sample was not retained for further processing. Samples were stored in 70% ethanol before processing.

Mysid processing

For each sample, species identifications of all mysids were conducted according to Kathman et al. (1986) using a 40× dissecting microscope. Mysids were measured from the tip of the rostrum to the tip of the telson and the species, sex, maturity, and gravidity were identified. Mysids were designated as “Unknown” species if they could not be identified to species due to a damaged or absent telson. Mysids were assigned a sex based on the presence of a brood sac for females or a reproductive appendage for males (gonopod), and mysids that did not have either characteristic were designated as ‘non-brooded’. The non-brooded group therefore includes both juvenile mysids of both sexes and non-brooded adult female mysids, which were indistinguishable in this study. The characteristics of mysids in samples with high mysid counts (>1,000 mysids) were approximated by visually estimating the total count of mysids in the sample, collecting a subsample of 100 mysids from that sample, and identifying the species, sex, length, maturity, and gravidity of the subsample. The demographic composition of the subsample was then used to characterize the full sample. The samples that were characterized via subsampling were included in all subsequent analyses and can be inspected in our publicly available data repository (Allyn, Scordino & Akmajian, 2023). Other non-mysid organisms (including some species of crab larvae known to be gray whale prey items) collected during sampling were identified to the lowest taxonomic level possible using Shanks (2001) but were not included in this analysis; the non-mysid data are available in Supplement 1 and the Mendeley data repository (Allyn, Scordino & Akmajian, 2023).

Gray whale surveys

Gray whale sightings were recorded throughout the study period using the methods described in Scordino et al. (2017). In summary, the time and location were recorded at each sighting and the observed gray whales were photographed with the goal of collecting photographs for photo-identification along the dorsal flank of both sides using digital SLR cameras with 100–400 mm lenses. Photographs of gray whales were sent to Cascadia Research Collective (Olympia, WA, USA) for comparison to their catalog of individually identified gray whales from the west coast of the contiguous USA and Canada. Each whale photographed was processed according to established photo identification procedures and was either matched to an existing whale and identification number in the catalog, assigned a new identification number if no match could be made, or left unidentified if photos of the whale were of insufficient quality (Calambokidis et al., 2002; Calambokidis, Pérez & Laake, 2019). Individual identifications were used to determine the total number of unique whales observed in the survey area in a given time period, since whales can be sighted more than once during the same day or on subsequent days throughout the season. Surveys were conducted under Marine Mammal Protection Act research permits granted by NMFS to author JJS (23970 and 19430).

Data exploration

To explore potential patterns between mysid and whale presence, we performed standardization to account for different levels of effort between study months and between types of data collection (whale surveys and mysid samples). We calculated the average number of uniquely identified whales observed per km surveyed per day. We eliminated any whales that were observed engaging in behavior that was not foraging (e.g., traveling or socializing). This filtering excluded whales where we could be reasonably certain the whale was not engaging in foraging and therefore may not be responsive to the available prey reserves in the area. Whale sightings were split into two separate regions: the Strait region encompassed the area from Cape Flattery to just east of the Chito Beach mysid sampling site (48°17.74′N, 124°23.79′W), and the Ocean region encompassed the area from Cape Flattery south to approximately Sand Point (48°6.79N, 124°44.30W; Fig. 1). The average number of unique whales identified in each region was calculated per study month.

The weight of each sampled mysid was estimated using an established length-weight relationship developed in the San Francisco Bay estuary for ethanol-preserved mysids (Burdi et al., 2021). Mysid counts and biomass were calculated per sample and averaged for each region and day for each study month.

We used data visualization tools to explore patterns in this small dataset. Data analysis and visualization was conducted using Microsoft Excel and R version 4.3.1 (R Core Team, 2023). R packages dplyr (Wickham et al., 2021), psych (Revelle, 2021), and tidyr (Wickham & Girlich, 2021) were used to summarize data for analysis. Data visualization and figure development was conducted with ggplot2 (Wickham, 2016), ggmap (Kahle & Wickham, 2013), ggimage (Yu, 2023), cowplot (Wilke, 2021), gridExtra (Augie & Antonov, 2017), and ggrepel (Slowikowski, 2021). All prey sample data underlying the results presented in the study are available from Allyn, Scordino & Akmajian (2023). Those interested in whale sighting data can make a request to the Makah Fisheries Management Department of the Makah Tribe (see Data Declaration for contact information).

Mysid sample composition

While we aimed to collect one sample per month, sampling effort was highly variable due to a number of factors, including lost or malfunctioning equipment, inclement weather, and the impacts of the Covid-19 pandemic on staff availability (Table 1). In the collected samples, we identified prey species from 53 taxonomic groups (Table S1), including seven species of mysids (Table S2). In 2019, 11,305 mysids were collected in 72 samples from June–November, with an average of 157 mysids collected per sample. In 2020, we collected 4,878 mysids in 78 samples from June–September, with an average of 63 mysids collected per sample. In 2019, mysids were sampled in the greatest numbers in September–November, while in 2020 there was high sampled abundance in July followed by low sampled abundance for the rest of the sampling period (Fig. 2). In both years, the samples with the greatest counts of mysids were collected in the vicinity of Sail River in the Strait of Juan de Fuca, and lower mysid counts were recorded on the Pacific Ocean side of the survey area (Fig. 3).

Most of the mysids we sampled were identified as Holmesimysis sculpta (70.3%), followed by Neomysis rayii (26.8%; Fig. 2). Mysids that could not be identified to species due to damaged or missing telsons made up 1.2% of total samples. All other species each made up less than 1% of total mysids caught, including Columbiaemysis ignota, Telacanthomysis columbiae, Hippacanthomysis platypoda, Eucopia grimaldii, and Exacanthomysis davisi. H. sculpta composed a greater proportion of the mysids caught in 2020 (99.1%) than in 2019 (58.1%).

Overall, sex could be determined for 24.2% of the mysids collected in this study. In both years, more than two-thirds of the sexed mysids were male. Of the mysids that were identified as brooded females, over half were gravid (Table 2). In every month where gravid females were present in samples (all but June and July 2019 and August 2020), at least 81% of them were identified as H. sculpta. Only seven gravid N. rayii were collected in total in September, October, and November of 2019. Mysids collected in 2019 averaged 4.7 mm in total length compared to 13.3 mm in 2020 (Fig. 4).

Mysid and gray whale co-occurrence

We conducted a total of 41 gray whale surveys during June through November of 2019 and 26 surveys during June through September of 2020 (Table 1). In both years, the peaks in whale counts occurred during months when mysids were also abundant in prey samples (Fig. 3). Most of the mysids (84.9%) collected during this study came from just one section of the Strait region that encompasses the Seal and Sail Rocks, Sail River, and Bullman Beach sites. Of the 239 whale sightings that were documented during this study, 17.6% occurred within the immediate vicinity of the Seal and Sail Rocks, Sail River, and Bullman Beach sites, even though this is a relatively small section of our full survey area. 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 times when counts of sampled mysids were also higher (Fig. 3). In the Strait, the number of unique whales observed per km was generally higher in months when mysid biomass was higher (Fig. 5). The range of mysid biomass observed in the Ocean was too constrained (consisted of zero and near-zero counts) to explore patterns with whale presence.