Ecological Research on Rockfishes in Puget Sound
We used oceanographic modeling, acoustic tracking technology and stable isotope analyses to examine basic research questions for rockfish in Puget Sound.
Could Larval Dispersal Explain Population Structure?
Yelloweye Rockfish living in the Puget Sound/Georgia Basin region are genetically different from Yelloweye Rockfish living on North America's outer coast. Meanwhile, Canary Rockfish show no population structure between these regions (Andrews et al. 2018). These characteristics played a role in determining each species listing status under the U.S. Endangered Species Act. These results also raised several questions about the connectivity of each population and how that might affect the management and recovery of these species.
In this project, we wanted to quantify the relative connectivity, pathways and settlement locations of Yelloweye and Canary rockfish larval dispersal among basins in the Puget Sound region (Fig. 1). We had two goals:
- To understand where significant sources of rockfish larvae are most likely to help increase explored larval dispersal patterns.
- To test whether larval dispersal patterns explained the differences in population structure between the two species.
We used a 3-D oceanographic model to track the pathways of 100,000 larvae of each species released from 19 sites inside and outside the Puget Sound/Georgia Basin region. Yelloweye larvae were released in the summer months and Canary larvae were released in the winter months based on known reproductive timing of each species in other locations. Some pathways were quite expansive, while other pathways were restricted to relatively small ranges (Fig. 2).
Overall, results were generally similar for both species. After 120 days of dispersal, we found larvae in high proportions in the same region (inside or outside the Puget Sound/Georgia Basin) or basin where they were released (Fig. 3).
Exceptions included relatively high cross-boundary dispersal for both species from release sites nearest the geographic boundary between the Puget Sound/Georgia Basin and the Strait of Juan de Fuca (shown in Fig. 3 as the line between the "JDF" and “SJI” basins in Fig. 3).
These results suggest rockfish populations in several basins will be dependent on adults within the basin to supply larvae for future recovery. However, adults in the San Juan Islands may serve as a source of larvae for multiple basins.
We also found that the proportion of cross-boundary dispersal for both species is likely high enough to allow gene flow across boundaries (Fig. 4). These results suggest larval dispersal may explain the lack of population structure observed in Canary Rockfish. Still, those results conflict with the differentiated population structure we observed in Yelloweye Rockfish between inside and outside the Puget Sound/Georgia Basin region.
Understanding the potential larval dispersal pathways within and across management boundaries informs spatial management and recovery of important and protected species.
Movement Behavior of Adult Yelloweye Rockfish
The Puget Sound Rockfish Recovery Plan identifies threats to the recovery of two rockfish species listed under the Endangered Species Act (ESA; Fig. 5). Changing environmental conditions is one threat that may inhibit the recovery of these species or slow the recovery process.
In this project, we wanted to understand whether Yelloweye Rockfish behavior changes under different environmental conditions. Behavioral differences could indicate changes in foraging, resting, spawning, and reproductive behavior.
We externally tagged 15 Yelloweye Rockfish with acoustic transmitters (Fig. 6) at three sites in Hood Canal, WA. We established an array of acoustic receivers at each site to monitor each of the tagged individuals for one year (Fig. 7). The transmitters emit a unique code that is recorded by each receiver and allows us to follow individual fish. These transmissions include the fish's location, depth, water temperature at depth of fish, and the acceleration rate of the fish every 3 – 5 minutes.
We have not analyzed the detection data yet, but plan to collaborate with an undergraduate NOAA Hollings Scholar in the summer of 2021 to answer several questions:
1) Do Yelloweye Rockfish undergo diel or seasonal patterns of vertical migration?
2) What is the home range for Yelloweye Rockfish in the Hood Canal?
3) Do Yelloweye Rockfish show diel or seasonal patterns in foraging activities (using the acceleration data as a proxy for foraging events)?
4) Are any of these patterns altered during periods of low dissolved oxygen or high bottom water temperatures?
Relative Importance of Pelagic, Kelp Forest and Eelgrass Habitats to Rockfish
In the Puget Sound, bull kelp (Nereocystis luetkeana) and eelgrass (Zostera marina) are important biogenic habitats providing shelter for various valued species. As such, they are the focus of restoration efforts. While highly productive, it is unclear how these habitats, or pelagic habitats, contribute to the diets of rockfish in Puget Sound.
In this project, we wanted to understand the proportion of young-of-year and juvenile rockfish diets resulting from pelagic organic matter, kelp forests, and eelgrass beds. We collected water samples, kelp fronds, and eelgrass blades from habitats in northern and southern Puget Sound (Fig. 8).
We then collected young-of-year and juvenile rockfish, along with other invertebrate species, using SCUBA at these same sites (Fig. 9).
We used these tissue samples to quantify stable isotopes of carbon and nitrogen. This analysis can tell us what level of the food chain each individual has been feeding at (nitrogen) and what type of carbon is the base of their food chain (carbon). Overall, we found that most species examined had similar carbon signatures as red algae growing on eelgrass blades (Fig. 10).
For rockfish, we found that these red algae epiphytes contributed the most to the diets of juveniles and adults for two species (Fig. 11a & b). Interestingly, kelp habitats contributed the most to the diet of young-of-year (< 2 months old) rockfish (Fig. 11c).
Given the reported declines of bull kelp in Puget Sound, quantifying this and other primary producers' value to the functioning of the Puget Sound ecosystem and species of conservation concern is necessary for effective management and recovery.