Predator/prey relationships
River Otter Diet and Predation Project
In 2012, SeaDoc is funding a River Otter diet and predation study by Monique Lance of the Washington Department of Fish & Wildlife.
The study aims to describe the diet of river otters and investigate the potential effect they have on rockfish and salmon populations in the San Juan Islands.
Rockfish and salmon are currently listed under the Federal Endangered Species Act. Because of the dramatic decline in these species over the past several decades, Marine Protected Areas have been created to assist in their recovery.
However, little attention has been paid to how predators respond to increased prey densities. Therefore it's important to investigate the diet of potential predators.
Otter scat samples were collected in the spring, summer and fall of 2008 on Fidalgo, San Juan and Orcas Islands. The scat samples have been cleaned and invertebrates in them have been identified. The project now allows for the analysis of fish remains in the scat, as well as examinations of spatial variation among the project sites and temporal variations through 2008. Data will also be compared to diet data from Alaska and British Columbia. The project will also compare diet among various predators, including harbor seals and Steller sea lions.
Photo by Michael Ransburg: Creative Commons license.
2011 Rockfish Recovery Workshop recap
This past week (June 28 & 29, 2011) SeaDoc co-hosted a Rockfish Recovery Workshop in Seattle with the State Department of Wildlife and NOAA Fisheries.
Nearly 100 scientists, fisheries managers, fishers and SCUBA divers attended the 2-day workshop to discuss the current state of knowledge on rockfish and to identify future needs related to recovering depleted rockfish populations in the Salish Sea.
There are 28 species of rockfish in the Salish Sea. Thirteen (13) are listed as species of concern and recently 3 species were listed under the US Endangered Species Act.
In addition to helping organize the workshop, SeaDoc also helped bring in Canadians to share their perspective on what has and has not worked with rockfish recovery on the other side of the border.
A lot of the research SeaDoc funded over the last 10 years was presented and plans were laid for moving rockfish recovery forward. The meeting proceedings will be published soon and will be available here on the SeaDoc website.
In the meantime, here's a recap:
(Please note that this summary is taken from my notes and if there are errors or misstatements they are mine, not the researchers/presenters! -Joe T.)
Historical Context Session
Wayne Palsson spoke on the biology and assessment of rockfishes in Puget Sound. Rockfishes are a diverse group of species with different life histories. They require various habitat during different life stages. They are adapted for slow growth, long survival, late maturity, low natural mortality rates, and high habitat fidelity. These are all factors that make recovery tough. There's a lack of long-term data that makes it hard to create conventional age-structure population models and biomass dynamic models.
Chris Harvey reviewed the ecological history of rockfish exploitation in Puget Sound. Rockfish bones have been found in middens dating back 1,500 years. Much of the fishing pressure on rockfish began after the Boldt decision in 1974, which required that harvests in Puget Sound be coequally managed by the State government and the Treaty Tribes of Washington. It's also been influenced by demographic trends and by the promotion of the fishery by State government. (Unfortunately, as covered in Wayne Palsson's talk, it wasn't until 1982 that scientists learned that rockfish were generally 2 to 3 times longer lived than they'd thought, which meant the existing population models were not accurate.) By the time management efforts were deemed necessary, the greatest harvests had already occurred.
Anne Beaudreau discussed her work to reconstruct historical trends in rockfish abundance. The lack of data on historical populations of rockfish is a major barrier to developing sustainable fisheries. Beaudreau and colleagues interviewed 101 individuals ranging in age from 24 to 90 years to try to derive trends in the abundance of rockfish from 1940 to the present. Of particular interest was the evidence of "shifting baselines." To a statistically significant degree, each age group of respondents interpreted the conditions at the beginning of their awareness as "abundant" and saw declines from there, but what was "declining" to an older person was "abundant" to a younger person.
Benthic Habitat Surveys/Rockfish Abundance Estimates Session
Gary Greene presented the Salish Sea sea floor mapping project, which has produced bathymetric and habitat maps of the San Juan Islands area. Rockfish prefer particular habitat types, and the multibeam echosounders used by Greene and his colleagues allows these potential habitat areas to be identified. (Other participants were very interested in having these maps for other areas in the Salish Sea.
Bob Pacunski spoke on work to use non-lethal methods to survey rockfish populations. Traditional trawl or long-line sampling results in fish mortality, but using a small remotely-operated vehicle has been shown to be effective at providing population surveys.
Stressors Session
Joan Drinkwin of the Northwest Straits Foundation spoke on the threat posed to rockfish by derelict fishing gear, including both nets and traps. The Northwest Straits Initiative has removed 3,860 nets from Puget Sound, all at less than 105 feet deep. There are 950 shallow-water nets still in the water, and at least 70 in deeper water. Based on studies of net mortality by the SeaDoc Society, approximately 1,600 rockfish per year are captured and killed in derelict nets each year in the United States portion of the Salish Sea.
...More coming soon...
New research from Alaska: Steller Sea Lions attacking Harbor Seals
A new paper just released in the Journal of Marine Mammalogy shows that Steller Sea Lion predation on Harbor Seals in Glacier Bay, AK could be responsible for the decline in Harbor Seals in that region.
While there is no evidence that Stellers are predating harbor seals in the Salish Sea region, it will be interesting to watch for this type of activity in the future.
In the Salish Sea, our Harbor Seal population is at carrying capacity and we are seeing more and more Steller Sea Lions in the region in the fall, winter and spring.
The Mystery of Surplus Predation
In the Salish Sea, some transient killer whales seem to be killing sea lions for no particular reason. Joe Gaydos provided background for a Kitsap Sun piece by Christopher Dunagan discussing the four sea lions apparently killed by blunt trauma from orcas in a single month.
A 2005 publication by Gaydos discussed five harbor seal pups found decapitated by killer whales, but not eaten.
The article discusses possible causes of this "surplus predation" and reinforces the importance of quick reporting of stranded and dead marine mammals, because the faster the animals can be recovered, the more we can learn from them.
Kitsap Sun: http://pugetsoundblogs.com/waterways/2010/05/14/transient-orcas-may-be-l...
Photo: Stranding Network volunteer Maria Webster standing by one of the dead Steller sea lions on North Beach of Orcas just before SeaDoc necropsied it. Credit: Joe Gaydos.
What's the Deal with Rockfish and Seals?
Harbor seal populations have exploded nearly tenfold in the Salish Sea since the 1970s, while at the same time many rockfish species have plummeted. Some fishermen blame that on the increase in hungry seals. But coincidence is not science. So who ya gonna call?
In the second year, 12% of the samples contained rockfish remains, particularly in the winter when there are fewer salmon locally. With all the seals in the Salish Sea, those numbers could impact rockfish recovery. That doesn't mean, however, that we should start controlling seal populations: this study showed seals also have a taste for dogfish, another major fish predator. So fewer seals could mean more dogfish. The real answer is to remember that all parts of our ecosystem are tightly intertwined. Rockfish recovery depends upon ecosystem recovery, including salmon recovery, herring recovery and so on around the Sea.
More information on this topic:
- The original report on this study (so you can see what we've learned since the first round)
- The Puget Sound Partnership's resource page on rockfish.
- SeaDoc's comments on the proposed (12/2009) rockfish conservation plan by the WA Dept of Fish & Wildlife.
- All the info on our site about rockfish (automatic search)
- All the info on the site about predator/prey relationships (automatic search)
(You can do these last two searches yourself by choosing "browse by species" or "browse by issue" from the Salish Sea menu at left.)
Here's a video (aimed at kids) that gives a behind-the-scenes look at how this kind of research is done.
Urban Orca: killer whales in Puget Sound
Killer whales kill and then abandon a harbor porpoise off San Juan Island
On September 9, 2009, researchers who were tracking southern resident killer whales in Haro Strait noticed whales from J- and K-pods interacting with a harbor porpoise.
Three specific whales (J31, J36 and J39) spent the most of the time with the porpoise and J-31 was seen using its head to throw the harbor porpoise in air on two occasions. The entire interaction lasted over 30 minutes.
.thumbnail.jpg "Harbor porpoise killed by southern resident killer whales")
When Food Can Kill: Salmon and Killer Whale Populations
A study published in Environmental Toxicology and Chemistry explains the process by which threatened and endangered killer whales in the waters of northwestern North America become contaminated with persistent organic pollutants, particularly polychlorinated biphenyls (PCBs), that are present in chinook salmon, their primary food source.
Two years of this study were funded as part of the SeaDoc Society's competitive grants program.
Two main observations were made. First, nearly 100% of the contaminants in chinook salmon were acquired while they lived in the ocean. Second, southern killer whales are contaminated with higher concentrations of chemicals because 1) they eat more salmon than their northern counterparts and 2) southern salmon have higher levels of contaminants than northern salmon because the southern waters are more contaminated than the northern waters. In fact, PCB concentrations in southern salmon were almost four times those in northern salmon.
It is known that salmon can lose as much as 80% of their lipid (fat) stores as they journey back to their natal streams. Salmon stop eating during this time and draw energy from their lipid stores. Thus, they are less nutritious to whales than they would be otherwise. Furthermore, southern salmon were found to have lower lipid content than northern salmon. Whales therefore eat larger amounts of salmon and consequently are exposed to larger amounts of chemicals in the salmon. Southern whales, in particular, consume as much as 50% more salmon to compensate for the fact that their food is, per unit, less nutritious.
Salmon paradoxically help killer whales (food) and harm them (contamination). This study is important because it illustrates the increasing amount of damage to fragile ecosystems that occurs as industries continue to dump waste into rivers and oceans.
To read the entire review, Persistent Organic Pollutants in Chinook Salmon (Oncorhynchus tshawytscha): Implications for Resident Killer Whales of British Columbia and Adjacent Waters (Vol. 28(1):148–161), visit http://www.allenpress.com/pdf/ENTC_28.1_148.pdf
