Predator/prey relationships

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?
 
SeaDoc-funded scientists rolled up their sleeves and collected almost 1,000 samples of seal scat in the San Juan Islands. Detailed analysis of these samples revealed that herring make up nearly 60% of these harbor seals' diets, with their next favorite meals being salmon, pollock and cod-like fish. The seal's total menu, though, was surprisingly diverse, with seals chowing down on at least 35 species of fish! Still, through the entire first year of the study, less than 3% of samples contained rockfish bones. Case closed? Not so fast...
 
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.)

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.

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