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?
 
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.)

Here's a video (aimed at kids) that gives a behind-the-scenes look at how this kind of research is done.