Where do the fish go?

I’m worried that once again I’ve missed my window of opportunity this fall. You see, over the past few years I’ve been spending a bunch of time in the late summer/early fall down in Shelburne Bay swimming with a big school of common carp (Cyprinus carpio). And I mean big in both the number of fish (between 10 and 30 are shoaling at any given time) and size of the individual fish (the smallest is about 24″ and 7 or 8 pounds). I’ve wanted to capture a video of the carp so people don’t think I’m telling fish stories (video), but my underwater camera shorted out and needed repairs and I didn’t get around to getting it fixed until the beginning of October. And yeah yeah yeah, I know how it sounds, but by the time I finally did get in the water with my camera, the carp seemed to have disappeared.

Location of both the shipwreck and the school of carp

And it wasn’t just the carp that had gone for the winter. The bay seemed like a desert. Well, not quite a desert, as you can see from the video below of The Admiral shipwreck, there’s still plenty of vegetation around. But it was very nearly devoid of fish, save for a few yellow perch, a largemouth bass, and a lone long-nosed gar (my first gar!! very excited). So did I miss them? Where do carp go in the fall?

Where the fish go!

While carp aren’t strictly solitary during the warmer months, they also aren’t exactly schooling fish in the summer. But as is true for so many of our “gregarious” winter resident species (e.g. grackles, crows, and robins), this changes in the fall as resources dwindle and predators become a greater threat. When water temperatures dip to about 50ºF (the lake is about 54ºF right now), carp shift their home ranges and meet up in larger and larger groups to staggering numbers (seine nets can capture thousands of these carps: link). These groups don’t seem to have any particular fidelity to a favorite spot from year to year, though I have consistently noticed the carp in the shallows at Shelburne Bay in August through October for several years now. They also reportedly use deeper waters than in the summer, but this is conflicted from both fishing accounts and other studies. These schools also seem to jump around quite a bit, which is possibly why I’ve missed them on my last couple swims. I’ll be back in the water soon and will update if/when I get these beautiful fish on camera. 

Judas goats Scout fish

Europeans first introduced common carp to North America in 1877 (mostly because European-Americans had devasted native fish stocks due to overharvesting and habitat destruction), and during the intervening century and a half they have become invasive across the continent. Conservationists spend millions annually in attempts to control their populations, and knowing when populations are at their lowest and where they’re most concentrated is critical. As for most species, carp populations are lowest in the winter, but finding these concentrations of carp when they’re the most concentrated in the winter can be tricky business.

As far back as 1977, researchers have been capturing, tagging, and using radio telemetry to follow carp movements during the fall and winter (source). In fact, this may be the first case of researchers using this technique to track down conspecifics. A carp collared in the late summer can lead researchers to these large mobile schools of carp in a lake ecosystem. These scout animals (a new term that is slowly replacing the antiquated “Judas goat” terminology) are essential conservation tools for locating reclusive or itinerant herd animals. In the 1977 study, the researchers gave the winter coordinates of the carp schools to fishermen who then harvested just over 100,000 pounds of carp (source). The table below shows the percent of carp populations that were culled by using scout carp to locate larger schools. 

Using the Judas technique to locate and remove wintertime aggregations of invasive common carp (https://core.ac.uk/download/pdf/33142481.pdf)
An old term

In stockyards, trained goats would lead goats to the slaughterhouse, much like Judas betrayed Jesus and lead the Romans to him. Similarly, in conservation, so-called Judas “goats,” animals captured and affixed with a transponder so researchers can follow the animal’s movements, have been used to locate populations of invasive species (like nutria in the Chesapeke Bay or pythons on the everglades). Noting the anti-Semitic undertones of the term (Judas was Jewish and Jews, by extension, have been associated with his deviousness and blamed for Jesus’s death), researchers working on a the python project co-authored a paper on finding a more neutral term (it is a truly excellent read: link). The researchers had been tipped off to the connection after an article on their work attracted the attention of a local rabbi who pointed out the anti-Semitic nature of the association between Judas and deception (see the American Jewish Committee’s glossary of anti-Semitic terms, themes, and memes link). The researchers also changed the name of their project.

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