Why such an early close date on the South Fork of the Salmon

Question:

I just read that the SF of the Salmon will close on the 9th. Only 2 1/2 weeks after the season has opened. This year there were 273,000 plus Salmon over Bonneville. 118,000 more that last year. There were 87,000 over Lower Granite. 47,000 more than last year. According to your catch numbers the catchable fish on the South Fork this year close to 750 fish. This is roughly 1.6% of the fish over Granite. Given the huge return numbers closing the South Fork this early seems to be a closure based on something other than numbers. I am wondering what the motivation is behind this decision.

Answer:

We're talking "apples and oranges" when you mix the total number of Chinook coming over Bonneville Dam and Lower Granite with the angler sport harvest on the South Fork Salmon River.

First, lets talk about the numbers of fish coming over the dams.  These numbers represent a mixture of fish returning to rivers all over the northwest.  They are a combination of hatchery and wild fish.  Big numbers of returning Chinook at Bonneville are an indicator of a strong return to Idaho - but not necessarily a strong return of Chinook to the South Fork of the Salmon River.

Here is how we arrive at the quota for the South Fork Salmon River:

Background - before the juvenile fish leave our hatchery in McCall and are transported to the headwaters of the South Fork Salmon, a small percentage of fish have a PIT tag inserted into their bodies.  Each tag, when passing a "reader" in the bed of the stream or on a dam, will transmit a number programed into the tag.  It's like a social security number.  Using math, we extrapolate population mortality on their downstream migration to the ocean and gain a "picture" of how many fish survived to Bonneville Dam from a particular hatchery release.

Adult quota - as the adult fish begin returning to the Columbia River, we can detect those same PIT tags at Bonneville and Lower Granite Dam.  As we detect individual fish, we can again extrapolate the expected number of South Fork Salmon fish that pass Bonneville Dam and monitor mortality from the dams and downstream fisheries as the fish journey up the Columbia River and into the Snake River.  Lower Granite Dam is the last major dam they cross where we detect the PIT tagged fish headed to the South Fork Salmon River and can extrapolate how many hatchery-produced fish will return to our trap.  To arrive at the quota, 1) we take the estimated number of hatchery fish returning to the South Fork Salmon River at Lower Granite Dam - based on PIT tag detections; 2) subtract the anticipated mortality from predators, anglers, diseases, etc. on the journey from Lower Granite to the South Fork Salmon River trap; and 3) subtract the number of adult brood stock fish we need to meet our hatchery production needs.  The number of fish left is our "harvestable surplus" of returning South Fork Salmon River hatchery Chinook salmon.  We then need to divide that number in half because legal rulings have determined that Idaho Indian tribes are entitled to half the harvestable surplus of Chinook returning to the South Fork.

As an example, if we estimate 5,000 adult South Fork Salmon Chinook pass Lower Granite Dam and there is a 10% mortality between the dam and South Fork trap- we have 4,500 fish remaining.  We need 2,000 Chinook for brood stock at the South Fork Salmon River trap - so we subtract that number and reach 2,500 fish in the harvestable surplus.  We then divide that number in half to reach the sport fishing quota (1,250) with the tribes being able to harvest the other 1,250 fish.