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Monitoring Fish Production
Several techniques are used to collect the data we use to develop fish production estimates:
techniques used with adult Chinook salmon
Photo: J Selke, CDFW
A monitoring method used to estimate the number of adult salmon that spawn in a river. With this technique, recently dead salmon carcasses are collected, and colored tags are attached to the carcasses. The tagged carcasses are returned to the river. The ratio of tagged to untagged carcasses which are observed during subsequent surveys is used to develop a fish abundance estimate.
- Ladder counts
A monitoring method used to estimate in-river spawner abundance by counting adult salmon returning upstream to spawn as they pass a fish ladder.
"Fish steps" would be more accurate than "fish ladder." The many steps in the "ladder" provide areas for fish to jump up and deep pools for resting.
Fish Ladder from Battle Creek
Coleman National Fish Hatchery
Photo: M Gingras, CDFW
A monitoring method using divers with snorkels to estimate in-river spawner abundance. Divers visually count adult salmon (normally spring-run Chinook) prior to spawning. This underwater survey method is used as an index of fish abundance, not an absolute count.
- aerial redd counts
A monitoring method that uses an airplane or helicopter to count the number of redds in a river. Redds are gravel spawning nests. The aerial redd counts are then used to estimate the number of adult salmon that spawn in that river.
- counts of hatchery returns
A monitoring method that counts the number of natural and hatchery-produced adult fish entering a fish hatchery.
The number of naturally produced fish that enter hatcheries is added to the estimated number of fish that naturally spawn in a river to estimate the total in-river run for a particular watershed.
Clipping fin of hatchery-raised fish
Fisheries biologists routinely mark hatchery fish by clipping the adipose fin. The adipose fin is a small fleshy fin on the back, just in front of the caudal (tail) fin. Marking juvenile hatchery fish provides an ability to discriminate between wild (unmarked) and hatchery (marked) fish when surveys of adult fish are made.
- angler surveys
Also known as a creel census, an angler survey is a monitoring method used to estimate the number of fish harvested by sports anglers.
- ocean harvest surveys
A monitoring method used by the California Department of Fish & Game to estimate the number of adult fish harvested in the ocean by sport and commercial fishing.
technique used with adult striped bass, green sturgeon, and white sturgeon
- Mark-recapture surveys
For striped bass, the mark-recapture technique uses gill nets and fyke traps (See Glossary) to capture, tag and recapture striped bass during spring migration. The ratio of marked to unmarked fish that are collected in the nets and traps is used to estimate of the number of fish. For sturgeon, the mark-recapture technique is used in the fall when white sturgeon and green sturgeon are captured in trammel nets (See Glossary).
technique used to develop an index of the abundance of juvenile American shad
- Midwater trawl surveys
A monitoring method that uses a boat to tow a net that captures fish in the upper 6 feet of the water column behind the boat.
technique used with juvenile Chinook salmon
- Rotary screw traps
An in-river fish sampling tool for sampling juvenile salmonids that are swimming downstream. Also called a rotary screw trap.
The trap consist of a six to eight foot funnel suspended between two pontoons. As water enters the funnel, the internal screw core rotates. Fish are trapped in pockets of water that are forced into a box at the rear of the trap.
Photo: Harry Mossman, USFWS