WAHLE, VREELAND, and LANDER: BIOECONOMIC CONTRIBUTION 



closed pocket is found to be 6.060. The total 

 number of fish in that pond is then estimated as 

 6.060/0.101 = 60.000. Suppose further that of 

 the 6.060 fish retained by the pocket. 606 fish 

 are found to be marked. Then 606/6.060 = 10% 

 of the estimated 60.000 fish in the pond, or 6,000 

 fish are estimated to be marked and 54.000 un- 

 marked. The total release, numbers marked and 

 unmarked, and proportion marked were esti- 

 mated for a hatchery by summing data from all 

 ponds. Finally, estimates of the foregoing quan- 

 tities for all fish released into a given river sec- 

 tion were obtained by summing the estimates 

 for appropriate hatcheries. 



To estimate actual recoveries with a certain 

 mark during a specific sampling period in a 

 given fishery, the total catch (of marked and 

 unmarked fish) during that period was multi- 

 plied by the fraction of sampled fish observed 

 with that mark during the same period. For 

 example. 16 Ad-RM marks were detected dur- 

 ing June 1-30, 1968, from 9,827 coho salmon 

 examined at Crescent City, Calif., in a catch of 

 31,082 from the commercial troll fishery. Thus. 

 16/9.827 (approximately 0.2%) of the sample 

 had Ad-RM marks. The total marked catch for 

 that period and landing port was estimated to 

 be about 0.2% of the 31.082 fish caught or 62 

 Ad-RM marks (actual calculations were carried 

 to eight places to avoid rounding errors). Simi- 

 lar calculations were made for each period. The 

 results then were summed for all periods and 

 appropriate landing locations to estimate the 

 seasonal recovery of a certain mark in the given 

 fishery (e.g.. Ad-RM marks in the California 

 troll fishery during 1968). 



The catch of unmarked hatchery fish for each 

 ocean sport and commercial fishery, and the 

 Columbia River fisheries, was estimated for 

 each year and brood by dividing the estimated 

 catch of fish having a specific mark by the 

 expected marked/unmarked ratio. The latter 

 was calculated from the ratio at release and 

 the estimated relative survival of marked fish. 

 Suppose an estimated 2,000 1965-brood Ad-RM 

 marks were recovered in 1968 in the California 

 ocean sport fishery, the marked/unmarked ratio 

 was 0.1 for all hatcheries where Ad-RM marks 

 were released, and the survival of marked fish 

 was estimated to be 80% that of unmarked fish; 

 then the estimated catch of unmarked fish 

 would be (2,000)/(0.1 X 0.80) = 25,000 fish. 



The catch of hatchery fish released from a 

 given river section was estimated by summing 

 estimates for marked and unmarked hatchery 

 fish from each type of fishery. Nondetection of 

 certain marks in ocean fisheries complicated 

 the estimation of the hatchery contribution. 



The relative survival of marked fish was 

 estimated by comparing marked/unmarked 

 ratios at release and return, as noted earlier for 

 tetracycline (internal) and finclip (external) 

 marking at the Big Creek and Eagle Creek sta- 

 tions. At the Big Creek station for the 1965 

 brood, for example, the pond sampling pro- 

 cedures just described gave an estimated 

 marked/unmarked ratio at release of 0.12083 

 (for the Ad-RM finclip). Enumeration at the 

 hatchery of all internally- and externally- 

 marked returns ("jacks" or age 2 males in 1967 

 plus age 3 males and females in 1968) gave a 

 marked/unmarked ratio of 0.09885. The sur- 

 vival of marked fish between the time of 

 release and at return to the hatchery was there- 

 fore estimated to be 0.09885/0.12083 = 

 0.8181 that of unmarked fish. 



With estimates of all these quantities at hand, 

 it remained only to estimate the average weight 

 and unit value of fish caught to calculate their 

 total economic value. Weight data were collected 

 from a predetermined number of fish through- 

 out the season at different landing locations 

 from which a given fishery operated. Resulting 

 means were assumed to be representative. 



Assumptions 



The foregoing method of estimating catches 

 of hatchery fish requires certain assumptions. 

 These are considered after presentation of the 

 data (see "Bias Associated with Marks" and 

 "SUMMARY"). The main assumptions are: 



1. A marked fish is identifiable as a 

 marked fish throughout life. 



2. All fish detected and reported with the 

 kind of mark applied are hatchery fish. 



3. All coho salmon sampled in ocean fish- 

 eries are in their third and final year of life. 



4. Marked and unmarked fish have 

 the same maturity schedules. 



5. The same proportion of releases is marked 

 at each hatcherj' in a given river section. 



6. Marked and unmarked fish from a given 

 river section are equally vulnerable to capture 



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