Estimating the number of fish that return to spawn using capture-recapture methods.
The commercial catch of millions of fish is worth tens of millions of dollars to the economy of British Columbia in the form of employment for fishermen, cannery workers and related industries. Commercial fisheries originating from both Canada and the United States exploit salmon returning to the Fraser River. The proportion of the estimated forecast of sockeye salmon, as well as other species of salmon (chum, pink, chinook and coho), harvested by the commercial sectors of each country are determined through the Pacific Salmon Treaty, which is negotiated between the United States and Canada. The size of the commercial harvest of sockeye, pink, and chum is closely monitored by each nation, and reported to the Pacific Salmon Commission (PSC) which publishes harvest information weekly.
Historically, the Sockeye salmon has been an important food source of the indigenous people of British Columbia. As the sockeye migrate upstream through the Fraser River, they are further exploited by a number of native fisheries. The number of salmon taken by each native fishery is determined prior to the start of the fishing season through bi-lateral negotiations between individual Indian Bands and the DFO. Native harvests of salmon are monitored by either Native Organizations or the DFO, depending upon the area of the Fraser River being surveyed. Once fishing begins, weekly summaries of native harvests of salmon are submitted to the DFO every week, until the end of the fishing season.
For an excellent synopsis of this species please visit the
Alaska Department of Fish and Game Homepage
(http://www.state.ak.us/local/akpages/FISH.GAME/notebook/sockeye.htm).
The life cycle of a Fraser River Sockeye salmon is four years. Every spring the salmon eggs hatch in gravel beds of streams and rivers which feed into the Fraser River. The young fish, known as fry, hatch in such well-known B.C. rivers such as the Adams, Chilko and Stellako.
Typically, the fry mature in calm, nutrient-rich lakes for one to two years prior to proceeding to the Fraser River and then on to the salt water of the Pacific Ocean. Three large salmon fry rearing lakes are the Shuswap, Harrison and Stuart.
The Sockeye salmon spend the next two to three years in the North Pacific Ocean feeding (Figure 1). There they grow to sizes exceeding 20 kilograms.
Each fall, the mature salmon return from the open ocean and pool at the mouth of the Fraser River. The salmon will remain here for two to six weeks before ascending the river to spawn (or reproduce). Much of the commercial harvesting takes place at this point as the salmon are highly concentrated in this small area.
The salmon that escape this harvesting (the escapement) leave the salt water for the fresh water of the Fraser River. Incredibly, the adult salmon return to the tributaries of the Fraser from which they were hatched (Figure 2). In gravel-bed nests, called redds, the adult Sockeye salmon spawn in pairs. Each mating pair then defends its redd until it dies. The cycle is repeated by the young of each pair four years later.
This latter estimate is the most crucial, as the number of salmon that return to spawn determines the number of eggs laid and, to a great extent, the size of the next generation. The method commonly used to estimate the escapement is the focus of this workshop.
The escapement is often estimated using a technique called the Petersen method.
n1 = | 2,213 | number of fish marked with tags |
n2 = | 59,665 | number of carcasses examined |
m2 = | 505 | number of tags recovered from carcasses examined |
Known Statistics | Unknown Parameters |
---|---|
n1 = number of fish initially tagged | p1 = proportion of all fish that are tagged |
n2 = number of carcasses examined | p2 = proportion of all carcasses that are searched |
m2 = number of tags recovered from carcasses examined | N = total number of fish returning to spawn (the escapement) |
We wish to obtain an estimate of N, the total number of spawning fish, which we call .