Estimates of population size and instream mortality rate of American eel elvers in a Nova Scotia river


B M. Jessop

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American eel, Anguilla, Anguilla rostrata, capture, COUNTS, density, DIFFERENCE, eel, eels, elver, Fisheries, mark-recapture, migration, mortalities, mortality, mortality rates, Natural mortality, Nova Scotia, NOVA-SCOTIA, pH, population, population estimates, population size, POPULATION-SIZE, predation, RATES, recapture, Recruitment, river, size, stream, streams, TIME, trap, upstream

Journal or Book Title

Transactions of the American Fisheries Society


The recruitment of American eel Anguilla rostrata elvers to the East River, Chester, Nova Scotia, during May through July was estimated by trap (Irish type) counts at 209,190 elvers and by mark-recapture (with a tail-clip mark) at 960,000 elvers. The population size Following migration to recapture sites about 1.3 km upstream during late July-October was 1,145 elvers by trap count and 2,894 elvers by mark-recapture using a Bismark Brown Y mark. Finite mortality rates (M) were 0.9945 (trap counts) and 0.9968 (mark-recapture). Instantaneous daily mortality rates (Z) were 0.0612 (trap counts) and 0.0675 (mark-recapture). The 4.6 times difference between population estimates at the river mouth by trap count and by mark-recapture and the 2.5 times difference at the upriver capture sites resulted from the different duration of each mark-recapture experiment and from higher mortality for marked than for unmarked elvers. High mortality for unmarked elvers was attributed to the effects of low pH (4.7-5.0), high initial elver density (4.7 elvers/m(2)), and predation by resident eels. Elver fisheries may be most justified in locations where the natural mortality rate for elvers is high, as in low-pH streams. [References: 57]







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