© 2003 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Variability of demographic parameters and population dynamics of Atlantic salmon Salmo salar L. in a south-west French river
a INRA, Ecologie Comportementale des Poissons, Station d'Hydrobiologie B.P. 3, 64310 St-Pée-sur-Nivelle, France
b IFREMER, Laboratoire Halieutique d'Aquitaine, Technopole Izarbel Côte Basque Maison du Parc, 64210 Bidart, France
*Correspondence to Jacques Dumas. e-mail: dumas{at}st-pee.inra.fr; patrick.prouzet{at}ifremer.fr.
The abundance of the salmon population in the Nivelle River was assessed for 11 cohorts during all the stages of their life cycle, from eggs to spawners. A stochastic life history model was used to simulate the changes in numbers at each stage over several years and to evaluate the parameters of a Ricker-type Stock and Recruitment (S–R) relationship. Parameters necessary for managing the exploitation of the species were also estimated. The results indicated that an average deposition of 611 700 eggs (values varying in a proportion of 1 to 3, depending on the year) produced 4870 0+ parr in autumn (variation from 1 to 5.6); 71.8% of which belonged to the group of future 1-year old smolts. The age 1+ parr were eight times less numerous. Survival from egg to 0+ parr was on average 0.97%, but highly variable (varying from 1 to 15). It was density-dependent and followed Ricker S–R model with an optimum of 7800 parr for a survival rate of 3%. During their second year, the survival of 1+ parr reached 53.4% and varied little. The adult runs of complete cohorts amounted to 196 maiden salmon (range, 88 to 382) and previous spawners comprised only 0.9% of adults. Grilse (1 year in the sea) constituted the majority (88.7%). The overall survival rates from 0+ parr to adult returns (6.2% on average) varied three-fold. The majority was females among the grilse (56.2%) and 2-sea-year salmon (88.6%); all 3-sea-year adults were female. Eggs deposited per female averaged 4200, 8500 and 12 750 eggs in each age group, respectively. Simulations of population abundances at various life stages were in good agreement with the observed data. The S–R relationship revealed the low productivity and the vulnerability of this stock, mainly due to the low survival rate of the young during their first year of life. The maximum yield of 12.2% of recruits could be obtained from a deposition of 1 424 000 eggs, which is twice the present average level.
Keywords: fisheries, life history, modelling, population dynamics, salmon
Received 7 May 2002; accepted 28 December 2002.
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