Selective exploitation of early running fish may induce genetic and phenotypic changes in Atlantic salmon

Abstract

Genetic evidence for the selective exploitation by anglers of early running fish was examined in four Iberian Atlantic salmon populations using protein and mtDNA markers. The populations studied had been exploited exclusively by anglers since 1949 during a fixed fishing season that ran approximately from March to July. Genetic variation at six protein loci was small and was accounted for by the MEP-2* and MDH-3,4* polymorphisms, which generally remained stable over time and were in Castle-Hardy-Weinberg equilibrium during the fishing season. Early running fish that had spent multiple winters at sea (MSW) generally had higher frequencies of the common MEP-2* (100) allele than did late running, one sea winter (1SW) grilse that were significantly smaller and tended to escape the fishery. Spawners differed from angler caught fish in their mtDNA frequencies and consistently had a lower seaage and a smaller body size. Spawners also smolted at an older age and displayed lower frequencies of the MEP-2* (100) allele in three of the four populations studied. These results suggest that in these rivers anglers selectively exploit a distinct component of the population and inadvertently cause a differential mortality of genetic types that is likely to be detrimental to population viability. # 2005 The Fisheries Society of the British Isles

Growth-history perspective on the decreasing age and size at maturation of exploited Atlantic salmon

“…During the period 1996-2000, salmon spawners (mostly late-running) sampled in the River Asón were found to be on average 0.5 SW younger and 7 cm smaller than salmon caught by anglers (early-running) during the same season (Consuegra et al 2005a). However, it is unknown whether such differences in age and size at maturation of early-and late-running salmon have changed over time.…”

“…1a), suggesting that the abundance of at least the earlyrunning component of the population has declined markedly. In the River Asón, early-running salmon are predominantly large MSW salmon, whereas small 1SW salmon (grilse) are common among late-running individuals that enter the river only during summer and early autumn (Consuegra et al 2005a). Therefore, fishing in the Asón is to some extent age-and sizeselective, as it tends to target early-running fish (Consuegra et al 2005a).…”

“…Nevertheless, there are a few reasons to believe that the life-history patterns seen in early-running salmon might still apply for the entire population: (1) patterns in smolt length and post-smolt growth were similar among all sea-age classes (Fig. 4), suggesting that these changes were due to common conditions experienced by all juveniles, and (2) previous molecular genetic studies focusing on this population have suggested a genetic basis for variation in run timing (Consuegra et al 2005a), and indicated that a decrease in size and age at maturation might result from anglers selectively removing large early-running fish from the population during a fixed fishing season (García de Leániz et al 1992;Consuegra et al 2005a). Overexploitation of large MSW fish entering the river early in the season will inevitably cause small, late-running fish to be better represented among the spawning stock.…”

“…The river has supported fisheries for hundreds of years. However, since 1949 the exploitation in the river has been restricted to rod and line only, over a constant fishing season from mid-March to mid-July (Consuegra et al 2005a). Complete and accurate annual rod and line salmon catches have been reported since 1949, and despite some inter-annual variation there has been a clear declining trend in catches since 1970 (Consuegra et al 2005b;Fig.…”

Spawning-Age Differences and their Temporal Trends in Wild and Sea-Ranched Atlantic Salmon Stocks, from Stock Mixture Data

“…Hybridization of reared and wild fish may also change the spawning age and other lifehistory traits of the stocks [21][22][23][24]. Domestication selection in hatcheries tends to favor a high growth rate and early sexual maturity [21,25,26], and fisheries usually select against a large size and later spawning age [27][28][29]. A trend towards younger spawners was observed in the 20 th century for many salmon stocks, as a marked decline in either the proportion of older, multi-sea-winter (MSW), or of repeatedly spawning salmon [30][31][32].…”