Recruitment of the threatened mussel Margaritifera margaritifera in relation to mussel population size, mussel density and host density

ABSTRACT Anthropogenic, abiotic factors are considered the main cause of recruitment failure of unionid mussels, including the freshwater pearl mussel (Margaritifera margaritifera). This large‐scale investigation instead examined the relationship between biotic factors and mussel recruitment. Mussel population size and density were positively related to recruitment and to juvenile mussel density. Juvenile mussel density appeared to be the more accurate measure of recruitment of the two. Host fish density of young‐of‐the‐year and older brown trout (Salmo trutta) were positively related to recruitment. Moreover, the mean density of both age classes of fish, when grouped into density classes, was positively related to juvenile mussel density, an effect that decreased at trout densities above 10 trout 100 m‐2. Mussel population size and density were relatively more important to recruitment than trout density. To increase recruitment of juvenile mussels, managers should apply measures that increase mussel density, and trout density up to about 10 trout 100 m‐2 in proximity to mussel beds. Mussel beds may also be managed; one possible measure within small and sparse mussel populations may be to concentrate the remaining mussels in areas where trout density is high. Likewise, young‐of‐the‐year trout may also be moved to areas of high mussel density, as young trout individuals tend not to move far during their first year. This may increase mussel larval infection rates and mussel recruitment. Copyright © 2012 John Wiley & Sons, Ltd.