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1.The signal crayfish (Pacifastacus leniusculus) is a large, polytrophic crustacean which has invaded waterways across much of Europe. Crayfish prey on the eggs of several fish species and egg predation, especially by invasive crayfish, is cited as a likely cause of population decline and a serious concern for the conservation of some fish species including at least one salmonid. 2.Numerically, crayfish populations may be dominated by small individuals, but most studies have investigated egg predation by large crayfish. Evidence for crayfish gaining access to buried fish eggs is equivocal. 3.The ability of signal crayfish of a range of age groups (and hence, sizes) to prey on unburied and buried Atlantic salmon (Salmo salar) eggs was investigated in laboratory experiments. 4.Only Age 2+ and older crayfish (24–43 mm carapace length (CL)) significantly reduced egg survival in unburied egg experiments, although some evidence of egg predation was observed with Age 1+ crayfish (16–22 mm CL). Age 0+ crayfish (8–14 mm CL) did not prey on salmon eggs. No evidence of substantial excavations or predation upon buried eggs by crayfish of any size class was observed. Binomial logistic regression of egg survival against crayfish CL indicated that mean egg recovery fell below control levels when CL exceeded 16.3 mm. 5.These results suggest that large signal crayfish are likely to pose the greatest threat to salmonid eggs, but that crayfish larger than 16.3 mm CL have the potential to prey on eggs. 6.Further research is needed before manual removal or harvesting of signal crayfish are used as conservation measures for Atlantic salmon spawning areas in which signal crayfish occur. Restoration of high-quality salmonid spawning habitat might, however, help to prevent signal crayfish predation of salmonid eggs in addition to broader benefits for salmonid conservation.

Populations of anadromous Atlantic salmon Salmo salar (‘salmon’) have decreased dramatically across much of their geographic range in recent decades. It is posited this is being driven by low marine survival, resulting in reduced adult return rates to natal rivers. We hypothesised that marine return rates (MRR) of adult salmon are positively influenced by their body lengths as seaward-migrating juveniles (‘smolts’), so larger smolts will be more likely to survive at sea and return as spawning adults to the natal river. We also suggest that the magnitude of this effect decreases with latitude, with its effect being weaker in more northerly populations. These were tested using individual capture-mark-recapture data from seven salmon populations in Western Europe spanning 13 degrees of latitude and 12 years of smolt cohorts. We used a multi-state state-space model to test whether MRR was influenced by smolt body length and latitude, and their interaction. Across the seven populations, smolt length had a significant positive effect on the probability of adult return, which was stronger in southern versus northern populations. Atlantic salmon smolt body length is thus an important indicator of their MRR, especially among populations in the warmer, southern part of their geographic range, with this having important implications for conserving declining salmon stocks. It suggests that management in their freshwater juvenile stages can increase MRR and should focus not only on the numbers of seaward-migrating juveniles, but also their individual quality, as indicated by their body length.