The role and identity of host and non-host chemical cues (semiochemicals) in host location and non-host avoidance for copepodid larvae of sea lice, Lepeophtheirus salmonis, was investigated using Y-tube behavioural bioassays, solid-phase extraction (SPE), and coupled gas chromatography – mass spectrometry (GC–MS). Using artificial seawater conditioned with the preferred salmonid host, Salmo salar, L. salmonis displayed high activation and directional responses in Y-tube assays to salmon-conditioned water (SCW), to an extract of SCW prepared by SPE, and to a vacuum distillate of the SPE extract. Similar responses were observed to two chemicals identified from SCW by coupled GC–MS: isophorone and 6-methyl-5-hepten-2-one. Dose-response studies with isophorone showed that copepodid responses across the range tested were maximised at 0.01 and 0.1 mg·mL–1. A mixture of isophorone and 6-methyl-5-hepten-2-one also induced high activation and directional responses. Semiochemicals were also isolated from the non-host fish, turbot (Scophthalmus maximus (Rafinesque)), by SPE and analysed by GC–MS. Two non-host-specific chemicals were identified as 2-aminoacetophenone and 4-methylquinazoline. When SCW was mixed with either of the non-host chemicals, activation and directional responses to SCW were eliminated in the Y tube.

Assessing the effects of invading species on native community structure is often confounded by environmental factors and weakened by lack of replicated, longterm pre-and post-invasion monitoring. Here, we uncouple the community effects of a freshwater amphipod invader from environmental differences. In Irish rivers, the introduced Gammarus pulex replaces the native Gammarus duebeni celticus . However, the River Lissan in Northern Ireland is dissected by a weir that has slowed the upstream invasion by G. pulex . This allowed us in 2000 to sample three contiguous 150-m reaches that were (1) G. pulex dominated; (2) mixed Gammarus spp.; and (3) G. duebeni celticus only. In 2003, we resampled these reaches and one additional of mixed Gammarus species and one with only G. duebeni celticus further upstream. In temperature, conductivity, and pH, there were statistically significant but no biologically relevant differences among the five reaches of 2003, and between the three reaches surveyed in both years. Although there was evidence of recovery in macroinvertebrate diversity and richness in invaded reaches between years, continued upstream invasion was associated with sustained reductions in these community metrics as compared to un-invaded sites. Community ordination indicated (1) different associations of community composition attributed to the distribution, abundance, and biomass of the invader; and (2) increasing similarity of invaded communities over time. The impact mechanisms of G. pulex on macroinvertebrate community composition may include predation and competition. The consequences of the observed community changes for ecosystem functioning require further investigation.

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