CONTENTS 0. B. STABELL reliable data are usually those from the river of release. Tags, on the other hand, give advantages both with respect to data on straying rate and high-sea fishing. Straying rate (SR) here means the ratio:where RR are the number of fishes recovered in streams other than the one of release, and TR represents the total number of fishes recovered in streams. (It should be noted that straying rate equals the percentage of fish recovered in streams other than the one of release from the total stream catch. The expression percentage is, however, omitted to avoid confusion with percentages of recovery from those released.) Table I presents data, found in the literature, from 'homing' experiments on native, wild Atlantic salmon. As can be seen, the recovery of fishes in their native stream varies between I yo and 3.4 Yo of those released, the only clear exception being experiment no. 7.Very little straying is registrated in the above experiments, the exception again being experiment no. 7. Concerning the straying rate stated in experiment no. 14, Table I , it should be noted that the cases registered were all within the native watershed. A maximum straying rate of three appears to occur in Canadian and Norwegian rivers. This figure also agrees with data from rivers on the west coast of Sweden where straying rate is less than one (Carlin, 1969a), and with data from Britain and Ireland where straying rate averages 2.7 and 3.0, respectively (Thorpe & Mitchell, 1981). Data on Baltic salmon (S. salar L.) show a straying rate of approximately two (Carlin, BODZNICK, D. (1975). The relationship of the olfactory EEG evoked by naturally-occurring stream waters to the BODZNICK, D. (1978). Characterization of olfactory bulb units of sockeye salmon with behaviorally relevant stimuli. (Salmo trutta L.) : evidence of distinct sympatric populations. Hereditas 83, 73-82. stock of cutthroat trout (Salmo clarci). Transactions of the American Fisheries Society xog, 537-543. and Aquatic Sciences 38, 1523-1538. Report 58, 5-26. on the olfactory sense in grilse of Baltic salmon (Salmo salar). Behaviour 35, 235-241. of Ichthyology 21, 35-47. homing behavior of sockeye salmon (0. nerka). Comparatiue Biochemistry and Physiology 52 A, 487-495.

Some water fleas (Daphnia spp.) undergo phenotypic changes when exposed to chemical signals from predators. The chemical signals have been assumed to be of predator origin (i.e. kairomones), since juices of crushed Daphnia have been found ineffective. We speculated that latent alarm signals could be present in Daphnia, to be activated in predators following ingestion. Accordingly, fish predators were fed earthworms for 10 weeks to remove Daphnia remains from their gastro-intestinal tracts. Following another 6 days of earthworm feeding, water conditioned by fish induced no morphological changes in D. galeata. When fish were alternatively fed Daphnia for 6 days, changes were induced with fish-conditioned water. Extracts made from intestines of earthworm-fed fish, homogenized with earthworms, gave no morphological changes, but intestines of the same origin homogenized with Daphnia did. Similar results were found when earthworms and Daphnia were homogenized with fish liver. Freshly frozen extracts of homogenized Daphnia gave no detectable changes at first instar stage in test animals, whereas extracts of Daphnia that had been kept at room temperature did induce such changes. Our results suggest that Daphnia respond to latent conspecific alarm signals (i.e. 'dormant' pheromones) that are activated by intestinal or bacterial enzymes in predators or in the water.

No abstract

2004. Antipredator behaviour mediated by chemical cues: the role of conspecific alarm signalling and predator labelling in the avoidance response of a marine gastropod. -Oikos 104: 43 -50.Gastropods represent a challenge in the understanding of alarm signalling. We studied predator avoidance (climbing behaviour) of the marine snail Tegula funebralis in laboratory experiments. Snails were exposed to crude extract of conspecifics, and to water conditioned by actively feeding or non-feeding predatory crabs. Crabs had previously been maintained on different diets, and were accordingly labelled by chemical cues of various origins. Tegula-extract alone released climbing behaviour in May, but not in June. However, during both these months, snails responded to chemical cues from crabs that were actively feeding on Tegula. Crabs labelled by Tegula-diet, and actively feeding on Tegula, also caused more climbing responses compared to crabs labelled by other diets. Chemical cues derived from crabs actively feeding on another snail species, or from non-feeding crabs, did not induce snail climbing no matter the previous feeding history of the predators. When snails received Tegula-extract combined with water conditioned with a non-feeding, Tegulalabelled crab, no climbing occurred. However, when the non-feeding, Tegula-labelled crab was present in the solution of Tegula-extract, moderate climbing responses were obtained. The results imply that climbing responses of T. funebralis are in general caused by the action of a two-component system. This system seems to be a mixture of chemical cues leaking from the tissue of conspecifics when being eaten, and latent conspecific chemicals that are modified in crabs and presumably released with the urine of chemically labelled predators. The modified chemical labels appear to be fully released by crabs when feeding, and moderately released when detecting food. The responses obtained in May with crude extract alone may result from a seasonal change in alarm signalling properties, or a change in behavioural responsiveness of snails exposed to a variable predator regime.

No abstract