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Shoaling could increase the accuracy of homing on migration, since the mean direction or route is likely to be a more accurate estimate of the correct destination than any individual's choice (Larkin and Walton, 1969). A similar effect may operate in bird flocks, but there appears to have been no critical test of this idea, which ought to be strongly dependent upon group size. There is now both direct and indirect evidence supporting the migration-enhancement hypothesis.
The first indirect evidence is that adult coho salmon home more accurately to their natal river at higher densities (Quinn and Fresh, 1984). The evidence is based on statistical analysis of tagging returns along with population size estimates, and the figures are subject to considerable variance. The point could be tested directly using the techniques pioneered by Hasler (1983) in proving the home-stream hypothesis.
Secondly, social transmission of information about diurnal migration routes between refuge and feeding sites has been demonstrated in grunt shoals (Helfman and Schultz, 1984). Transplant experiments proved that individuals rapidly acquired knowledge from local residents about routes and shoaling sites. After only a short period with their new conspecifics, isolated transplantees travelled the residents' routes to the correct locations. Such cultural transmission had not been shown in fish before, and is clearly a variation on the information centre idea.
Wijffels et al. (1967) showed that groups of eight Barbus tincto swam more quickly through a maze than individuals, but careful controls did not confirm Welty' s (1934) conclusion that fish in groups learn routes faster than isolates. The fish in Wijffels' study performed better in a group if they had learned the maze in a group, and better alone if they had learned it alone, a general confirmation of the 'context' learning phenomenon analysed by Hinde and Stevenson-Hinde (1973).
Direct support for the migration-enhancement hypothesis comes from painstaking and elegant work by Kils (1986 and pers. comm.), most of which has unfortunately not yet been published. Kils performs field experiments on Baltic herring schools from a floating glass-bottomed laboratory (a converted night club) moored near the entrance to the Kiel canal, through which the herring migrate on their way to spawning grounds. The herring use salinity (and probably temperature) cues to orientate their migration to the entrance of the canal. Kils' video measurements on fish in travelling schools, which he can manipulate by introducing water jets beneath his floating laboratory, show how minor course adjustments by neighbouring individuals act to spread the most effective direction of movement through the group.
Kils has extended the idea of more effective course adjustment of fish in groups to include other environmental factors such as patches of planktonic food. He has introduced the new term 'synchrokinesis' to express this hypothesis (Fig. 12.18; Kils, 1986). The experimental work has not yet shown that course corrections are quantitatively more effective in larger groups, a critical test of the main hypothesis.
Fig. 12.18 Diagram illustrating Kil's mechanism of synchrokinesis: small movements of individuals copied through the shoal provide an accurate movement towards better conditions. Left column (times labelled from bottom to top) shows fish moving into a favourable area, right column shows them avoiding an unfavourable area. Schematics taken from video frames of herring swimming near the Kiel canal. Reproduced with permission from Kils (1986).
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