On selective tidal transport in the migration of North Sea Plaice (pleuronectes platessa) and other flatfish species

Evidence is given for selective tidal transport in North Sea plaice and other flatfish species. Data collected during midwater plankton surveys in two Dutch Wadden Sea inlets in early spring in the years 1970 to 1977 show that plaice, sole, flounder and dab demonstrate pelagic swimming behaviour in that period. Plaice, sole and flounder show a clear selective tidal transport mechanism to re-enter the Wadden Sea after staying in the open sea in winter whereas dab tends to leave the Wadden Sea. Data on by-catches of semi-pelagic pair trawl fishery for gadoids in the Dutch coastal area show that small quantities of plaice may be caught off the bottom throughout the whole year whereas sole moves to higher levels in two distinct periods only. The first period lower left diagram) demonstrate a strong preference for northgoing currents. This is also the direction towards the areas from which the plaice used had been transported to the laboratory and therefore this preference for northgoing tides might be interpreted as a clear case of homing to the Silverpits and Borkum areas.Also the rate of dispersion of the cluster of displaced tagged plaice in general (Fig. 18, lower right diagram) is much faster in the first months after release than that of the local plaice. Thus displaced plaice are more strongly motivated to migrate than non-displaced fish. This may well have been the case too in the plaice equiped with the transponding acoustic tags. This together with the release in an area with strong currents and narrow tidal ellipses means that the plaice should be considered to have been in an exceptional situation, extremely suited for travelling at great speeds in a fixed direction. Fig. 18 also shows the average surface tidal current velocities, derived from the 13 maps with hourly data on water circulation in the Stroomatlas Nederland (1963). It is obvious that the Lowestoft plaice, mostly released nearer to the coast within the release area, encountered the highest average current velocities observed in the North Sea. It cannot be proven that these circumstances explain completely the vast speed of the Lowestoft plaice but it certainly demonstrates that there is a bias in these experiments which could easily affect the results. When drawing conclusions in respect to the “normal” picture of plaice migration this bias should not be ignored.In the common sole selective tidal transport is observed in juveniles when entering the Wadden Sea in spring. In adult soles this mechanism is a common feature in March to May during the spawning migration but no evidence for the use of this mechanism is available during other periods of the year. The sole thus is an example of a flatfish species using selective tidal transport in a restricted period and for special purposes only.In plaice the use of selective tidal transport in the Wadden Sea inlets has been shown for juveniles. In adult plaice the need for using the mechanism might be much less although plaice have been seen swimming at the surface in spring (van den Ende, 1849) during day time quite in contrast with their normal behaviour. In spring nights, among soles swimming at the surface, plaice too have been observed (together with some dabs and turbots) but in much lower numbers than the sole (de Veen, 1967).On the whole there is not much evidence for a strong continuous use of selective tidal migration in plaice. No serious drops in the catch rates during specific tidal phases in the bottom trawl fisheries have been recorded by the results of the UK 81 otter trawl fishery. The coincides with the re-entering of soles in the Wadden Sea and with the well-known phenomenon of soles swimming at the surface. During this period the soles use selective tidal transport to reach the spawning grounds near the continental coasts.Data on catch per unit effort of a Dutch ottertrawler in 1958 to 1960, constantly fishing in one restricted area near Terschellingerbank lightvessel, reveal significant differences between day and night catch rates and a change from a preference for flood tide in the first half of the year to a preference for ebb tide in the second half.The available evidence for selective tidal transport in plaice is discussed in relation to the patterns of migration in 4 plaice sub-stocks studied by means of tagging experiments, and in relation to results of displacement experiments with mature and immature plaice. The tagging and displacement experiments present a picture of migration in which the directional swimming is a weak component superimposed on largely random movements and the speeds of travelling are low. The plaice react to displacement by trying to return to their original grounds and this tendency to home lasts for a full year.This picture about selective tidal transport contrasts with results obtained by the Lowestoft Laboratory. Plaice equiped with transponding acoustic tags did show largely directed and fast tidal migration before and after spawning. Arguments are given that these plaice were, however, in an exceptional position and may have shown homing behaviour. Notwithstanding this reservation to the Lowestoft experiments, these results and the present ones are explained if it is assumed that apparently only a fraction of the population uses the mechanism of selective tidal migration, thus saving energy and achieving quick transport when necessary. It is likely that in plaice as in sole the mechanism will be used in special cases only, probably for short periods, in certain areas.