The distribution and movements of juvenile plaice Pleuronectes platessa and their potential predators and competitors were recorded in a small microtidal (-20 cm) bay on the west coast of Sweden uslng underwater television and conventional nettlng techniques. There was generally close correspondence between the results obtalned by the 2 methods. Young plaice moved upshore at dusk and returned to deeper water at dawn. Larger predatory fishes also moved upshore at night but not as far as the plaice; they also entered shallow water later and left for deeper water earlier than the plaice. One interpretation of these nocturnal upshore migrations by plalce, therefore, is that they minimse predation risk. Although stomach fullness was greater at night, the availability of suitable food items for the plaice throughout the bay makes it unl~kely that the upshore movements were solely related to feeding. Because there was a strong diel temperature fluctuation In the bay, such movements also ensured that the fish remained in a fairly constant temperature. The main competitor of the plaice for food in July, the shrimp Crangon crangon, showed no such migratory movements and was mostly active during the day. The other dominant large crustacean (Carcinus maenas) was nocturnally active and did not markedly change its depth distribution over the 24 h period. The observed patterns of movement suggest that the timing of immigration and emgration is mainly triggered by changes in light intens~ty because only in C. rnaenas was any relationship found between activity and the phase of the tidal cycle.

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Concentration of juveniles of marine fishes in nurseries may act as a bottleneck during the life cycle, where quantity and quality of nurseries determine population size. Macroalgal blooms have become a common phenomenon in eutrophic shallow waters worldwide, and matforming algae may now cover many essential nursery habitats. In this investigation, the aim was to assess the quantitative effect of algal mats on the recruitment of plaice (Pleuronectes platessa) from nurseries in the Swedish Skagerrak archipelago. A model was constructed using data on nursery size, settling density, and mortality of plaice combined with data on algal distribution. Recruitment of 0-group plaice from nurseries could be reduced by 30%-40% due to algae. The largest negative effect occurred during high settlement, reducing the important influence of strong year classes on stock size. The model predicted a reduction of juveniles due to algae of 45-46 × 10 6 individuals at high settlement. This amounts to 68% of the output at medium settlement and equals the amount of plaice produced during 5 years of low settlement. Up to 75% of the total reduction could occur in one quarter of the study area. With limiting resources, management actions should not be generally applied but rather be concentrated to optimize the cost-benefit of measures taken.Résumé : La concentration des jeunes poissons marins dans les nurseries peut agir comme un goulet dans les cycles biologiques au point où la qualité et la quantité des nurseries déterminent la taille des populations. Les proliférations de macro-algues sont devenues courantes dans les eaux eutrophes peu profondes à l'échelle globale et les tapis d'algues peuvent maintenant couvrir plusieurs habitats essentiels dans les nurseries. L'objectif de notre étude est d'évaluer les effets quantitatifs des tapis d'algues sur le recrutement de la plie (Pleuronectes platessa) dans des nurseries de l'archipel suédois de Skaggerrak. Notre modèle incorpore des données sur la taille des nurseries, le taux de colonisation et la mortalité de la plie et les combine à des données sur la répartition des algues. La présence d'algues dans les nurseries peut réduire de 30-40 % le recrutement des plies d'âge 0. L'effet négatif le plus significatif se produit lors des périodes de forte colonisation des plies, ce qui réduit l'influence considérable des cohortes annuelles fortes sur la taille des stocks. Le modèle prédit une réduction de 45-46 × 10 6 jeunes plies à cause des algues lors de colonisations importantes. Cela représente 68 % de la production lors de colonisations moyennes et l'ensemble des plies produites pendant cinq ans de colonisations faibles. Jusqu'à 75 % de la réduction pourrait se produire dans un quart de la zone d'étude. Étant donné les ressources limitées, les actions d'aménagement ne devraient pas être appliquées de façon générale, mais plutôt être concentrées de manière à optimiser les coûts et les bénéfices des gestes posés.[Traduit par la Rédaction] Pihl et al. 1193