Ultrasonic telemetry was used to follow the movements of two groups of smolts in the estuaries of two small Scottish rivers. Hatchery reared smolts released into a typical wedge flow, partially mixed estuary had movements which were dominated by the influence of tide on the direction of water flow. The net movements of wild native smolts in a two layer flow estuary, in which freshwater flow dominated, was downstream but were intermittent consisting of short steps and numerous long pauses. The hatchery reared smolts escaped from the estuary within a tidal cycle, moving out on an ebb tide. The wild smolts remained in the estuary for periods up to 108 h, none escaping within one tidal cycle.

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Measurements of oxygen consumption and heart rate showed that the range of variation in metabolic rate for any given heart rate in fish is too wide for any significant correlation to be used as a measure of metabolism as in some other vertebrates. By defining the maximum oxygen pulse a precise relationship can be established between maximum metabolic rate and heart rate. From field measurements of heart rate by telemetry useful information can be gained on metabolic rates.

Previous morphological and histological studies of the early developmental stages of the halibut (Blaxter et al., 1983) suggested that the mouth was not formed and the gut was not patent at the time of hatching. Apparently the mouth opened 21 days after hatching and it was only after a further 7 days that the rectum developed and a lumen first appeared. Furthermore, the gut was first fully patent 10 days later. This evidence would suggest that halibut larvae are incapable of drinking before the 38-day post-hatch stage. Recent work (Tytler & Blaxter, in prep.) has found that the larvae of herring, Clupea harengus L., plaice, Pleuronectes platessa L. and cod, Gadus morhua L. drink as part of the process of osmoregulation. Since halibut larvae can osmoregulate (Riis-Vestergaard, 1982) it is not clear how water balance is maintained in the yolk-sac stage. The purpose of this study was to examine alternative routes of water intake.Yolk-sac stage larvae, 3-7 days post-hatching, were placed in 40ml of a fluorescein isothiocynate dextran (Sigma, Mean MW 40 000) solution made up to a concentration of 7 mg ml-' with 32Yt sea water. The larvae had been held in 36% at 7.5" C but were acclimated to 32Yt for 16 h before transfer to the fluorescent marker solution. After an exposure period of 6.5 h at 7.5" C the larvae were removed, washed in 32Yt sea water for 5 min, and anaesthetized in a bath containing benzocaine (500 mg 1-I). As soon as they were immobile, individual larvae were placed on a cavity slide and examined by UV (450-490 nm) epifluorescence microscopy. Bright fluorescent patches were observed within the head region, dorsal to the heart and in the region posterior to the yolk sac (Fig. 1). These two patches were connected by a thin line of fluorescence contained in the lumen of the gut lying dorsal to the yolk sac. Figure 1(A) is a photomicrograph taken of the posterior fluorescent patch with UV and background transmitted-light illumination, to show the location of the fluorescence with respect to key structures. Figure 1(B) was taken of the same area in the same larva with UV illumination only to show that the fluorescent marker is confined to the lumen of the gut. Both were taken immediately after the larva voided some of the gut contents, producing an external, fluorescent halo round the anus.It is clear that the gut in yolk-sac larvae is patent soon after hatching. The mouth (stomodeum) and anus (proctodeum) have functional sphincters before they are detectable by gross anatomical studies. The presence of the fluorescent marker in the gut demonstrates that the larvae of halibut can drink, whereas previously it was thought that the gut was insufficiently developed to perform this function. This finding is relevant for aquaculturists at present developing a technology for rearing halibut larvae to metamorphosis. Halibut larvae often sink to the bottom of rearing containers, even in elevated salinities. The fact that they can drink from the earliest post-hatching stages suggests that their osmoregulation and bu...