No abstract

A high concentration of certain planktonic animals was found in a frontal region in the English Channel. Temperature, salinity and current measurements and direct visual observations (underwater) describe the nature of the front. It is shown that water depth, season, strong tidal mixing and residence time are important factors leading to the formation and maintenance of a turbulent convergent tidal front. I N T R O D U C T I O NThe hydrography of the English Channel has been studied extensively for nearly a century. It is therefore somewhat surprising that no descriptions of persistent convergent fronts have been recorded. Fishermen have recognized for many years that oceanic fronts are areas where fish tend to congregate (Uda, 1938). Similarly fronts in the English Channel are found to contain concentrations of certain planktonic animals.Hydrographic studies in the open ocean have shown that fronts coincide with sharp temperature discontinuities (Cromwell & Reid, 1956). In the front recorded here (Fig. 1) an abrupt change of temperature of 1 °C was observed in crossing the front and its position also included the 16 °C isotherm. Although reports of fronts are not uncommon (Knauss, 1957;Voorhis & Hersey, 1964;Katz, 1969;Voorhis, 1969) the nature of mixing processes at the boundary remain obscure. In this paper current measurements and associated density structure reveal the nature of the convergence and its associated turbulent structure that results in the concentration of planktonic animals. Fig. 1 shows the position of a persistent front between Guernsey and Jersey. It is of at least 10 nautical miles extent and easily identifiable by the rafts of floating sea-weed, debris and oil lumps held in the boundary. Dimensions of these (rafts) might be typically 20 m long and | m deep but varied according to the wind, usually making up a ragged rather than a strictly linear boundary. Puffins, shearwaters and terns were seen apparently feeding along the line of the front. Indeed the appearance was similar to other oceanic fronts (Beebe, 1926;Uda, 1938;Amos et al., 1972). OBSERVATIONS AND RESULTS Description of a front

This work was undertaken at the suggestion of Mr F. S. Russell that little was known about the natural history of even so familiar a species as Leander serratus, the common prawn of the English Channel, which may at times form a valuable natural resource to the inshore fishermen. Attention was therefore chiefly concentrated on its growth rate and breeding biology. In spite of the long breeding season the statistical method of studying growth was adopted when preliminary results appeared successful. By this method additional data could be obtained on distribution and migration. In the closely related genus Crangon, growth has been shown to be retarded under laboratory conditions (Nouvel & van Rysselberge, 1937), so that an experimental study could not be relied on to give a picture of the normal life history. Höglund's recent monograph (1943) has treated Leander squilla in great detail. However, this species differs considerably in size and distribution from L. serratus, and it was felt that a similar study in the Plymouth area would not be mere repetition; in fact the biology of L. squilla itself differs somewhat in the two districts.

A tagging investigation on the ormer (Haliotis tuberculata L.) has been carried out for the Guernsey States Sea Fisheries Committee. Cementing plastic discs to the shell has proved a satisfactory technique since substantial numbers of 40—50% were recovered after 1 and 2 years in the open sea off the west coast of Guernsey. From the increase in shell size of the recovered specimens the mean growth rate has been shown to be about 15 mm of shell length per year up to a shell size of 50 mm, and thereafter decreasing until at 100 mm length growth is negligible. The growth parameters K and L∞ have been calculated. Annual rings were frequently, though not invariably, found on the shell. From the frequencies of the shell-ring lengths in the smaller ormers estimates of the shell size reached 1 and 2 years after settlement have been made. A single annual ring was also found with the majority of the larger shells (> 50 mm), though when growth did not exceed 4 mm in a year the rings were not included owing to the difficulty of separating annual rings from possible disturbance rings caused by the process of tagging. The position of the annual ring varied, being closer to the outer edge of the shell with increasing size. Measurements of the length increase from one annual ring to the next indicated rates of growth closely comparable to those obtained from tagging measurements. Damage to the shells by the boring sponge Cliona lobata Hancock is very widespread, particularly in the larger ormers living below the tide marks.