ALBERT

Notes: Abstract Acoustic Doppler Current Profilers (ADCPs) are now used routinely by physical oceanographers. A by-product of the signal processing for calculating the Doppler shift used to determine current velocity is the intensity of the acoustic backscatter, and this, in turn, is a measure of biomass. ADCPs can therefore potentially provide biological oceanographers with quantitative information, collected routinely and non-intrusively in near real time. If these acoustic measurements are obtained concurrently with hydrographic data, the possibility of collecting quantitative biological information on the same time and space scales as physicochemical data becomes a reality. ADCPs have been routinely operated on NERC ships for several years. Examples of the types of biological information which can be obtained are described here in the hope that this will stimulate other users to make use of a potentially valuable source of data.

Notes: Abstract Feeding in relation to temporal changes in the depth distribution of predator and prey is described for 9 species of mesopelagic decapods from an examination of 268 foreguts. Intensive nighttime feeding appears to be the rule in all species. The smaller decapods Sergestes (Sergestes) atlanticus, Sergestes (Sergestes) sargassi and Sergestes (Sergestes) pectinatus exploit the smaller prey, principally copepods and to a lesser extent ostracods. Larger decapod species Sergestes (Sergestes) henseni, Sergestes (Sergestes) curvatus, Sergestes (Sergia) grandis, Systellaspis debilis, and Acanthephyra purpurea mainly prey on macrozooplankton and micronekton, i.e., chaetognaths, euphausiids, decapods and fish, but copepods also occur in the foreguts. Gennadas valens is exceptional for the high incidence of foraminiferal remains, and a predator-prey relationship seems probable. All 9 decapod species have mixed diets, and pronounced feeding preferences are not evident. However, a high incidence of “secondary” feeding or “dietary contamination” has been deduced from the frequent occurrence of remains of the copepods Pleuromamma spp. and Oncaea spp. in the foreguts of the larger decapod species. Direct feeding cannot have occurred, since the depth distributions of these copepods and decapods are disjunct by day and night. It is concluded that the remains of Pleuromamma probably represent the food of the larger prey such as chaetognaths etc. which are eaten by the decapods. The presence of Oncaea is speculatively attributed to a possible ectoparasitic relationship with the larger prey items, but confirmatory evidence is required. These anomalies suggest that caution must be exercised in deducing predator-prey relationships simply from gut contents without consideration of distributional factors.

Notes: Abstract Diurnal changes in abundance caused by vertical migrations have been examined in populations of copepods, ostracods, euphausiids, amphipods, decapods, chaetognaths, siphonophores and fish. The animals were taken in a series of hauls made over a 24 h period with an opening-closing midwater trawl system (RMT 1+8), consisting of a net of 1 m2 mouth area combined in the same frame as one of 8 m2 mouth area. The samples were taken at 250 m depth in a position 30°N; 23°W on 7/8 April 1972. The specific composition of the community and the numbers of individuals changed continuously with time. The numbers of fish, decapods and chaetognaths increased at night, but those of copepods, ostracods and euphausiids decreased. More species of fish, decapods and copepods were present by night than by day, whereas the numbers of species per haul for other groups remained fairly constant. The relative abundances of groups caught by the RMT 1 have been analysed, but similar treatment of the RMT 8 samples was impossible as only 3 groups were taken from this net. Non-migrants were a minority in every group except chaetognaths. Migrant species have been put into one of 6 transitory categories according to their patterns of abundance and hence migrations. Within each category, migratory behaviour varied both inter- and intraspecifically. The patterns of abundance of many species were smooth and continuous, suggesting slow migratory cycles of small amplitude. Conversely, extensive migrants had discontinuous patterns and presumably more rapid movements. Few migrants had a steady numerical plateau between their upward and downward migrations, and most apparently moved up or down continuously. The presence of migratory species in the sampled layer depended upon the time of day or night. It is concluded that, in a vertical series of hauls, the depths of occurrence of migrants will vary with the sampling time. Further-more, a vertical series will show a species minimum migration range but not necessarily its maximum. Individuals of some species were out of phase with the migrations of their main populations. There is evidence that the distributions and migrations of some species of decapods, euphausiids, copepods and fish could be related to the distribution of underwater light. Three pairs of congeneric copepod species were both spatially and temporally segregated for at least part of their diurnal cycles. Such an orderly arrangement could provide a means of reducing competition between species. Some species, however, overtook others on their migrations and the pattern of underwater light cannot, therefore, regulate the distribution of all species in the same way.

Notes: Abstract A new multiple rectangular midwater trawl (the RMT 1+8M) is described. It consists of three 1 m2 and three 8 m2 mouth area nets combined within the same frame and is based upon the Institute of Oceanographic Sciences RMT 1+8. It is acoustically operated and controlled and has been successfully used to a depth of 4500 m. The condition of the catch is excellent and use of the multiple net has resulted in considerable saving in ship's time. Some modifications to the existing RMT 1+8 are also described.

Notes: Abstract Stomach contents were analysed from the 7 most numerous species of mesopelagic fish caught in a series of 11 hauls over a 24 h period at 230 to 266 m depth in the eastern North Atlantic Ocean. The numerical abundance of organisms per filled stomach and the frequency of occurrence of empty stomachs were used to indicate feeding periodicity. The ecological significance of the feeding periodicity was considered by examining it in connection with an investigation of the day-night vertical distribution of zooplankton and micronekton to 2000 m at the same station. Additional dietary evidence on the 7 species considered was also obtained from the vertical series. Feeding selectivity was examined by comparing the composition of the zooplankton population, sampled separately but simultaneously with the micronekton, with that from the overall stomach contents of the species examined. Feeding periodicity was demonstrated for 6 species, of which 3 were found to be feeding selectively: Valenciennellus tripunctulatus on calanoid copepods, Argyropelecus aculeatus on ostracods, and Lampanyctus cuprarius on amphipods and possibly euphausiids. The limited data available on the other 3 species suggested that they were either random feeders (A. hemigymnus and Lobianchia dofleini) or perhaps selecting against a particular group (Notolychnus valdiviae). No indication of feeding periodicity or selectivity was found for Chauliodus danae. The overall pattern of results confirmed the supposed close correlation between vertical migration and feeding in mesopelagic fish.