Projections of world population and world food supply

This paper considers projections made about world population levels and world food production from 1945 to 1970, attempting to evaluate them in terms of actual levels of 1975. Most underestimated the population explosion of the past two decades; short-term estimates were more reliable than long-term ones; there was little variation in population estimates at any one time; and U. N. figures were generally the most useful. Projections of world food production were less satisfactory than those of population levels, as both data and measures were debatable. The research suggests some of the problems in evaluating the danger of world famine by examining projections of population and food supply mechanically and in isolation from the complex processes which affect these factors.     

Surfacing activity and food utilisation in the obligatory air-breathing fish ophiocephalus striatus as a function of body weight

Reared in cylindrical aquaria containing different depths of water (2.5 to 70 cm) the obligatory air-breathing fish Ophiocephalus striatus, belonging to different weight classes (0.1, 0.75, 10, 20 and 41 g), was forced to swim vertically a longer or shorter distance per surfacing. Surfacing frequency was a depth-dependent, activity in individuals weighing less than 20 g in all weight classes, the frequency was nearly 2 times more in the series fed ad libitum on fish muscle, than in the one where the fish were starved. Owing to the sustained surfacing activity and the consequent fatigue, the test individuals hung to the surface for a definite period. Neither frequency nor duration of hanging was depth-dependent. Mean hanging durations for the feeding series were 1.1, 3.8, 5.9, 7.9 and 8.8 hr/day in the 0.1, 0.75, 10, 20 and 41 g weight classes, respectively; the corresponding values for the starving series were 1.3, 15.0, 13.0, 12.7 and 12.8 hr/day. The distance swum by the feeding fish increased from 57 to 681 m/day and from 61 to 507 m/day in the 0.1 and 41 g individuals exposed to the minimum and maximum aquarium depths. Feeding rate, which was a depth-dependent activity, decreased from 280 to 113 g cal/g live fish/day with increasing weight. Rate and efficiency of conversion also decreased with increasing body weight; in larger fish conversion was dependent on volume rather than on depth of the aquarium. O₂ uptake of feeding fish was about 6 times higher than the starving ones of the tested weight classes at different aquarium depths.This paper is part of a thesis submitted by the author (from A.P.A. College, PALNI) to Madurai University in partial fulfillment of the requirement of the Ph. D. degree. Contribution No. 12 under a research scheme granted to Dr. T. J. Pandian by the UGC (New Delhi). The author is grateful to Dr. T. J. Pandian for valuable guidance, financial support and encouragements.     

Une etude sur clarias senegalensis val. dans la region du lac kossou (cote d'ivoire)

A study on Clarias senegalensis Val. in the region of Lake Kossou (Ivory Coast). Clarias senegalensis is the dominant silurid of the man-made lake Kossou and some smaller impoundments in the same region. Comparison of data spread over a five year period starting from 1970, some months before the dam was completed, reveals that the density of C. senegalensis dropped to the original level that existed in the river after reaching a peak in the years following the damming. This phenomenon which coincides with the impoverishment of the lake waters, is explained by the fact that C. senegalensis has its optimal habitat in poorly oxygenated waters, rich in organic matter. It is able to colonize this kind of productive waters thanks to the suprabranchial organ. In these optimal conditions it mainly feeds on aquatic insects and plankton.The fecundity of C. senegalensis may reach 20o ooo ripening eggs per female. Reproduction occurs during the rainy season in flooded savannas. Growth rates are very high and the age determinations show that, in the region of Kossou, C. senegalensis reaches an average standard length of 620 mm (2.7 kg) within three years.Our results are compared to literature data on other related species of the genus Clarias and the position of C. senegalensis within this generic complex is discussed.

     

Possible occurrence of character displacement in the sympatric skates Raja erinacea and R. ocellata (Pisces: Rajidae)

Synopsis Raja erinacea and R. ocellata are sibling species which are positively correlated with each other by occurrence and numerical abundance. In sympatry the species undergo interactive segregation; R. erinacea feeds on a higher percentage of epifauna and R. ocellata feeds on a higher percentage of infauna.An isolated allopatric population of R. ocellata occurs in the Gulf of St. Lawrence which is phenotypically intermediate between the sympatric populations of R. erinacea and R. ocellata in characters related to feeding e.g. size, number of tooth rows in the upper jaw, and shape of the upper jaw. It appears probable that the allopatric population represents the morphological state of R. ocellata before it became sympatric with R. erinacea; divergence in size, number of tooth rows, and shape of the upper jaw between the two species developed after establishment of sympatry. These divergences in character traits, related to feeding, reduced competition between the two sympatric species and permitted the present wide overlap in their ranges.Character displacement is evidently rare in demersal fishes inhabiting the flat and soft bottoms of the northwestern Atlantic because the three other pairs of sibling species that occur there are parapatrically distributed and thus would not compete for resources. Raja erinacea and R. ocellata may have been restrained from establishing parapatry by another species pair of skates (R. senta and R. radiata) which have a complementary distribution and similar feeding habits of R. erinacea and R. ocellata but which occur in deeper water.     

Diel food selection of pelagic Arctic charr, Salvelinus alpinus (L.), and brown trout, Salmo trutta L., in Lake Atnsjø, SE Norway

Patterns of diel food selection in pelagic Arctic charr, Salvelinus alpinus (L.) and brown trout, Salmo trutta L. were investigated in Lake Atnsjo, SE Norway, by gillnet sampling during July-September 1985. Arctic charr feed almost exclusively on zooplankton both day and night, while brown trout had a diurnal shift in diet. For this species zooplankton made up a considerable part of the diet in the daytime, while at night the diet consisted mainly of surface insect and chironomid pupae. Both species had a selective feeding mode on zooplankton during the day and night. Arctic charr had a higher gill raker number and a denser gill raker spacing compared with brown trout. Still, the differences in prey size between the two species were small. We argue that the observed differences in food selection between Arctic charr and brown trout can be explained by differing abilities to detect food items under low light conditions.     

The role of Acanthospermum hispidum in the phenology of Euschistus heros and of Nezara viridula

Laboratory, greenhouse, and field studies with Euschistus heros (F.) and Nezara viridula (L.) (Heteroptera: Pentatomidae) in northern Paraná, Brazil, demonstrated greater survival and longevity with E. heros than N. viridula when fed the weed Acanthospermum hispidum DC. (Compositae). E. heros was better adapted to A. hispidum than N. viridula but neither species reproduced on this plant. Stink bugs moved from soybean to A. hispidum mainly during mid-March to mid-April when soybean plants matured. The infestation reached 60 and 100% with a maximum of ca. 3 and 10 stink bugs/plant in 1988 and 1989, respectively. Surprisingly, E. heros and N. viridula, which are normally seed-feeders, strongly preferred the high moisture stems of A. hispidum. Both species demonstrated similar feeding frequencies. These results suggest that in northern Paraná, the common weed, A. hispidum functions as a temporary host providing water and nutrients to those two pentatomid pests of soybean. Finally, both species do not seem to recognize A. hispidum as an unsuitable or perhaps toxic plant.     

Effects of potassium levels over the range occurring in host plants on the growth of an insect herbivore (Manduca sexta)

Third instar tobacco hornworms (Manduca sexta L.: Sphingidae) on low dietary potassium had a lower relative growth rate than individuals on diets with potassium concentrations reflecting those in host-plants, due to decreased consumption rate, lower efficiencies of conversion of ingested and digested food (ECI and ECD), and a prolonged growth/feeding phase. Furthermore, these larvae, when placed on a diet with a moderate potassium concentration through the fourth stadium, ended up being smaller due to lower ECI and less biomass gained, and had a prolonged growth phase, which suggest an irreversible cost of the previous low potassium diet. Third instar hornworms on high potassium diets had lower ECI and ECD, and they had a prolonged growth phase. These individuals, when placed on a moderate potassium diet in the fourth stadium, gained less biomass, than those previously offered hostplant-like-potassium diets. Body potassium concentrations (% dw) at the end of the third stadium were similar among treatment groups. With increasing potassium concentrations in the diet, utilization efficiencies of potassium decreased and potassium concentrations in the frass increased. Correspondingly, water content (% fw) of the newly-molted fourth instar larvae declined with increasing potassium, indicating a passive loss of water during potassium excretion. Low and high dietary potassium reduced survivorship of third instar larvae; fourth instar caterpillars previously fed the low potassium diet also had poor survivorship. We conclude that, within the normal range of potassium concentrations in the hostplants, caterpillar performance is largely unaffected by potassium concentration, but that potassium-poor and potassium-rich diets, such as those hornworms may sometimes experience, can reduce growth and survivorship.     

Worldwide distribution of commercial resources of seaweeds including Gelidium

The commercial exploitation of seaweeds for use as food and for the production of agar, alginate and carrageenan is outlined. The quantities of seaweed harvested for each purpose are tabulated and discussed. Seaweeds for food are derived chiefly from China, Japan and Korea, with almost 94% obtained by cultivation. Alginophytes are collected in 15 countries but six of these account for more than 80% of the total harvest; all are from natural stocks except for a large quantity of Laminaria cultivated in China. Natural carrageenophytes, from 12 countries, now account for only 20% of the total harvest; the remainder is cultivated Eucheuma species, 99% of which is produced in only two countries, the Philippines and Indonesia. Of the four categories of commercial resources of seaweeds considered, agarophytes are spread more evenly over a greater number of countries; they come from 20 countries and only five of these are minor contributors to the total. Gelidium species are particularly important because of the high quality agar they yield; their distribution and location are discussed.     

Mechanisms of food selection in Daphnia

A conceptual behavioural and mechanistic Holling-type model of food selection in Daphnia pulicaria is derived from SEM observations with animals feeding on mixtures of spherical-cylindrical diatoms, oblongate green algae, and filamentous cyanobacteria, as well as ultrafine particles. The algae used were Stephanodiscus hantzschii (<- 6 µm length), Monoraphidium setiforme ( 20 µm), and Oscillatoria aghardii (strands, >- 80 µm). Cell (strand) selection can occur at any or all of three stages: (i) interception from the feeding currents, (ii) collection and channeling to the food groove, and (iii) compaction and transport to the mouth. During each stage, given equal initial cell densities, elongate cells are more likely to escape collection than spherical cells and are more likely to be rejected. In addition, filaments require increased handling time at stages (ii) and (iii) and promote entanglement with limb 5 and the postabdominal claw. Food is collected primarily with the aid of limbs 3 (and 4), but limbs 1 and 2 also intervene. Neither the leaky sieve hypothesis alone nor any other single-process hypothesis explains the observations on examined in corpore positions, morphology, and derived movements of the feeding limbs. Attachment and mucus appear to be important for the ingestion of bacteria and ultrafine particles.The model is consistent with many experimental results of differential feeding by Daphnia pulicaria on mixtures of variously shaped algae and other observations on Daphnia feeding behaviour. The paradigm of invariate, nonselective feeding by Daphnia is rejected.