AN ANALYSIS OF AGGRESSIVE BEHAVIOR, GROWTH, AND COMPETITION FOR FOOD AND SPACE IN MEDAKA (ORYZIAS LATIPES (PISCES, CYPRINODONTIDAE))

The Midas cichlid (Cichlasoma citrincllum) is an abundant fish in the lakes of Nicaragua. Many populations are polychromatic, about 7 to 10% of the adults being variously white, yellow, orange, or red, and without the species-typical markings. These are termed 'golds' because the most frequent color is yellow-orange. The common cryptically colored morphs are called 'normals'. In the experiment, 6 equal sized small fish had to approach a feeder guarded by a Midas cichlid twice their mean weight. In the experimental groups, 3 of the small Midas cichlids were gold and 3 were normal (mixed color groups). The control groups were of two types, one with all 6 small fish gold, the other with all 6 normal (pure-color groups). In half of all trials, the large fish was gold, and in the other half normal in color. Data were gathered on Days I and 3. I) Differences in the behavior of large fish toward the small ones were not statistically significant. However, the large gold attacked the small fish more often than did the large normal, although the rate of attacking was remarkably low. The large gold fish also fed on average much more often (9.7/hr.) than did the large normal (0.7/hr.), whose mean rate was about half that for all small fish. 2) There was little difference in rate of feeding among the small fish. However, on Day 3, only, the small fish fed significantly less in the presence of a large fish whose color they shared. 3) The spread in weights within each group of small fish increased 1.4 to 2.4-fold during the four days of the experiment (growth depensation). 4) The distribution of the fish within the arena was recorded at 30-sec. intervals, and analyzed with regard to Near (to the feeder), Mid, and Far Regions, adjusting the data to fish per m2 per hr. There were no significant differences in distribution in relation to fish coloration. The small fish occurred at about the same rate in the Near and Mid Regions, but more frequently in the Far. The shelters in the Far Region had an effect equivalent to adding more space. 5) Across all experiments and groups of small fish, the rate of attacking was, in the Near, Mid, and Far Regions, respectively, 9.1, 19.4, and 14.7 per m2 per hr. When these data were modified to take into consideration the fish available for attack in the same region, the scores became 10.5, 22.4, and 15.0 per m2 per hr., respectively. Small fish, therefore, were about half as likely to attack in the Near Region, with the feeder and the large fish, as in the Mid Region. 6) The small fish attacked significantly less those small fish that shared the color of the large fish. 7) Small golds, in mixed color groups, were attacked less than were small normals. 8) Attacks by golds on normals were compared to those on golds by region and by day. Likewise, the attacks by normals on golds were compared to their attacks on fellow normals. In total, there were 24 such pairs of comparisons for the mixed color groups. Of these, 50% were significantly different. In 11 of these 12 cases, golds were attacked less than were normals. And 9 of these 11 cases had the large fish gold. Thus sharing the color of the large fish conferred some immunity from attack. When both the small and large fish were gold the effects summated in the favor of small golds to produce significant differences. Conversely, when the large fish was normal the effects of gold coloration and that of not sharing the color of the large fish cancelled one another. 9) All the small golds had had prior experience with normal colored siblings, but most of the small normals had never been with golds before. The attack suppressing effect of gold color on normals was pronounced on Day I but weak on Day 3. This suggests that the effect of gold coloration is enhanced by infrequent exposure. If so, golds should experience a lessened advantage in aggressive interactions in direct proportion to their abundance in a population. 10) We hypothesize that gold coloration decreases the readiness to attack in the perceiver by stimulating incompatible fear responses.

Size‐assortative shoaling is a widespread phenomenon potentially linked to competition for resources among shoal members and anti‐predator benefits gained from living in shoals. This paper investigates the influence of both factors on size‐assortative shoaling in chub (Leuciscus cephalus). In a first experiment food competition was studied by manipulating the proportions of small and large chub in shoals of 20 fish. Large fish were generally more competitive than small ones. However, the competitive effect of large fish was strongly dependent on their number in the shoal. This has important consequences for the theory of competition because it makes it difficult to predict stable combinations of different phenotypes when foragers divide between food patches. Despite the strong effects of food competition no significant trend for size‐assortativeness was observed in free‐swimming shoals when undisturbed. This changed after the shoal had been frightened by introducing Schreckstoff (an alarm substance) to the test tank. Large and small fish still remained in the same shoal but showed a strong preference to be close to neighbours of their own size class. Large fish were observed mainly in the centre of the shoal and small fish were found more in the periphery. It is unclear whether these position differences are due to aggressive behaviour of large fish towards small fish which excludes them from the potentially safe centre of the shoal. These results indicate that prédation plays a more important role than food competition for size‐assortative shoaling in chub.

Glycogen is a vital energy substrate for anaerobic organisms, and the size of glycogen stores can be a limiting factor for anoxia tolerance of animals. To this end, glycogen stores in 12 different tissues of the crucian carp (Carassius carassius L.), an anoxia-tolerant fish species, were examined. Glycogen content of different tissues was 2-10 times higher in winter (0.68-18.20% of tissue wet weight) than in summer (0.12-4.23%). In scale, bone and brain glycogen stores were strongly dependent on body mass (range between 0.6 and 785 g), small fish having significantly more glycogen than large fish (p < 0.05). In fin and skin, size dependence was evident in winter, but not in summer, while in other tissues (ventricle, atrium, intestine, liver, muscle, and spleen), no size dependence was found. The liver was much bigger in small than large fish (p < 0.001), and there was a prominent enlargement of the liver in winter irrespective of fish size. As a consequence, the whole body glycogen reserves, measured as a sum of glycogen from different tissues, varied from 6.1% of the body mass in the 1-g fish to 2.0% in the 800-g fish. Since anaerobic metabolic rate scales down with body size, the whole body glycogen reserves could provide energy for approximately 79 and 88 days of anoxia in small and large fish, respectively. There was, however, a drastic difference in tissue distribution of glycogen between large and small fish: in the small fish, the liver was the major glycogen store (68% of the stores), while in the large fish, the white myotomal muscle was the principal deposit of glycogen (57%). Since muscle glycogen is considered to be unavailable for blood glucose regulation, its usefulness in anoxia tolerance of the large crucian carp might be limited, although not excluded. Therefore, mobilization of muscle glycogen under anoxia needs to be rigorously tested.

Feeding rate and behaviour of juvenile roach in a non-depleting patch consisting of Daphnia magna of two size classes (optimal and much smaller) were studied in 20-min experiments. The medium size group (standard length 26-28 mm), large (28-32 mm), and small (24-26 mm) fish differed significantly in the proportion of small prey eaten. During a short initial period of burst feeding (1-5 min) only large prey were selected by all fish. The switch from large to small prey occurred much earlier in small than in large roach. During the initial period small fish had higher feeding rates. In small fish, small prey made up about 30% of the food biomass consumed during the whole 20-min feeding period, whereas in large fish small prey played a minor role (about 2%). Within the medium-sized group, one subgroup of fish, characterized by early switching to small prey, showed a feeding behaviour similar to small fish, whereas the other subgroup, which switched later, behaved like large fish. Neither the apparent prey size model nor optimal foraging theory, even in its dynamic version, were supported by the data. It is suggested that vigilance against predators changes the initial energy-maximizing strategy in hungry fish to a time-minimizing strategy in partially satiated fish. Intra-cohort variations in prey size selection might be related to social status of the fish. Social status may affect prey size selection and feeding rate, and thus result in different growth rates and size differentiation.

Vision plays a crucial role in the life of the vast majority of vertebrate species. The spatial arrangement of retinal ganglion cells has been reported to be related to a species' visual behavior. There are many studies focusing on the ganglion cell topography in bony fish species. However, there are still large gaps in our knowledge on the subject. We studied the topography of retinal ganglion cells (GCs) in the Japanese smelt Hypomesus nipponensis, a highly visual teleostean fish with a complex life cycle. DAPI labeling was used to visualize cell nuclei in the ganglion cell and inner plexiform layers. The ganglion cell layer was relatively thin (about 6-8μm), even in areas of increased cell density (area retinae temporalis), and was normally composed of a single layer of cells. In all retinal regions, rare cells occurred in the inner plexiform layer. Nissl-stained retinae were used to estimate the proportion of displaced amacrine cells and glia in different retinal regions. In all retinal regions, about 84.5% of cells in the GC layer were found to be ganglion cells. The density of GCs varied across the retina in a regular way. It was minimum (3990 and 2380 cells/mm2 in the smaller and larger fish, respectively) in the dorsal and ventral periphery. It gradually increased centripetally and reached a maximum of 14,275 and 10,960 cells/mm2 (in the smaller and larger fish, respectively) in the temporal retina, where a pronounced area retinae temporalis was detected. The total number of GCs varied from 177×103 (smaller fish) to 212×103 cells (larger fish). The theoretical anatomical spatial resolution (the anatomical estimate of the upper limit of visual acuity calculated from the density of GCs and eye geometry and expressed in cycles per degree) was minimum in the ventral periphery (smaller fish, 1.46cpd; larger fish, 1.26cpd) and maximum in area retinae temporalis (smaller fish, 2.83cpd; larger fish, 2.75cpd). The relatively high density of GCs and the presence of area retinae temporalis in the Japanese smelt are consistent with its highly visual behavior. The present findings contribute to our understanding of the factors affecting the topography of retinal ganglion cells and visual acuity in fish.

Carbohydrate utilization versus body size in tilapia Oreochromis niloticus × O. aureus

Field observations quantified the effects of fish size and time of day on activity patterns, intraspecific encounters, and foraging styles in mutton snapper,Lutjanus analis, during the spring and winter of 1991. Fish ranged in size from 15 to 65 cm fork length (FL), and were associated with an artificial patch reef system located on a shallow seagrass meadow in the Exuma Cays, Bahamas. The most common, non-resting diurnal activities were intraspecific chasing and displacing, and feeding. Intraspecific displacing was significantly higher during midday compared to morning or evening. The highest proportion of intraspecific encounters (combined chasing and displacing events) occurred among medium (25–35 cm FL) and large (> 35 cm FL) fish. The few large fish observed (13% of population) initiated the same proportion of encounters as the predominant (50%) medium fish. The remaining (37%) small fish (> 25 cm FL) were the least aggressive. Dark barred and dark nape color patterns were associated with displacing and chasing, respectively. Fish exhibited considerable variability in feeding behavior, Proportionally fewer fish fed during midday compared to morning or evening, although small fish fed proportionally more often than medium or large fish despite time of day. Picking was the primary feeding mode and was observed during all times of day. Winnowing was observed during midday and evening, whereas midwater strikes were confined to morning and evening. Small fish displayed proportionally higher picking and midwater strikes during morning and evening, respectively, compared to medium or large fish. However, large fish winnowed proportionally more often than small or medium fish during evening. Dark barred color patterns were associated with feeding on the substrate, whereas no color changes occurred during midwater strikes. Our results indicate thatL. analis forms dominance hierarchies and that high variability in foraging styles, according to fish size and time of day, may be a means to reduce intraspecific competition.

Replacement channel catfish Ictalurus punctatus are typically stocked as fingerlings into commercial grow-out ponds following partial harvest of large fish. To determine if timing of subsequent stocking of fingerlings into mixed-size populations influences fingerling survival, we examined the effects of stocking large and small fish 2 weeks after initial stocking of large fish. Fifty large (mean weight = 374 g) and 50 small (25 g) catfish were stocked into two 1,323-L tanks. Tank 1 was stocked with large fish and maintained for 14 d; on day 14, small fish were added. Tank 2 was stocked with large and small fish simultaneously on day 14. A third tank was stocked with 100 small fish only on day 14. Three ponds (0.05-ha each) were also used in the same design with large fish (240 g) and small fish (30 g). On day 15, all fish were harvested and counted to determine 24-h survival. This procedure was replicated over three trials in both the tank and pond studies. Survival of small fish stocked 14 d after stocking of large fish in tanks and ponds was significantly less (P ≤ 0.05) than survival of small fish stocked concurrently with large fish or stocked alone. These results suggest that replacement fingerlings should be stocked as soon as possible following partial harvest.

The effects of food competition and relative size of group members on growth of sea bream, Sparus aurata were tested in groups composed of four fish. Each group consisted of a medium sized test fish (approximately 250 mg) and three fish larger or smaller than the test fish. In the first experiment food competition was addressed directly by contrasting its presence with its absence. Neither food competition nor relative size of school members had a significant effect on growth. However, there was a highly significant interaction between these two main effects. Under conditions of food competition the medium sized test fish had a three times higher specific growth rate when surrounded by small vs large fish. Under conditions of no food competition when the test fish were separated during feeding a similar but reverse pattern occurred due to differences in food conversion efficiency. In the second experiment, the effects of food quantity and relative size of group members were tested. Both main effects and their interaction were highly significant. Under conditions of low food competition (i.e., surplus food supply) fish grew again better when surrounded by larger fish as compared to their growth rate when surrounded by smaller ones. Size matching via a differential growth rate in a schooling species such as sea bream was suggested to increase the confusion effect by reducing the ability of a predator to focus on an odd sized fish. The implications of direct competition for food for the efficient culture of this species are discussed.

Summary Sea bass with approximate average weights of 5 and 20 g were treated against Ceratothoa oestroides infection with: (i) medicated pellets of diflubenzuron PC90 at a dosage of 3 mg kg−1 body weight (BW) per day for 14 days. Lice were counted at the beginning of treatment and 19 days after treatment. The drug cleared all lice in the treated group; in the control group, infection remained high 30 days after beginning the experiment. It was concluded that medicated pellets containing 3 mg kg−1 BW diflubenzuron effectively cleared pre-adult and adult stages of the isopod parasite over a 14-day period. No adverse effects were recorded in treated sea bass during the trials and no reinfection occurred 15 days after end of the treatment. (ii) Deltamethrin by means of bath treatments in infected sea bass kept in experimental tanks at 20°C. Before treatment, toxicity on healthy fish was preliminarily assessed by testing five fish from each size group at concentrations of 30, 10, 5, 3, 1, 0.1, 0.05 and 0.01 mg L−1 for 30 min. The therapeutic concentrations tested were: 10, 5, 3, 0.15, 0.1, and 0.05 μg L−1 and assessed at 1, 24 and 48 h. Best results were achieved with the 10 μg L−1 (0.01 mg L−1) dose, where prevalence was reduced from 100 to 0% over 24 h in both large and small fish. No parasite recovery was observed at 48 h. The dose of 5 μg L−1 reduced prevalence from 100 to 11.7% and to 0% for small and large fish, respectively. Finally, with the 3 μg L−1 dose, prevalence was reduced from 100 to 37.5% (small fish) and to 13.3% (large fish). Lower doses were ineffective on the parasites at either 24 or 48 h.

The effects of photoperiod, temperature, and fish size on oxygen consumption (OC) in the black porgy Acanthopagrus schlegeli, a euryhaline marine teleost, were studied using a closed recirculating seawater system with a respiratory chamber. Fish reared in indoor recirculating seawater tanks were divided into two groups: small (15.7-55.8 g, mean 38.1<TEX>$\pm$</TEX>15.9 g) and large (108.7-238.8 g, mean 181.8<TEX>$\pm$</TEX>54.9 g) fish. The OC of the fish showed a clear diel rhythm, with higher values in the daytime and lower values at night, in accordance with light (09:00-20:59 h) and dark (21:00-08:59 h) cycles. The OC of the fish increased linearly with the water temperature. The OC was the highest at 10:00 h, one hour after the onset of daylight and was the lowest at 03:00 h, six hours after dusk. The average OC at <TEX>$20^{\circ}C$</TEX> during the light period was as high as 219.8 mg <TEX>$O_2$</TEX>/kg/h in the small fish and 156.3 mg <TEX>$O_2$</TEX>/kg/h in the large fish, while during the dark period it was as low as 130.5 and 110.4 mg <TEX>$O_2$</TEX>/kg/h, respectively. The OC during the dark period, which showed limited variation, could be regarded as the resting OC, and was 107.6, 130.5, and 219.8 mg <TEX>$O_2$</TEX>/kg/h at 15, 20, and <TEX>$25^{\circ}C$</TEX>, respectively, in small fish, and 52.3, 110.4, and 171.0 mg <TEX>$O_2$</TEX>/kg/h in large fish. As the body weight of black porgy increased, the OC decreased exponentially and the relationship was expressed as OC=1,222.8<TEX>$BW^{-0.567}$</TEX>, OC=1,113.2<TEX>$BW^{-0.448}$</TEX>, and OC=1,495.3<TEX>$BW^{-0.468}$</TEX> at 15, 20, and <TEX>$25^{\circ}C$</TEX>, respectively. At a fish density of 14.5 g/L at <TEX>$20^{\circ}C$</TEX>, black porgy had the highest OC per breath compared to fish at the same density at 15 or <TEX>$25^{\circ}C$</TEX>. This suggests that the black porgy responds to the stocking density (15 kg/<TEX>$m^3$</TEX>) and water temperature (<TEX>$20^{\circ}C$</TEX>) conditions commonly observed in intensive aquaculture with the deepest breath and the highest metabolic activity.

New insights into the factors mediating the onset of puberty in sea bass

Muscle ornithine decarboxylase activity as an indication of recent growth in pre-smolt Atlantic salmon, Salmo salar

On the relation between size of necrosis and diameter of tumor spheroids

The effects of dietary protein-energy levels on the growth rate, proximate composition and production were examined in Nile tilapia, Oreochromis niloticus, at two starting weights (22.9 and 39.8 g) reared in concrete ponds for 180 days. The highest weight gain (183.1 g) was obtained with fish fed a 30% protein and 10.5 kJ g−1 diet for the small initial size and 180.2 g for a diet containing 25% protein and 12.6 kJ g−1 for the large initial size. Dressed yields (edible mass) and fillets increased to 56.9% and 52.5% in fish fed diet with 25% protein and 10.5 kJ g−1 at the initial size of 22.9 g, while fish started at 39.8 g exhibited the best values (56.5% and 52.1%) when fed the 30% protein and 10.5 kJ g−1 diet. Proximate composition of soft tissue (wet weight basis) in small fish was significantly influenced by dietary protein-energy levels. Protein was 26.1±0.3% in fish fed the high protein (30%) and low energy (10.5 kJ g−1 diet), while lipid content was 6.4±0.3% at diet containing 20% protein and 14.7 kJ g−1 diet. Large initial size fish fed the diet with 25% protein and 14.7 kJ g−1 had the highest body protein (32.0±0.4%) and lowest lipid content (2.2±0.3%). Feed conversion ratio (FCR) and protein efficiency ratio varied with different dietary protein-energy levels and initial fish sizes. Feed conversion ratio increased with increasing protein and decreasing energy level in the diet, and values in small fish were higher than values in large fish. Protein efficiency ratio decreased with increasing dietary protein level and decreasing energy level. The maximum total production (7.6 tons feddan−1) was with dietary high protein (30%) and low energy (10.5 kJ g−1) for small-sized fish, while large initial fish had the highest production (3.7 tons feddan−1) when fed the 25% protein and 12.6 kJ g−1 diet energy. Starting with 22.9 g fish was more advantageous than the initial size of 39.8 g for rearing Nile tilapia. Small fish required a high-protein and low-energy diet, whereas large fish required a low-protein and high-energy diet to achieve highest production.