Ronald W. Davies

INVERTEBRATE PREDATION AND LOTIC PREY COMMUNITIES: EVALUATION OF IN SITU ENCLOSURE/EXCLOSURE EXPERIMENTS'

The influence of the stonefly Kogotus nonus on prey communities was assessed by varying the predator density within small enclosures in a first-order stream in southwestern Alberta. Experimental containers with natural densities of prey, standardized substrate and detritus, and zero, one, two, or three Kogotus were buried in a riffle for 10 d during three periods: July 1981, June 1982, and July 1982. Kogotus depressed the densities of Thienemaniella and some other Orthocladiinae, but had no effect on the densities of Corynoneura (Orthocladiinae), Stempellinella (Tanytarsini), and Baetis tricaudatus (Ephemeroptera). Significant reduction of the densities of Thienemaniella and other Orthocladiinae occurred only when these prey items were present at high densities. The extent to which prey densities were depressed was found to be dependent on predator density within the enclosures: increased predator densities resulted in lower prey densities. However, when predator density in the containers exceeded that commonly observed in the riffle, predator efficiency dropped, probably because of interference between predators, the existence of prey refuges, or changes in predator search effort. An experimental design in which containers that excluded predators were compared with controls accessible to predators was used to determine if results would be similar to results from the enclosure experiments. Using this design, observed results could not clearly be attributed to the presence or absence of the predator, and were more likely due to differences in abiotic conditions between the experimental and control containers. It was concluded that observation of predator effects in field manipulative experiments may be strongly dependent on the selection of an experimental design in which predator effects are not confounded with container effects.

The effects of chronic cadmium stress on energy acquisition and allocation in a freshwater benthic invertebrate predator

Abstract The effects of chronic exposure to cadmium (0 (control), 5 (low), 10 (medium), or 50 (high) μg Cd 1−1) were examined in terms of changes in energy acquisition and allocation (somatic growth, reproductive growth, faeces plus mucus, active respiration, resting respiration, activity time) in the predatory leech Nephelopsis obscura. Leeches in the high Cd treatment had significantly lower biomass production, survivorship and ingestion rate than leeches in the other treatments or control. Assimilation efficiency, reproductive investment, resting respiration and active respiration were unaffected by Cd but leeches in the high Cd treatment were significantly less active and produced more mucus. Decreased ingestion and increased mucus production in the high Cd treatment contributed to a lower growth rate. Although Cd had no significant effects on resting or active respiration, there were significant effects on total respiration as leeches exposed to high Cd exhibited a significant decrease in time spent active and thus in total respiration. Significant mortality in the high Cd treatment indicates that leeches are unable to compensate. As reproductive investment did not change with Cd exposure, the only physiological tradeoffs occurred between growth of somatic tissue and survival mechanisms.

Observations on the Age, Growth, Reproduction, and Feeding of Mountain Whitefish (Prosopium williamsoni) in the Sheep River, Alberta

Abstract The ecology of mountain whitefish (Prosopium williamsoni) in the upper Sheep River was studied during 1972 and 1973. Mountain whitefish spawned from late September to mid-October in shallow, fast, midstream areas. The number of spawning fish on each spawning site was small (two to 20 fish per site). Absolute fecundity (eggs/fish) increased as the females became older; however, relative fecundity (eggs/g of fish) remained comparatively stable for all age classes. The linear regression of absolute fecundity (Y) on fork length (X, in mm) is described by the formula: Y = 4347.6 + 50.7 (X – 301.5). Growth is rapid except in the older age classes of 7 and 8. The weight (Y, in g) length (X) relationship (sexes combined) is described by the formula: log Y = 2.1967 + 3.2108 (log X – 2.3710). The relationship of body (Y) to scale (X) growth is described by: log Y = 2.0891 + 0.9542 (log X – 2.4367). Recruitment into the population of the upper Sheep River appeared to be at 3 years of age with an annual natu...

The effects of argulosis-saprolehniasis on the growth and production of Cyprinus carpoi

Growth profiles of male Cyprinus carpio, 12 uninfected (healthy) and 12 infected with the crustacean Argulus indicus and associated fungus Saprolegnia sp. are presented. The fish were maintained under controlled conditions and provided with ad libitum food for 8 weeks. The instantaneous (G) and relative (h) rates of growth, and production (P) in uninfected carp (0.003 to 0.036 g.d−1, 0.29 to 3.68%, 0.25 to 3.23 g respectively) were much higher than those of infected carp ( − 0.007 to 0.021 g.d −1, 0.71 to 2.17%, 0.59 to 1.76 g). The condition factor (K) for uninfected carp was 4.5 to 4.8 compared to 2.6 to 3.0 for infected carp. During each of the 8 wk the weekly change in weight of infected carp was about 4% less than that of uninfected carp. These data show that Argulus-Saprolegnia infections significantly suppress carp growth and indicate that comparative growth rates can be used as indicators of parasite stress.

A SEROLOGICAL STUDY OF PREY SELECTION BY HELOBDELLA STAGNALIS (HIRUDINOIDEA)

SUMMARY (1) The feeding of Helobdella stagnalis in the field was investigated using specific rabbit antisera against Chironomidae, Copepoda/Cladocera, Oligochaeta, Amphipoda and Gastropoda. (2) Prey utilization was similar in all three field sites, with Chironomidae and Oligochaeta most heavily utilized, Amphipoda and Gastropoda moderately utilized and Copepoda/Cladocera slightly utilized. (3) On any given sampling date all weight classes of Helobdella stagnalis showed the same relative frequency of prey utilization. (4) Prey utilization differed between months with the Chironomidae most heavily utilized in April, and Oligochaeta most heavily utilized in May and June. During other parts of the year there are no significant differences in the pattern of prey utilization. (5) Feeding activity was highest in the spring and summer and lowest in the fall and winter. (6) Annually the 0.0-7.9 mg weight range utilized the Oligochaeta most heavily followed by the Chironomidae. The contribution of the Copepoda/Cladocera to the overall diet was approximately equal to the contribution of the Amphipoda and Gastropoda. The 8.0-20 + mg weight range utilized the Chironomidae most heavily followed by the Oligochaeta. The Copepoda/Cladocera were an insignificant contributor to the overall diet.

Chemotherapy of six ectoparasitic diseases of cultured fish.

Abstract Periodic observations on ectoparasite infections of Saprolegnia, Trichodina, Myxobolus, Hemiclepsis, Argulus , and Posthodiplostomum on cultured fish were made at the Central Inland Fisheries Research Institute, Karnal and the Haryana Government Fish Seed Farm, Jyotisar (Kurukshetra) from August 1982 through July 1983. Chemotherapeutic control of the ectoparasites by gammexane (1-6-hexachlorocyclohexane), sodium chloride, formalin, potassium permanganate, malachite green, glacial acetic acid, calcium oxide, calcium hydroxide and picric acid was investigated. Malachite green and sodium chloride solutions were most effective on Saprolegnia ; sodium chloride, acetic acid or formalin on Trichodina ; calcium hydroxide, calcium oxide, potassium permanganate or formalin on Myxobolus ; glacial acetic acid and potassium permanganate on Hemiclepsis ; and gammexane on Argulus . None of the chemicals tested was effective on fish heavily infected with Posthodiplostomum although picric acid was successful on less heavily infected fish.

A comparison of the life-cycle of Helobdella stagnalis (Linn. 1758) (Hirudinoidea) in two different geographical areas in Canada

Helobdella stagnalis (Linn.) is a widely distributed species of freshwater leech recorded from every continent except Australia (Sawyer 1972). With the exception of the Yukon it has been recorded from all Provinces and Territories in Canada (Davies 1973) and is also abundant throughout the northern United States of America (Sawyer 1972). Despite its widespread and common occurrence few studies have been made of the life-history of H. stagnalis. In Great Britain the life-history has been studied by Mann (1957a) and Learner & Potter (1974). In North America the only study is that of Tillman & Barnes (1973) in Utah Lake, Utah. This paper on the life-history, growth and age structure of H. stagnalis in British Columbia and Alberta is the first study in Canada.

Effects of Chlorinated Sewage Effluents on Fish in the Sheep River, Alberta

Abstract From bioassays with caged rainbow trout Salmo gairdneri and quantitative sampling of fish populations, the effects of a chlorinated sewage effluent on fish in the Sheep River, Alberta were determined. The data obtained contradict recent studies that indicate fish congregate below sewage outfalls but support the contention that fish avoid chlorinated effluents. However, fish moved into and out of the chlorinated effluent, presumably to forage.

The bioenergetic costs of specific dynamic action and ammonia excretion in a freshwater predatory leech Nephelopsis obscura

The energy utilized by the predatory freshwater leech for specific dynamic action (SDA) and exogenous excretion, and the proportion of absorbed energy available for growth and/or activity were determined at 5, 10, 15, 20 and 25 degrees C. SDA and exogenous ammonia excretion increased with increases in temperature to 20 degrees C but decreased at 25 degrees C. The duration of SDA decreased from 19 hr at 5 degrees C to 11 hr at 25 degrees C, while the duration of exogenous excretion decreased from 22 hr at 5 degrees C to 11 hr at 25 degrees C. The proportion of absorbed energy utilized for SDA and exogenous excretion decreased from 5 degrees C to 15 degrees C and increased at 25 degrees C; however the energy available for growth and activity increased from 7.4 J/day at 5 degrees C to a maximum of 32.6 J/day at 15 degrees C, decreasing to 12.7 J/day at 25 degrees C.

Chemosensory detection of prey by Nephelopsis obscura (Hirudinoidea: Erpobdellidae)

The chemosensory detection of prey by the freshwater leech Nephelopsis obscura (Erpobdellidae) was investigated in the laboratory. Without direct tactile contact N. obscura of all three size classes tested were unable to detect and react to any of the prey types (Gammarus lacustris, Chironomus sp., Tubifex sp) or prey conditions (live, asphyxiated, homogenate) tested. The length of the starvation period (0–56 days) did not affect these results, nor did it increase activity. N. obscura thus appears to be an opportunistic wanderer rather than a stalker-hunter.