A modified drop-net method for sampling mobile epifauna on marine shallow sandy bottoms

The seasonal variations in numbers and biomass of the mobile epifauna of a shallow, sandy bay in Gullmar Fjord (Sweden) was investigated during 1976–1977 using the drop-net technique. Juvenile plaice, Pleuronectes platessa L., sand goby, Pomatoschistus minutus (Pallas), and brown shrimp Crangon crangon L. are the dominant epifaunal species. A net enclosing an area of 100 m² was used, and the organisms captured were collected with a small trawl. Three stations with a water depth≤ 1 m were regularly sampled with a total number of 73 drops. Maximum abundance was for plaice in June (6 ind m^-2), for the shrimp in July-August (61 ind m^-2), and for sand goby in September (2 ind m^-2). Standing stocks (dry wt) were 0.2, 1.5 and 0.2 g m^-2 respectively. Production estimates were based on seasonal changes in size frequency and information on specific growth rates. The production rate was for plaice (0-group) 0.3 g m^-2 yr^-1, for the shrimp 1.5, and for sand goby 0.2 g m^-2 yr^-1. Reliable estimates of the absolute abundance can be derived from the mean catch per unit area only when the efficiency of the fishing gear is known. After 6 hauls with the trawl in the drop-net enclosure more than 85% of the shrimps were caught and after the fifteenth haul less than 1% were still left. The fishing efficiency of a push-net in relation to the drop-net was for the shrimp 50%, for plaice 35% and for sand goby 17%.     

Oxygen consumption by eggs and larvae of striped mullet, Mugil cephalus, in relation to development, salinity and temperature

Oxygen consumption rates during embryonic and the first 38 days of larval development of the striped mullet were measured at 24° C by differential respirometry. Measurements were obtained at the blastula, gastrula and four embryonic stages, and at the yolk-sac, preflexion, flexion and post-flexion larval stages.
Oxygen uptake rates of eggs increased linearly from 0.024 μl O₂ per egg h^-1 (0·323 μl O₂ mg^-1 dry wt h^-1) by blastulae to 0·177 μlO₂ per egg h^-1 (2·516 μlO₂mg ¹dry wth^-1) by embryos prior to hatching. Respiration rates did not vary significantly among four salinities (20,25, 30, 35%₀).
Larval oxygen consumption increased in a curvilinear manner from 0·243 μl O₂ per larva h^-1 shortly after hatching to 18·880 μl O₂ per larva h^-1 on day 38. Oxygen consumption varied in direct proportion to dry weight. Mass-specific oxygen consumption rates of preflexion, flexion, and postflexion larvae did not change with age (10·838 μl O₂ mg ¹dry wt h^-1).
Larval oxygen consumption rates did not vary significantly among salinities 10–35%. Acute temperature increases elicited significant increases in oxygen consumption, these being relatively greater in yolk-sac larvae ( Q₁₀ = 2·75) than in postflexion larvae ( Q₁₀ = 1·40).     

Growth and production of five species of Ephemeroptera larvae from an experimental recirculating stream

SUMMARY. The production and growth of the larvae of five species of Ephemeroptera in an experimental recirculating stream-channel are described. The most abundant species were Baetis rhodani , which achieved a maximum population density of c. 29,000 m^-2 in August, and Ephemerella ignita which reached a density of c. 13,000 m^-2 in the spring. Three other common speeies, Paraleptophlebia submarginata, Centroptilum luteolum and Caenis rivulorum were present at lower densities (< 1000 m^-2). B. rhodani had five cohorts in a year whilst C. luteolum had two or possibly three. E. ignita, C. rivulorum and P. submarginata were univoltine. Mean speeifie growth rates were calculated for each species, the maximum being 3.26±0.49% length day^-1 for E. ignita (3 April—15 May). Production of each of these five species of Ephemeroptera was calculated giving a total annual production of 10.2 g dry wt m^-2 (computational method).     

Seasonal changes in biom ass of macrophytes on shaded and unshaded sections of the River Lambourn, England

SUMMARY. Monthly changes in the biomass of Ranunculus, Berula and Callitriche were recorded on two 50-m sites on the River Lambourn at Bagnor between March 1971 and October 1973. On the unshaded site. Ranunculus was the dominant macrophyte and characteristically its biomass changed from 20–40 g dry wt m^-2 in March to around 400 g dry wt m^-2 at the end of the growing season. On the shaded site, where Berula was the dominant macrophyte. the biomass of Ranunculus was lower than on the unshaded site. There was no significant difference between the mean biomasses of Berula on these two sites but Callitriche had a signifieantly higher biomass on the shaded site. Estimates of the total biomass of macrophyte on each site were calculated and the biomass of macrophyte removed during cutting operations each summer was also assessed.     

The growth and mortality of larval herring, Clupea harengus L., in the River Blackwater Estuary, 1978–1980

Herring larvae were sampled in the Outer Thames Estuary and the River Blackwater Estuary in the springs of 1978, 1979 and 1980. Data were collected on larval stage, yolk sac and post yolk sac, larval length and total larval numbers. Newly hatched larvae were 6.8±0.5 mm long and the growth rate of yolk sac larvae was estimated at 0.18 mm d⁻¹ ( L = 6.8±0.186 t ). The growth rate of post yolk sac larvae increased to 0.43mm d⁻¹ ( L = 11±48.0±43 t ). Mortality estimates, derived from total numbers in the summed estuary segments, varied between the years and the cohorts within the year. In 1979 the mortality rates were 0.061 d⁻¹ and 0.074 d⁻¹ for the two cohorts. The mean size of the larval population was estimated at 2.48×10⁹ (1.63–3.77 × 10⁹) which agreed well with population size estimates from egg laying and from catch in numbers at age together with estimated fishing mortality rates.     

Oxygen uptake in developing eggs and larvae of the cod, Gadus morhua L.

Unfertilised cod eggs showed a mean oxygen uptake rate at 5°C of 0.089 μl O₂, dry wt.⁻¹ h⁻¹; this gradually rose to 0.768 μl O₂ mg dry wt.⁻¹ h⁻¹ in eggs about to hatch. From hatching to complete yolk absorption larvae respired at 1.6 μl O₂, mg dry wt.⁻¹ h⁻¹. During starvation following yolk absorption, uptake fell significantly to 1.1 μl O₂, mg dry ⁻¹ h⁻¹. Much of this decrease in oxygen consumption was shown to be caused by reduction in activity. Loss of weight during the embryo and larval phases could not easily be reconciled with total oxygen consumption; it is suggested that cod embryos and larvae may not rely solely upon endogenous energy reserves during development.     

The effect of light intensity and spectrum on the incidence of first feeding by larval haddock

Under full–spectrum white light, feeding success of haddock Melanogrammus aeglefinus first feeding larvae, as measured both by proportion of larvae feeding and mean prey consumed, peaked at 1·7-18 μmol s^-1 m^-2. Feeding was significantly reduced at lower and higher intensities. A similar result was observed for larvae feeding under blue (470 nm) light, with significantly greater feeding success at intermediate light intensity (1·8 μmol s^-1 m^-2). When different light qualities were compared, larvae had significantly greater feeding success when exposed to blue (470 nm) light than either full-spectrum white or green (530 nm) light. Haddock larvae were capable of prey capture under all light treatments tested, indicating a necessary degree of adaptive flexibility in feeding response. The results are consistent with predisposition of haddock larvae to optimal feeding in a visual environment comparable with open ocean nursery grounds. Information on the impact of light on haddock first feeding can be incorporated into models of larval growth, survival, year-class strength and recruitment, and assist in developing husbandry protocols to maximize larval survival in aquaculture.     

Litter decomposition by slugs in mixed deciduous woodland

For the one year period considered, it is calculated that 683.5 g m^-2 plant litter entered the study area site. Of the six slugs studied, Arion intermedius Normand, the smallest species, had the highest overall consumption rate on the nine test foods (29.1 ± 0.9 mg dry wt g live wt^-1 d^-1) and the highest overall mean assimilation efficiency (72.0 + 0.9%). The mean faecal production for each of the six slug species was very similar, ranging from 8.4 to 9.9 mg dry wt g live wt^-1 d^-1. Using regression equations of food consumed on faeces produced, it is estimated that the slug fauna each year consumes 8.4% of the leaf litter input and 6.5% of the total available plant litter. Of the plant material consumed, 13.8 g dry wt m^-2 are deposited annually as faeces. The role of slugs in decomposition processes is discussed and compared with that of other soil organisms.     

Carbon, nitrogen and caloric content of eggs, larvae, and juveniles of the walleye pollock, Theragra chalcogramma

Measurements of dry weight (wt), carbon (C), nitrogen (N) and calories were made on walleye pollock eggs (0.24 mg, 35.3% C, 8.3% N, and 4.6 kcal g⁻¹ dry wt), larvae (0.16 g, 42.9% C, 11.1% N and 5.1 kcal g⁻¹ dry wt) and juveniles (22.4 g, 47.2% C, 9.0% N and 5.6 kcal g⁻¹ dry wt). For juvenile fish (9–360 g wet wt) the measured values were related to dry weight and Fulton's condition factor index (CFI) by regression models. The CFI was a better predictor of body composition than dry weight. As CFI improved from a minimum starvation level of 0.42 to a maximum of 1.16, body caloric content, percentage C, and the C/N ratio increased (kcal g⁻¹ dry wt = 4.4 CFI + 1.7, percentage carbon = 49.7 CFI^0.5, C/N ratio = 5.0 CFI + 0.9), while percentage N and percentage ash decreased (percentage N =−3.5 CFI + 12.1; percentage ash = 9.1 CFI^−1.4). The results of this study suggest that seasonal C, N and caloric content of young pollock can be estimated from measurements of Fulton's condition factor index.     

Production of epipelic algae before and during lake fertilization in a subarctic lake

Epipelic production was studied over three years in Lake Gunillajaure, a small, subarctic lake in northern Sweden. In 1977 the lake was in its natural state and in 1978 and 1979 it was continuously fertilized with phosphorus and nitrogen. The fertilizations in 1978 and 1979 did not increase the annual epipelic production measured with ¹⁴C technique. Also the seasonal production pattern was the same during the three years. The variation in production with depth was similar all three years with mean values of 85 mg C m^-2 d^-1 at 2 m and 1 mg C m^-2 d^-1 at 11 m. Light penetration and water temperature were found to be the most important environmental factors affecting epipelic production. A long turnover time, between I months and 14 yr at different depths, of the probably well adapted epipelic community is suggested to be an explanation to the absence of a response to the fertilization.     

Temporal and spatial distribution of planktic cyanobacteria in Lake Kastoria, Greece, a shallow, urban lake

The temporal and spatial distribution of planktic cyanobacteria and some environmental parameters were studied in the shallow, urban Lake Kastoria, Greece from June 1996 to June 1997. Water temperature varied from 6–27 °C, pH from 7.5–8.9 and dissolved O₂ concentration from 0.7–12 mg m^-3 10^-3. The mean annual Chl a concentration was 83 mg Chl a m^-3 indicative of the eutrophic-hypertrophic state of the lake. Cyanobacterial biomass ranged from 11–238 g FW m^-3, constituting about 90% of the total phytoplankton biomass throughout the year. Cyanobacterial biomass was non-uniformly distributed both vertically and horizontally from August to November 1996 and resulted mainly from the distribution of Microcystis. Seven cyanobacterial taxa were reported for the first time in Lake Kastoria. Six taxa were dominant: Microcystis aeruginosa, M. flos-aquae, M. novacekii. Limnothrix redekei, Anabaena sp. and Cylindrospermopsis raciborskii. The dominant cyanobacterial taxa can be grouped on the basis of their distribution patterns (1) Microcystis species: maximum biomass occurring at pH > 8, temperature 12–17 °C, depth < 0.2 m; (2) Anabaena sp. and Cylindrospermopsis raciborskii : maximum biomass at temperatures 23–26 °C; (3) Limnothrix redekei : maximum biomass at temperatures 6–27 °C. Usually, non-uniform, vertical distributions of cyanobacterial biomass were associated with the formation of temperature, pH and O₂ gradients. L. redekei was considered to be a key lake organism since it contributed up to 59% of the cyanobacterial biomass. Interestingly, three of the dominant cyanobacteria Microcystis aeruginosa, Anabaena sp. and Cylindrospermopsis raciborskii belong to genera that include toxin-producing species.     

Effects of light and temperature on the cell size and some biochemical components in two freshwater cryptophytes

The effects of light and temperature on cell size and cellular composition (chlorophyll, protein, carbohydrate) of two freshwater cryptophytes were studied with batch cultures. Neither of the species had a constant cell size but the size varied with growth conditions. At each temperature the smallest cells were recorded at the lowest experimental photon flux density. The smallest cells of Cryptomonas 979/67 had an average volume of 232 μm³ and the largest ones 1 020 μm³. In Cryptomonas 979/62 the smallest and largest cells measured 4 306 μm³ and 12 450 μm³. Both species increased their cellular chlorophyll content when PFB dropped below 110–120 μmol m^-2 s^-1. The highest and lowest chlorophyll contents of 979/67 were 7.45 fg μm^-3 and 0.55 fg μm^-2 respectively. For 979/62 the corresponding values were 10.23 fg μm^-3 and 0.93 fg μm^-3. In both species the protein content remained stable at PFDs higher than 110–120 μmol m^-2 S^-1. The highest content of protein measured in 979/67 was 638 fg μm^-3 and the lowest 147 fg μm^-3. For 979/62 these values were 1 036 fg μm^-3 and 148 fg μm^-3 respectively. The carbohydrate results were less clear and no pattern either in response to photon flux density or temperature was obvious. The lowest and highest contents recorded for 979/67 were 62 fg μm^-3 and 409 fg μ^-3 and for 979162, 36 fg μm^-3 and 329 fg μm^-3     

Diel abundance, migration and feeding of fish larvae (Eleotridae) in a floodplain billabong

Eleotrid larvae (2.1–16 mm) were collected from surface waters of a billabong in south-eastern Australia. Estimates of larval density in plankton net samples at night averaged 148.3 larvae per m³ and 16.6 larvae per m³ during the day. In contrast, pump samples provided density estimates of 8.3 larvae per m³ at night and 0.9 larvae per m³ during the day. Larval densities did not differ between open water, snag (fallen tree) and Typha habitats, but Typha habitats yielded larger larvae than other habitats. 32.9% of larvae in pump samples were damaged and unmeasurable, creating a bias favouring larger larvae. The modal length of larvae in net samples at night was 5–6 mm, compared with 3–4 mm during the day, reflecting both greater net avoidance by larger larvae during the daytime and dispersal of smaller larvae from the surface at night. Dispersion patterns of larvae suggest that classes of larvae smaller than, and larger than 5.0 mm exhibit reciprocal diel vertical migration behaviour linked to ontogenetic changes in diet. Larvae less than 5 mm fed only during the day and preyed exclusively on rotifers, whereas larger larvae continued to feed at night and consumed mostly planktonic crustaceans.     

Effects of development, temperature and salinity on metabolism in eggs and yolk-sac larvae of milkfish, Chanos chanos (Forsskål)

Oxygen uptake rates and yolk-inclusive dry weiGhts were measured during the egg and yolk-sac larval stages of milkfish, Chanos chanos (Forsskal). Oxygen uptake by eggs and yolk-sac larvae was measured to assess the effects of four salinities (20,25,30,35 ppt) at 28°C. The effects of three temperatures (23,28,33°C) on oxygen uptake by yolk-sac larvae were determined at a salinity of 35 ppt. Dry weights were measured throughout embryonic development at 28°C and the yolk-sac stage at 23.28 and 33°C.
Oxygen uptake rates of eggs increased more than fivefold during embryogenesis (0.07±0.03 to 0.40 ± 03 μl O₂ egg ⁻¹ h ⁻¹;blastula to prehatch stage). Larval oxygen uptake did not change with age but was affected by rearing temperature (0.33 ± 0.08, 0.44 ± 0.07 and 0.63 ± 0.13 μl O₂ larva ⁻¹ h⁻¹ at 23, 28 and 33°C, respectively; Q₁₀= 1.93). Acute temperature changes from 28 to 33°C caused significant increases in oxygen uptake by embryos (Q ₁₀= 1.69–3.58) and yolk-sac larvae (Q ₁₀=2.55). Salinity did not affect metabolic rates.
Dry weight of eggs incubated at 28°C decreased 13% from fertilization to hatching. Incubation temperatures from 23–33°C did not affect dry weights at hatching. Rearing temperatures significantly affected the rate of larval yolk absorption (Q ₁₀= 2.25).     

The periphyton communities of Nyumba ya Mungu reservoir, Tanzania

The periphyton communities established in Nyumba ya Mungu were investigated between July and September 1974.
One hundred and ninety-two taxa were identified. One hundred and fifteen taxa were collected in the southern end of the lake, where 97 taxa were non-selectively distributed in the littorine epiphytic, epilithic, and epixylic environments. The offshore epixylon supported only 53 taxa. The epipelon and epipsammon were poorly developed at the southern end of the lake but were well developed in the northern inflow regions.
Diatoms and cyanophytes were the dominant phyla which contributed over 80% of the algal numbers. Chlorophytes were not common. Taxonomic compositions of periphyton on artificial and natural surfaces were similar, but artificial substrata accumulated greater proportional representations of diatoms.
The mean net production rates of periphyton on artificial substrata located at depths from 0-0.65 m for 28 days ranged from 140 mg organic dry weight m^-2 day^-1 on shoots in the macrophyte zone to 820 mg organic dry weight m^-2 day^-1 on glass surfaces on a rocky shore. The epiphytic environments produced the least standing crops of less than 250, 000 individuals cm^-2, but maximum densities of over 2,000,000 individuals cm^-2 were recorded in the epilithon.     

Drinking rate, uptake of bacteria and microalgae in turbot larvae

The drinking rate of turbot larvae increased from 14 to 120 nl larva^-1 h^-1 from day 2 to 11 after hatching, which gave a slightly increased specific drinking rate (calculated per biomass) from day 2 to 7 (0·8–1·9 nl μg carbon^-1 h^-1. The clearance rate of both algae and bacteria was 10–100 times higher than the drinking rate, which indicated that the larvae had an active uptake of both algae and bacteria. On day 2 and 4 after hatching highest clearance rate was observed for Tetraselmis sp. On day 6 about the same clearance rate was observed for bacteria, Isochrysis galbana and Tetraselmis sp. Until day 4 the turbot larvae had a higher ingestion rate of Tetraselmis sp. than of I. galbana , whereas on day 6 the rates were similar (28–41 ng carbon larvae^-1 h^-1). The assimilation efficiency was somewhat higher for I. galbana than for Tetraselmis sp., and on day 6 the assimilated algae constituted 1·5 and 0·9% of the larval biomass for I. galbana and Tetraselmis sp., respectively.     

Life history and bionomics of Arctopsyche grandis (Trichoptera) in a Central Idaho stream

Arctopsyche grandis was studied for two years In a cold stream in central Idaho. Larvae have a one year life cycle consisting of five instars. Three to four instars are present in most months. Growth is rapid in summer with instar I and II passing to IV and V in 30 d or less. Little or no growth occurs during winter. Maximum larval densities were 547 (1977) and 2060 m⁻² (1978) of planar rock surface area. Populations fitted a negative exponential survivorship curve with similar exponents (−0.0078, −0.0103) in both years. Larval growth was generally isometric with head capsule width (403–2197 μm) and weight (0.05–16.23 mg AFDW) increasing by a factor of 1.5 between instars. Net dimensions correlated with head capsule width and nets were made by all instars and maintained throughout the year. Larvae fed preferentially on animals and diatoms avoiding vascular plant detritus. No differences were detected in diet among the instars.     

LONG-TERM TRENDS IN ABUNDANCE AND DISTRIBUTION OF MANATEES (TRICHECHUS MANATUS) IN THE NORTHERN BANANA RIVER, BREVARD COUNTY, FLORIDA

Four aerial survey projects were conducted between 1977 and 1986 to determine the abundance, density and distribution of West Indian manatees (Trichechus manatus), in the northern Banana River, Kennedy Space Center, Florida. Manatee density and distribution within selected portions of the 78.5 km² study area were determined. Peak numbers of manatees occurred in spring of each year. The maximum counts increased from 56 in 1978 to 297 in 1986. Manatee abundance was lowest in the winter of each year. Mean density per flight increased from 0.52 manatees/km2 in 1977–78 to 2.73/km² in 1984–86. This increase may reflect increases in the east coast population or shifts in the population distribution. Distributional changes were observed in the study area through time, with a lower percentage of manatees occurring in industrial areas and a correspondingly higher percentage of manatees in nonindustrial areas by 1985.     

Growth, daily ration, and gastric evacuation rates of milkfish (Chanos chanos) fed supplemental diet and natural food

Growth, daily ration, and gastric evacuation rates of milkfish ( Chanos chanos ) that fed on natural food and supplement diet were evaluated. Milkfish fingerlings (5.5g) were stocked at 1.5 fish/m² in ten 12 m² concrete tanks layered with 15-cm thick earthen bottoms. All tanks were regularly fertilized (16–20–0 and chicken manure) to maintain natural food production; 4 of the tanks additionally received a supplemental diet containing 34.3% protein and 4290 kcal/kg gross energy. Estimates or daily ration (based on dry weight of stomach contents) were calculated using the E lliot and P erson (1978) and E ggers 1977) models. Gastric evacuation rate was lower in fish that fed on natural food (1.57) compared to fish fed a supplemental diet (1.79). Consequently, the lower rate resulted in lower food intake and slower fish growth. When fish were provided a high quality supplemental diet, daily rations for fingerlings (35 g) to marketable size (116 g) ranged approximately from 0.60 to 19.68 kcal/fish/day. The deviation in daily ration (kcal/fish/day) from the above estimates may indicate the insufficient quantity of dietary energy taken by fish from natural food alone, which could be provided by supplemental diet.     

Observations on the growth and production of chub Leuciscus cephalus and dace Leuciscus leuciscus in a small lowland river in southeast England

SUMMARY. The River Eden (Kent, England) holds a mixed coarse fish population in which minnows ( Phoxinus phoxinus ), gudgeon ( Gobio gobio ), chub ( Leuciscus cephalus ) and dace ( Leuciscus leuciscus ) are numerically predominant. Chub and dace provide the major interest to anglers and their growth and production were studied. Observed growth rates of both species were marginally below recorded averages from other British habitats. Back-calculations showed that year-class strength and relative growth rates varied between years. Instantaneous mortality rate Z was 0.147 for chub aged 1–10, 0.438 for chub aged 10 and over, and 0.172 for dace. Exclusive of the 0-group, fish numbers, biomass (wet wt) and production were found to be 0.1305 m^-2, 19.12 gm^-2 and 5.38 gm^-2 year^-1, respectively, for chub and 0.0968 m^-2, 4.33 gm^-2 and 1.69 gm^-2 year^-1 for dace. Smoothing of data produced theoretical production figures of 6.69 gm^-2 year^-1 for chub and 1.15 gm^-2 year^-1 for dace.