Fish production in the Jarama River, Central Spain

Fish production was estimated at three sites on the Jarama River, a small, typical upland river in Central Spain. Population estimates were made by the successive removal method of electrofishing. The same six species were recorded at each of the sites: Chondrostoma polylepis, Barbus barbus bocagei, Leuciscus cephalus pyraenaicus, Salmo trutta m. fario, Gobio gobio and Cobitis paludicola , with the first three species always dominant. Density, biomass, production (assuming that N_o is the total number of eggs spawned), and available production were, respectively: 13502-85776 ind. ha⁻¹, 178.6–221.3 kg ha⁻¹, 221.7–583.6 kg ha⁻¹ yr⁻¹, 118.1–271.9 kg ha⁻¹ yr⁻¹. Production estimates based on mortality curves were 7.9–19.5% (mean: 13.7) lower than those based on estimated from the number of eggs laid. Production per unit of area was highest at the widest and deepest site. Brown trout production contributed only 2–4% of the total production for all sites.     

Life cycle, growth, mortality and production of Ephemerella major Klapalek (Ephemeroptera) in a trout stream in Belgium

SUMMARY. 1. The life cycle of Ephemerella major Klapalek in a chalk trout stream in Belgium took 1 year. Emergence was highly synchronized with a flight period from mid-May to mid-June. Tiny nymphs occurred from June to late August.
2. The mean instantaneous growth rate was high in autumn (3.6% wet wt day⁻¹), very low from November to February (0.8% wet wt day⁻¹) and high until emergence (2.3% wet wt day⁻¹); short day length seemed to be the major factor reducing growth rate during winter.
3. Mortality was close to zero during winter and 1.4—1.7% day⁻¹during other seasons. Total mortality from egg to adult was 99.6%.
4. The annual production was about 9g wet wt m⁻² year⁻¹ and the annual P/ ratio was 7.5. There was good agreement between the production values estimated by four methods. Production rate was highest in May (13 mg wet wt m⁻² day⁻¹) and zero in February.     

The population biology of pike, Esox lucius L., in two gravel pit lakes, with special reference to early life history

The population biology of pike in two gravel pit lakes of contrasting habitat type were studied. The density of pike (1 + and over) in November 1987 in the Main Lake was 7.7 fish ha⁻¹ (19 kg ha⁻¹) and in St Peter's was 71 fish ha'(53 kg ha⁻¹). The age distribution showed that recruitment was consistent in St Peter's but variable in the Main Lake. The main factor causing this difference in recruitment is abundance of aquatic vegetation. Survival up to November 1987 of 0-group pike in the Main Lake was 0.00175% from potential egg deposition, producing 6.6 fish ha⁻¹ (0.83 kg ha⁻¹). In St Peter's survival was 0.00315%, producing 23 fish ha⁻¹ (3.04 kg ha ¹).     

Concentrations and annual loading of dissolved organic matter in a small moorland stream

SUMMARY. 1. Dissolved organic matter (DOM) was measured at 8h intervals over a 1 year period in a stream draining 51 ha of moorland with peaty soils.
2. DOM concentrations increased with increasing stream discharge from low flow values of 0–3 mg 1⁻¹ to maximum values of 30 mg 1⁻¹. There were also seasonal differences of up to 13mgl⁻¹ between August (maximum) and February, and differences of about 2.5 mg l⁻¹ between rising and falling stage samples.
3. Seasonal variation was closely related to mean temperature.
4. Total loss over the year was 168 kg ha⁻¹ DOM (84 kg ha⁻¹ C), larger than previous estimates for upland sites.     

The fate of a roach Rutilus rutilus stock under an extremely strong fishing pressure and its predicted development after the cessation of mass removal

The effects of very intensive exploitation on a roach stock over 5 years are estimated. The total roach catch of the mass removal carried out with trawls was 507 metric tons (195 kg ha⁻¹, 54% of the total catch). According to the virtual population analysis the biomass of the stock has decreased from 180 kg ha⁻¹ in 1989 to c . 50 kg ha⁻¹ in 1993. The production of the stock has fallen from 64 to 33 kg ha⁻¹. The stock is dominated by younger age groups than before the mass removal. The future development of the stock is predicted by considering different levels of natural and fishing mortality. If the exploitation is stopped totally after 1993, roach biomass is predicted to double in 3 years. A constant fishing mortality rate of 0.3 (yearly catches 45–50 metric tons) would prevent the recovery of the stock, providing that 1–year–old and older roaches are exploited.     

The distribution and production of Gammarus pseudolimnaeus (Crustacea: Amphipoda) along a reach of the Credit River, Ontario

SUMMARY. The population density of the univoltine amphipod Gammarus pseudolimnaeus was estimated monthly from April 1977 to June 1978 along a 600-m reach using a form of removal sampling at 0.5, 1.5 and 2.5 m from the bank. The probability of catching an amphipod with three sweeps over an area of 0.15 m² was 0.95. Drift and upstream migration were measured for 24 h, also monthly. Population density varied from 1600 m⁻² (newly-hatched young in July) to 26 m⁻² (adults) within 3 m of the bank with 95% confidence limits of 40–60% of the mean; density was highest < 1 m from the bank and beyond 3 m it declined to insignificant levels. No significant difference could be shown between the total catches in the drift and upstream nets over a year and there appeared to be no net migration of amphipods. Mean drift was 2 amphipods day⁻¹.
Growth was estimated from the length composition of monthly samples. Annual production ( P ) was 29.4 kg ha⁻¹ for one generation of the amphipod with a P/B ratio of 4.65. Egg production accounted for only 2% of this estimate. Recruitment was calculated from regressions relating female length to clutch size and egg development time to temperature in the laboratory; mean temperature was measured in the field.
Mortality was greatest in July at the beginning of the generation. It was high again in September and during winter; at these later times it was suggested that starvation occurred in addition to predation. Annual consumption ( C ) of organic matter for the generation calculated from published data on feeding rate was 1547 kg ha⁻¹ which results in a P/C ratio of 1.9%.     

Fish production in Oued Sebaou, a seasonal river in North Algeria

Fish were sampled by electrofishing at three contiguous sites 26 km from the source of Oued Sebaou, a seasonal river in North Algeria. River discharge ranges from 300 m³s⁻¹ in the peak of the rainy season to 0.1 m³s⁻¹ at the end of the dry season. Extensive gravel extraction from the river bed takes place during the dry season.
Two species were recorded at each site: Barbus callensis , which is heavily fished locally, and Anguilla anguilla . The three sample maximum likelihood Zippin method was used to estimate mean density and standing crops, which were: barbel, 452 fish ha⁻¹, 5.33 kg ha⁻¹; eels, 149 fish ha⁻¹, 3.04 kg ha⁻¹. Estimated average biomass and production were: barbel, 2.31 kg ha⁻¹, 4.27 kg ha⁻¹ year⁻¹; eels, 1.51 kg ha⁻¹, 2.60 kg ha⁻¹ year⁻¹.     

Population dynamics and production of the pelagic amphipod Hyalella montezuma in a thermally constant system

SUMMARY. 1. The population dynamics and annual production of the multivoltine. pelagic amphipod Hyalella montezuma were studied over a 3-year period in the thermally constant environment of Montezuma Well. Arizona.
2. H. montezuma showed two maxima which coincided with spring and autumn phytoplankton maxima. Juveniles comprised over 85% of the population in the pelagic zone compared to 37% in the littoral vegetation and there were significantly more females in the littoral vegetation. It appears that juvenile and adult H. montezuma show different habitat preferences.
3. Total annual mean production for H. montezuma calculated by the size frequency method and adjusted for multiple generations, was 357 kg ha⁻¹ yr⁻¹, which is higher than single-species production estimates reported for most zoobenthic amphipods and multivoltine planktonic crustaceans. Average energy production for H. montezuma was 4640 kJ ha⁻¹ yr⁻¹ in the pelagic zone and 1072 kJ ha⁻¹ yr⁻¹ in the littoral vegetation.
4. Average cohort P / ratios for H. montezuma were higher in the pelagic zone (5.5) than in the litttoral vegetation (3.7). Juveniles had higher cohort P ratios than adults in the pelagic zone, while the reverse relationship was true in the littoral vegetation. We propose that different size-selective predators may contribute to the differences in P ratios for juveniles and adults in these two habitats.     

The population density, growth rate and production of roach Rutilus mtilus (L.) in Tjeukemeer, The Netherlands

A method of estimating the population density of roach in Tjeukemeer (21.3 km²) using 20 529 introduced fin-clipped fish is described. Fyke nets proved to be an effective method of sampling the population for marked fish during the spawning season. A total of 20 277 roach were processed during the recapturing period. The population density of roach (⋝ 14 cm) was estimated to be 1 246 458. The growth rate of roach in the lake although relatively poor (von Bertalanffy's L ∞ for males and females, 22 and 26 cm respectively) for the species was similar to that recorded in three other Friesian lakes. The mortality rate of males was higher than that of females. Logarithmic length-weight regression analyses showed that the value of the coefficient varied both within and between the sexes, that of females being higher (range 3.03–3.375) than that of males (range 2.76–3.254). Seasonal changes in the size of the coefficient were due to a disproportionate loss of gonad weight in larger fish. The fecundity of the population was comparatively high for the species.
The total production of the population was estimated to be 95 hg ha⁻¹ of which 39 kg ha⁻¹ was contributed by fry. In older (⋝ IV) fish the production of females (12 kg ha⁻¹) exceeded that (2 kg ha⁻¹) of males, due to differences in their growth and mortality rates. The relatively poor performance of roach in Tjeukemeer, in terms of biomass and production was due to a scarcity of zoobenthos and competition from other species offish. There is no evidence either from this study or the literature that the productive potential of roach in lakes is high, even though macrophytes and detritus can be consumed in significant amounts.     

An estimate of the sustainable rate of shell extraction from the Dutch Wadden Sea

1. Shell production by cockles Cerastoderma edule was studied to examine whether or not the present licensed rate of shell extraction in the Dutch Wadden Sea exceeds the current rate of shell addition to the exploitable stocks.
2. Long-term data on numbers of cockles and weights of their shells were used to estimate their annual production on Balgzand, a 50-km² tidal flat area in the western-most part of the Wadden Sea. During the 1969–97 period, it amounted to an average of 125 g m^–2, including 107 g m^–2 of shells large enough to be exploitable for shell-lime fishery.
3. The very irregular annual recruitment of cockles was the main cause of the wide 95% confidence limits (74 and 140 g m^–2 year^–1) of this 28-year estimate. Moreover, high mortality rates in severe winters substantially reduced production per recruit in some year classes.
4. About one-third of the estimated production does not reach exploitable stocks, because it is fragmented by birds (particularly eider ducks), permanently buried in the sediment, or removed by the fishery for live cockles.
5. During the last few decades, the estimated mean amount added annually to the exploitable stocks was 88 million kg or 132 000 m³ of large cockle shells. This amount compares favourably with the current annual level of removal of 134 000 m³ of shells, three-quarters of which are cockles.
6. Even at temporarily lower production rates, the exploitation of shell stocks at its present rate is not expected to lead to a rapid exhaustion of the existing stocks in the tidal inlets of the Dutch Wadden Sea, as these stocks will be in the order of a few million m³.     

Grasshopper egg mortality mediated by oviposition tactics and fire intensity

Abstract.  1. It is commonly assumed that arthropod species living or hibernating in the soil would not be affected by grassland fires, even though burning results in elevated surface and below-ground soil temperatures. The importance of elevated below-ground soil temperatures during fires on the survival of grasshopper eggs had not been examined.
2. The effects of simulated autumn grassland fires of varying intensities on below-ground egg mortality were examined with grasshopper species laying shallow egg pods ( Ageneotettix deorum ) and deeper egg pods ( Melanoplus sanguinipes ) to test the hypothesis that exposure to heat during fires was the mechanism responsible for population reductions in A. deorum following fire.
3. Species-specific oviposition characteristics mediated the effects of fire intensity on below-ground egg mortality. The results indicate that fires occurring in areas with at least 3100 kg ha⁻¹ standing crop biomass would be expected to significantly reduce populations of A. deorum , but not M. sanguinipes . No A. deorum eggs hatched in 12 of the 14 oviposition containers subjected to simulated fires approximating a standing crop biomass of 4500 kg ha⁻¹. This is the first study to link field observations of rangeland insect populations following fire to mechanisms related to below-ground egg mortality.     

An examination of the fish population in the River Trent, Nottinghamshire using angler catches

During this study, anglers caught over 580 kg of fish, comprising over 29 500 individuals of 14 species. Nine times as many gudgeon, Gobio gobio , as roach, Rutilus rutilus , were caught. Gudgeon represented 35.94% of the total catch weight compared with 44.64% for roach. The mean total catch per man-hour was 8.9 fish and the mean catch rate was 176 g per man-hour. Population estimates for fish longer than 12 cm for all species other than gudgeon and bleak, Alburnus alburnus , gave fish densities of 0.21 fish m⁻², the roach population was 0.15 m⁻², and that for dace Leuciscus leuciscus , 0.02 m⁻². The population density for all sizes of all species caught was estimated to be 2.25 fish m⁻².
The fish biomass available to the angler was 447 kg ha⁻². The rate of exploitation of the fishery over the period of sampling was high at 17%, representing an annual exploitation rate of 94%.     

Sediment processing by gizzard shad, Dorosoma cepedianum (Lesueur), in Acton Lake, Ohio, U.S.A.

Gizzard shad are primarily detritivorous in Acton Lake, a 253-ha impoundment in southwestern Ohio, U.S.A. To determine the magnitude of sediment utilization by the gizzard shad population in Acton Lake. I used data on population density and age structure, daily ration, and feeding selectivity in estimating the mass of sediments processed by shad daily from April through November. At densities of 4595–10 645 fish ha⁻¹(wet weight biomass = 90–121 kgha ¹), gizzard shad could process 3.8–23.0 kg of dry sediments ha⁻¹ day ¹. On average throughout the growing season, gizzard shad could process a dry mass of sediments each day equivalent to 13% of shad wet weight biomass. Because of the high rate of sedimentation (> 700 kg dry sediment ha⁻¹ day⁻¹) in Acton Lake, gizzard shad can process < 4% of the freshly deposited sediments each day, and therefore are likely to have little effect on benthic community dynamics in the system.     

Population ecology of pike-cichlid, Crenicichla lepidota, in two streams of the Brazilian Pampa subject to a severe drought

Scales were valid for age determination of pike-cichlid Crenicichla lepidota Heckel in two Brazilian streams, since, for every age-class, annual rings (annuli) were formed once a year, in August. Back-calculated Jengths-for-age coincided with those obtained by inspecting the monthly length-frequency distributions. Only 11 reproductive females were captured (length range 9.6–16 cm, age 1+ and 2 + ) and their fecundity ( F ) increased with fish length ( L ) according to: F= 2.4027 L^2.3631. Average annual density and biomass in four localities of Zanga do Campus were 2194, 1676, 1824 and 1071 individuals ha⁻¹ and 17.2, 12.1, 13.8 and 18.7 kg ha⁻¹and about 1000 individuals ha⁻¹ and 8.5 kg ha⁻¹ in three combined sites of Zanga do Barbará. The annual production ranged between 18.5 and 32.7 kg ha⁻¹ a ⁻¹ in the first stream and 8.7 kg ha ⁻¹a ⁻¹ in the other. Feeding tactics differed between streams and age-classes (0+ v . 1+/2+). In winter, the 0+ age-class in Zanga do Campus fed mainly upon Ephemeroptera (Baetidae) but in other months also fed on zooplankton. In winter, these age-classes in Zanga do Barbara preyed upon benthic species but the contribution of different taxa varied between months. In January, these fish preyed exclusively upon Chydoridae and fish. The effects of a severe drought (November-February) upon these parameters are discussed.     

Wood decomposition in an abandoned beech and oak coppiced woodland in SE England

Stand structure and rates and patterns of wood litterfall, between 23 June 1977 and 27 June 1979, are described for two sites In a coppiced beech Fagus sylvatica L. and oak Quercus petraea (Mattuschka) Liebl. woodland in south-east England.
Total wood fall (excluding trunks) was estimated at 1264 kg ha⁻¹ and 593 kg ha⁻¹ for the two years of sampling at one site and 776 kg ha⁻¹ during the second year on the other site. The majority of leaf fall occurred in autumn; the maximum twig (wood < 1 cm diameter) and branch (wood ≥ 1 cm diameter) fall, occurred in winter at times generally corresponding with high wind speeds. Many oak twigs, however, abscissed in early November irrespective of wind speed.
Branch characteristics at fall (size, state of decay and bark cover) were documented. The most significant feature observed was that a large amount of decay frequently occurred in branches whilst still in the canopy.     

Angulate and leopard tortoises in the Thicket Biome, Eastern Cape, South Africa: populations and biomass estimates

The population parameters (morphometrics, population size and biomass) were determined for leopard and angulate tortoises in the Thicket Biome, Eastern Cape, South Africa. Leopard tortoises had a mean total length of 306.47mm (SD = 94.65mm, range 59–496mm, n = 89). Males were smaller (mean total length 277.31mm, SD = 71.85mm, range 73–377mm, n = 36) than females (mean total length = 332.33mm, SD = 97.64mm, range 59–496mm, n = 53). Angulate tortoises had a mean total length of 155.17mm (SD = 33.89mm, range 59–194mm, n = 18). Males were larger (mean total length = 186mm, SD = 8.40mm, range 174–194mm, n = 5) than females (mean total length = 143.31mm, SD = 32.48mm, range 59–178mm, n = 13). The estimated population density for leopard tortoises was 0.85 tortoises ha⁻¹ with an estimated biomass of 6.02kg ⁻¹. Angulate tortoise population density was estimated at 0.12 tortoises ha⁻¹ with a mean biomass of 0.06kg ha⁻¹. The possible consequences of this high density and biomass are discussed.     

Population dynamics of the gastropod Theodoxus jordani (Sowerby) in the Shatt Al-Arab river

SUMMARY. 1. Measurements of the population density of Theodoxus jordani were made for the period July 1980 until June 1981. Numbers showed a well-defined pattern of fluctuation and varied between 390 in February and 1804 snails m⁻² in May after recruitment.
2. The population age structure revealed that the snail was semelparous, and that one generation dominated the population for most of the year.
3. The breeding season was long. Snails laid eggs from early December to the end of April. Eggs were laid in protective capsules and one snail developed per capsule. Total population natality estimated from field samples was 9813 snails m⁻².
4. Age specific mortality was high immediately after recruitment, low during the juvenile stages, and increased sharply at the adult stage with the onset of reproduction.     

Physiological indicators of nitrogen response in a short rotation sycamore plantation. I. CO2 assimilation, photosynthetic pigments and soluble carbohydrates

American sycamore ( Platanus occidentalis L.) seedlings were grown in the field under different urea-nitrogen fertilization regimes to identify physiological variables that characterize the growth response. Treatments included fertilization at the beginning of the growing season with 50, 150, 450 kg N ha⁻¹, fertilization 3 times each at 37.5 kg N ha⁻¹ and unfertilized control. The greatest aboveground biomass accumulation (3× that of control) occurred in plots fertilized with 450 kg N ha⁻¹, but nearly as much growth occurred when 37.5 kg N ha⁻¹ was added periodically. Photosynthesis, chlorophyll concentrations and growth increased rapidly after the midseason application of 37.5 kg N ha⁻¹ but not after the late-season application. Although nitrogen fertilization increased leaf area per plant, leaf nitrogen concentration did not differ between treatments. There was no evidence to indicate that fertilization extended the physiologically active season or increased susceptibility to drought or cold. Sycamore leaves accumulated sucrose and mannose in response to water stress in all treatments. Photosynthetic pigment concentrations and net photosynthetic rate were the most sensitive indicators of growth response to nitrogen fertilization in the first growing season. Careful timing (based on physiological indicators) of low level applications of nitrogen fertilizer can optimize growth.     

The population density, growth rate and production of bream, Abramis brama, in Tjeukemeer, the Netherlands

A method of estimating the population density of bream in Tjeukemeer (21.3 km²) using 16 690 introduced fish (fin-clipped and opercular tagged) is described. Gill nets of the winter fishery proved to be a more effective method of sampling the population for marked fish than fyke nets. The population density of bream (⋝25 cm) was estimated to be 180 000. There was no significant difference between the estimates derived from fin-clipped and opercular tagged fish. The growth rate of bream in Tjeukemeer (L_∞=41 cm) is poor compared with that of bream in other waters, due to its high density and the scarcity of zoobenthos available to it.
The production of bream (I–XV) is estimated to be 34 kg ha⁻¹ of which 25% is contributed by two (1959, 1963) strong year classes. Of a total biomass of 37.5 kg ha⁻¹ available to the fishery in 1969 only 2.7 kg ha^−l was removed as yield. The average P/B ratio for the population was low (0.39). It is concluded that a major flow of energy to bream is through zooplankton.     

Terrestrial export of highly bioavailable carbon from small boreal catchments in spring floods

1. We assessed the terrestrial export of organic carbon, which effectively supported aquatic bacterial production (BP), from small boreal catchments during spring flood. We analysed stream runoff from nine small catchments with different proportions of peat mires and coniferous forests by monitoring the dissolved organic carbon (DOC) flux in combination with conducting bacterial bioassays.
2. Multiple linear regression analysis showed that BP during 7-day-dark bioassays (BP₇; μg C L⁻¹day⁻¹) was explained by both the quantity and quality (low-molecular weight fractions) of the DOC. BP₇ can be used as a measure of export of terrestrial organic carbon that is highly bioavailable.
3. Total export of DOC during spring flood from the different catchments ranged from 20 to 27 kg ha⁻¹ and was negatively correlated to forest cover (%). However, the export of BP₇ carbon was positively correlated to forest cover and varied from about 0.1 kg ha⁻¹ in mire-dominated streams to about 0.2 kg ha⁻¹ in forest-dominated streams.
4. The high bioavailability of forest carbon suggests that forests are the main contributors of BP-supporting carbon in boreal streams although mires have higher area-specific export of DOC.