Effects of five years of intensive trawling on a previously unexploited smelt stock

In the years 1989–1993, over 1000 metric tons of fish (392 kg ha⁻¹) was removed by trawls from the Enonselkä basin in Lake Vesijärvi in order to enhance the water quality. Roach (52%) and smelt (28%) were the most abundant species in the catches. In this study, the effects of the mass removal on the previously unexploited smelt stock was assessed. The total smelt catch was c . 282 metric tons. Due to the mass removal, the density of the stock collapsed from c . 46 000 fish ha⁻¹ in 1989 to c . 14 000 fish ha⁻¹ in 1993 ( M =1.0). The biomass of the stock decreased from 75 kg ha⁻¹ in 1989 to 12 kg ha⁻¹ in 1993. The age group composition of the stock changed clearly towards the dominance of younger age groups. The male/female relationship in the samples changed from clear dominance of females to the dominance of males and the infection rate by the sporozoan parasite Glugea hertwigi declined. The growth rate of the smelt did not change considerably during the years of mass removal. The results suggest that in biomanipulated lakes several mechanisms may prevent the exploited stock from responding to the increased 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%.     

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.     

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.     

A preliminary assessment of the biomass of the pelagic sardine Limnothrissa miodon in Lake Kariba

The sardine Limnothrissa miodon was very patchily distributed in Lake Kariba in 1981–1983 and its mean biomass was 59 23 kg ha⁻¹, declining from 90–91 kg ha⁻¹ in 1981 to 38–66 kg ha⁻¹ in 1983. In addition, the mean size of the fish fell from 55–2 mm in 1981 to 49–9 mm in 1983. These changes are attributed to the effects of drought and reduced river flows as well as to the high level of commercial fishing for this species. There was a correlation between commercial catches and biomass estimates which allowed estimation of biomass in previous years from fishing statistics. Fishing effort appeared to be a major influence on sardine abundance: total biomass in 1985 was about 10% of what it was in 1974 when the fishery began. When expressed in terms of unit area, as is usual for African lakes, the pelagic biomass appeared to be greater in Lake Tanganyika, where Limnothrissa is endemic, than in Lake Kariba, although there is little difference in their primary productivity, but consideration in terms of volume rather than area shows that this was not the     

Effects of mass removal and variable recruitment on nutrient excretion by a planktivorous roach stock

The effects of intensive fishing and fish recruitment on the nutrient excretion rate by a planktivorous roach Rutilus rutilus stock in Lake Vesijärvi were estimated using a bioenergetics model. The values were compared with those given by the model assuming that no mass removal occurred or that the mass removal ceased after 2 years. According to the model, during June-August 1989 phosphorus (P) excretion by the pelagic roach stock corresponded to 18% of the annual external P loading and showed a maximum of 0.4 mg m⁻² day⁻¹. As a result of mass removal, the maximum daily P excretion declined to 0.18 mg m⁻² by 1990 and to 0.10 mg m⁻² by 1992. The reduction was due to the lower number of roach, while the excretion rates of individual fish were unaffected. A 2-year mass removal would have temporarily reduced the P excretion rate, but in 1991 and 1992 the highest daily excretion rates would have been doubled compared with values after the 4-year mass removal. Assuming no fishing mortality, interannual differences were diminished and the excretion rates in 1991 and 1992 were 2.5 times higher than the values after the 4-year mass removal. The results suggest that nutrient recycling by fish stocks may be affected substantially by fisheries management. However, the effects are dependent on the timing of fishing due to the variations in the year class strength of fish. In Lake Vesijärvi the reduction in nutrient excretion was amplified coincidentally by the natural variation in recruitment of the roach stock.     

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 structure, growth, mortality and estimated stock size of the introduced tench, Tinca tinca (L.), population in Lake Beyşehir, Turkey

Population structure, growth, length–weight relationship, mortality and stock size of tench, Tinca tinca (L.), was studied in Lake Beyşehir, Turkey in 2005. Totals of 3360 tench (1865 males; 1795 females) were captured with gill- and trammel-nets of various mesh sizes. Male to female ratio was 1.04 : 1. The study covered length year classes. Fork lengths and total weights ranged from 9 to 37 cm and 13 to 815 g. For all individuals, the von Bertalanffy growth equation and length–weight relationship were L _t = 54.2[1−exp(−0.1350( t  + 1.0281)] and W  = 0.0151  L ^2.9993, respectively. Growth performance index and mean condition factor of the tench population were 2.598 and 1.513, respectively. Mortality rates were Z  = 1.97 year⁻¹, M  = 0.29 year⁻¹ and F  = 1.68 year⁻¹ for total, natural, and fishing mortality, respectively. The exploitation rate was E  = 0.85, and the percentage of surviving fish was 13.9%. Tench stock was assessed as about 6–7 million individuals and 1450–1500 tonnes in biomass. It was determined that maximum sustainable yield could be obtained with an 80% level of the current fishing effort.     

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.     

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 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.     

Age, growth, mortality and population structure of the oyster, Crassostrea madrasensis, in the Moheskhali Channel (southeastern coast of Bangladesh)

The population structure, age, growth, mortality and harvest intensity of the oyster Crassostrea madrasensis were examined in the Moheskhali Channel, Bangladesh between June 2003 and May 2004. The channel is a representative habitat for the area. C. madrasensis monthly length frequency data were analyzed using FiSAT software for estimating population parameters, including asymptotic length ( L _∝ ), growth co-efficient ( K ) and recruitment pattern to assess the status of the stock. Asymptotic length ( L∝ ) and growth co-efficient ( K ) were 20.88 cm and 0.35 year⁻¹, respectively. The growth performance index (φ') was calculated with 2.18. The growth pattern showed negative allometric growth ( b  < 3), with an asymptotic weight ( W _∝ ) of about 1124.6 g. The oyster attained an average length of 6.17 cm at the end of 1 year. Total mortality ( Z ) by length-converted catch curve was estimated at 1.78 year⁻¹, fishing mortality ( F ) at 0.77 year⁻¹, and natural mortality ( M ) at 1.01 year⁻¹. The exploitation level ( E ) of C. madrasensis was 0.43, while the maximum allowable limit of exploitation ( E _max) was 0.45 for the highest yield. The recruitment pattern was continuous, displaying a single major peak event per year. Habitat temperatures were 25.5–31.0°C (mean ± SD, 29 ± 1.62°C); salinity range was from 12.36 to 26.0 ppt (mean ± SD, 19.6 ± 4.7 ppt). The exploitation level (0.43) indicated that the oyster stock was exploited at almost maximum yield in this channel.     

Population biology and stock assessment of Pristipomoides filamentosus on the Mahe Plateau, Seychelles

The population of P. filamentosus around the periphery of the Mahe Plateau was studied using data derived from commercial handline fishing operations between November 1989 and December 1990. Sex ratio was close to one. Fork length was in the range 25.6–79.8 cm: at first sexual maturity, 36–38 cm for females and 40–42 cm for males. Spawning occurred from October to April peaking between February and April and in November.
The length ( f.l. , cm)–weight (kg) relationship was W =0.00005353 × f.l. ^2.7004. The von Bertalanffy growth parameters, estimated from length frequency data, were K =0.2875 and L _inf=81.7 cm. Mortality rates, estimated from length converted catch curve analysis, were Z =0.811, F =0.277 with M =0.534 derived from Pauly's empirical formula. Jones' length cohort analysis gave an estimate of F= 0.294 for the fully exploited part of the stock, and a density of 2.59 t km⁻² for the periphery of the plateau from 75 to 150 m depth. The Lesley constant catchability model, applied to the results of intensive fishing around isolated banks, derived a mean density estimate of 2.99t km⁻². The sustainable yield was estimated to be 233–268 tonnes per annum and the catch in 1990 was 200 tonnes.     

The global relationship between forest productivity and biomass

Aim  We aim to determine the empirical relationship between above-ground forest productivity and biomass. There are theoretical reasons to assume a relationship between forest structure and function, as both may be influenced by similar ecological factors such as moisture supply. Also, dynamic global vegetation model simulations imply that any increase in forest productivity driven by climate change will result in increases in biomass and therefore carbon storage. However, few studies have explored the strength and form of the relationship between forest productivity and biomass, whether in space or time.
Location Global scale.
Methods  We collated a large data set of above-ground biomass (AGB) and above-ground net primary productivity (ANPP) and tested the extent to which spatial variation in forest biomass across the Earth can be predicted from forest productivity.
Results  The global ANPP–AGB relationship differs fundamentally from the strongly positive, linear relationship reported in earlier analyses, which mostly lacked tropical sites. AGB begins to peak at c . 15–20 Mg ha⁻¹ year⁻¹ ANPP, plateaus at ANPP > 20–25 Mg ha⁻¹ year⁻¹, and may actually decline at higher levels of production.
Main conclusions  High turnover rates in high-productivity forests may limit AGB by promoting the dominance of species with a low wood density. Predicted increases in ANPP will not necessarily favour increases in forest carbon storage, especially if changes in productivity are accompanied by compositional shifts.     

A study of the dynamics of a natural population of the freshwater crayfish, Austropotamobius pallipes

SUMMARY. 1. Population size, survival and recruitment were estimated from mark—recapture data at frequent intervals.
2. Two types of mortality were identified: (1) moult-associated—an intense but short duration mortality, and (2) overwinter mortality, which operated at a lower estimated weekly rate but over some 30 weeks.
3. Population size reached a mid-summer peak as recruitment into the catchable size classes occurred. Thereafter numbers declined slowly to the end of the growth season, followed by an overwinter decline of some 50% of total.
4. Annual production was estimated at 196.5 kg wet wt ha⁻¹ in 1977, 116.8 kg wet wt ha⁻¹ in 1978 and 87.79 kg wet wt ha⁻¹ in 1979.     

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 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).     

Age, growth and mortality of the Mediterranean amberjack Seriola dumerili (Risso 1810) from the south-eastern Adriatic Sea

The age, growth and mortality of the Mediterranean amberjack Seriola dumerili (Risso 1810) were determined in 298 specimens collected in the south-eastern Adriatic Sea (Donji Molunat Bay) from 17 May to 26 June 1997. The total length ranged from 32.0 to 160.0 cm and weight from 0.5 to 46.5 kg. Ten age classes, ranging from 1° to 10° years were defined by scale readings. Mean total length and weight-at-age data were used to estimate the growth parameters of the von Bertalanffy equation: L _∞=174.6 cm, K =0.190, t ₀=–0.314; W _∞=79.01 kg, K =0.139 and t ₀=–0.746. The length–weight relationship was estimated at: W =0.000123 ×  L ^2.847. The overall sex ratio was 1 : 1.06 in favour of males. Total ( Z ) and natural ( M ) mortality were found to be 0.41 years⁻¹ and 0.30 years^–1, respectively. The exploitation ratio ( E =0.27) indicates that the fishing pressure on the Mediterranean amberjack was low in the investigated region.     

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 fish fauna of the Jordan reservoir, one of the oldest man-made lakes in central Europe

The Jordan reservoir, Czechoslovakia, was constructed in 1492 and the last full drainage and fish removal was in 1830. In this 'mature' fish community of 20 species (plus three hybrids), the roach, Rutilus rutilus (L.) (1446 fish ha^–1) and beam, Abramis brama (L.) (1074 fish ha^–1) dominated the fish stock (total of all species = 3628 fish ha^–1). Predatory species made a small contribution to the total (maximum 7.7%, obligatory predators 2%). The contribution (12%) by perch, Perca fluviatilis L., was also low, and its numbers in successive age-groups showed a steady decrease. The high variability and time-synchronization in the year-class strengths of the principal cyprinid species may indicate unstable environmental conditions or intra-community cycles. The growth rates of non-predatory species were low. There was a high fishing rate on predatory species but there was little impact by fishery management on the whole 'mature' community. Intensive eutrophication from both communal waters and agriculture probably caused the high biomass level (all species) and the low abundance of zander, Stizostedion lucioperca (L.).