Changes in the species composition of anglers' catches in the River Trent (England) between 1969 and 1984

Data on anglers' catches on the River Trent were collected for the seasons 1969/70 to 1983/84 using postal questionnaires. During this period 60.4t of fish were caught during a total fishing effort of 526 871 man-h or 60.15 man-years, and more than 20 species of fish were recorded. The percentage of anglers making a catch (70–80%) and the catch per unit effort, mean 114.7g man-h ⁻¹, were comparable with those reported for other waters.
Roach and gudgeon were the most commonly captured species with chub, bleak, bream, eels and dace also forming major components of anglers' catches. Since 1969 chub, bream, eels and perch have made an increasingly greater contribution to catches, coupled with a decline in numbers of roach and dace. It is suggested that improvement in water quality and its implication on interspecific competition was the most likely cause of the changes.     

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.     

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 efficiency of a new electrofishing technique in determining fish numbers in a large river in Central Poland

A new method of determining fish numbers in a large river, which involved electrofishing from boats downstream to an AC electrical barrier, produced capture efficiencies for different species ranging from 28 to 82% when successive pairs of catches were combined. Estimates of population density, biomass and production for the 18 species in a 2.538 ha segment of the Pilica River, Poland revealed a decline in total numbers of the fish in species diversity between 1963 and 1980. This is attributed to increased fishing pressure, and to a loss in habitat diversity following the loss of many water mills and associated dams. The total production estimate of 0.85 g m⁻² year⁻¹ is low compared with the few published estimates for other large rivers. Roach, dace, chub, gudgeon and bream were the most numerous fish and they constituted 75% of the total population estimate, and 68% of the standing crop and annual production.     

Biomass and production estimates of a fish community in a small South African estuary

The fish community of the small (17·5 ha) intermittently open East Kleinemonde estuary was sampled between 1994 and 1997 to estimate population size, standing stock, growth and productivity. The estuarine-spawning species were numerically more abundant ( n c . 750 000) but due to their small size contributed only 11·7% to the total biomass. The total annual productivity of all fishes in the estuary ( n c . 890 000), with a standing stock of 28·44 g m ⁻², was calculated at 55·89 g m⁻² year⁻¹. The small sparid Rhabdosargus holubi with a production estimate of 41·35 g m⁻² year⁻¹ accounted for <74% of the total fish production in this estuary.     

Long-term study of the ecology of wild Atlantic salmon smolts in a small Norwegian river

Backcalculated lengths at the end of the first growth season in wild Atlantic salmon Salmo salar differed significantly between parr smolting at age 1, 2 and 3 years over a period of 11 years (i.e. 1983–1993). Mean body lengths of the respective age groups at the end of the first growth period were 11·1, 6·2 and 4·7 cm, respectively. The mean percentage distribution of fish smolting at age 1, 2 and 3 was 14, 78 and 7%, and the mean smolt age was 1·95 years. Mean lengths at smolting of age groups 1, 2 and 3 were 13·6, 15·8 and 17·5 cm, respectively. Females outnumbered males among the downstream migrating smolts with a mean sex ratio (females/ males) estimated at 1·61, with a significant female surplus in 7 of the 11 years sampled. Of the smolts sampled, 14% exhibited enlarged gonads indicative of parr maturation, and all were males (37% of the parr males sampled). Mean annual smolt density from 1975 to 1996 was 13·4 individuals 100 m⁻² ranging between 0·3–31 smolts 100 m⁻². Mean densities (100 m⁻²) of the smolts aged 1, 2 and 3 years were 1·5, 9·3 and 0·9 fish, respectively. Mean annual biomass for the 22-year period (1975–1996) was estimated at 437 g 100 m⁻², with a range of variation from 136 to 683 g 100 m⁻². Smolt age 2 made up 81% of the mean annual biomass (355 g 100 m⁻²) and smolt age 1 and 3, 8% (35 g 100 m⁻²) and 11% (47 g 100 m⁻²), respectively.     

Ecological changes in a lake fishery and their effects on a stunted roach Rutilus rutilus population

Data for two sampling years (1969/70 and 1970/71) is presented to illustrate the changing composition, density and biomass of the macroinvertebrate fauna in a South Lancashire lake. Six faunal groupings are recognised for a non-parametric analysis of the data and significance levels are given for the between-station and between-year differences recorded. The mean density of macroinvertebrates (1.2 mm mesh sieve) estimated for the lake as a whole increased by 28.5% in the second year (2519 m⁻²-3237 m⁻²) whereas the estimated mean biomass in g m⁻² increased by 86% wet weight (8.61−16.01) and 196% dry weight (1.74−5.15). Molluscs contributed substantially to the increased standing crop which was accompanied by increased development of macrophytes.
A comparative study of the diets of roach in two size groups in winter and summer is described. The variety of food items consumed was found to be greater than in a preliminary study in May 1969 and, for the roach sampled, the average quantity of food consumed per fish increased after the 1969/70 winter. The possible relationships between invertebrate standing crop, roach diet and previously reported changes in the growth of the roach are discussed and an explanation is offered concerning the sequence of biological changes observed in the lake.     

The loss of submerged plants with eutrophication II. Relationships between fish and zooplankton in a set of experimental ponds, and conclusions

SUMMARY. 1. In 1982 and 1983 sets of experimental ponds were left with their submerged plant communities intact (plant ponds) or were cleared manually of them (cleared ponds). The ponds were all fertilized with ammonium nitrate and with variable amounts of phosphate. In 1982 fish were removed from the ponds. Zooplankton communities were dominated by large Cladocera with Daphnia prominent in the cleared ponds and Simocephalus in the plant ponds. There was no detectable effect of differential phosphorus additions on zooplankton communities or populations.
2. In 1983 zooplanktivorous fish (mainly roach) were stocked in the ponds. In the plant ponds the fish did not survive, probably through severe deoxygenation and the zooplankton community again included large-bodied Simocephalus. Fish survival was variable in the cleared ponds. Where fish stocks were absent or low (0.5–1 g m⁻²) a Daphnia- dominated community persisted; at intermediate fish stocks (18.1 g m⁻²) Eudiaptomus gracilis was predominant and where fish stock was high (22.8–29.1 g m⁻²) Bosmina longirostris , and cyclopoid copepods dominated the communities. Mean biomass of the zooplankton community declined with increase in fish stock to between 5.1 and 18.1 g m⁻² then increased.
3. On the basis of results from the experimental ponds and elsewhere, a new hypothesis is put forward to account for the switch from aquatic plant to phytoplankton dominance in eutrophicated shallow lakes. It envisages dominance by either group to be possible as alternative states over a wide range of high nutrient loadings. It suggests that each state is buffered against increased loading by mechanisms involving plant and algal physiology and zooplankton grazer populations. The nature of the buffers and the reasons by which one state may be switched to the other are, discussed.     

The production of brown trout, Salmo trutta in tributaries of the Upper Wye, Wales

The population density, age structure, biomass, growth and production of brown trout were investigated in four tributaries of the upper River Wye. The populations at each site were largely maintained by immigration from nursery areas. Abundance of separate year classes at sites on the three largest tributaries reached a peak at age 2+. On the smallest stream numbers reached a peak at 1+. Recruitment occurred throughout the year but decreased with age of year classes. Maximum O+ densities ranged from 0.04 to 0.89 m⁻², and >0+ densities from 0.13 to 0.59 m⁻². Average total biomass in 1975 ranged from 2.6 to 14.2 g m⁻². Within the study sites annual trout production in 1975 ranged from 2.9–19.7 g m⁻². Production values were dependent on age structure and population mobility at the study sites. In the three largest streams 2+ and 3+ fish contributed 66.3–88.3% of total production whilst 1+ and 2+ fish contributed 74.5–84.5 % of the total in the smallest stream. The mobile (non-resident) component of the population accounted for up to 60–70% of production at certain times of the year, but over the year (1976) accounted for =30 % of total production. The resident component of the highest annual production value (19.7 g m⁻²) was estimated to be between 15.0 and 18.2 g m⁻².     

Growth and production of some important species of fish in a Mexican coastal lagoon system

In terms of biomass, the three dominant fish species in the Huizache-Caimanero lagoon system during 1975–76 were the mullet Mugil curema , the anchovy, Anchoa panamensis and the catfish, Galeichthys caerulescens . Peaks in the abundance of the secondary predators occurred at the end of the wet season (September-October), while pelagic forms and members of the centropomid-gerreid association (Warburton 1968a ) were most common during November-February and December-February respectively. Twelve species, including the ten having the highest overall mean biomass, were selected for growth and production studies. Lagoon-specific differences in the patterns of growth and recruitment of several species were observed. The total estimated annual fish production in Caimanero lagoon was 34·48 g m⁻² y⁻¹, with M. curema contributing 9·36 g m⁻², A. panamensis 9·24 g m⁻² and G. caerulescens 6·15 g m⁻² (maximum estimate). Mean turnover ratios (annual production: mean biomass) were 3·25 (demersal species), 8·44 (pelagic species) and 4·48 (all species). Implications for possible fishery and aquacultural expansion are discussed.     

Relative abundance, sex ratio and population structure of the Japanese eel Anguilla japonica in the Tanshui River system of northern Taiwan

From the Gong-Shy-Tyan (GST) Stream and the Tanshui River, Taiwan, eels ranged from 5.53 cm t.l . (elvers) to 72.5 cm t.l . Anguilla japonica accounted for 93-99%, A. marmorata 1-7% and A. bicolor pacifica less than 1% of all eels caught. Mean eel lengths increased from 10 cm t.l . at the downstream sites to 50 cm t.l . at the upstream sites. Females made up 92.8% of the sex-determined eels. Population density, averaged approximately 0.14 eel m⁻² (2.42 g m⁻²) and 0.25 eel m⁻² (0.92 g m⁻²) in downstream sites of the GST and Tanshui River, respectively, and decreased substantially with upstream distance. Eels were rarely found in the heavily polluted and dam constructed areas in the midstream site of the Tanshui River.     

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.     

The role of salinity in structuring the fish assemblages in a tropical estuary

The present study describes the seasonal changes of the fish species composition in three areas of the main channel of the Caeté River estuary, Brazil. The fish faunas of each habitat differed in density, biomass and species composition. Mean fish density and biomass for the Caeté River estuary channel was 0·25 individuals m⁻² and 0·9 g m⁻² respectively. Analysis of catch data showed that the number of species, total density and total biomass differed significantly between areas and seasons. For the most important species, the mean density of Cathorops spixii, Aspredinichthys filamentosus, Aspredo sp. 2, Pimelodus blochii, Pseudauchnipterus nodosus and Macrodon ancylodon , differed significantly between seasons while the mean density of Stellifer rastrifer , Stellifer microps, Aspredo aspredo , Aspredo sp. 1 and Cynoscion acoupa did not. The mean biomass of these species, with exception of S. microps and Aspredo sp. 1, also differed significantly between seasons. In the Caeté estuary seasonal salinity fluctuations appeared to be the main factor that structured the fish assemblage in the entire estuarine system. At least 85% of the species captured by the artisanal and subsistence fisheries in the Bragantine region required estuarine conditions to complete their life cycle.     

Structural, technological and productivity analyses of rainbow trout (Oncorhynchus mykiss) farms in the Marmara region, Turkey

This study analyses the structural, technological performance components and the overall productivity of inland rainbow trout ( Oncorhynchus mykiss ) farms in the Marmara region, Turkey. Of 81 active farms, 36 were small scale (1–10 t year⁻¹), 32 medium-sized (11–30 t year⁻¹) and 13 large production units (over 30 t year⁻¹). Sufficient data for this study were collected from 70 farms for inclusion in the analyses; 59.3% were so-called combined farms (hatchery and grow-out), 37.9% on-growing farms (for market production) and 2.9% producing juvenile fish only for stocking or for supply to grow-out farms. Juvenile fish production was largely conducted on large farms. Capacity utilization (102.8%) of the large farms was most similar to their projected capacity. Many large farms (43.9%) use high-tech utilities and tools. Small farms mostly use concrete ponds whereas most medium and large-scale farms use fiberglass tanks. Medium and large-scale farms were found to be more successful in broodstock management, fertilization, hatching success and survival rate of juvenile fish. Fish stocking density in medium-sized farms (21.8 kg m⁻³) was higher than in small-scale (14.5 kg m⁻³) and large-scale farms (15.5 kg m⁻³). Overall feed conversion ratio for all farms was estimated as 1.2. From the survey results it appears that production capacity planning was calculated more accurately in the projection phase for small and large-scale farms rather than for medium-sized farms. Medium and large-scale farms were more successful in terms of performance (good experience, good structural and technological capabilities).     

Vertical distribution and abundance of the Peruvian anchovy, Engraulis ringens, and sardine, Sardinops sagax, larvae during November 1977

Horizontal samples taken with the BIONESS at various depths on 13 stations within 115 km of the coast of Peru showed nine species of fish larvae present with the most common of these being, in order of abundance, Leuroglossus stilbius, Sardinops sagax, Diogenichthys laternatus, Merluccius gayi and Engraulis ringens . All the species except S. sagax and E. ringens showed a diurnal migration to the upper 30 m of water at night. The two above species were present in the top 30 m at all times. The size of the larvae of all species did not vary with time, depth or sample location. The numbers of larvae increased with zooplankton m⁻³ up to 1000 copepods m⁻³ above which no further increase in larvae occurred. A significant linear relationship was found between the total number of larvae m⁻² and biomass and zooplankton m⁻², but not between individual species of larvae and zooplankton biomass. Significant correlations were found between E. ringens and seven species of copepods found in the upper 50 m of water.
Polymodal analysis showed that the larval populations of all species, with the possible exception of S. sagax , were made up of more than one sub-population. From this analysis the length of the spawning season forE. ringens was estimated at 54–68 days.
A significant correlation was found between the numbers of E. ringens larvae m⁻² and the maximum concentration of chlorophyll α, as measured with Batfish, in the area of the BIONESS stations. This showed the largest numbers of larvae in areas of the highest chlorophyll concentrations.     

Litterfall in two Canadian deciduous woods: quality, quantity and timing

Litterfall in two deciduous woods in Ontario, Canada, was caught in bag traps and by new screen traps for two years. The beech-maple wood produced 818 g m⁻² (8.2 t ha⁻¹) and the poplar wood produced only 450 g m⁻² (4.5 t ha⁻¹). Of these totals, canopy leaves alone made up 386 g m⁻² for the beech-maple wood and 270 g m⁻² for the poplar wood leaving 47% and 34%, espectively, for other litter components. Data are presented for seven types of litter in addition to canopy leaves.
Redistribution of litter on the ground causes spatial heterogeneity of substrate and of habitats for decomposers and hence of decomposition.
Temporal distributions of various types of litterfall differ between the two woods in relation to their vegetation structure. The diversity of litter types combines with extensive temporal distribution of litterfall in the beech-maple woodland to give that wood the potential for nearly continuous decomposition activity. In contrast, the poplar woodland has low diversity of litter and short duration of litterfall.     

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.     

The production of juvenile Atlantic salmon, Salmo salar in the upper Wye, Wales

The production of juvenile Atlantic salmon ( Sulmo salur L.) was investigated in 16 study sites in the upper Wye catchment during the period February 1975 to November 1976. The population structure was characterised by large numbers of 0+ fish whose abundance decreased with respect to time so that the numbers of each of the older year-classes was a function of the year-class strength of the original fry. The range of parr densities was similar to that recorded for other rivers: the estimates of 2+ smolts did not exceed about 0.04 m⁻². Production ranged from 0.3 to 11.0 gm⁻² a^−l; Of and 1+ fish contributed over 72% and 3+ fish less than 1 % of the total annug production. The relationship between production P (gm⁻² a⁻¹) and mean biomass B (gm⁻²) for all yeatclasses considered separately and in combination can be expressed by the relationship P=a^b . The values for a vary with age-composition and season but the values for b are not significantly different. Differences in growth rate of salmon between sites are small and differences in production result principally from differences in standing crop (biomass density). In most cases low values for salmon production can be attributed to poor spawning.     

Seasonal biomass and diversity of estuarine fishes coupled with tropical habitat heterogeneity (southern Gulf of Mexico)

Nekton dynamics were studied in two contrasting habitats in Terminos Lagoon, Mexico. Over an annual cycle, a total of 83 fish species used the high-salinity fringing mangrove/ Thalassia testudinum habitat and biomass ranged from 0.43 to 3.43 g m⁻². The highest biomass occurred during the dry season when aquatic primary production was highest (i.e. 333 g C m⁻² year⁻¹). By contrast, 65 species used the freshwater and low-salinity riverine mangrove/Crassostrea virginica/Vallisneria habitat and biomass ranged from 0.57 to 1.48 g m⁻² with the highest biomass occurring during the wet season, the time of highest primary production in this habitat (i.e. 219 g C m⁻² year⁻¹). The high- and low-salinity habitats serve as ecological bridges between freshwater areas and the sea. Fish life histories have evolved to utilize these habitats for spawning, feeding and nursery grounds in a manner which generally leads to the use of different habitats during the periods of highest primary productivity.     

Productivity of a herbivorous pupfish population (Cyprinodon nevadensis) in a warm desert stream

Production and food intake by an herbivorous pupfish population ( Cyprinodon nevadensis amargosae ) living in the outflow of a thermal artesian well (Tecopa Bore) near Death Valley, California is described. Water issues from the ground at 47.5° C and cools 8.12° C before leaving the study area 300 m from the source. High stream temperatures restricted the pupfish population to some 41 % of the study area, with a resulting mean density of 89 individuals m⁻² (range = 13 to 196m⁻²). Biomass estimates ranged from 7 kcal m⁻² to 42 kcal m⁻². The mean annual standing crop of pupfishes (24 kcal m⁻²) turned over about five times annually. Growth rates were highest in juveniles (♂= 9.5% day⁻¹) and slowest in large adults (♀= 08% day⁻¹). Monthly production ranged from 22 kcal m⁻² in September to 3 kcal m⁻² in July and August. Pupfish in Tecopa Bore fed on algae and detritus, ingesting 1941 kcal m⁻² yr⁻¹ or 17.5% of the annual net primary production. 119 kcal m⁻² yr⁻¹ was deposited in growth. This latter value is approximately ten times greater than values previously reported for large carnivorous fishes but is comparable lo values reported for herbivorous fishes under pond culture.