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.     

On the biology of bream, Abramis brama (L.) in Lake Peipsi in 1994

The population of bream in L. Peipsi was studied with respect to age, growth rate, condition factor (according to Fulton) and length-weight relationship in 1994. That autumn the bream population in L. Peipsi consisted of fishes aged from 0+ to 15+. During the first year bream reached an average body length of 7.9 cm (the commercial legal size (30 cm) was usually attained by the end of the 5th–6th year. The condition of bream in this lake was above the average of Estonian lakes. The relatively good growth rate and condition of bream in the lake indicates that the waterbody is appropriate for this fish.     

Differences in the exploitation of bream in three shallow lake systems and their relation to water quality

SUMMARY 1. The development of bream populations, water transparency, chlorophyll‐a concentration, extent of submerged vegetation and densities of the zebra mussel, Dreissena polymorpha, were analysed in three shallow eutrophic lake systems subject to different fish management. 2. In Lake Veluwemeer, the bream population was reduced from c. 100 to 20 kg ha^?1 after 5 years of fishing. The mortality caused by the fishery was estimated at 38% of bream >15 cm in addition to a 13% natural mortality of bream >17 cm. The decline was followed by an expansion of the Chara beds present in the shallow parts, an increase in water transparency in the open‐water zone, an increase in the density of zebra mussels and a decrease in chlorophyll‐a concentrations. 3. The newly created Lake Volkerak showed trends opposite to those in Lake Veluwemeer. Bream colonised the lake in 1988 and reached a biomass of c. 140 kg ha^?1 in 1998. The water transparency decreased from a maximum of 3 m to c. 1 m and the chlorophyll‐a concentration increased from 5 to 45 μg L^?1. Submerged vegetation colonised up to 20% of the total lake area in the first 5 years after creation of the lake in 1987 but decreased to 10% as turbidity increased. 4. Seine fishery in the Frisian lake system did not appear to affect the bream population despite annual catches as high as 40–50 kg ha^?1. The estimated natural mortality of fish >15 cm was 15% and mortality by fishery was 26%. The high loss was apparently compensated by good recruitment and high growth rates resulting from a c. 1 °C higher water temperature during the years when bream were removed by fishing. There was only a slight decrease in chlorophyll‐a concentrations and a slight increase in water transparency. 5. The results of this study suggest that the effects of bream exploitation in eutrophic lakes can vary depending on the efficiency of the fishery, recruitment success and temperature regime. In the absence of fishery, bream dominated the fish community in the study lakes and apparently prevented D. polymorpha and submerged vegetation from establishing because of physical disturbance, enhanced internal P‐loading and resettling of resuspended sediments.     

Restoration by biomanipulation of lake Bleiswijkse Zoom (The Netherlands): First results

In 1987, the Bleiswijkse Zoom, a small, shallow lake in The Netherlands, was divided into two compartments to investigate the possible use of biomanipulation as a tool for restoring the water quality of hypertrophic lakes. The density of the fish stock before restoration was about 650 kg.ha^–1, composed mainly of bream, white bream and carp. Pikeperch was the main fish predator in the lake. In April, 1987, in one compartment (Galgje) all planktivorous bream and white bream and about 85% of the benthivorous bream and carp were removed. Advanced pikeperch fry were introduced as predator during the transient period. The other compartment (Zeeltje) was used as a reference. Removal of the fish in Galgje resulted in low concentrations of chlorophyll-a, total phosphorus, nitrogen and suspended solids. The absence of bottom-stirring activity by benthivorous fish and the low chlorophyll-a concentrations led to an increase in the Secchi disk transparency from 20 to 110 cm. Within two months after removal of the fish, macrophytes, mainly Characeae, became abundant. Until July the high density of large zooplankton species caused low algal biomass. From June onwards, the zooplankton densities decreased, but the algal concentrations remained low. This is probably because of nutrient limitation or depression of algal growth by macrophytes or both. Compared with the non-treated compartment the number of fish species in the treated compartment was higher. Perch, rudd and roach, i.e. the species associated with aquatic vegetation, were found in the samples. The survival of the O⁺ pikeperch was poor. The pikeperch could not prevent the growth of young cyprinids. Within two months after the removal of the fish a habitat for northern pike was created.     

Recent invasion by a non-native cyprinid (common bream Abramis brama) is followed by major changes in the ecological quality of a shallow lake in southern Europe

We present an example of how an invasion by a non-native cyprinid (common bream, Abramis brama (Pisces: Cyprinidae), hereafter bream) in a natural shallow lake in southern Europe (Lake Montorfano, northern Italy) may have adversely affected the state of the lake’s ecosystem. In less than two decades, bream became the most abundant species and characterized by a stunted population with asymptotic length 33.5 cm, an estimated mean length at first maturity of 19.6 cm, a total mortality rate of 0.64 year^?1 and a diet overwhelmingly dominated by microcrustaceans. Following bream establishment, nutrients and phytoplankton biomass rose, the proportion of Cyanobacteria by numbers increased markedly and water transparency decreased. Total zooplankton abundance increased with a marked increase in small cladocerans and copepods, whereas the abundance of large herbivorous cladocerans did not change. The coverage of submerged macrophytes declined, as did the abundance of native pelagic zooplanktivorous fish. The composition of the fish community shifted towards a higher proportion of zoobenthivorous species, such as bream and pumpkinseed (Lepomis gibbosus). Our results indicate that bream affected water quality through bottom-up mechanisms, while top-down effects were comparatively weak. Selective removal of bream and perhaps stocking of native piscivores might improve the ecological status of the lake.     

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 population dynamics of Nemipterus marginatus (Cuvier & Val.) off Western Kalimantan, South China Sea

The following parameter values are derived for a West Kalimantan (West Borneo) stock of the threadfin bream, Nemipterus marginatus :
L _∞= 24·5 cm(L.T.), W_∞= 210g, K = 0·42, t = -0·41 andM= 1·73.
Yield-per-recruit curves for N. marginatus are presented. Catch per hour of N. marginatus is very low, even in the area of highest density (=3·5 kg/h at 45 m depth). Single-species exploitation of a stock of the type represented by N. marginatus appears unfeasible and the conclusion is reached that the traditional single-species approach in fish population dynamics—such as exemplified in this paper—does not help in formulating rational exploitation schemes.     

Migration of bream between the main channel and floodplain lakes along the lower River Rhine during the connection phase

Mature bream (>30 cm L_F) dominated the fish communities in floodplain lakes along the lower River Rhine in The Netherlands, in terms of biomass, in all lakes at all times. In some lakes bream made up 99% of the total fish biomass. While immature, medium–sized bream (10-30 cm) and white bream and roach were abundant in the main channel of the river, these species were almost absent from the floodplain lakes. Inundation of the floodplains did not lead to spatial homogenization of species and length distributions. Small bream (<10 cm) left the floodplains to become resident in the main channel until maturity. Once mature (c. 30 cm), they returned to the lakes during the next inundation and stayed there. Mature bream that moved into the lakes were significantly smaller and had a lower condition than the resident bream. Turbid clay-pits were probably important sources of bream for the lower Rhine and the recruitment of the 0+ year group of bream to the riverine population was influenced by their discharge pattern.     

The dynamics and exploitation of the brown trout population of Øvre Heimdalsvatn

The very dense trout population in the lake was substantially reduced by heavy fishing. The resulting changes in growth, age distribution and population density were examined. Fishing efforts with 32 mm nets gave the highest sustainable yield. With a yearly effort of 1600 nights with these nets the predicted sustained yield would be 5.7 kg ha⁻¹ of trout with a mean weight of 269 g.     

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.     

Role of organophosphates in adaptations of an obligate water-breathing teleost (Tilapia nilotica L.) to hypoxia

Dry matter, total carbon (C), nitrogen (N) and phosphorus (P) content of mature bream from Lake Balaton were investigated and the quantities of N and P stored in the bream population and their possible removal by fishery were estimated. Carbon made up 43.3–44.8% of dry weight, N made up on average 10.6% of the dry weight of bream and P accounted for a further 2.7%. About 3.3 kg N ha^–1 and 0.9 kg P ha^–l are stored in the bream population. Approximately 0.5 kg N ha^–1 and 0.1 kg P ha^–1 are removed from the lake by bream harvest. Taking into account the total fish yield, the N removal is 2.1% and P removal 3.4% of the amount entering the lake.     

Diet of Atlantic cod in the southern Gulf of St Lawrence as an index of ecosystem change, 1959–2000

Major temporal changes in the importance of euphausiids and Atlantic herring Clupea harengus in the diet of Atlantic cod Gadus morhua , 10–75 cm L _F, occurred in shallow waters (<100 m depths) of the southern Gulf of St Lawrence between 1959 and 2000. Euphausiids represented 6–70% of prey mass for Atlantic cod 31–60 cm L _F from 1959 to 1987 but only trace amounts were detected in stomachs collected from 1990 to 2000. Mysids and gammarid amphipods (for Atlantic cod ≤45 cm L _F), and Atlantic herring (for Atlantic cod >45 cm L _F) largely replaced euphausiids in the Atlantic cod diets from 1990 to 2000. This diet change suggested there has been a major perturbation of the food web of the southern Gulf of St Lawrence. The importance of fishes (mostly Atlantic herring) in the diet of Atlantic cod >45 cm L _F increased significantly between the periods 1959–1980 and 1987–2000. Atlantic herring comprised 0–4% (mean 1.3%) of prey mass of Atlantic cod 46–60 cm L _F from 1959 to 1980 and increased to 2–42% (mean 19.6%) of the diet from 1987 to 2000. Atlantic herring comprised 0–25% (mean 9.4%) of the prey mass of Atlantic cod 61–75 cm L _F from 1959 to 1980 and increased to 42–81% (mean 54.2%) of prey mass from 1987 to 2000. This increased consumption of Atlantic herring was consistent with observed changes in abundance of Atlantic herring in the ecosystem between the late 1970s and 2000. The large changes in consumption of euphausiids and Atlantic herring represent highly significant changes and would need to be included in the development and interpretation of ecosystem-based management models for this ecosystem.     

Age and growth analysis of the central mudminnow, Umbra limi (Kirtland, 1840)

A rudimentary understanding of age, growth, and life-span is lacking for many non-game fishes. Growth characteristics of the central mudminnow ( Umbra limi ) have not yet been accurately described using reliable hard part analysis. The utility of scales and otoliths as ageing structures and quantified growth was examined in one lake and one stream population of central mudminnow. Scales were found to be of no utility in determining age due to inconsistent formation of yearly annuli and a high incidence of regenerated scales, while otoliths were easily extracted and considered to be an accurate ageing structure. Ages determined from scales were low compared to those from otoliths, and the difference in age interpreted from the two structures increased with fish age. A power function was fitted to describe the length-weight relationship for this species ( a  = 0.0069, b  = 3.175). Von Bertalanffy growth parameters were estimated and compared for each population (Lake: L _∞ = 114.20 mm, K  = 0.30, t ₀ = −0.93; Stream: L _∞ = 77.59 mm, K  = 0.63, t ₀ = −0.76). The lake population showed greater size at age compared to the stream population, especially at older ages, and achieved a larger maximum size. Growth rate was also greater in the lake population (Lake: 1.74; Stream: 1.09 g year⁻¹). Females were larger at age than males in both populations, however all individuals greater than age 3 were males. This work represents the first successful account of central mudminnow growth using hard part analysis.     

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.     

Growth of silver hake within the U.S. continental shelf ecosystem of the northwest Atlantic Ocean

Von Bertalanffy growth parameters were estimated for silver hake Merluccius bilinearis within four regions of the United States continental shelf ecosystem of the northwest Atlantic Ocean during 1975–1980, 1982–1987 and 1988–1992. Differences in silver hake growth were found between the Middle Atlantic and southern Georges Bank regions ( P <0–01) over all three time periods. Growth also differed between northern Georges Bank and the Gulf of Maine ( P <0–01) during 1975–1980 and 1982–1987, and significant differences between southern Georges Bank and northern Georges Bank were found only during 1982–1987. Silver hake in the Gulf of Maine exhibited larger asymptotic sizes, but attained their asymptotic size at slower rates (L₀₀= 47 cm and K =0–33) in comparison to silver hake in the Middle Atlantic ( L₀₀= 43 cm and K =0–51), while growth parameter estimates for silver hake on Georges Bank were intermediate between these. Although significant differences in growth of silver hake between the various regions are consistent with separate stocks in the U.S. continental shelf ecosystem, the growth characteristics from the different regions are inconsistent with the presently assigned stock boundaries. (c) 1996 The Fisheries Society of the British Isles     

Size-dependent mortality of red sea bream, Pagrus major, juveniles released with fluorescent otolith-tags in News Bay, Japan

Totals of 2^.67 x 10⁵ and 7^.56 x 10⁵ juvenile red sea bream of three size groups (10, 20 and 40 mm t.l.) marked with a fluorescent substance in the otolith were released in News Bay, Oita Prefecture, Japan, in July 1987 and June 1988, respectively; the aim was to estimate growth and mortality of different developmental stages.
Of fish released in 1987 and 1988, 10 618 and 4413, respectively were recaptured during those two years. Released fish remained in the bay until the end of summer, and afterwards migrated out towards open waters. Fish of the 40-mm group released in 1987 grew to over 200 mm t.l. in one year. Mean growth rate for 19 days after release was higher in the 40-mm group (0^.87 mm day⁻¹) than in the 20-mm group (0^.74mm day⁻¹). Survival rates over 19 days were 59^.0 and 10^.1 % for 40-mm and 20-mm fish, respectively, in 1987, and those over 30 days were 69^.2, 3^.3 and 0^.0% for 40-mm, 20-mm and 10-mm fish, respectively, in 1988.
Cannibalism was indicated by the presence of marked otoliths for 20-mm fish in the stomachs of a few 40-mm individuals recaptured 2 days after release. Size-dependent growth and size-selective mortality were both noted in juvenile red sea bream, i.e. the relative size differential between larger and smaller individuals was maintained in the period between marking and recapture, and mortality was inversely proportional to size.     

Spawning period and growth of yellowfin bream, Acanthopagrus australis (Günther), in Moreton Bay, Australia

Monthly changes in the gonosomatic index indicate that yellowfin bream, Acanthopagrus australis , in Moreton Bay have a short spawning season with a peak during the period July to August each year. The growth rate of A. australis was estimated from tag-recapture data and by length-frequency analysis. Modes representing age classes 0, I and II often appeared in length-frequency distributions, however modes for older age classes were difficult to identify. The von Bertalanffy growth curve parameters calculated from release–recapture data are: L_∞ = 29.5 cm; K = 0.51; t ₀=– 0.32 years.     

An in situ estimate of food consumption of five cyprinid species in Lake Balaton

Daily rations of five cyprinid species, bream Abramis brama , silver bream Blicca bjoerkna , roach Rutilus rutilus , gibel Carassius auratus gibelio and carp Cyprinus carpio , in Lake Balaton, a large shallow lake, estimated by the Eggers model differed from that of the Elliott – Persson model by only − 4.3 to +7.3% (the differences were insignificant). Daily rations varied within the range of 0.23–0.69 in bream, 0.55–3.61 in silver bream, 0.69–4.65 in roach, 0.38–3.16 in gibel and 0.50–9.74 g dry 100 g wet fish mass⁻¹ day ⁻¹ in carp at temperatures ranging from 8.7–25.8% C. Daily ration was related exponentially with temperature in silver bream, roach, gibel and carp. For bream, a significant relationship was obtained only when a daily ration value was excluded from the analysis. Annual rations were assessed using the relationships between the daily ration estimates from the Elliott—Persson model and water temperature, and the long-term averages of the monthly water temperature data. From these estimates the bream population consumed 104%, silver bream 424%, roach 487%, gibel 363% and carp 913% dry mass of food of its wet biomass annually.     

Mackerel icefish size and age differences and long‐term change at South Georgia and Shag Rocks

This study analysed the total length ( L _T)‐frequency distribution of mackerel icefish Champsocephalus gunnari at South Georgia and Shag Rocks from nine bottom trawl surveys at South Georgia and eight at Shag Rocks between 1987 and 2002. The estimated mean L _T of age‐classes 1+, 2+, 3+ and 4+ years during January were, respectively, 14·7, 23·5, 29·8 and 35·1 cm at South Georgia. Age‐classes 1+, 2+ and 3+ years were 18·3, 26·2 and 33·8 cm at Shag Rocks. The derived Bertalanffy growth parameters for South Georgia were: L _∞ = 51·7 cm, k  = 0·27 and t ₀ = −0·26. The mean L _T of each age‐class of C. gunnari at Shag Rocks was significantly larger than at South Georgia, equivalent to c . 5 months growth, although the annual growth in L _T was similar. This is further evidence that C. gunnari hatched earlier at Shag Rocks. At South Georgia, the mean L _T of age‐classes 1+ and 3+ years were correlated, and significantly decreased between 1987 and 2002, and were smaller following warmer summers. This decrease in the size of C. gunnari may be the result of reduced food availability linked to climate warming.     

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.