Gillnets Of Mesh Sizes Research Articles

Biodiversity and abundance of fish and plankton of Dan-Zaria Dam, Niger State, Nigeria

An assessment of fish and plankton biodiversity and abundance of Dan-Zaria Dam were carried out over a period of one year. Sampling of fish and plankton was conducted monthly. Five (5) sampling stations were randomly established based on their importance to the dam. Fish sampling was conducted using cast net, fleets of gill nets of various mesh sizes, ranging from one inch to seven inches. Fishes caught were identified, counted, weighed and lengths taken and recorded. Results indicated that five fish species existed in the dam during the study period, about three of the species viz. Oreochromis niloticus, Tilapia zilli and Clarias gariepinus were the most common, while the two other species, Laboe senegalensis and Alestes nurse were less common. A total of 2,010 fishes were caught. Of the three common species, O. niloticus dominated with an estimated total catch of about 1,609 accounting for 80.05%. The total catch for, T. zilli was 178 representing 8.86%, that of, C. gariepinus was 137 representing 6.82%, the others represented about 4%. Water and plankton samples were taken simultaneously with fish samples. Results showed that Four (4) common taxa existed in the water body at the time of study viz. Chlorophyceae, Cyanophyceae, Desmidiaceae and Bacillariophyceae. In all about 28 species of phytoplankton were identified. About forty (40) species belonging to 4 taxa of zooplankton, the protozoa with about 26 species, Cladoceran (3 species); Copepoda (4 species) and Rotifers comprising about 7 species %. It was therefore concluded that the biodiversity of fish species and abundance were of a short range however with a good yield. While for plankton, it was concluded that their biodiversity and abundance were great at the time of this study. Key word: Biodiversity, Fish, Plankton, Abundance, Dan-Zaria dam Journal of Aquatic Sciences Vol.18(2) 2003: 141-148

Dinâmica reprodutiva de Oligosarcus jenynsii e O. robustus (Characiformes, Characidae) na lagoa Fortaleza, Rio Grande do Sul, Brasil

Oligosarcus jenynsii (Gunther, 1864) and O. robustus Menezes, 1969 are fish species distributed in Rio Grande do Sul, Brazil, Uruguay, and northern Argentina. The reproductive period and recruitment, sex ratio, absolute and relative fecundity, and body length at first gonadal maturation of the two carnivorous species from Fortaleza Lagoon were analized. The specimens were sampled monthly, from May 2000 to April 2001, with fishing effort of 24 hours/month, using stationary gillnets of different mesh sizes and seine net (three samples per edge). The records of each individual included total length, total weight, gonad weight, sex and gonadal maturity stage. The reproductive period of both O. jenynsii and O. robustus ranges from May/June to November/December, according to the bimonthly variation of the mean values of gonosomatic index, and the relative frequencies of the gonadal maturation stages. Recruitment of new individuals to the population occurs from November/December to March/April. The sex ratio is different from 1:1 for O. jenynsii and similar to 1:1 for O. robustus. The mean absolute fecundity, calculated by counting sub-sampled oocytes from mature females, was 14,483 oocytes for O. jenynsii, and 16,308 oocytes for O. robustus. The first maturation curve shows that O. jenynsii begins to reproduce between 84 mm and 104 mm (total length), and O. robustus between 126 mm and 146 mm, probably at similar ages.

Arctic charr in Lake Myvatn: The centennial catch record in the light of recent stock estimates

In Lake Myvatn, Arctic charr (Salvelinus alpinus) is economically the most important fish species. It is fast growing fish and its size at maturity is 35–50 cm at the age of 4 to 5 years. Catch statistics that are available for the whole 20th century show considerable long-term variation with the highest catch in the 1920s. The catches after 1970 are about 40% lower than the average from 1930–1969 and the annual catches for the last decade are the lowest. Stock size during the winter fishing season fluctuated considerably between years, with average annual fishing mortality of 83.9%. The Arctic charr population has been monitored annually since 1986, using standard series of gill nets of different mesh sizes. In 1988 one to three-year-old fish were heavily reduced in numbers during the summer months. There are indications of a similar event in 1997. In both years the charr changed its main diet from Cladocera and chironomid midges to three-spined sticklebacks Gasterosteus aculeatus and snails. At the same time its condition deteriorated. The catch in the monitoring fishery in Lake Myvatn correlates with the stock size in the beginning of the following winter fishing season deduced from Leslie's method and can give prospects for the fishery in the successive fishing season. An index of abundance of young charr also correlates with the number of chironomids and cladocerans and also with ducklings that feed on the same food as the charr. Large fluctuations in the Arctic charr population in Lake Myvatn seem to be related to changes in the main food species. The catch records available from Lake Myvatn can to a large extent be used as a measure of changes of the Arctic charr population in the lake for the past century.

Fleet Dynamics of the Commercial Lake Trout Fishery in Michigan Waters of Lake Superior during 1929–1961

Understanding fishing fleet dynamics is important when using fishery dependent data to infer the status of fish stocks. We analyzed data from mandatory catch reports from the commercial lake trout ( Salvelinus namaycush) fishery in Michigan waters of Lake Superior during 1929–1961, a period when lake trout populations collapsed through the combined effects of overfishing and sea lamprey ( Petromyzon marinus) predation. The number of full-time fishermen increased during 1933–1943 and then decreased during 1943–1957. Addition of new fishermen was related to past yield, market prices, World War II draft exemptions, and lost fishing opportunities in Lake Huron and Lake Michigan. Loss of existing fishermen was related to declining lake trout density. Large mesh (≥ 114-mm stretch-measure) gill net effort increased during 1929–1951 because fishermen fished more net inshore as lake trout density declined, even though catch per effort (CPE) was often higher in deeper waters. The most common gill net mesh size increased from 114-mm to 120-mm stretch-measure during 1929–1957, as lake trout growth increased. More effort was fished inshore than offshore and the amount of inshore effort was less variable over time than offshore effort. Relatively stable yield was maintained by increasing gill net effort and by moving some effort to better grounds. Because fishing-up caused yield and CPE to remain high despite declining lake trout abundance, caution must be used when basing goals for lake trout restoration on historical fishery indices.

Linking fishery management and conservation in a tropical estuarine lagoon: biological and physical effects of an artisanal fishing gear

Information coming from fishery monitoring, surveys and experimental fishing with participation of fishers was employed to determine the impact of an artisanal gear, ‘boliche’, on the biodiversity of the Ciénaga Grande de Santa Marta (CGSM), an estuarine lagoon on the Caribbean coast of Colombia. Fishery monitoring (catch data) included landings before (1968 and 1978) and after (1994–1996) the introduction of the boliche in the CGSM (1985), whereas surveys were conducted seasonally during 1993–1994. Fishing experiments involved evaluating different mesh sizes and the short-term effect of physical disturbance by the boliche. Monitoring suggested potential trophic effects of this fishing gear: the catch of large, long-lived, carnivorous species declined after the introduction of the boliche in the CGSM, whereas catch rates of smaller, shorter-lived, and lower trophic level species increased. Surveys revealed that the boliche retained 41 species. The by-catch made up 62% of the total catch and the remaining 38% involved the three target species Eugerres plumieri, Mugil incilis and Cathorops spixii. Selectivity experiments showed that 2.5 in. stretched mesh size gill nets caught more species than the 3.0-in. mesh. The smaller mesh also increased the risk of a critical reduction in the spawning stock of target species (notably E. plumieri); a situation that could affect the fish community if mesh sizes lower than 2.5 in. were intensively used. Suspended particulate matter significantly increased after fishing activity, with higher resuspension on mud-shells and mud substrata, whereas dissolved oxygen showed no appreciable changes after fishing operations. Notwithstanding, the activity of the boliche would generate sediment resuspension between 382 and 470 t day −1, which could lead to potential cascade impacts on water quality. We propose a framework of redundancy in management measures in order to simultaneously reach management and conservation goals.

Gill net and longlines fisheries in Cyclades waters (Aegean Sea): species composition and gear competition

Species composition and gear competition for the small-scale fisheries in the Cyclades waters (off Naxos Island, Aegean Sea, Hellas) was studied based on samples collected during 1997–1998 with gill nets of different mesh sizes and longlines with hooks of different sizes. Comparisons used univariate and multivariate techniques based on species compositions by number and weight and the commercial/total catch ratio in terms of weight and number. The results of the various techniques were in agreement and suggested that the different gear/size/season combinations fall into two groups (all gill nets and all longlines), which differed from each other in terms of species composition, species diversity and dominance. In contrast, the commercial/total catch ratio did not differ either within and between gears. The implications of the results for the management of the Mediterranean fisheries resources are also discussed.

Variations in sex, length and age compositions of commercial catches of Platycephalus fuscus (Pisces : Platycephalidae) in New South Wales, Australia

Commercial landings of dusky flathead (Platycephalus fuscus) from four estuaries in New South Wales (NSW), Australia, were sampled for data on sex, length and age composition between February and July each year for 2–3 years between 1995 and 1997. Landings primarily contained female fish, ranging from 55% to 93% by number for different estuaries. Flathead sampled in commercial catches ranged from 30 cm to 96 cm total length (TL), but the majority were 33–50 cm TL. Fish >40 cm TL were primarily female and male fish >45 cm TL were uncommon. The length composition of catches differed between gillnets of different mesh sizes, with the average length of fish being least in the smallest allowed mesh size of 70 mm. Fish were aged by otolith interpretation and the analysis of marginal increments indicated that one opaque and one translucent growth zone was formed each year; the opaque zone being deposited in June–August (winter) and first observed in September–October (spring). Commercial landings included fish aged 2–11+ years, but fish aged 2–4+ years dominated landings in all estuaries. The total mortality of dusky flathead in each estuary was estimated by catch curve analysis and was relatively high, ranging from 0.45 to 1.64. The data indicate that dusky flathead may be heavily exploited in NSW.

Reconstructing the historical changes in Daphnia mean size and planktivorous fish abundance in lakes from the size of Daphnia ephippia in the sediment

The zooplankton community structure in lakes is highly influenced by size-selective predation by fish, with small zooplankton species dominating at high predation pressure. Remains of cladocerans are preserved in the sediment and may be used to trace historical changes in fish predation. We determined how contemporary data on planktivorous fish were related to the size of Daphnia ephippia (dorsal length) in the surface sediment (0-1 cm) of 52 mainly shallow lakes with contrasting densities of fish and nutrients (TP: 0.002-0.60 mg P l-1). Density of fish expressed as catch per unit effort, in terms of numbers in multiple mesh-sized gill nets (CPUEn), decreased significantly with increasing mean size of ephippia. The relationship was improved by adding TP as an independent variable, now explaining 90% of the variation in CPUEn on the full data set covering lakes in Denmark, Greenland and New Zealand, and 78% if only data on Danish lakes were used. CPUE by weight of planktivorous fish and mean weight of Daphnia in the pelagial during summer were also related to ephippial size. By including contemporary data on established relationships between the sizes of egg-bearing female Daphnia and ephippia, we inferred changes in the CPUEn, mean size of ephippia-bearing Daphnia and summer mean body weight of Daphnia from ephippial size in four lakes during the past 1-2 centuries. In a hypertrophic lake subject to periodic fish kills, Daphnia mean body weight was high and CPUEn was low compared with those in two eutrophic lakes, while CPUEn was low and Daphnia body weight was high in the least eutrophic, clearwater lake. Estimated CPUEn and Daphnia mean weight in the surface sediment of these four lakes corresponded well with contemporary data. Only small changes in ephippial size with time were observed in the clearwater lake and in one of the lakes that had suffered early eutrophication, while major changes occurred in the two other lakes that had been subjected to a major increase in nutrient input or fish kills. We conclude that Daphnia ephippia preserved in the surface sediments of lakes may be a useful and efficient method to quantify the present-day abundance of planktivorous fish and Daphnia mean size. The method is particularly valid in surveys aimed to give a general picture of the fish stock and the ecological state in a set of lakes in a region rather than a precise estimate for a single lake. Though some evidence is provided, more work is needed to evaluate whether the equations are valid for hind-casting in down-core palaeoecological studies.

Catch and selectivity parameters of the anastomid fish Schizodon nasutus using gillnets in the Jurumirim reservoir (São Paulo, Brazil)

Catch and selectivity parameters using gillnets were calculated for Schizodon nasutus in the Jurumirim reservoir on the Parana‐panema River in southern Brazil. The simple gillnets used were made of monofilament nylon of 30 to 140 mm mesh size and of different heights. The relative abundance of the S. nasutus caught was analysed monthly and a positive relationship was found be tween catches per unit effort in number and biomass. The relative catches were more abundant for the nets of 40 and 50 mm mesh size. Catches were characterized by clear temporal variations and a predominance of medium size fish throughout the study period. The selectivity curve of gillnets for S. nasutus in this ecosystem was obtained. The catch obtained was compared with the avail able catch for each mesh size, and the available catch for a cer tain length of fish was calculated starting from the theoretical se lectivity curve. The 50 mm mesh size gillnet showed the best fit between real and available captures and was considered the most suitable, because a smaller mesh size resulted in a catch of pre dominantly immature specimens. It is expected that these results will contribute to improving management strategies for conserva tion of resources. Parameters such as the minimum catch length advisable or the mesh size permitted for a given species are indis pensable for such decision‐making.

Changes in the abundance of planktivorous fish in Lake Skanderborg during the past two centuries—a palaeoecological approach

Fish have a profound effect on food-web structure and water quality in lakes. Quantitative historical data for fish are however scarce, which render it difficult to interpret palaeoecological records in the sediment. To reconstruct the historical changes of the fish community structure and abundance of Lake Skanderborg during the past 150 yr we conducted a stratigraphic analysis of selected zooplankton remains in the lake sediment. From these data we inferred the abundance of planktivorous (expressed as catch net −l night −l in multi mesh-sized gillnets, CPUE) and percentage piscivores using a previously developed transfer functions for Danish lakes. Inferred CPUE was high throughout the study period, being highest before 1900 after which it declined following introduction of pikeperch ( Stizostedion lucioperca) in 1903–1904. Further reductions occurred after a major reduction in external P-loading in 1974 and fish removal in 1983–1985. The inferred CPUE in the surface sediment is in accordance with recent monitoring data. Good correspondence was also obtained between measured zooplankton:phytoplankton biomass ratio and Daphnia mean body weight versus that calculated from inferred CPUE using equations established earlier from contemporary data on Danish lakes. At present, the fish stock is numerically dominated by roach ( Rutilus rutilus) bream ( Abramis brama) and small perch ( Perca fluviatilis), while pikeperch ( Stizostedion lucioperca) is the most important piscivore. In accordance with the inferred data, earlier fish investigations and angler descriptions dated as far back as to 1927 indicate little changes in fish community structure. The early dominance of plankti-benthivorous fish most likely reflects an early eutrophication of the lake due to early urban development in the lake catchment. The development of a transfer function for planktivores fish have improved the possibilities of discovering changes in trophic structure and ecological state of lakes in the past.

Interaction between prey availability and feeding behaviour of age-1 and age-2 perch ( Perca fluviatilis L.) in a biomanipulated lake (Bautzen Reservoir, Germany)

In this study, we have analysed the interaction between prey availability and feeding behaviour of age-1 and 2 perch in the top-down manipulated Bautzen Reservoir from 1986 to 1997. We focused on predation on daphnids and age-0 fish. Age-1 and 2 perch in Bautzen Reservoir were sampled with multi mesh size gill nets (10 to 32 mm bar mesh size). Stomach contents of age-1 and age-2 perch were analysed. To determine the year-class strength of age-0 fish, gill-nets with 6, 7 and 10 mm bar mesh size were used. Sampling of zooplankton was done using nets with mesh sizes of 500 and 750 μm and a 2-L Ruttner-sampler. Feeding behaviour of age-1 perch was strongly determined by the abundance of daphnids. There was a clear functional relationship between diet composition of age-1 perch and availability of D. galeata, but age-1 perch could not be described as important predators of daphnids due to their low abundance. This age group was at least partly piscivorous in all years analysed. Age-2 perch fed on D. galeata only at a low level. They were mainly piscivorous. Our study indicates that probably the good growth of perch in response to the high proportion of piscivores in the lake contributes to the strong overall piscivory of the perch population and therefore enhances the efficacy of a long-term biomanipulation experiment.

Seasonal dynamics of fishery at the Piracicaba River (Brazil)

This study describes and analyses the small-scale fishery in the Piracicaba River, located at the southeastern Brazil, with regard to the diversity, quantity and composition of fish catches both by seasons and gillnets mesh sizes. We recorded fish landings monthly during a year, collecting data on the weight of fish caught (kg per species) and on the quantity of nets utilized (on square meters). During the study period the fishermen used gillnets, capturing 8382 kg of fish from 43 species. Of these, only 22 fish species were of commercial value, and six dominated the catches year round (90% of total weight). The species composition and quantity of fish caught varied between seasons and gillnets mesh sizes. During the autumn and winter, fishermen relied more on the medium sized mesh gillnets, exploiting closer fishing grounds and capturing a greater diversity of fish, including those of low commercial value. In the spring and summer, catches were of lower diversity, and more specialized large mesh gillnets were used, harvesting large and more desired fish in more distant fishing grounds. Such seasonal variation in fishing strategy reduces fishing pressure among several fish species along the year, and should be important in reducing fish stock depletion at the Piracicaba and at other impounded and altered tropical river basins.

GILL NET SAMPLING STANDARDISATION IN LARGE RIVERS OF FRENCH GUIANA (SOUTH AMERICA).

In this paper is analyzed the sampling efficiency of repeated batteries of mixed gillnets mesh size from 15 to 70 mm. The study was conducted in three French Guiana rivers : the Sinnamary, Comte and Malmanoury rivers. Efficiency of sampling methods is one of the problems faced by freshwater ichthyologists. If the choice of one approach must consider its efficiency and cost/profit rates for a determinate level of accuracy, the abiotic characteristics of the environment sampled has also an important role in this process. Large rivers, like those sampled in French Guiana, present specific abiotic characteristics that prevent the large use of non-selective sampling methods like electrofishing or rotenone poisoning. Differently, other sampling gear such as gillnets is used frequently. This kind of gear presents known bias problems related to fish size or species. However, bias can be diminished through the use of mixed gillnets mesh size, which efficiency was previously determined. Results of Principal Component Analysis (PCA) suggest that gillnet mesh size 15 to 35 mm are the most efficient in sampling fish fauna in the rivers sampled. Regarding the number of repeated batteries, the analysis of Catch Per Unit of Effort (CPUE), species richness, sample structure (rank correlation) and the examination of their variance suggest that the use of three batteries gives a good estimation for the fraction of the fish community sampled.

Horizontal distribution of cladocerans in arctic Greenland lakes – impact of macrophytes and fish

Submerged macrophytes may play an important role as a refuge for zooplankton against predators. However, a recent study suggests that their importance depends on the trophic state of the lake. We studied the impact of fish and macrophytes on the horizontal distribution of pelagic cladocerans in 56 oligotrophic arctic Greenland lakes. In north-east and western Greenland, zooplankton was sampled in the near-shore (littoral) and central (pelagial) part of all lakes and fish were sampled with multiple mesh-sized gill nets. Macrophytes were visually estimated in the littoral. In north-east Greenland, 5 taxa of cladocerans were found, while 14 taxa were recorded in western Greenland. Daphnia pulex occurred only in fishless lakes in both northeast and western Greenland and avoided the near-shore areas in the shallow and deep lakes. Bosmina spp. and Holopedium gibberum were evenly distributed between the littoral and the pelagial in the deep and shallow fishless lakes. However, their near-shore density was lowest in the presence of fish. Macrophyte-related and benthic cladocerans concentrated either in the littoral or were evenly distributed between the littoral and the pelagial, irrespective of depth and fish presence or absence. Macrophytes had no impact on the horizontal distribution of pelagic cladocerans. Thus, it is concluded that horizontal heterogeneity of Bosmina spp. and Holopedium gibberum might be affected by the presence of fish.

Indirect Estimate of Gill Net Mesh Size Selectivity for Landlocked Alewives

ABSTRACT During summer 1999, 5,004 landlocked alewives (Alosa pseudoharengus) were captured from 12 Connecticut lakes using horizontal gill nets with mesh sizes of 9.5, 12.7, 15.9, 19.1, 25.4, and 31.8 mm (bar measure). The most effective mesh size was 12.7-mm, capturing 3,035 alewives. The modal length of alewives caught in each mesh size increased as the mesh size increased from 9.5 to 25.4 mm. No alewives less than 70 mm total length or greater than 245 mm were captured. Modal lengths of selectivity curves estimated indirectly using the Holt model were 85, 114, 143, 172, and 228 mm for the 9.5-, 12.7–15.9-, 19. 1-, and 25.4-mm mesh sizes, respectively. Adding a 22.3-mm mesh and removing the 31.8-mm mesh is suggested to sample landlocked alewives between 100 and 245 mm in Connecticut lakes.

Trophic structure, species richness and biodiversity in Danish lakes: changes along a phosphorus gradient

1. Using data from 71, mainly shallow (an average mean depth of 3 m), Danish lakes with contrasting total phosphorus concentrations (summer mean 0.02–1.0 mg P L−l), we describe how species richness, biodiversity and trophic structure change along a total phosphorus (TP) gradient divided into five TP classes (class 1–5: <0.05, 0.05–0.1, 0.1–0.2, 0.2–0.4,> 0.4 mg P L−1). 2. With increasing TP, a significant decline was observed in the species richness of zooplankton and submerged macrophytes, while for fish, phytoplankton and floating‐leaved macrophytes, species richness was unimodally related to TP, all peaking at 0.1–0.4 mg P L−1. The Shannon–Wiener and the Hurlbert probability of inter‐specific encounter (PIE) diversity indices showed significant unimodal relationships to TP for zooplankton, phytoplankton and fish. Mean depth also contributed positively to the relationship for rotifers, phytoplankton and fish. 3. At low nutrient concentrations, piscivorous fish (particularly perch, Perca fluviatilis) were abundant and the biomass ratio of piscivores to plankti‐benthivorous cyprinids was high and the density of cyprinids low. Concurrently, the zooplankton was dominated by large‐bodied forms and the biomass ratio of zooplankton to phytoplankton and the calculated grazing pressure on phytoplankton were high. Phytoplankton biomass was low and submerged macrophyte abundance high. 4. With increasing TP, a major shift occurred in trophic structure. Catches of cyprinids in multiple mesh size gill nets increased 10‐fold from class 1 to class 5 and the weight ratio of piscivores to planktivores decreased from 0.6 in class 1 to 0.10–0.15 in classes 3–5. In addition, the mean body weight of dominant cyprinids (roach, Rutilus rutilus, and bream, Abramis brama) decreased two–threefold. Simultaneously, small cladocerans gradually became more important, and among copepods, a shift occurred from calanoid to cyclopoids. Mean body weight of cladocerans decreased from 5.1 μg in class 1 to 1.5 μg in class 5, and the biomass ratio of zooplankton to phytoplankton from 0.46 in class 1 to 0.08–0.15 in classes 3–5. Conversely, phytoplankton biomass and chlorophyll a increased 15‐fold from class 1 to 5 and submerged macrophytes disappeared from most lakes. 5. The suggestion that fish have a significant structuring role in eutrophic lakes is supported by data from three lakes in which major changes in the abundance of planktivorous fish occurred following fish kill or fish manipulation. In these lakes, studied for 8 years, a reduction in planktivores resulted in a major increase in cladoceran mean size and in the biomass ratio of zooplankton to phytoplankton, while chlorophyll a declined substantially. In comparison, no significant changes were observed in 33 ‘control’ lakes studied during the same period.

Evaluation of Paddlefish Length Distributions and Catch Rates in Three Mesh Sizes of Gill Nets

Abstract –We evaluated the length distribution and catch rates of paddlefish Polyodon spathula collected in three mesh sizes of gill nets in Keystone Reservoir, Oklahoma, during the winters of 1996–1998. In all, 1,454 paddlefish were collected in monofilament gill nets with bar measures of 127, 152, and 203 mm. Mean lengths of paddlefish increased with increasing mesh size. The smallest and the largest mesh sizes caught age-0 paddlefish in 1996, a year when 152-mm mesh was not used and the only year we collected age-0 paddlefish. Median catch rates for all sizes of paddlefish were highest in 152-mm-mesh net and lowest in 127-mm-mesh nets. For population size and age structure sampling, we recommend a range of mesh sizes (e.g., 127–203 mm) to collect the broadest size range of paddlefish. To collect sexually mature fish for broodstock, we recommend using large mesh sizes (e.g., 152 and 203 mm) to increase catch rates of adult paddlefish and reduce bycatch of other species. Net height (4.6–9.1 m) and durati...

Fish morphology and estimating selectivity by gillnets

A fleet of gillnets of various mesh sizes was fished in the Patos Lagoon, Brazil, to investigate how measurements of a fish's body can best be used to determine the selectivity by gillnets. Catch data were analysed for four morphologically dissimilar species: Micropogonias furnieri and Menticirrhus americanus (Sciaenidae), Mugil platanus (Mugilidae) and Brevoortia pectinata (Clupeidae). Fish enmeshed at several positions along their bodies were used to elucidate the mechanism by which they are caught in relation to mesh size. Capture was found to be most efficient when the ratio of the girth at the position where the fish was caught to the mesh perimeter lies in the range 1–1.1. In these four species, fish with similar maximum girths in the available length range were most likely to be caught by a net of a particular mesh size, provided that enmeshing rather than tangling was the main mode of capture. In this context, gillnets can be considered to be girth-specific rather than species-specific gears. However, total length is closely related to girth, and both measurements can be used to describe the selectivity of a particular net.

Ecological Consequences of Alternative Gill Net Fisheries for Nile Perch in Lake Victoria

Both harvest rates and food web interactions may be altered through use of different gill net mesh sizes in the fisheries of Lake Victoria, East Africa. Thus, conservation of the native ichthyofauna is intimately linked to patterns of exploitation in the fisheries. We used bioenergetics modeling to estimate the harvest rates of introduced Nile perch (Lates cf. niloticus) and changes in their predation rates on other important fishes that would result from the implementation of various restrictions on gill net mesh size. Total mass of harvested Nile perch was predicted to vary more than three‐fold over the range of minimum gill net mesh size from 3 to 16 inches. Maximum harvest is expected to result from minimum mesh size restrictions between 6 and 10 inches. Predation rates on all prey fishes would increase with all increases in minimum mesh size. Universal enforcement of a 5‐inch minimum mesh size would reduce both Nile perch cannibalism and predation on other important fishes, such as tilapiines, haplochromine cichlids, and Rastrineobola argentea by as much as 44%, with little decrease (10%) in Nile perch yield. But, if fishers respond to a 5‐inch minimum mesh size by concentrating all fishing effort on the smallest gill nets (5‐ and 6‐inch mesh), a substantially larger reduction in predation on native haplochromines will result, but at the expense of large (~35%) decreases in Nile perch harvest. Our analyses demonstrate that fisheries in Lake Victoria will play an active role in the restoration of a diverse fish community and fishery resource, thus benefiting both many native fish species and the sustainability of the fishery.Consecuencias Ecológicas de Pesquerìas de Redes Agalleras Alternativas para la Perca del Nilo en el Lago VictoriaTanto las tasas de cosecha como las interacciones de la cadena alimenticia pueden ser alteradas mediante el uso de diferentes tamaños de luz de malla de las redes agalleras en las pesquerías del lago Victoria, en el este de Africa. Por lo tanto, la conservación de la ictiofauna nativa esta íntimamente ligada a los patrones de explotación de las pesquerías. Utilizamos un modelado bioenergético para estimar las tasa de cosecha de la perca del Nilo introducida (Lates cf. niloticus) y los cambios en sus tasas de depredación sobre otras especies de peces importantes, mismos que podrìan resultar de la implementación de varias restricciones en el tamaño de la luz de malla de redes agalleras. Se predijo que la masa total cosechada de perca del Nilo variará mas de tres veces sobre un rango mìnimo de luz de malla de entre 3 y 16 pulgadas. Las cosechas máximas se esperan como resultado de restricciones en el tamaño mìnimo de luz de malla entre 6 y 10 pulgadas. Las tasas de depredación en todas las presas de peces podría incrementarse con todos los incrementos en el tamaño de luz de malla. La aplicación estricta de un mínimo de luz de malla de 5 pulgadas podría reducir tanto el canibalismo de la perca del Nilo así como su depredación sobre otras especies importantes de peces, como son las tilapias, los cíclidos haplocrominos y Rastrineobola argenta por hasta un 44%, con una pequeña disminución (10%) en la cosecha de la perca. Sin embargo, si los pescadores responden a un tamaño mínimo de luz de malla de 5 pulgadas al concentrar todo el esfuerzo pesquero en las redes mas pequeñas (i.e., 5 y 6 pulgadas de luz de malla), podría obtenerse una reducción sustancialmente grande en la depredación de los peces haplocrominos, pero a expensas de un disminución considerable (~35%) en la cosecha de la perca. Nuestro análisis demuestra que las pesquerías del lago Victoria jugarán un papel activo en la restauración de la comunidad diversa de peces y recursos pesqueros,beneficiando tanto a varias especies nativas de peces, como a la sustentabilidad de la pesquería.

Gillnet selectivity of Ompok bimaculatus (Siluridae) and Puntius dorsalis (Cyprinidae) in a small-scale riverine fishery

Gillnet selectivity of two indigenous fish species, Ompok bimaculatus and Puntius dorsalis was determined in a small-scale fishery at Uruwal Oya, in the Gampaha district. The. optimal lengths (Lopt) of the two species for different mesh sizes were determined by the Baranov-Holt method. The relationships between Lnptand mesh size (M) of gillnet for the two species are described by the following equations: 0. bimaculatus: Lopt = 4.076 + 3.694 M (r = 0.992; p < 0.01) P. dorsalis: Lon, = 2.445 + 3.410 M (r = 0.994; p < 0.001) The mean sizes of maturity of 0. bimaculatus (21.6 cm for males and 26.1 cm for females) and P. dorsalis (13.4 cm for males and 14.8 cm for females) approximately correspond to Lo,, in stretched gillnet mesh sizes of 5.8 cm and 3.8 cm respectively according to the above relationships. Therefore there seems to be a need for regulating mesh size of gillnets in this fishery in order to prevent immature 0, bimaculatus and P. dorsalis being caught.