M.A.M. Machiels

Declining stocks of Lake Tana's endemic Barbus species flock (Pisces, Cyprinidae): natural variation or human impact?

The only remaining species flock of endemic, large cyprinid fishes is found in Lake Tana, Ethiopia. A monthly experimental trawl program was conducted in 1991-1993 and 1999-2001, sampling 12 stations distributed over three habitats differing in depth and distance to shore. The aim was to compare the total abundance, spatial distribution and proportion of juveniles of the most common Barbus species in the Bahar Dar Gulf between both periods. We found a sharp reduction (75%) in total abundance, both in number and biomass of the Barbus species and even more (90%) in the number of juveniles between the two periods. However, the spatial distribution of the different Barbus species over the three habitats had not changed. High natural variability in fish stocks might be expected in environmentally unstable lakes. Although strongly pulsed (seasonal), Lake Tana is a relatively stable system. No major differences were found in abiotic parameters in 1990s that could have caused the dramatic changes in abundance. The most likely explanation is the negative impact of the motorised, commercial gillnet fishery targeting the spawning aggregations of these barbs. The drastic decline in juveniles points especially towards serious recruitment over-fishing. The results stress the need for the immediate development of a sound management plan focussing on fishing effort restrictions during the Bat-bus breeding season

A dynamic simulation model for the blooming of Oscillatoria agardhii in a monomictic lake

Abstract The occurrence of phytoplankton blooms is a common problem in water bodies. The quantitative description of the algal population growth is of primary importance to understand the mechanisms which lead to these phytoplankton blooms, hence also for the development of a successful water-quality management. Due to the high degree of complexity of phytoplankton blooms, dynamic simulation models may be a powerful tool to study and elucidate the mechanisms leading to these blooms. In the present study, a simulation model for the blooming of a natural population of Oscillatoria agardhii in Lake Vechten (lat. 52°04′N, long. 5°05′E, the Netherlands) using measured temperature, filament length and nutrient concentrations as external drives, is presented. The lake was sampled weekly from January to the middle of May, 1992. Temperature, nutrient concentration (nitrate and phosphate in the water) and filament length were recorded. Because cell size was constant, filament length could be converted to number of cells per litre to estimate algal biomass. To implement the model additional information, e.g. some algal growth parameters, was obtained from the literature. The model simulated well the growth of the Oscillatoria agardhii population in the lake during the research period. According to the model, temperature was the most important rate-limitating factor for growth at the beginning of the year. Nutrient limitation became increasingly important while the importance of temperature limitation decreased during the season. During the whole period, light was strongly limiting and became the most important rate-limiting factor in the second week of April. Via sensitivity analysis, the model showed to be highly affected by temperature-related factors, and it was useful in the identification of research needs.

Effect of snood length and hanging ratio on efficiency and selectivity of bottom-set gillnets for pikeperch, Stizostedion lucioperca L., and bream, Abramis brama

Abstract Experiments were conducted with multi- and monofilament bottom-set gillnets of various mesh sizes (50, 55 and 65 mm bar mesh), snood length (0.6 and 1.1 m) and hanging ratio, E (0.25, 0.33, 0.5) to study the influence of gear construction on the selectivity of the nets for catching pikeperch ( Stizostedion lucioperca ) and bream ( Abramis brama ). Catches were always higher in the conventional gillnets with 1.1 m snoods, but on an exposed net area basis, catches of bream were higher in the nets with 0.6 m snoods. Catches in the gillnets with the lowest hanging ratio ( E = 0.25) were always higher than in the conventional gillnets ( E = 0.50). Catches of pikeperch were higher in the multifilament nets, those of bream were higher in the monofilament nets. Lowering the hanging ratio in the conventional nets to 0.25 always resulted in a higher mean length of fish caught.

Length at maturity and gillnet selectivity of Lake Tana's Barbus species (Ethiopia): Implications for management and conservation

Lake Tanas 15 large Barbus species form the only known intact endemic cyprinid species flock left in the world. The barbs contribute around one third of the total annual catch of the motorised commercial gillnet fishery which was introduced in 1986. A dramatic reduction of the adult Barbus stocks and the even lower proportion of recruits at the end of the 1990s, show the necessity for the development, implementation and control of fisheries legislation in Lake Tana. The reproductive biology of the Barbus species, essential for fishery management, is poorly known. This paper presents results on size at maturity, size at harvest and gillnet selectivity curves, which can be used to provide a scientific base for management proposals. Size at maturity varied widely among the Barbus species, ranging from 18.8 cm in Barbus brevicephalus to 44.3 cm in Barbus crassibarbis . Males matured at smaller size and reached smaller maximum length than females. Estimated selectivity curves fitted closely or were slightly larger than the observed length-frequency distribution of the commercial catch. The vast majority (85%) of barbs landed by the commercial gillnet fishery were mature. Fishing pressure on juvenile, immature fish is unlikely to be the cause of the observed decrease in Barbus stocks. Size control regulations like mesh size restrictions, intended to protect the immature part of fish populations are expected to have little positive effects on the Barbus stocks and are therefore not recommended. The drastic reduction in barbs during the 1990s is most like due to recruitment overfishing, that is, poorly regulated high fishing effort by the commercial gillnet fishery on the spawning aggregations of adult barbs during their annual breeding migration in river mouths and surrounding floodplains. Only effort control regulations limiting the gillnet fishery in the spawning season and/or areas will prevent a total collapse of the Barbus stocks as has happened to other cyprinids in African lakes. Such measures have to be implemented urgently to guarantee the conservation of Lake Tanas unique biodiversity as a sustainable source of cheap protein and as a natural laboratory to study the evolutionary processes underlying speciation in freshwater fish.

Trends in an Indonesian coastal fishery based on catch and effort statistics and implications for the perception of the state of the stocks by fisheries officials

Indonesias capture fisheries are monitored in each district of all 27 provinces with a comprehensive catch and effort data recording system that was installed in 1976. The annual data are sent to the Indonesian Directorate General of Fisheries (DGF) in Jakarta, where these are aggregated for nine regions. Catches for the 45 recognised fish categories are accumulated in four major fish groups and analysed with conventional fisheries surplus models to estimate maximum sustainable yields (MSYs). These estimates have been used by DGF to determine the number of fishing licenses for each region in the nations economic exclusive zone (EEZ). This paper discusses the effect of data aggregation and accumulation on the variance around trends in fisheries data of the South Sulawesi province. Simple regression techniques are applied to time series of catch, effort, catch-per-unit-effort, and numbers of boats. At the lowest level of data aggregation and accumulation we find the highest variance. Although high variance obscures the perception on the state of fish stocks at the lowest levels, perceptions at the highest levels are not necessarily more useful for fisheries management. Bias caused by motorisation of the fleet by using CpUE as an indicator of fisheries mortality and by combining data from administrative units that have no ecological or biological meaning obscures the detection of trends.

Characteristics of a Sri Lankan reservoir fishery and consequences for the estimation of annual yield

Abstract A quantitative analysis of dynamics in catch, effort and catch per unit of effort (CpUE) was applied to indicate how a system for the collection of catch and effort data could be optimised and how the accuracy of fisheries statistics could be improved. Temporal and spatial patterns in the commercial fisheries were studied in Tissawewa, a representative irrigation reservoir in Sri Lanka. Considerable fluctuations of water level in shallow waters are important characteristics of the irrigation reservoirs in Sri lanka. Reservoir water level and surface area in Tissawewa fluctuated around means of 2.30 m and 175 ha respectively. The reservoir almost ran dry halfway through the study period. After 2 months the reservoir filled up again. The total annual catch from Tissawewa was estimated at 42 431 kg year −1 over the first year, and 11 011 kg year −1 over the second. The annual yield was estimated to range from 150 to 250 kg ha −1 year −1 depending on the occurrence of dry periods. The main fishing methods were normal (stationary) gillnetting, cast netting, seining with gillnets and ‘water beating’. Normal gillnets, usually with 64 or 70 mm stretched mesh, were by far the most important gear. The CpUE was defined as the catch of a single fishing trip by an outrigger canoe. Strata in the CpUE data set were identified to enable the development of an efficient catch and effort data recording system (CEDRS). The overall mean catch per trip was not significantly different between fishing methods or between fishing areas. Significant differences, explaining 30% of the total variance in CpUE, were observed between sampling months. A CEDRS with an accuracy of 28% for the estimated annual catch, 17% for the catch per unit of effort, and 11% for the effort was proposed for the reservoir fishery. Not more than one sampling day per month is needed to achieve this accuracy in a reservoir like Tissawewa. This is a 50% reduction in sampling investments compared with the former official CEDRS for Sri Lankan reservoirs. Application of the proposed CEDRS will remove the bias which presently occurs in the official data, which are based on the number of registered canoes and not on actual recordings of effort.

A size- and age-structured simulation model for evaluating management strategies in a multispecies gill net fishery

Abstract A size- and age-structured simulation model is presented for evaluation of management strategies for a multispecies gill net fishery, using a commercial gill net fishery for pikeperch (Stizostedion lucioperca) and perch (Perca fluviatilis) as an example. The model takes size distributions within age groups into account. Growth for each sex is determined by length and temperature and dispersion in size distributions is controlled by the ‘fractional boxcar train’ method. The model is applied to evaluate the integrated short-term and long-term effects of management measures such as mesh size regulations, fishing effort limitations and combinations of both on the biomass, the size- and age-structure of the stock and of the yield. Changes in yield during the transitional period after a management measure has been implemented, can be quantified.

In search of a better unit of effort in the coastal liftnet fishery with lights for small pelagics in Indonesia

Abstract Despite major criticism, catch per unit effort (CPUE) is still widely used as a measure for the size of the exploited stock, but its indicative value is affected by selection of a proper unit of effort. The unit of effort used in the Indonesian fisheries statistical system is poorly standardised with one trip made by a fishing unit of any type. Therefore, changes in catchability, induced by technical innovations cannot be accounted for in official statistics and thus bias the perception of fisheries authorities on the state of stocks. This study shows that differences in technical characteristics in a coastal liftnet fishery on small pelagics around Ambon, such as unit size and type of lamps used to attract fish, influence both effort allocation and catch per trip. Larger fishing units (more windlasses) are more commercially orientated. They have a larger nonzero catch per trip—1.5 times larger nonzero catch per added windlass—and ignore potentially small catches, as shows from their higher proportion of zero-catches. The shift from kerosene to electric lamps does not lead to a higher catch per trip, but the use of electric lamps is less laborious, inviting fishermen to fish also under more unfavourable light conditions such as during full moon. Model simulations of average catch per trip of two fishing areas show that technical innovations in the last 20 years have increased the mean catch per trip by a factor of 4.1 in the fishing area with the largest technical improvements. Technical improvements explain 80% of the change in catch per trip in the official statistics over the last 15 years. Furthermore, the variance in catch per trip among individual fishing units has increased and so has the uncertainty in the average CPUE, as only part of the fishermen implemented technical improvements. Both developments stress the poor indicator value of catch per trip for developments in fish stock biomass and the importance to standardise the unit of effort. The characteristic high variability in catch in this fishery will always obscure effects of technical improvements in more local situations and smaller timeframes. Because of the inconsistent effects of the type of lamps used on the catch rate, it is questionable whether standardising the unit of effort by the type of lamp used will enlarge the precision in the CPUE. Nevertheless, standardising the unit of effort by the size of the unit will enlarge the precision in, and the indicator value of the CPUE in more aggregated catch data as used on higher levels in the fisheries administration of Indonesia.

A size-structured simulation model for evaluating management strategies in gillnet fisheries exploiting spatially differentiated populations

Abstract A length-structured simulation model is presented as a tool in decision making for gillnet fisheries management. The analytical model simulates the fish population dynamics and impact of the fisheries, taking into account size-dependent spatial distribution patterns and migration of the fish. The daily recruitment of fish, at a theoretical starting length of 0 cm, is simulated with a variable temporal pattern, giving the model a high degree of generality and making it applicable to tropical situations where recruitment is not restricted to a short period in time. The model is sex-differentiated, the growth of the fish is determined by their length and the dispersion over the length-classes is controlled by the ‘fractional boxcar train method’. Natural mortality is assumed constant for all exploited size classes. Fishing mortality is a function of fish length, based on a combination of gillnet selectivity curves for various mesh sizes. The fishing mortality for the most vulnerable size class is calibrated using estimates for the fishing mortality per length-class from length-based cohort analysis. Spatial distribution of the fish over different habitats is based on observed patterns in size-dependent habitat occupation. The realistic model provides predictions on the progressive effects of management regulations on numbers, size distributions and total biomass of the population and the catch in the various habitats. The commercial gillnet fishery for Oreochromis mossambicus (Cichlidae) in Tissawewa, a Sri Lankan reservoir, is used as an example. Simulation results correspond with independently estimated values for output parameters. Based on model predictions, conservation of this fishery is recommended by re-enforcement of the legal minimum mesh size of 76 mm stretched mesh, and a limitation of fishing effort to the present level. The model also provides predictions on the effects of subsidiary small-meshed fisheries for minor cyprinids on the existing fishery for O. mossambicus . Model predictions take into account the size-dependent spatial distribution patterns of the fish and the spatial allocation of the fishing effort.

Catch, effort and sampling strategies in the highly variable sardine fisheries around East Java, Indonesia.

Abstract Temporal and spatial patterns in the fishery for Sardinella spp. around East Java, Indonesia, were studied in an attempt to develop an efficient catch and effort sampling strategy for this highly variable fishery. The inter-annual and monthly variation in catch, effort and catch per unit of effort (cpue) are quantified and discussed, using official catch reports and reports from local village cooperatives. In 1993 the total catch of Sardinella spp. in East Java was 56 000 tonnes and the total effort was equivalent to 5000 units of purse seine. A maximum sustainable yield (MSY) of 40 000 tonnes of sardines is estimated, at an optimum effort of 3000 units of purse seine. The present study focuses on the purse seine fishery since this fishery accounts for more than 50% of all sardines landed in East Java. Purse seiners use lamps to attract fish and peak catches are landed around the new moon. Catches are smaller towards the full moon, when the fishery is inactive during a period of 10 days. Considerable differences exist between monthly catches from peak seasons and monthly catches from off-seasons. The fishery is further characterized by a relatively large amount of zero catches. A catch and effort data recording system (CEDRS) is developed based on the analysis of temporal variation in cpue and effort in the purse seine fishery for sardines. A purse seine fleet, with a variable size of 40 to 70 vessels, was sampled in Probolinggo harbour at the Madura Strait, during 4 to 6 days per month from March 1990 until March 1991. The overall mean effort of the fleet was 36.1 trips per day, of which on average 12.6 trips per day resulted in non-zero catches. The geometric mean cpue over the non-zero catches was 404 kg per trip. The proposed CEDRS has an accuracy of 30% for the estimated annual catch, 21% for the cpue and 9% for the effort (non-zero catches). A minimum required number of 4 sampling days per month is calculated for the temporally stratified CEDRS, with months used as strata within years, and lunar phases as strata within months. For this purpose, the lunar cycle is divided in three lunar phases, one around the full moon, one around half moon and one around the new moon. Within the proposed CEDRS, no sampling takes place in the lunar phase around full moon, when the fishery is inactive. The sample of 4 days per month is proportionally allocated over the other two lunar phases.