Improving productivity in tropical lakes and reservoirs

Freshwater aquaculture, Inland fisheries

Balanced exploitation and coexistence of interacting, size-structured, fish species

This paper examines some effects of exploitation on a simple ecosystem containing two interacting fish species, with life histories similar to mackerel (Scomber scombrus) and cod (Gadus morhua), using a dynamic, size-spectrum model. Such models internalize body growth and mortality from predation, allowing bookkeeping of biomass at a detailed level of individual predation and growth and enabling scaling up to the mass balance of the ecosystem. Exploitation set independently for each species with knife-edge, size-at-entry fishing can lead to collapse of cod. Exploitation to achieve a fixed ratio of yield to productivity across species can also lead to collapse of cod. However, harvesting balanced to the overall productivity of species in the exploited ecosystem exerts a strong force countering such collapse. If balancing across species is applied to a fishery with knife-edge selection, size distributions are truncated, changing the structure of the system and reducing its resilience to perturbations. If balancing is applied on the basis of productivity at each body size as well as across species, there is less disruption to size-structure, resilience is increased, and substantially greater biomass yields are possible. We note an identity between the body size at which productivity is maximized and the age at which cohort biomass is maximized. In our numerical results based on detailed bookkeeping of biomass, cohort biomass reaches its maximum at body masse

Engaging local communities in aquatic resources research and activities: a technical manual

This document is part of a series of 5 technical manuals produced by the Challenge Program Project CP34 “Improved fisheries productivity and management in tropical reservoirs”. The objective of this technical manual is to relay the field experience of a group of scientists who have worked extensively in small fisheries in sub-Sahara Africa and Asia and lay out a series of simple and pragmatic pointers on how to establish and run initiatives for community catch assessment. The manual relies in particular on practical experience gained implementing Project 34 of the Challenge Programme on Water and Food: Improved Fisheries Productivity and Management in Tropical Reservoirs. (PDF contains 26 pages

Effects of ambient oxygen and size-selective mortality on growth and maturation in guppies

Growth, onset of maturity and investment in reproduction are key traits for understanding variation in life-history strategies. Many environmental factors affect variation in these traits, but for fish, hypoxia and size-dependent mortality have become increasingly important because of human activities, such as increased nutrient enrichment (eutrophication), climate warming and selective fishing. Here, we study experimentally the effect of oxygen availability on maturation and growth in guppies (Poecilia reticulata) from two different selected lines, one subjected to positive and the other negative size-dependent fishing. This is the first study to assess the effects of both reduced ambient oxygen and size-dependent mortality in fish. We show that reduced ambient oxygen led to stunting, early maturation and high reproductive investment. Likewise, lineages that had been exposed to high mortality of larger-sized individuals displayed earlier maturation at smaller size, greater investment in reproduction and faster growth. These life-history changes were particularly evident for males. The widely reported trends towards earlier maturation in wild fish populations are often interpreted as resulting from size-selective fishing. Our results highlight that reduced ambient oxygen, which has received little experimental investigation to date, can lead to similar phenotypic changes. Thus, changes in ambient oxygen levels can be a confounding factor that occurs in parallel with fishing, complicating the causal interpretation of changes in life-history traits. We believe that better disentangling of the effects of these two extrinsic factors, which increasingly affect many freshwater and marine ecosystems, is important for making more informed management decisions

Review of catch trends and changes in fish species composition of the Volta lake during its 45 years of existence

Existing catch data for the whole of the Volta lake from 1969 to 2004 and those for certain segments of the lake (Strata II, III, and IV) covering various periods were analysed in relation to lake levels, to find out changes in catch trends and species composition. The study was undertaken as a baseline activity aimed at enhancing fisheries productivity and  management of the lake. Higher catches were made at the inception of the lake, with about 65,000 t being recorded in 1969. This dropped and  fluctuated between 36,000 anct48,000 t from 1971 to 1994. Catches kept increasing rapidly from 1995 with about 80,000 t being recorded in 1999, the highest in the history of the lake. In relation to annual fake water level fluctuations, high catches were made during periods of low water level compared to periods of high water level. On the long term, decreasing lake water level corresponded with higher fish catches. There was a change frominsectivorous fish species (e.g., Chrysichthys, Schilhe and Synodontis) at the initial stages of the formation of the lake to those with vegetarian food habits dominated by the tilapias. The dominance oftilapias in catches, which persisted till the early parts of the 1990s, has given way to Chrysichthys spp., suggesting that the composition of fish species in the lake is still undergoing changes 45 years after its formation

Engaging local communities in aquatic resources research and activities: a technical manual

This document is part of a series of 5 technical manuals produced by the Challenge Program Project CP34 ôImproved fisheries productivity and management in tropical reservoirsö. The objective of this technical manual is to relay the field experience of a group of scientists who have worked extensively in small fisheries in sub-Sahara Africa and Asia and lay out a series of simple and pragmatic pointers on how to establish and run initiatives for community catch assessment. The manual relies in particular on practical experience gained implementing Project 34 of the Challenge Programme on Water and Food: Improved Fisheries Productivity and Management in Tropical Reservoirs.Research, Fishery data

Sustainable fishing of inland waters

Sustainability in fisheries has over the past decades evolved from a single species maximization concept to covering ecosystem and biodiversity considerations. This expansion of the notion, together with increased evidence that the targeted removal of selected components of the fish community may have adverse ecological consequences, poses a serious dilemma to the conventional fisheries management approach of protecting juveniles and targeting adults. Recently, the idea of balanced harvest, harvesting all components in the ecosystem in proportion to their productivity, has been promoted as a unifying solution in accordance the ecosystem approach to fisheries, but this will require a fundamental change to management. In this paper, we review theoretical background, and practicalities of securing high yielding fisheries in inland waters, with empirical examples freshwater fisheries which satisfy the extended objectives of minimal impact on community and ecosystem structure. We propose framework of ecological indicators to assess these objectives

Catch composition and efficiency of major fishing gears used in stratum II of the Volta lake– implications for managing the fisheries

Catch composition and efficiency of major fishing gears viz; gillnets, basket traps, net traps and ‘atidza’ (brush park), deployed by fishers on the Volta lake at Dzemeni (Stratum II), was assessed over a 16-month period to determine how their deployment can impact on the fish stocks. Fifty thousand seven hundred and ninety four individual specimens were recorded during the study period. This represented 13 taxonomic families, 21 genera and 32 fish species. Only five genera constituted 95 per cent and 84 per cent by number and weight, respectively, of the total catch. Chrysichthys spp. was the most abundant genus (59.1%) followed by the tilapias (15.7%), Hydrocynus spp. (9.7%), Synodontis spp. (8.8%) and Bagrus spp. (2.6%). Catches by ‘atidza’ and basket traps were predominantly Tilapias and Chrysichthys spp., respectively, while that by gillnets and net traps were more heterogeneous. The highest catch of 20 t during the period was made by basket traps, while that from the other gears ranged from 0.33 to 6 t indicating that basket traps were very efficient. Knowledge about the catch composition by the various gears and their efficiency will help to regulate their use when it comes to formulating measures to manage the fisheries of the lake

Growth, mortality, maturity and length-weight parameters of fishes in Lake Kariba, Africa

This preliminary compilation presents vital parameters for 22 species of freshwater fish from Lake Kariba. The majority of the growth parameters are derived from tables in Balon and Coche's "Lake Kariba: a man-made tropical ecosystem in central Africa". The rest of the parameters are compiled from more recent sources and unpublished data

Water level fluctuations and the ecosystem functioning of lakes

Hydrological regimes are key drivers of productivity and structure in freshwater ecosystems but are increasingly impacted by human activity. Using 17 published food web models of 13 African lakes as a case study, we explored relationships between seasonal and interannual water level fluctuations and 15 attributes related to ecosystem function. We interpreted our results in the context of Odum's ecosystem maturity hypothesis, as systems with higher magnitude fluctuations may be kept at an earlier maturity stage than those that are relatively stable. The data we compiled indicate that long-term changes in the hydrological regimes of African lakes have already taken place. We used Least Absolute Shrinkage and Selection Operator (LASSO) regression to examine relationships between ecosystem attributes and seven physical characteristics. Of these characteristics, interannual water level fluctuation magnitude was the most frequently retained predictor in the regression models. Our results indicate that interannual water level fluctuations are positively correlated with primary and overall production, but negatively correlated with fish diversity, transfer efficiency, and food chain length. These trends are opposite those expected with increasing ecosystem maturity. Interestingly, we found seasonal water level fluctuations to be positively correlated with biomass. An increase in standing biomass is generally associated with more mature ecosystems. However, we found that less production and biomass occurred at high trophic levels in highly fluctuating compared to relatively stable systems. This synthesis provides evidence that water level fluctuations are a key process influencing ecosystem structure and function in lakes.publishedVersio