The Technical Consultation on the Enhancement of Small Water Body Fisheries in Southern Africa was organized by ALCOM under project GCP/RAF/277/BEL “Utilization of Small Water Bodies for Aquaculture and Fisheries”. It was held at the Harare Holiday Inn, Harare, Zimbabwe, 25–29 January 1993, and attended by 52 participants and observers from Belgium, Botswana, Finland, Lesotho, Malawi, Mozambique, Namibia, The Netherlands, Swaziland, Sweden, Tanzania, Zambia, Zimbabwe, FAO headquarters in Rome, the SADC Fisheries Sector Coordinating Unit and the ICLARM Africa Aquaculture Project. The list of participants is given in Appendix 1.
The objective of the technical consultation was to review the status of small water body fisheries and estimates of their potential in Southern Africa, and suggest management action which will bring exploitation closer to resource potential and benefit local communities. The agenda is given in Appendix 2 and the list of documents in Appendix 3.
Mr. A. Andreasson, Programme Director, ALCOM, described the background to the small water bodies technical consultation. The ALCOM Technical Consultation on Aquaculture in Rural Development (1987) identified the utilization of small water bodies as a target area. The aim would be to demonstrate practices for increased production and methods for distribution of benefits within communities in the member states of SADC, the Southern Africa Development Community. ALCOM's small water body activities began on a modest scale in late 1989 and included appraisal of small water bodies (biologial, physical, fishery, community), development of production enhancement methods, assistance to communities to improve management practices, monitoring of production and community participation, and studies on water borne human diseases and disease vectors. In September 1992 Belgium generously funded the project “Utilisation of Small Water Bodies for Aquaculture and Fisheries - A Project of ALCOM”, which aims at development of small water body fisheries and management techniques which will enable local communities to fully benefit through higher nutrition and/or incomes. During the next five years the project will address: economics and socio-economics of small water body fisheries; fishery management through community organizations and enterprises; fishing gears and methods; and, enhancement of fish production. Mr. Andreasson noted that the consultation starts off the Belgium-funded project and that ALCOM will use its recommendations when formulating activities to achieve project objectives. He also looks forward to wider use of the consultation's findings by governments, other institutions and projects.
Mr. A. Stockseth, Programme Officer (APO), Office of the FAO Representative in Zimbabwe, welcomed the participants on behalf of FAO. He noted that over the past decade FAO has strengthened its assistance to member nations to understand, develop and manage the fisheries of small water bodies. For example, the Committee for Inland Fisheries of Africa, an FAO regional body, found in 1987 that the use of small water bodies in Africa needs improvement by effective development planning, stocking with fish, control of over-exploitation, research on socio-economic aspects and dissemination of traditional fisheries techniques. In this respect, ALCOM, an FAO executed programme, is well placed to assist member governments address small water body fisheries as one of its target areas. This is even more relevant in the light of the severe drought which has recently plagued Southern Africa. He thanked the Government of Belgium for supporting ALCOM's small water bodies activities and wished the delegates fruitful deliberations.
Dr. Cecil Machena, Acting Chief Ecologist (Aquatic), Department of National Parks and Wildlife Management, Zimbabwe, on behalf of his Director, formally opened the technical consultation. In his speech he emphasized the importance of small water body resources in the Southern African region in terms of food production and income generation in rural areas, especially in times of drought. In Zimbabwe, with its many thousands of water bodies, the Government is giving appropriate authority to local government units to manage small water body resources for local communities. The Department of National Parks and Wildlife Management, and the Department of Agricultural, Technical and Extension Services (AGRITEX), will continue to assess the small water body resources and advise on their management. He saw particular need to address issues concerning resource assessment, appropriate gear for exploiting the resource, and community-based management systems. He looked forward to the outcome of the technical consultation and wished the delegates an enjoyable stay in Zimbabwe.
Dr. O.V. Lindqvist, Professor, Department of Applied Zoology, University of Kuopio, Finland, in his role as Chairman of the consultation, delivered the keynote address.
There are between 50,000 and 100,000 small, often man-made, water bodies in eastern and southern Africa. Many of them are far from consumption centres, and considering their fishery potential, are often underutilized. Their exploitation for aquaculture and fisheries will depend on the potential benefits and perceived risks involved with producing and/or harvesting fish. In Southern Africa, benefits and risks are especially influenced by: precipitation patterns which are highly variable and give generally low annual water yields; plentiful land; scarce labour; poorly developed markets for fish; avoidance of fish consumption in some areas or under certain conditions; and, water-related health risks.
It is especially important to see man-made small water bodies as new ecosystems which are highly dynamic, unlike older natural lakes. Characteristics with respect to the aquatic environment, fish community structure, ecological energetics, nutrient cycling, and succession patterns can be identified which will facilitate the sustainable and enhanced use of the dynamic small water body resource.
The small water body resource in Southern Africa is unique, at both the biological and social level, and local solutions are needed. Since extension services are limited by the available resources, solutions should to the extent possible come from local communities and be easily disseminated through existing education and extension networks.
Aspects of small reservoir resources in Southern Africa were reviewed in four papers and eight posters from seven countries. It was accepted that the major small water body resource in Southern Africa comprises man-made reservoirs, although coastal lagoons are potentially important in the four SADC maritime states. The state of knowledge of small water bodies in the region is uneven, especially for water bodies at the low end of the size range. Inventories are available for all or part of Botswana, Lesotho, Malawi, Namibia, Tanzania, Zambia and Zimbabwe.
Small water bodies may be usefully classified by a number of measurable criteria: age, morphology, size, and potential management techniques. Classification by management intervention related to size is particularly useful for determining the extent and potential output of the resource.
Dr. P.-D. Plisnier suggested that, based on data from the literature, biological fish production is most closely correlated with the level of primary production (and its indicator chlorophyll a), fish biomass, and total phosphorous. Morphology has especially strong positive and negative impacts on primary production with respect to nutrients, temperature and light. Studies are needed to more closely relate primary production with chlorophyll a, turbidity and biological fish production.
Dr. D.F. Thys van den Audenaerde pointed out that the dynamic nature of the small water body environment extends to the composition and size of fish populations. Small water bodies generally have a good semi-pelagic environment which can favour the Cichlids, large and small Cyprinids, Mormyrids, Schilbeids and other catfishes found in Southern Africa. It is difficult to predict the number of species which will establish themselves. More information on the reproductive biology of the small species would be useful. It is clear, however, that efficient predators should not be introduced into the small water bodies of Southern Africa.
Ms. C. Mangwaya raised several issues concerning the introduction of Chinese carps. While grass carp may be useful for clearing macrophytes, the main advantage of silver and bighead carps is their fast rate of growth. However, large fish are not necessarily preferred by consumers, and hatchery production of fingerling would be required since these fishes are not known to breed in the wild outside their native ranges.
Methods for assessing the physical, biological, economic and social resources of small water bodies and their surrounding environment were presented in seven papers. The findings and recommendations of Working Group 1 highlight important issues from the presentations and discussions during this session.
Methods for management and expoitation of small water bodies were presented in 12 papers. The findings and recommendations of Working Groups 2, 3 and 4 highlight important issues from the presentations and discussions during this session.
To facilitate discussion and formulation of recommendations, four working groups were formed, each to consider one main topic. Each working group was requested to prepare a brief report based on the consultation presentations, discussions, and knowledge of group members giving a brief overview of the topic; a statement of central issues and problems with a focus on Southern Africa; and, priorities for action by Governments, ALCOM, FAO (regional and interregional aspects), and other institutions in Southern Africa.
The group considered the status of knowledge about small water bodies (SWBs) in four broad categories: physical information, fishery resources, fishery potential and information on fisheries (Table 1). Each country represented by a group member provided the information to complete the table. Where a country was not represented, the information was obtained by interview. The general status of information and short and medium term recommendations are given below for each category.
Physical knowledge
Generally, knowledge about numbers and locations is good for only a few countries; however, the situation is unclear about others because the fisheries departments may not have some essential information. The situation on surface areas, their variations and permanence is even less certain.
The ultimate objective is that each country should have its own SWB database. Further, the databases should be “transparent” so that each one is open to all of the others and so that ALCOM can undertake regional analyses. Each country should develop its own approach to acquiring the information for its database. In the region the database effort by AGRITEX Zimbabwe is a good example in that it combines a database with a Geographical Information System (GIS). The latter can be used to characterize SWBs in a broad way in the context of their physical and economic environments. In Mozambique where SWB information seems to be largely lacking, the existing satellite remote sensing coverage could be used for an inexpensive and rapid inventory.
Recommendation: Before some of the more difficult information aspects such as surface water variation are dealt with, ALCOM should as a first step initiate a search of data among government records. This should be an inventory of all of the information available and should not be limited to physical characteristics alone. This should be done by someone in each country with access to or the ability to locate the pertinent records. In practical terms this person should understand fishery and aquaculture considerations in SWBs.
Knowledge about Fishery Resources
Knowledge about fish species distribution, biology and life history were considered and found to be generally insufficient for Southern Africa.
Recommendation: A first step to better define the situation is a review of the literature on species distribution, biology and life history of the fishes already known to be important in SWB fisheries. Initially, use should be made of FAO's Aquatic Science and Fisheries Abstracts (ASFA) and ICLARM's FISHBASE. This should be followed by a search within the SADC region. The databases can be provided by FAO and ICLARM, and the compilation and synthesis should be done on an author's contract within the region. Universities are usually involved in this kind of investigation and their resources should be considered along with those of fisheries departments.
Fishery Yield Potential
Predictive models and on-site investigations were considered. In short, none of the existing predictive models have been adequately tested on SWBs in the region although some have been applied. Although AGRITEX Zimbabwe has developed a procedure to assess the impact of exploitation on SWBs, it is not usable for estimating fishery potential.
A high priority is for an index of fishery potential that can be used in this region until a better approach can be developed. The basic data should be fish catch and any other complementary information that can be obtained.
Recommendation: ALCOM should undertake a compilation of existing catch data and accompanying information on a country by country basis. An author's contract could be the vehicle. In making this recommendation it is recognized that there is likely to be only a small quantity of reliable data. Therefore, the data on which to build a predictive model are eventually going to have to be obtained in the region.
Recommendation: Looking ahead to this need, but for the present, ALCOM should examine ways to promote a field research programme on SWBs aimed at developing a model for predicting fisheries potential. Among the items to consider early on are the kinds of predictions that are needed for management and development and the parameters that should be measured to realize them. In Table 2 some bio-physical parameters of potential importance in predicting SWB productivity are proposed, including a rating of the difficulty in measuring them. A partnership between universities and fisheries departments could provide a pathway to realize this objective.
Recommendation: Regarding the potential for enhancement of SWBs, not enough attention has been paid to the opportunity to use aquaculture facilities as sources of data and for experimentation to understand the behaviour of SWB fish communities. ALCOM should investigate the potential use of aquaculture facilities for this purpose and liaise with governments and universities to develop programmes to use these facilities.
An important need is to characterize SWBs as to their best aquatic uses whether for fisheries, aquaculture, or a combination of the two. GIS can be a useful tool to help with the decision-making. With a broad approach it is possible to view SWBs in their physical and economic contexts using criteria that bear on their suitability for one or other use.
Recommendation: Although the FAO GIS for Southern Africa has been aimed primarily at looking at aquaculture potential in purpose-built ponds, it should be broadened to include SWB fishery and aquaculture potential. Likewise, the potential of the AGRITEX Zimbabwe GIS to perform this same kind of analysis at the national level should be encouraged.
Status of Information about Fisheries
There is very little information about SWB fisheries in their broadest contexts. There are two fundamental problems. The first is that fishery statistical systems in most countries do not function well even for the larger water bodies. SWBs represent a large burden because of their dispersed nature and because they are so numerous. It is not reasonable to assume that countries can quickly move to implement comprehensive fishery statistical systems on SWBs. Therefore, an alternative has to be found. Given the example of AGRITEX in Zimbabwe, it seems that encouraging the fishermen themselves to provide the statistical information is the path to follow.
A key factor in acquiring this information is the rationalization of fishery legislation. A fundamental step is to cover all fishing gears by legislation and to liberalize gear restrictions. If penalties are reduced, fishermen will be more straightforward in giving information; and with the right extension approach, they can see the advantage of keeping information that will be useful to themselves and to government as well -- for implementing management measures.
Unlike information about catch, effort and gears that varies from water body to water body, economic, cultural and social information that bears on SWB development and management can be acquired from larger areas such as districts and wards. However, from time to time it will be necessary to verify and to supplement this information by sampling at the level of the SWB community. Rapid Rural Appraisal could be one of the latter techniques.
The group considered the preconditions and use of five key enhancement mechanisms and made recommendations on their use.
Improvement of habitat
Recommendation: Authorities/projects should assist communities to understand and be involved in activities such as creation of brush parks.
Improvement of existing gear/Introduce appropriate fishing techniques
Nutrient input (fertiliser and feeding)
Aquaculture (cage and pen culture)
Stocking
Overview
Conflicts between fisheries and other uses of SWBs are common. Management of SWBs can encompass measures ranging from encouragement of the utilization of underexploited reservoirs to regulation of access and effort where fishing intensity is high, which may affect the sustainability of the fishery.
Selection of SWBs for management
The first category of reservoirs should have priority because of the possibility of spreading benefits among a large number of people. Government-owned dams can also provide benefits to a large number of people but often have other more important uses than fisheries (e.g. domestic water supply, irrigation). Privately-owned reservoirs are few and generally benefit a limited socio-economic group.
Natural factors -- such as, for example, low primary production or high risk of siltation -- can exclude some reservoirs from being the focus of fisheries management,. The level of local economic development is also important: labour will be plentiful in economically depressed areas and heavy fishing pressure is likely.
Objectives for SWB management
In each specific SWB situation, one or several objectives may apply.
A three-point framework for accomplishing SWB management objectives is suggested which encompasses fishery management, regulation and extension.
Potential Fishery Management Regimes
Three major fishery management options can be considered for community-and government-owned SWBs: community-based management (CBM), direct government management and private management.
Compared with direct government management, CBM is likely to meet community needs, be more equitable to interest groups, be less expensive to government (in the long run), and be better understood and complied with by fishermen and communities at large. However, under CBM, conflicts within the community may delay decision-making, more effort and time is needed for its establishment, and lack of managerial and technical skills may result in poor performance. Linkages between community-based management structures and government administrative systems should be established as communities often lack necessary skills to carry out effective management. Governments also need to have influence over community-based management processes so as to ensure that they are not destructive to the fisheries resource in the long run and do not conflict with other reservoir uses.
Direct government management is simpler and more straight-forward to implement, but it lacks the positive distributive and community-oriented advantages of CBM. Government management schemes can work efficiently only when sufficient resources are available. When resources are lacking, government priority should be given to managing heavily fished SWBs, which are likely to be in heavily populated areas.
Private management can take the form of licensed enterprises with a part or whole fishing concession. They may also be entrusted responsibility for managing the fishery. Communities not interested in fishing could still benefit directly or indirectly from license or lease fees.
The possibility of allowing short-term overfishing and then recovery of the fish stocks, with a large but discrete output rather than a low continuous output, should be considered.
Catch data are needed to evaluate the economics of any management scheme.
Regulatory measures
CBM and government management differ in the mechanism used for implementing fishing regulations. Whereas government relies on its staff to enforce regulations, communities are self-regulatory through CBM structures. In the case of private management, regulation is incorporated in the terms of the license or lease to the operator and is subject to review by the issuing authority.
Extension approaches to SWB fisheries
Extension approaches for SWBs are influenced by: low potential yield; wide geographical distribution and distance from major fishing areas; and, beneficiaries who are usually farmers. The first two characteristics make it difficult to justify specialized extension services. The fact that the users are mostly farmers makes integration with agricultural extension services logical.
The extension approach chosen would differ according to the characteristics of recipient farmers and existing local extension structures. The extension message would have to be adapted to each specific small water body, depending on its size, the fish resource, characteristics of surrounding communities, and the management and regulatory framework. Since non-governmental organizations (NGOs) are localized, they can only complement national extension systems. The use of farmer-to-farmer information transfer as an extension tool is also of limited value, as SWBs are usually separated by quite large geographical distances. It may work within SWB communities, but interaction between different SWB communities will be limited.
In order to provide SWB fisheries information through existing extension services in the SADC countries, it is necessary to train extension workers. This can be accomplished either through their regular training or in special courses. There is also a need for a technical service to support extension agents when it comes to more complicated fisheries matters, to carry out research, and to advise on the use of credit when available.
Brief overview of the topic
The construction of SWBs has had ecological impacts on the environment. The most important impacts are: hydrological aspects, flooding of large areas, changing of flowing water into stagnant water, change in river discharge downstream, change in ground water levels, erosion control, water pollution (accumulation of agrochemicals), destruction of farm lands and forests, introduction of human waterborne diseases, and displacement of people. Most of these factors have impacts on the biology of the fishes as well.
Stocking programmes in SWBs are often carried out without considering the ecological aspects of transferring or introducing new fish species.
Some of the most important and widespread diseases in the tropics, such as arboviruses, schistosomiasis, malaria and filariasis, have vectors that directly depend on water for their development or as habitat. Therefore, SWBs represent an increased health risk for people living nearby.
Statement of central issues and problems with a focus on Southern Africa
Current fishing activities in SWBs have no specific environmental impacts. The general rule is that the less intensive the management the less the environmental impact. Management and enhancement of fisheries imply that the data we need to build the management plan are present. This is most often not the case with SWBs in Southern Africa. The biology, especially the reproduction cycles, of most fish species present, are not known. There is a lot of fishing going on with different gear types and efforts, but until we know better the biology of the fish species, the interaction among the species and with the limnological environment, we cannot make sound recommendations concerning either fishing level, gear and restrictions or on transfer or introduction of new fish species.
There is currently no specific code of practice for the transfer or introduction of new fish species in the region. The national fisheries acts are often flouted as authority is not obtained to transfer aquatic species from one SWB to another, or from one water basin to another.
The increased risk of serious health problems created by the SWBs is evident and well documented.
Priorities for action in Southern Africa
Governments and donors are recommended to give priority to research on fish biology and aquatic ecology in the SWBs. When new SWBs are being planned, thorough pre-impoundment investigations should be made.
All transport of live aquatic species should respect local laws and regulations and follow the Codes of Practice and Manual of Procedures for Consideration of Introductions and Transfers of Marine and Freshwater Organisms (EIFAC Occasional Paper No. 23, FAO Rome, 1988). FAO and ALCOM should assist the governments to implement the codes of practice. In countries where the Fisheries Act is not clear as regards live fish movements, FAO could assist in defining these rules.
Biological and environmental control of bilharzia, malaria and other water borne diseases should be given priority, as all available chemical methods are harmful to fish and other aquatic organisms. Awareness campaigns to inform people about existing disease risks and ways to avoid these diseases should be carried out at SWBs.
Governments, agricultural institutions and farmers should be made aware of the implications and risks of using pesticides and other agricultural inputs in the watershed of SWBs.
In his concluding remarks, the Chairman thanked the participants for their contributions and lively discussions. He then gave a brief summary of the deliberations.
Since small water bodies in Southern Africa are generally rather young man-made reservoirs, they represent dynamic ecosystems and fish production sites that are not well, if at all, researched and documented. Biologically, SWBs are still “black boxes” in many respects, especially with respect to fish stock dynamics and limnological processes. Although their fishery and aquaculture potential is not documented, it can be estimated that the fish production potential of SWBs in sub-Saharan Africa is roughly 3 million tons of fish annually, which exceeds the current annual fish production of all other African inland fisheries (rivers, lakes and large reservoirs) combined.
Since SWBs are built for purposes other than fisheries, the appropriate administrative structure for fisheries management and extension is usually not present and needs to be determined and implemented. This issue is compounded by the widely scattered nature of SWBs and the variable nature of fish catches, which has implications for the transaction costs of servicing the SWBs and the local and wider markets for fish harvested. Community-based management schemes may be the most profitable and feasible management method under these conditions and at this stage of development.
An urgent problem of wide ecological and economic implications is the question of introduction of exotic fish species for aquaculture and/or capture fisheries. First preference should go to indigenous fish species which have potential for reasonably good production and are acceptable on the local market.
SWBs can have important positive and negative impacts on the environment: they may serve as indicators of the environmental processes and environmental quality in a given area; they may be a source of human and animal disease vectors; and, they can be used locally to improve the environment by providing habitat diversity which will support more animal and plant species, contributing to the maintenance or overall biodiversity.
In the long run, the exploitation of SWBs will move from capture fishery management towards aquaculture, which holds the key to vastly increased fish production. This movement must be assisted by research and extension services, whose technical and field resources can be better applied and allocated through planning frameworks which utilize insights gained from geographical information systems incorporating the physical, biological, economic and social factors critical to the SWB environment.
The papers presented at the technical consultation will be published separately.
TABLE 1: Status of knowledge about small water bodies in Southern Africa
| Level of knowledge | Physical | Fishery Resources | Fishery potential | Status of Info. on Fisheries | |||||||
| Number location | Surfaces | Surfaces variation and permanence | Environment | Species composition | Bio-Data Basic life hist. and ecology | Approxy (predictive models) | Specific (on site catch rec. sampling) | Number of persons, gears | Processing marketing | Economic/ Social/ Cultural | |
| Angola | |||||||||||
| National | |||||||||||
| Subnational | |||||||||||
| Botswana | |||||||||||
| National | - | + | - | + | |||||||
| Subnational | ++ | + | + | + | ? | + | |||||
| Lesotho | |||||||||||
| National | - | + | + | + | + | ? | + | ++ | + | ||
| Subnational | ++ | ++ | |||||||||
| Malawi | |||||||||||
| National | - | + | + | ++ | ++ | ||||||
| Subnational | +++ | +? | + | +? | |||||||
| Mozambique | |||||||||||
| National | - | - | - | - | - | + | - | - | |||
| Subnational | + | - | ? | ||||||||
| Namibia | |||||||||||
| National | |||||||||||
| Subnational | +++ | +++ | ++ | ? | +++ | ++ | ++ | - | - | + | - |
| Swaziland | |||||||||||
| National | + | + | + | + | |||||||
| Subnational | ++ | - | - | ||||||||
| Tanzania | |||||||||||
| National | + | ++ | - | ? | |||||||
| Subnational | ++ | +++ | - | ++ | +? | ||||||
| Zambia | |||||||||||
| National | ++ | ++ | + | + | ? | + | |||||
| Subnational | ++? | ++? | - | +? | ? | + | |||||
| Zimbabwe | |||||||||||
| National | ++ | ++ | +? | + | + | + | |||||
| Subnational | ++? | ++? | - | ||||||||
+ and - signs Indicate level of knowledge.
A question mark reflects uncertainty about the level Indicated.
- Rating based on work and cost involved in estimating the parameters where:
I = “all” SWB's;
II = wherever possible;
III = research level, only on selected SWB's.
| Parameter | Comment | Rating | ||
| Altitude and latitude | Influences water temperatures | I | ||
| Capacity of reservoir | For calculation of residence time, may be better correlate to yield rather than surface area | I | ||
| Catchment area | Related to in/out flow and productivity? | I | ||
| Geology & soil type in catchment | Will affect water quality and productivity | I | ||
| Mean depth | Related to productivity | I | ||
| Rainfall, annual & monthly averages | Prediction of retention time | I | ||
| River systems | Related to SWB species composition and productivity | I | ||
| Surface area | Related to total yield. Average, max .& min | I* | ||
| Temperature in air & water | Maximum, minimum & average | I/II* | ||
| Land use in drainage | E.g. intensive farming vs. forestry, related to allochthonous energy input | I/II | ||
| Annual in/out flow | Water residence time affects nutrient retention | II* | ||
| Annual draw-down pattern | E.g. to predict risk of increased fish mortality or fish kills due to low water levels | II* | ||
| Chlorophyll a | Correlated with primary production. Remote sensing (satellite images) also possible | II* | ||
| Conductivity | Correlated with nutrient concentration | II* | ||
| Gross primary production | Autochtonous energy fixation (internal photosynthesis). Closely correlated with fish production | II* | ||
| Macrophyte cover | Percentage cover | II | ||
| Maximum depth | Related to probability for stratification and anoxic hypolimnion | II | ||
| Numbers of fishers & gears | Related to effort and catch | II* | ||
| Number of households & livestock using reservoir | Presumably correlated with allochthonous energy input | II | ||
| pH | Limiting at extreme values | II | ||
| Previous fish species introductions | Prediction of SWB species assemblages | II | ||
| Sechi disk transparency | Correlated with primary production, turbidity may interfere | II* | ||
| Shoreline length | Influences allochthonous energy input | II | ||
| Species composition | II | |||
| Total alkalinity | Correlated with productivity | II* | ||
| Turbidity | Can be limiting for primary production if extremely high | II* | ||
| Fishery catch and effort | From creel census or existing statistics; indication of species composition. MSY may be estimated using e.g. the Schaeffer model. | II/III* | ||
| Biological fish production | Dominating species only. Requires detailed studies of population dynamics | III* | ||
| Experimental catch per unit effort by species | Index of relative abundance, seasonal fluctuations must be expected. Related to fishery yield/effort. Gives good information on species composition | III* | ||
| Fish biomass | Variable depending on e.g. fishing effort | III* | ||
| Ions | E.g. Ca2+, Na+, Mg2+, K+, NO3, SO42-, PO43-, C1- | III* | ||
| Nitrogen loading rate | May be limiting for productivity | III* | ||
| Phosphorous loading rate | Often limiting for productivity | III* |
