This technical paper deals with the species of tunas and billfishes of greatest commercial importance. The tunas include skipjack, Katsuwonus pelamis, yellowfin, Thunnus albacares, bigeye, T. obesus, albacore, T. alalunga, Atlantic northern bluefin, T. thynnus, Pacific northern bluefin, T. orientalis, and southern bluefin, T. maccoyii, and the billfishes include swordfish, Xiphias gladius, Atlantic blue marlin, Makaira nigricans, Indo-Pacific blue marlin, M. mazara, black marlin, M. indica, white marlin, Tetrapturus albidus, striped marlin, T. audax, Atlantic sailfish, Istiophorus albicans, and Indo-Pacific sailfish, I. platypterus. There are many other species of tuna, and several other species of billfish, of lesser economic importance. When general statements are made concerning tunas or billfishes they do not necessarily apply to species other than those listed above. They are often referred to collectively as "tunas and tuna-like fishes" in this report.
As the fisheries for tunas and tuna-like fishes have expanded during the decades since the late 1940s, regional fisheries bodies (RFBs) have been created to provide, at varying levels, for research and/or management of these resources, and for the ecosystems and species that are affected by the fisheries harvesting tunas and tuna-like fishes. These fisheries now extend throughout the tropical and temperate regions of the oceans and seas. The only area not covered at the time this report was written is the subject of the Multilateral High Level Conference to establish a regional fisheries body in the western and central Pacific.
In 1991, the Committee on Fisheries of the Food and Agriculture Organization (FAO) requested that FAO develop a Code of Conduct for Fisheries. Subsequently, FAO and the government of Mexico sponsored an International Conference on Responsible Fishing, held in Cancun in May 1992. Resolutions formulated in Cancun were presented at the United Nations Conference on Environment and Development (UNCED) in Rio de Janeiro in June 1992. The Rio meeting highlighted the importance of the Precautionary Approach in the Rio Declaration, and Agenda 21, Principle 15, of the Rio Declaration states that "in order to protect the environment, the precautionary approach shall be widely applied by States according to their capabilities. Where there are threats of serious or irreversible damage, lack of full scientific certainty shall not be used as a reason for postponing cost-effective measures to prevent environmental degradation."
Several binding and non-binding agreements embodying the Precautionary Approach were developed and concluded during 1991-1996. The most comprehensive of these is the Code of Conduct for Responsible Fisheries, concluded in late 1995. The Code of Conduct addresses six key themes: fisheries management, fishing operations, aquaculture development, integration of fisheries into coastal area management, post-harvest practices, trade, and fisheries research. While a Precautionary Approach is integral to all the themes, it is applied particularly to fisheries management. Article 7.5, paragraph 1 includes "States should apply the precautionary approach widely to conservation, management, and exploitation of living aquatic resources in order to protect them and preserve the aquatic environment." The paragraph also provides that the absence of adequate scientific information is not a reason for postponing or failing to take conservation and management measures.
The Code of Conduct is a voluntary, non-binding agreement. However, it contains sections that are similar to those in the Agreement for the Implementation of the Provisions of the United Nations Convention on the Law of the Sea of 10 December 1982 Relating to the Conservation and Management of Straddling Fish Stocks and Highly Migratory Fish Stocks which will, when it comes into effect, be a binding agreement (the UN Fish Stocks Agreement). These two agreements provide the formal basis for the Precautionary Approach to fisheries management.
The UN Fish Stocks Agreement has content and wording that is similar to the Code of Conduct, including those related to the Precautionary Approach and General Principles. The two Agreements incorporate the principal of reference points as important instruments for the application of the Precautionary Approach to fisheries management. Annex II of the UN Fish Stocks Agreement provides guidelines for the application of precautionary reference points. Paragraph 2 states, "Two types of precautionary reference points should be used: conservation, or limit, reference points and management, or target, reference points." Paragraph 5 stipulates, "Fishery management strategies shall ensure that the risk of exceeding limit reference points is very low," and imposes the further constraint that target reference points should not be exceeded on average. Paragraph 7 states that "The fishing mortality rate which generates maximum sustainable yield should be regarded as a minimum standard for limit reference points."
The Expert Consultation on Implications of the Precautionary Approach for Tuna Biological and technological research originated from a recommendation of the Tuna Symposium sponsored by the International Commission for the Conservation of Atlantic Tunas (ICCAT) and held in Punta Delgata, Sao Miguel, Azores, Portugal, in 1996, which recognized the existence of similar problems in the implementation of the Precautionary Approach for tuna fisheries on the global scale. Because of these similar problems it is appropriate to consider research implications for tuna as a group.
The tunas and tuna-like fishes are highly-migratory species that are fished world wide, both in coastal and oceanic zones, and by multiple fishing gears. This large-scale distribution and the biology of these species are such that the scientific knowledge and uncertainties upon these stocks can hardly be compared with other species. These peculiarities of the tunas and tuna-like fishes and of the fisheries for them should be taken into account in the Precautionary Approach.
The Precautionary Approach does not require that our knowledge of tuna stocks and fisheries improves before appropriate conservation and management measures are taken. It does, however, require that management measures be more cautious when information is uncertain, unreliable, or inadequate. The proper balance between acquiring knowledge and management action should be made objectively. On this basis, the output of the Consultation includes proposed research actions that elaborate the likely demands to characterize and reduce the uncertainties involved in the management of tuna fisheries. The Consultation considered how to take account of and describe the existing uncertainty in our understanding of the effects of fishing, and how the existing uncertainty might be reduced with appropriate research. Those two aspects are equally important for the application of the Precautionary Approach.
Under a Precautionary Approach, appropriately designed and implemented research can have direct benefits by reducing uncertainty, which may mitigate the problem. Conversely, not conducting such research might require more restrictive management than would be needed with additional information.
The Consultation considered situations for which there are quantitative techniques for describing the effect of fishing and those for which there are few data and only qualitative information on the stock and the fishery. Any quantitative method of assessment should include estimates of uncertainty, and consideration should be given as to how uncertainty can be included in qualitative descriptions of tuna fisheries.
This report deals with both the target and by-catch species caught by the fisheries for tunas, and also with the other species that share the habitat of the tunas and tuna-like fishes.
As more and more species are targeted by fisheries in the
world oceans, and as the capacities of the fishing fleets increase, it becomes
necessary to consider harvesting strategies that take into account the
interactions among species and environmental effects. With a reasonable
knowledge of an ecosystem, it may be possible to determine management strategies
that allow the utilization of resources in some optimal manner based on
management objectives. When that information is not available for the
pelagic ecosystems, however, that approach is not possible. In
this case, the Precautionary Approach suggests that the by-catches be maintained
at sustainable levels or reduced. Further ecosystem research, based on an
extensive collection of data on various important things, including by-catches,
will then be urgently needed to provide scientific understanding of the effects
of fishing on ecosystems.
The general acceptance of the need to consider the effect of fishing on ecosystems has not yet been expressed in practical management objectives analogous to those that have long been available for single species, such as optimizing catches subject to sustainability constraints. Until such objectives are developed, it will not be possible to give advice on how they can be met.
The Precautionary Approach can be followed regardless of the method of the assessment of the effect of fishing. For many of the stocks of the major market species of tuna, stock assessment is carried out via a quantitative model that estimates the response of the stock to fishing. Much of the Precautionary Approach described in the Code of Conduct and the UN Fish Stocks Agreement is based on this approach. However, there is not sufficient information available to do this for all stocks of tuna, and so less quantitative approaches (which at the minimum may simply be observing no signs of the effect of fishing at existing levels, e.g. the average size of fish or the catch rates are the same as at the initiation of the fishery) may be used.
Uncertainty in the results of stock assessment arises from observation error and model errors. Observation error includes errors due erroneous measurements or sampling errors. Model error may arise because of limitations in knowledge of the biology of the fish. Biological inputs to models include growth, mortality, longevity, reproduction, movements, stock structure, behaviour, and response to environmental conditions. Model error will also arise from an inability to model all the processes that affect the dynamics of a fish stock. Some of the uncertainty can be detected or measured through sampling theory, by lack of fit of the models, or by sensitivity analyses, and some may have to be assessed subjectively.
In addition, uncertainties associated with the implementation of management action must be considered when providing advice and assessments of associated risk.
Uncertainties may also arise from the use of new fishing technologies, whose effects could include changing the catchability of tunas, modifying the age distribution of those caught or killed, or modifying the environment and life history, which has been suggested as a possible effect of fish-aggregating devices (FADs).
Tunas and tuna-like fishes exhibit several specialized biological characteristics that present challenges to stock assessment and application of the Precautionary Approach.
Tunas are pelagic fishes inhabiting a wide range of ecosystems in the upper layers of all oceans. Most of them are confined to tropical waters, where they live primarily above the thermocline in the upper 200 m of the water column. Although most tunas are hatched in tropical waters, the distribution of the adult feeding grounds varies among species-some are predominantly tropical in distribution, while others occur in temperate or subtropical waters.
To varying degrees, all tunas can thermoregulate, using a specialised countercurrent heat exchange system called a rete mirabile. The relative development of the rete determines how much each species can regulate its body temperature relative to the ambient water temperature. For example, yellowfin tuna, which are predominantly tropical in distribution, have a much less advanced rete than do bluefin tunas, which range from tropical to sub-polar regions. Using behavioural and physiological thermoregulation, bigeye and the bluefin tunas can also extend their vertical ranges to well below the epipelagic zone, diving to depths in excess of 500 m. In contrast yellowfin, lacking the advanced rete, are confined largely to the upper 200 m of the water column.
Tunas exhibit a strong schooling behaviour (at least in the earlier stages of their life, until maturity), a large spawning potential, and very rapid growth, especially in their juvenile phase.
Billfishes are also pelagic fish, inhabiting tropical and temperate waters. Swordfish, however, use an advanced thermoregulation system to make extended dives to depths of more than 800 m). They also make seasonal migrations in temperate latitudes, extending their range further into temperate waters as they grow larger.
As adults, all tunas and tuna-like fishes are high-level apex predators. However, their highly versatile and opportunistic feeding behaviour and ontogenetic changes in their feeding habits mean that over their life history tunas and tuna-like fishes occupy more trophic levels than almost any other group of fishes. Their high trophic level may have important implications on the ecosystem stability as a whole, should the levels of tunas and tuna-like fishes biomass change as a result of fisheries or environmental variation.
For most widely-distributed species, there are data that suggest genetic heterogeneity among ocean basins. However, there is little evidence to suggest that biological characteristics, such as growth, movements, natural mortality, longevity, spawning conditions, schooling and feeding behaviour, or environmental preferences, vary significantly among oceans. Although this may reflect the maturity of our understanding of many of these characters (i.e. not enough is known to make objective comparisons), it seems reasonable to assume that observed responses of a species to exploitation in one ocean are likely to be similar to those in other oceans.
All of the major market species of tuna spawn in warm waters. In broad areas of the equatorial and tropical oceans, it is common for the larvae of several species of tuna to be caught in single plankton tows. This sympatry breaks down as the fish reach the end of their first year of life.
The tunas can be separated into three general groups based on their distribution, general biological characteristics, and habitat.
Tropical tunas: Skipjack and yellowfin spend their entire lives in tropical waters, or tropical water transported into temperate latitudes by currents. They are characterised by small to medium maximum size, poorly-developed retes, rapid growth, early age at first maturity, year-round spawning, short to medium life spans (<10 years), and high production to biomass (P:B) ratios.
Temperate tunas: Northern and southern bluefin can be classified as temperate species. From as early as the end of their first year of life they spend the majority of their life feeding in temperate latitudes, but return to warm waters to spawn. They are characterised by large maximum size, well-developed retes, slow growth, late age at first maturity, seasonal spawning, long life spans (>15 years), and low P:B ratios.
Subtropical tunas: Bigeye and albacore tunas have several characteristics that suggest that they are intermediate between the tropical and temperate tunas. Although they are widely distributed in tropical waters as adults, both are also widely distributed in subtropical and temperate waters. Albacore are anti-equatorial in the Pacific and Atlantic Oceans. Both bigeye and albacore are characterised by intermediate maximum size, have moderately developed retes, relatively fast growth, first maturity at 2 or 3 years of age, seasonal spawning, intermediate life spans (10-15 years), and P:B ratios between those of yellowfin and the bluefins.
Despite considerable research on the genetics of tunas, using a wide range of methods, there is little evidence in most tuna species for genetic heterogeneity within oceans. The exceptions are yellowfin and albacore in the Pacific Ocean and bluefin and albacore in the Atlantic Ocean. The lack of evidence for genetic heterogeneity within oceans is generally thought to be a reflection of genetically-significant exchange rates (i.e. adequate exchange across ocean basins occurs in each generation to swamp genetic divergence). It is important to note that genetically-significant exchange rates can be in the order of a few individuals per generation. Thus, in the context of fisheries assessments and management, the lack of genetic evidence for spatial structure is uninformative, and should not be used as a basis for assuming fish are homogenous across an ocean basin.
Mixing rates within and among areas of interest (e.g. ocean basins, management areas, etc.) are more important in the assessment and management contexts. With the possible exception of the skipjack and yellowfin populations in the Pacific, we have inadequate understanding of mixing patterns and rates in tunas.
The fact that most species are broadly distributed has led some tuna scientists to hypothesize the presence of unexploited or lightly exploited areas. These areas, linked with a relatively high viscosity of the resources, may lead to a permanent unavailable fraction of the biomass (for example, in the Arabian Sea or the eastern Indian Ocean until recently).
Spatial distribution
The populations of tunas and tuna-like fishes in all oceans occupy extensive areas (millions of square miles). Skipjack, yellowfin, and bigeye are ubiquitous in tropical oceans, and bigeye also occur in temperate waters. Albacore and bluefin spend most of their lives in temperate waters, but go to warmer waters to spawn. Swordfish are cosmopolitan and widely distributed in the oceans, and the adults typically migrate to temperate waters in the summer. Marlins are found throughout tropical and temperate waters, with seasonal latitudinal movements into temperate waters. Sailfish are distributed around tropical and subtropical waters, mainly in coastal waters, but also in oceanic waters.
Movements
Tagging studies have shown that all major species of tunas and tuna-like fishes are capable of large-scale movements (>1000 nm). The general patterns of movement vary significantly among species, and within species among different stages of the life history. For example, bluefin tunas are characterised throughout life by long-distance, directed migrations, while the movements of skipjack, despite their capacity to swim long distances, are usually more limited.
Most of what is known about the movements and migrations of tunas and tuna-like fishes is derived from conventional tagging and recapture experiments. As the distribution of releases and returns in most tagging experiments are restricted to areas in which commercial fisheries operate, these studies most often describe movements within or among fishing grounds, which are unlikely to be representative of the true range of movements or migrations of a species or population. Recent applications of archival and pop-up tag technology in tunas and tuna-like fishes have demonstrated some previously-undescribed patterns of movement and migration in these species, in particular cyclic migrations and seasonal movements into unfished areas.
Schooling behaviour
In the earlier stages of their life all tunas exhibit strong schooling behaviour. Schooling is less prevalent with increasing size in most species, except during spawning. Schooling fidelity is poorly understood in all species. Tagging studies have shown that individual skipjack tagged in the same school are often recaptured in separate schools shortly thereafter. However, tag returns for other species indicate that individuals may remain associated for periods of months to years. The relationship between abundance and school size may be particularly important to assessments that use catch rates.
Juvenile tropical tunas of different species, but the same size, are often caught in the same schools, but at larger sizes this is less frequent. The link between size and the extent of association among tuna species is very evident in the catches around FADs. A vertical heterogeneity (with juveniles on the top and adults at the bottom of the school) is also frequently observed on both free-swimming and FAD-associated schools). Little information exists on the schooling behaviour of billfishes, although it has been observed that sailfish may occur in small groups.
Horizontal and vertical spatial heterogeneity
Both horizontal and vertical heterogeneityin the sizes of tunas have been observed. The surface fisheries that catch juveniles in surface waters, and the longline fisheries that exploit adults well below the surface, do not interact with one another simultaneously. Vertical heterogeneity among stocks may be important to consider in dealing with uncertainty.
In general, despite assumptions of stability, few data exist on the variability of the biology of tunas or billfishes over time, or the extent to which key parameters vary geographically. Recent work on southern bluefin has demonstrated that the growth of juveniles has varied over a 30-year period, as a result of either environmental or density-dependent factors. Whether the same is true for other species should be investigated.
Longevity and natural mortality
Longevity varies significantly among the tuna species (Table 1). In many cases longevity has been estimated from tag return data.
Recent analyses of tagging data have provided evidence for large variations in natural mortality with size, the rates for juveniles being much higher (5 to 10 fold) than those for adults (Table 1). Estimates from tagging data have also indicated that the natural mortality is higher for the oldest fish of several species.
Estimates of the longevity of billfishes vary among species, although few age estimates are available. Little information is available on their natural mortality.
Growth
Growth is also highly variable among species (Table 1) - rapid for tropical tunas (skipjack and yellowfin), intermediate for bigeye and albacore, and slow for temperate tunas (northern and southern bluefin). Density-dependent effects seem to be common. Two-stanza growths have been observed for some species, from both otolith and tagging data, in the Atlantic and Pacific Oceans. Marlins and sailfishes have relatively rapid growth as juveniles (close to those of the tropical tunas), while swordfish grow more slowly, resembling the temperate tunas in this respect. Some tunas and tuna-like fishes, such as yellowfin, bigeye, and swordfish, have sex ratios that differ from 1:1 at large sizes, which could be due to differential growth or mortality.
Spawning and reproduction
High fecundity is characteristic of all tunas and tuna-like fishes, with females spawning several million eggs per year. All of the major market species of tuna spawn in warm waters. Tropical tunas spawn over wide areas, while bluefin have discrete spawning grounds in one to two relatively restricted areas.
Age and size at first maturity are variable (Table 1), from 1.5 year (45 cm) for skipjack to 12 years (147 cm) for southern bluefin. Spawning may occur throughout the year (skipjack) or during a limited period (2 months) for bluefin, with an intermediate situation for other tunas. Billfish spawning occurs in the warm tropical and subtropical waters throughout the year, with some seasonality at higher latitudes.
Recruitment
The spawner-recruit (S/R) relationship is generally poorly known, as estimates of the recruitment and spawning biomass are derived from catch data, which are not well suited for this purpose. For tropical tunas, the absolute levels of recruitment tend to be high, with relatively low variability among years (for instance in a 1:3 ratio between the highest and lowest recruitments). For the temperate tunas, the absolute recruitment levels tend to be lower. Long-term changes, such as cyclical (decadal) fluctuations and semi-cyclical (El Niño-Southern Oscillation (ENSO) events), due to environmental effects, have been often shown to influence the recruitment of both tropical and temperate tunas.
All regional tuna agencies represented at the meeting, except for the Secretariat for the Pacific Community (SPC), have management mandates. It should be noted, however, that a management arrangement for western and central Pacific tuna fisheries was being developed, with the Convention expected to be available for signature in August 2000.
The mandates and management strategies define the scope of the research and monitoring needed to address the Precautionary Approach for tunas and tuna-like fishes. The research includes stock assessment, ecological and biological characterizations of the fish and their environment, and the data collection and monitoring that are needed within that research to evaluating the expected performance of management strategies. If research is to help management in implementing the Precautionary Approach, then the management objectives (conservation constraints and targets) must be characterized adequately.
There are institutional differences in management objectives. For the Commission for the Conservation of Southern Bluefin Tuna (CCSBT) and Indian Ocean Tuna Commission (IOTC) the overall goals are to achieve "optimum utilization" and conserve the stocks. In a draft convention for the western central Pacific the goal is to achieve long-term conservation and sustainable use of the stocks. For both the Inter-American Tropical Tuna Commission (IATTC) and ICCAT, the stated general objective is to maintain stocks at levels that permit the maximum sustainable catch (yield). Within the constraints of their general goals, the agencies can also implement specific management plans to achieve shorter-term objectives. For example, both CCSBT and ICCAT have introduced plans to rebuild various stocks to given relative biomass levels by given years.
Table: Life history traits of selected tunas and tuna-like fishes
|
Species |
Spawning duration |
Length at maturity |
Weight at maturity |
Age at maturity |
Max length |
Max weight |
Max age |
Juvenile growth (%L¥-1) |
M Juveniles |
M Adults |
Min. SST |
|
Yellowfin |
6 |
105 |
25 |
2.8 |
170 |
176 |
10 |
22.1 |
3.5 (30-40) |
0.6 (60-70) |
18 |
|
Bigeye |
3 |
115 |
31 |
3.5 |
180 |
225 |
15 |
8.3 |
4.3 (<40) |
0.2 |
16 |
|
Skipjack |
12 |
45 |
1.7 |
1.5 |
75 |
23 |
4-5 |
40 |
9.0(<30) |
1.5 (50-60) |
20 |
|
Albacore |
3 |
90 |
15 |
4.5 |
120 |
80 |
10 |
16.7 |
|
|
15 |
|
Atlantic bluefin |
1.5 |
115 |
27.5 |
4.5 |
295 |
685 |
20 |
8.7 |
|
|
11 |
|
Southern bluefin |
6 |
147 |
65 |
8-12 |
220 |
220 |
22-40 |
5.6 |
0.22-0.29 |
0.1 |
9 |
Table: Life history traits of selected tunas and tuna-like fishes (continued)
|
Species |
Spawning duration |
Length at maturity |
Weight at maturity |
Age at maturity |
Max length |
Max weight |
Max age |
Juvenile growth (%L¥-1) |
M Juveniles |
M Adults |
Min. SST (oC) |
|
Atlantic little tuna |
12 |
42 |
- |
1.5 |
85 |
12 |
6 |
32.9 |
|
|
18 |
|
Swordfish |
3 |
175 |
70 |
5 |
290 |
650 |
17 |
12.1 |
|
|
15 |
|
Atlantic white marlin |
4 |
130 |
20 |
3 |
260 |
- |
15 |
16.7 |
|
|
20 |
|
Atlantic sailfish |
2 |
130 |
16 |
3 |
255 |
- |
18 |
17 |
|
|
22 |
Spawning duration: number of months per year during which spawning occurs.
Length, weight, and age at maturity: from literature.
Maximum length, weight and age: maximum length and weight from catch-at-length data collected during historical period (99% of distribution); maximum age from tagging (time at liberty + estimated age at release). (For southern bluefin the maximum age was estimated using hard parts, the maximum observed from tagging is 22 years).
Juvenile growth: means juvenile growth as a percentage of the maximum length (100*[length at maturity/maximum length]/age at maturity;
M for yellowfin, skipjack and bigeye from tagging by the SPC.
Minimum SST: from literature.
The focus of the Consultation was on the implementation of the Precautionary Approach in the context of international tuna management and RFBs. There may be other areas of interest or requirements for management at more local levels.
It is beyond the scope of this Consultation to review the frameworks that have been proposed by various international or national agencies for the implementation of the Precautionary Approach. Of more direct relevance to our Terms of Reference is the fact that all such agencies have highlighted the need for strengthened research due to either one of the two reasons. First, the international agreements call for increased monitoring and research in cases of large uncertainty (or data-poor situations). Second, as the Precautionary Approach calls for increased conservatism in the face of higher uncertainty, there are obvious benefits to be gained by investing in research that results in cost-effective reductions in uncertainty.
For example, it was concluded in the Report of the Eleventh Meeting of the Standing Committee on Tuna and Billfish (Honolulu, Hawaii, May 28-June 6, 1998) that: "The current levels of funding support for data collection, research and stock assessment of tuna fisheries of the western central Pacific are insufficient (< 1% of the value of the catch). Funding support will need to be increased substantially to allow management of this valuable fishery to be guided by good science.... One response to [this] uncertainty is to reduce catch or fishing effort in order to reduce the risk that a limit or target reference point is exceeded. A second, longer-term response to uncertainty is to reduce it by investment in carefully targeted data collection and research." Similarly, it was stated in the Report of the Meeting of the ICCAT Ad hoc Working Group on the Precautionary Approach (Dublin, Ireland, May 17-21, 1999) that: "Information underpins the precautionary approach, given that more caution is required when information is uncertain. In order to achieve an adequate balance between resource usage and precaution, increased funding at all levels (data collection, analysis, monitoring and enforcement) may be necessary," and went on to provide specific recommendations aimed at improving the data or knowledge base used in its stock assessments.
This section provides definitions of some key terms as they are used in this report. The list of definitions is not meant to be exhaustive.
Reference points are benchmarks against which an estimate about the stock can be measured, in order to determine its status and guide fisheries management. Two types of reference points are identified in the UN Fish Stocks Agreement. "Limit reference points set boundaries which are intended to constrain harvesting within safe biological limits within which the stocks can produce maximum sustainable yield. Target reference points are intended to meet management objectives."
Stock status is typically determined relative to two dimensions. One, is on a scale related to an overfished state, which means that the biomass (or an appropriate index of biomass) is "too low," i.e. below the biomass limit reference point that has been set as a benchmark to define the state of being overfished. The other dimension is on a scale related to the act of overfishing, which means that the fishing mortality rate (or an appropriate index of it) is "too high" relative to the relevant fishing mortality reference point. For a healthy stock, continued overfishing is expected to result eventually in an overfished state. For an overfished stock, continued overfishing represents an even greater risk of stock collapse.
The concept of risk due to uncertainty is fundamental to the implementation of the Precautionary Approach for fisheries management. In the context of the Precautionary Approach, risk has been as the "probability of something bad happening." In its guidelines for the application of reference points, the UN Fish Stocks Agreement states that "Fishery management strategies shall ensure that the risk of exceeding limit reference points is very low." An element that is shared by the various agencies that are developing frameworks for the application of the Precautionary Approach is that the definition of what "very low" means should be provided by fishery managers. However, it should be noted that, depending on the limit being considered and the life history characteristics of the stock in question, the biological consequences of exceeding the limit by a given amount may differ substantially. For this reason, the various agencies also consider that it must be scientists who provide the managers with adequate information so that they can, in turn, make informed decisions about acceptable levels of biological risk.
A management strategy encompasses the entire set of actions designed to achieve the goals of management. According to the UN Fish Stocks Agreement, "Fishery management strategies shall ensure that the risk of exceeding limit reference points is very low. If a stock falls below a limit reference point or is at risk of falling below such a reference point, conservation and management action should be initiated to facilitate stock recovery. Fishery management strategies shall ensure that target reference points are not exceeded on average." A control rule is a way of expressing actions to be taken under a management strategy, depending on the status of the stock. A type of control rule that has been proposed incorporates limit and target reference points into a scheme that shows how fishing mortality rates should be set, depending on the estimated biomass level. It should be noted, however, that the actual implementation of management actions is not achieved by setting a fishing mortality rate, but rather through fishery controls, such as effort limitation, catch quotas, closed areas or seasons, etc. Uncertainty is present in each of the many steps involved between the design of a management strategy and its actual implementation.
Research into issues related to the management of tunas and associated species was considered under four headings, stock assessment, biological and environmental research, and data collection and fisheries statistics. Most of the implications of technology research are also issues for the other three areas, so they are included in the chapters dealing with stock assessment, biological and environmental research, and data collection and fisheries statistics. Stock assessment is treated first, as it is the theme most directly linked to management. Biological and environmental research, and data collection and statistics support stock assessment. To some extent the scope of work in each area depends on requirements from those considered previously in the report.
