Effects of biological,economic and management factors on tuna and billfish stock status

Commercial tunas and billfishes (swordfish, marlins and sailfish) provide considerable catches and income in both developed and developing countries. These stocks vary in status from lightly exploited to rebuilding to severely depleted. Previous studies suggested that this variability could result from differences in life‐history characteristics and economic incentives, but differences in exploitation histories and management measures also have a strong effect on current stock status. Although the status (biomass and fishing mortality rate) of major tuna and billfish stocks is well documented, the effect of these diverse factors on current stock status and the effect of management measures in rebuilding stocks have not been analysed at the global level. Here, we show that, particularly for tunas, stocks were more depleted if they had high commercial value, were long‐lived species, had small pre‐fishing biomass and were subject to intense fishing pressure for a long time. In addition, implementing and enforcing total allowable catches (TACs) had the strongest positive influence on rebuilding overfished tuna and billfish stocks. Other control rules such as minimum size regulations or seasonal closures were also important in reducing fishing pressure, but stocks under TAC implementations showed the fastest increase of biomass. Lessons learned from this study can be applied in managing large industrial fisheries around the world. In particular, tuna regional fisheries management organizations should consider the relative effectiveness of management measures observed in this study for rebuilding depleted large pelagic stocks.     

A synthetic control approach to estimate the effect of total allowable catches in the high seas

Total allowable catch restrictions (hereafter referred to as catch quotas) play an important role in maintaining healthy fish stocks. While studies have identified a positive relationship between catch quota implementation and improved stock status, these methods are subject to selection bias as catch quotas are typically applied to stocks that are depleted. We address this challenge using the synthetic control method, which estimates the causal effect of catch quotas on fishing mortality and biomass by predicting a synthetic counterfactual outcome. We focus on high seas stocks (tunas, billfishes, and sharks) managed by tuna Regional Fisheries Management Organizations (tRFMOs), first providing an overview of stock status and current management measures in place. We find that implementation of catch quotas by tRFMOs has more than doubled over the past decade. Second, we predict the hypothetical fishing mortality and biomass trajectory for seven high seas quota-managed stocks in absence of a catch quota. These “synthetic non-quota stocks” are predicted using a weighted selection of high seas non-quota stocks. Credibility of the synthetic non-quota stocks is evaluated through diagnostic checks, and robustness tests assess sensitivity to study design. Five credible fishing mortality synthetic controls are predicted: three add support to the hypothesis that catch quotas successfully reduce fishing mortality, while two find that catch quotas increase fishing mortality. While our analysis is limited in scope, given that all seven quota-managed stocks are managed under a single tRFMO, we highlight the potential for the synthetic control method in fisheries management evaluation.     

Sea cucumber fisheries: global analysis of stocks,management measures and drivers of overfishing

Worldwide, most sea cucumber fisheries are ineffectively managed, leading to declining stocks and potentially eroding the resilience of fisheries. We analyse trends in catches, fishery status, fishing participation and regulatory measures among 77 sea cucumber fisheries through data from recent fishery reports and fishery managers. Critical gaps in fisheries biology knowledge of even commonly targeted species undermine the expected success of management strategies. Most tropical fisheries are small‐scale, older and typified by numerous (>8) species, whereas temperate fisheries are often emerging, mono‐specific and industrialized. Fisher participation data indicated about 3 million sea cucumber fishers worldwide. Fisher participation rates were significantly related to the average annual yield. permanova analysis showed that over‐exploited and depleted fisheries employed different sets of measures than fisheries with healthier stocks, and a non‐metric multidimensional scaling ordination illustrated that a broad set of regulatory measures typified sustainable fisheries. SIMPER and regression tree analyses identified that the dissimilarity was most related to enforcement capacity, number of species harvested, fleet (vessel) controls, limited entry controls and rotational closures. The national Human Development Index was significantly lower in countries with over‐exploited and depleted fisheries. Where possible, managers should limit the number of fishers and vessel size and establish short lists of permissible commercial species in multispecies fisheries. Our findings emphasize an imperative to support the enforcement capacity in low‐income countries, in which risk of biodiversity loss is exceptionally high. Solutions for greater resilience of sea cucumber stocks must be embedded within those for poverty reduction and alternative livelihood options.     

Density dependence in total egg production per spawner for marine fish

A paradigm of fisheries science holds that spawning stock biomass (SSB) is directly proportional to total egg production (TEP) of fish stocks. This “SSB–TEP proportionality” paradigm has been a basic premise underlying the spawner–recruitment models for fisheries management and numerous studies on recruitment mechanisms of fish. Studies on maternal effects on reproductive potential of a stock have progressed during the last few decades, leading to doubt concerning the paradigm. Nonetheless, a direct test of the paradigm at multidecadal scales has been difficult because of data limitations in the stock assessment systems worldwide. Here, we tested the paradigm for marine fish based on a novel combination of two independent 38‐year time series: fishery‐dependent stock assessment data and fishery‐independent egg survey data. Through this approach, we show that the SSB–TEP proportionality is distorted by density dependence in total egg production per spawner individual (TEPPS) or spawner unit weight (TEPPSW) at a multidecadal scale. The TEPPS/TEPPSW exponentially declined with biomass and thus was density‐dependent for Japanese sardine, a small pelagic species exhibiting a high level of population fluctuation, in the western North Pacific. By contrast, the TEPPS/TEPPSW was sardine‐density‐dependent for Japanese anchovy, another small pelagic species exhibiting a moderate level of population fluctuation well‐known for being out of phase with sardine. Our analysis revealed intraspecific (sardine) and interspecific (anchovy) density dependence in TEPPS/TEPPSW, which was previously unaccounted for in spawner–recruitment relationships. Such density‐dependent effects at the time of spawning should be considered in fisheries management and studies on recruitment mechanisms.     

Checking the pulse of the major commercial fisheries of lake Victoria Kenya,for sustainable management

The present study demonstrates the declining state of the major commercial fisheries of Lake Victoria, Kenya, a situation threatening sustainability of the lake's fishery. Data in the present study were derived from resource monitoring programmes that included hydro‐acoustics (2009–2018), trawl net fishing (2011–2018), frame surveys (2000–2016) and catch assessment surveys (2000–2015). The activities provided information on fish stocks and supported advice for fisheries management. The average fish stock densities for Tanzania, Uganda and Kenya of 8.92, 8.25 and 8.19 t/km², respectively, were relatively similar. Diplomatic and sustainable efforts for harmony in fish harvesting among the fishers of the riparian countries are encouraged given the interdependence of the lake. The Kenyan and River Kagera regions had a higher proportion (≈ 4% each) of big‐sized Nile perch (≥50 cm total length) in 2018, signifying the critical breeding areas for Nile perch. To sustain the fishery, there is need to enforce a 36%–44% effort reduction for all the major fisheries, and enforcement of gear limits to avoid harvesting of immature fish and destruction of the lake ecosystem.     

Forage fish fisheries management requires a tailored approach to balance trade‐offs

Ecosystem‐based fishery management requires considering the effects of actions on social, natural and economic systems. These considerations are important for forage fish fisheries, because these species provide ecosystem services as a key prey in food webs and support valuable commercial fisheries. Forage fish stocks fluctuate naturally, and fishing may make these fluctuations more pronounced, yet harvest strategies intended to ameliorate these effects might adversely affect fisheries and communities. Here, we evaluate trade‐offs among a diverse suite of management objectives by simulating outcomes from several harvest strategies on forage fish species. We demonstrate that some trade‐offs (like those between catches and minimizing collapse length) were universal among forage species and could not be eliminated by the use of different control rules. We also demonstrate that trade‐offs vary among forage fish species, with strong trade‐offs between stable, high catches and high‐biomass periods (“bonanzas”) for menhaden‐ and anchovy‐like fish, and counterintuitive trade‐offs for sardine‐like fish between shorter collapses and longer bonanzas. We find that harvest strategies designed to maintain stability in catches will result in more severe collapses. Finally, we show that the ability of assessments to detect rapid changes in population status greatly affects control rule performance and the degree and type of trade‐offs, increasing the risk and severity of collapses and reducing catches. Together, these results demonstrate that while default harvest strategies are useful in data‐poor situations, management strategy evaluations that are tailored to specific forage fish may better balance trade‐offs.     

Broadbill swordfish: status of established fisheries and lessons for developing fisheries

Guidelines for the assessment and management of developing swordfish fisheries are derived through an examination of five swordfish fisheries. As they develop, swordfish fisheries may be inclined to local depletion around underwater features, such as seamounts and banks. Few nations have applied the precautionary approach in managing their developing swordfish fisheries. Without controls, swordfish fisheries expand geographically and fishing effort increases, often overshooting optimum levels. However, it is difficult to distinguish clear evidence of fishery collapse; modern longliners harvest widely distributed tuna and swordfish and they are able to relocate to distant areas or switch between target species in response to fluctuations in species abundance and price. Furthermore, the wide distribution of swordfish combined with year‐round spawning and high growth rates amongst juveniles probably contribute to the apparent resilience of swordfish stocks to intensive harvesting. Over half the world’s swordfish catch is taken as an incidental catch of longliners fishing for tuna. In several areas, such as the North Atlantic, catch quotas have sometimes caused tuna longline fishers to discard swordfish. Minimum size limits have also resulted in discarding of swordfish in tuna fisheries and in dedicated swordfish fisheries. In addition to weakening the effectiveness of those management measures, bycatch and discarding add to the complexities of managing swordfish fisheries and to uncertainties in assessing the stocks. Longliners that target swordfish often fish at high latitudes where interactions with marine wildlife, such as seabird, are generally more frequent than at low latitudes. Concern over incidental catches of marine wildlife and other species is becoming a driving force in the management of several swordfish fisheries. Fishery management organisations will need to implement management measures to protect non‐target species and gather reliable data and information on the situation by placing observers on boats fishing for swordfish.     

Co-management without involvement: the plight of fishing communities

This paper discusses the role of fishing communities in the stewardship of their adjacent fish resources, and the benefits associated with community participation in co‐management. Contrary to the view of most fisheries management agencies, local communities are able to design institutions that can successfully restore equity and limit access to the fishery. The dismissal of local concerns may be at the root of biological and social crises in fisheries, and the privatization of common fishing rights world‐wide through individual transferable quotas (ITQs) is contributing to these problems. Community involvement that is embedded into a network of management at larger spatial scales would allow fishing communities to regain some control over their livelihoods. Meaningful co‐management arrangements must go beyond consultation by redirecting the flow of social and economic benefits from the fishery back into communities. Unless geographically defined communities are allowed to share power and responsibility with government fisheries managers, both fish stocks and fishing as a way of life are in danger of vanishing.     

Rebuilding global fisheries: the World Summit Goal,costs and benefits

Many of the world’s fish stocks are depleted as a result of overexploitation, pollution and habitat loss. The 2002 World Summit on Sustainable Development (WSSD) sets a target for fisheries to maintain or restore stocks to levels that can produce the maximum sustainable yield (MSY) by 2015. We assessed the global stock status and found that 68% were at or above the MSY level in 2008 and that the 2015 target is unlikely to be met. We compiled data for eight indicators to evaluate the sustainability of fisheries and the gap to meet the WSSD target. These indicators show that the overall condition of global fisheries is declining, long‐term benefits are being compromised, and pressures on fisheries are increasing despite fisheries policy and management actions being taken by coastal States. We develop a bio‐economic model to estimate the costs and benefits of restoring overfished stocks. Our results show that the global fishing capacity needs to be cut by 36–43% from the 2008 level, resulting in the loss of employment of 12–15 million fishers and costing US$96–358 billion for buybacks. On the other hand, meeting the WSSD goal will increase annual fishery production by 16.5 million tonnes, annual rent by US$32 billion and improve biodiversity and functioning of marine ecosystems. However, progress towards rebuilding has been hindered by an unwillingness or inability to accept the short‐term socio‐economic consequences associated with rebuilding fisheries. Thus, there is a pressing need for integration of rebuilding plans into national political and economic decision‐making.     

An interview‐based approach assessing interactions between seals and small‐scale fisheries informs the conservation strategy of the endangered Mediterranean monk seal

1. Small‐scale fisheries may pose a serious threat to the conservation of marine mammals. At the same time various factors have led to the decline of small‐scale fisheries, often making them unsustainable. Current rates of biodiversity loss and the reduction of fish stocks and fisheries dictate a thorough understanding of fisheries‐related issues and the implementation of effective management actions.
2. The Mediterranean monk seal is one of the most endangered marine mammals on Earth; its survival in the eastern Mediterranean Sea is threatened by negative interactions with fisheries. A nationwide questionnaire survey among fishers and port police authorities was carried out in Greece to describe the main characteristics of small‐scale fisheries, and to understand the nature and assess the magnitude of negative interactions between the monk seal and these fisheries. Questionnaire information was verified by a second round of interviews during landings.
3. The main attributes of the fishers, their fishing boats, and their practices were characteristic of the small‐scale fisheries sector. Overfishing was considered the main reason for fish‐stock reduction, and negative interactions with marine mammals was considered the main issue for the fishing sector.
4. Monk seals were present, caused damage, and got accidentally entangled in fishing gear throughout Greece. Damage to fishing gear was recorded mainly during spring and summer, and on average affected 21% of all fishing trips and 1% of nets deployed during a fishing trip.
5. Based on these results, the implementation of general and specific nationwide fishery management and conservation actions are proposed. These actions are mainly aimed at improving fish stock status, changing the behaviour of the fishers, and mitigating seal–fishery interactions in Greece, while promoting the recovery of the Mediterranean monk seal in the eastern Mediterranean Sea.

     

Toward an environmental predictor of tuna recruitment

Bigeye tuna are of global economic importance and are the primary target species of Hawaii's most valuable commercial fishery. Due to their high commercial value, bigeye tuna are relatively well studied and routinely assessed. Larval and adult bigeye surveys have been conducted for many years and are supported by ongoing research on their physiology and life history. Yet, modeling stock dynamics and estimating future catch rates remain challenging. Here, we show that an appropriately lagged measure of phytoplankton size is a robust predictor of catch rates in Hawaii's bigeye tuna fishery with a forecast window of four years. We present a fishery‐independent tool with the potential to improve stock assessments, aid dynamic fisheries management, and allow Hawaii's commercial longline fishing industry to better plan for the future.     

A comparative review of fisheries management experiences in the European Union and in other countries worldwide: Iceland,Australia, and New Zealand

This study compares the details and performance of fisheries management between the EU and a selection of other countries worldwide: Iceland, New Zealand, and Australia, which are considered in many respects to be among the most advanced in the world in fisheries management. Fisheries management in the EU, Iceland, Australia, and New Zealand has developed following different paths, despite being based on similar instruments and principles. Iceland, Australia, and New Zealand have been at the forefront of developing management practices such as stakeholder involvement, legally binding management targets (Australia, New Zealand), individual transferable quotas, and discard bans (Iceland, New Zealand). The EU has since the beginning of the 21st century taken significant steps to better involve stakeholders and establish quantitative targets through management plans, and a landing obligation is gradually being implemented from 2015 onward. The management of domestic fisheries resources in Australia, New Zealand, and Iceland has, overall, performed better than in the EU, in terms of conservation and economic efficiency. It should, however, be stressed that, compared to Australia, New Zealand, and Iceland, (i) initial over‐capacity was more of an issue in the EU when management measures became legally binding and also that (ii) EU has been progressive in developing common enforcement standards, on stocks shared by sovereign nations. The situation of EU fisheries has substantially improved over the period 2004–2013 in the northeast Atlantic, with fishery status getting close to that in the other jurisdictions, but the lack of recovery for Mediterranean fish stocks remains a concern.     

Dynamic angling effort influences the value of minimum‐length limits to prevent recruitment overfishing

Recruitment overfishing occurs when stocks are fished to a level where recruitment declines proportionally with adult abundance. Although typically considered a commercial fishery problem, recruitment overfishing can also occur in freshwater recreational fisheries. This study developed an age‐structured model to determine if minimum‐length limits can prevent recruitment overfishing in black crappie, Pomoxis nigromaculatus (LeSueur), and walleye, Sander vitreus (Mitchill) fisheries considering angling effort response to changes in fish abundance. Simulations showed that minimum‐length limits prevented recruitment overfishing of black crappie and walleye, but larger minimum‐length limits were required if angler effort showed only weak responses to changes in fish abundance. Low angler‐effort responsiveness caused fishing mortality rates to remain high when stock abundance declined. By contrast, at high effort responsiveness, anglers left the fishery in response to stock declines and allowed stocks to recover. Angler effort for black crappie and walleye fisheries suggested that angler effort could be highly responsive for some fisheries and relatively stable for others, thereby increasing the risk of recruitment overfishing in real fisheries. Recruitment overfishing should be considered seriously in freshwater recreational fisheries, and more studies are needed to evaluate the responsiveness of angler effort to changes in fish abundance.     

Integrating recreational fisheries data into stock assessment: implications for model performance and subsequent harvest strategies

Understanding the impacts of recreational fishing on commercially fished stocks is becoming increasingly relevant for fisheries managers. However, data from recreational fisheries are not commonly included in stock assessments of commercially fished stocks. Simulation models of two assessment methods employed in Australia's Commonwealth fisheries were used to explore how recreational fishery data can be included, and the likely consequences for management. In a data‐poor management strategy for blue eye trevalla, Hyperoglyphe antarctica (Carmichael), temporal trends in recreational catch most affected management outcomes. In a data‐rich age‐structured stock assessment for striped marlin, Kajikia audax (Philippi), estimates of stock status were biased when recreational catches were large or when the recreational fishery targeted different size classes than the commercial fishery and these data were not integrated into the assessment. Including data from recreational fishing can change perceptions of stock status and impact recommendations for harvest strategies and management action. An understanding of recreational fishery dynamics should be prioritised for some species.     

A scientific alternative to moratoria for rebuilding depleted international tuna stocks

There is considerable international concern and scientific debate about the current state and future of tuna stocks worldwide and the capacity of Regional Fisheries Management Organisations to manage the associated fisheries effectively. In some cases, this concern has extended to predictions of imminent collapse with minimal chances of recovery, even under a commercial catch moratorium. As a viable alternative to a full fishery closure, the Commission for the Conservation of Southern Bluefin Tuna (CCSBT) has adopted a scientifically tested, adaptive rebuilding strategy for the depleted southern bluefin tuna (Thunnus maccoyii) stock. The management procedure (MP) adopted involves a harvest control rule that fully specifies the total allowable catch as a function of key indicators of stock status, adjusting future harvest levels every three years so as to meet the rebuilding targets agreed by CCSBT. It was chosen from a subset of candidate MPs selected following extensive simulation testing. This involved first selecting a wide range of plausible scenarios for stock status and input data, ranging from pessimistic to optimistic, against which the alternative candidate MPs were tested to ensure that they were robust to important uncertainties. This is the first time that a comprehensively evaluated MP has been adopted for an internationally managed tuna stock. Both the process and the outcomes have broad applicability to other internationally managed stocks.     

Report card on ecosystem‐based fisheries management in tuna regional fisheries management organizations

International instruments of fisheries governance have set the core principles for the management of highly migratory fishes. We evaluated the progress of tuna Regional Fisheries Management Organizations (tRFMOs) in implementing the ecological component of ecosystem‐based fisheries management (EBFM). We first developed a best case tRFMO for EBFM implementation. Second, we developed criteria to evaluate progress in applying EBFM against this best case tRFMO. We assessed progress of the following four ecological components: target species, bycatch species, ecosystem properties and trophic relationships, and habitats. We found that many of the elements necessary for an operational EBFM are already present, yet they have been implemented in an ad hoc way, without a long‐term vision and a formalized plan. Overall, tRFMOs have made considerable progress monitoring the impacts of fisheries on target species, moderate progress for bycatch species, and little progress for ecosystem properties and trophic relationships and habitats. The tRFMOs appear to be halfway towards implementing the ecological component of EBFM, yet it is clear that the “low‐hanging fruit” has been plucked and the more difficult, but surmountable, issues remain, notably the sustainable management of bycatch. All tRFMOs share the same challenge of developing a formal mechanism to better integrate ecosystem science and advice into management decisions. We hope to further discussion across the tRFMOs to inform the development of operational EBFM plans.     

Guidelines for incorporating fish distribution shifts into a fisheries management context

The impacts of climate change have been demonstrated to influence fisheries resources. One way climate has affected fish stocks is via persistent shifts in spatio‐temporal distribution. Although examples of climate‐forced distribution shifts abound, it is unclear how these shifts are practically accounted for in the management of fish stocks. In particular, how can we take into account shifting stock distribution in the context of stock assessments and their management outputs? Here, we discuss examples of the types of fish stock distribution shifts that can occur. We then propose a decision tree framework of how shifting stock distributions can be addressed. Generally, the approaches for addressing such shifts fall into one of three main alternatives: re‐evaluate stock identification, re‐evaluate a stock unit area, or implement spatially explicit modelling. We conclude by asserting that the approach recommended here is feasible with existing information and as such fisheries managers should be able to begin addressing the role of changes in stock distribution in these fish stocks. The implications of not doing so could be notably undesirable.     

Shrinking baseline: the growth in juvenile fisheries,with the Hong Kong grouper fishery as a case study

Historic and current information on the grouper fishery from Hong Kong and adjacent waters reveals significant changes in species composition and fish sizes over the past 50 years in this important Asian centre for seafood consumption. Once dominant, large groupers are now rare and small species and sizes prevail in the present‐day fishery. Juveniles comprise over 80% of marketed fish by number among the most commonly retailed groupers, and reproductive‐sized fish are absent among larger species. Current fishery practices and the lack of management in Hong Kong and adjacent waters pose a significant threat to large species with limited geographic distribution such as Epinephelus akaara and Epinephelus bruneus, both now listed as threatened by the IUCN. The heavy reliance on juveniles, not only for groupers, but for an increasing diversity of desired fishes within Asia, potentially reduces stock spawning potential. The ‘shrinking baseline’ in terms of a progressive reduction in fish sizes being marketed in the region can seriously undermine fishery sustainability and recoverability of depleted fish stocks. Fishing pressure on groupers and other valuable food fishes within the Asia‐Pacific is intensifying, the declining long‐term trend of grouper landings in Hong Kong and the increasing focus on juveniles for immediate sale or for mariculture ‘grow‐out’ signal a worrying direction for regional fisheries. Moreover, the common appearance of small groupers for sale will influence public perception regarding what are ‘normal‐sized’ fish. Management attention is needed if these fisheries are to remain viable.     

Reducing sea turtle by-catch in pelagic longline fisheries

Reducing by‐catch of sea turtles in pelagic longline fisheries, in concert with activities to reduce other anthropogenic sources of mortality, may contribute to the recovery of marine turtle populations. Here, we review research on strategies to reduce sea turtle by‐catch. Due to the state of management regimes in most longline fisheries, strategies to reduce turtle interactions must not only be effective but also must be commercially viable. Because most research has been initiated only recently, many results are not yet peer‐reviewed, published or readily accessible. Moreover, most experiments have small sample sizes and have been conducted over only a few seasons in a small number of fisheries; many study designs preclude drawing conclusions about the independent effect of single factors on turtle by‐catch and target catch rates; and few studies consider effects on other by‐catch species. In the US North Atlantic longline swordfish fishery, 4.9‐cm wide circle hooks with fish bait significantly reduced sea turtle by‐catch rates and the proportion of hard‐shell turtles that swallowed hooks vs. being hooked in the mouth compared to 4.0‐cm wide J hooks with squid bait without compromising commercial viability for some target species. But these large circle hooks might not be effective or economically viable in other longline fisheries. The effectiveness and commercial viability of a turtle avoidance strategy may be fishery‐specific, depending on the size and species of turtles and target fish and other differences between fleets. Testing of turtle avoidance methods in individual fleets may therefore be necessary. It is a priority to conduct trials in longline fleets that set gear shallow, those overlapping the most threatened turtle populations and fleets overlapping high densities of turtles such as those fishing near breeding colonies. In addition to trials using large 4.9‐cm wide circle hooks in place of smaller J and Japan tuna hooks, other fishing strategies are under assessment. These include: (i) using small circle hooks (≤ 4.6‐cm narrowest width) in place of smaller J and Japan tuna hooks; (ii) setting gear below turtle‐abundant depths; (iii) single hooking fish bait vs. multiple hook threading; (iv) reducing gear soak time and retrieval during daytime; and (v) avoiding by‐catch hotspots through fleet communication programmes and area and seasonal closures.