Exotic species,fisheries and conservation of freshwater biodiversity in tropical Asia: the case of the Sepik River,Papua New Guinea

1. Development of effective conservation strategies for freshwater biodiversity must take account of the trade‐off between species preservation and human use of ecosystem goods and services. The latter cannot be prevented, and attempts to manage ecosystems that focus solely on maximizing biodiversity will fail. 2. A compromise position of management for ecosystem functioning and human livelihoods — rather than preservation of every species — will provide a better basis for biodiversity conservation. This has implications for the management of exotic species. 3. There are some situations, in lentic habitats in Sri Lanka, for example, where the establishment of exotics has increased fishery yields without apparent detriment to native biodiversity. The Sepik River in Papua New Guinea provides another illustration of potential compromises between human livelihoods and biodiversity conservation. 4. The Sepik supports a relatively unproductive fishery. Two fish stocking projects (in 1987–93 and 1993–97), representing a partnership between the Papua New Guinea Government and the United Nations (UNDP/FAO), led to the introduction of a suite of exotic fish into the Sepik. Species were selected on the basis of their potential to occupy niches not filled by native fish. Unfortunately, the outcomes of these introductions are poorly documented, although there is preliminary evidence both of increased human use of exotic fish as well as declines of some native species associated with the spread of exotics. 5. Better understanding of the results of the Sepik fish introductions is important, because the pressures of burgeoning human populations in most of tropical Asia make it impossible to preserve near‐pristine environments such as the Sepik. While attempts to conserve natural or near‐natural systems must remain a priority, there is a need to develop strategies for the management of damaged or degraded ecosystems, which may contain exotics, with the aim of maintaining ecosystem functioning and, if possible, maximizing the persistence of native biodiversity. Copyright © 2006 John Wiley & Sons, Ltd.     

Stocking strategies

Abstract  Stocking, transfer and introduction of fish are commonly used to mitigate loss of stocks, enhance recreational or commercial catches, restore fisheries or to create new fisheries. However, many stocking programmes are carried out without definition of objectives or evaluation of the potential or actual success of the exercise. This paper describes a strategic approach to stocking aimed at maximizing the potential benefits. A protocol is discussed which reviews factors such as source of fish, stocking density, age and size of fish at stocking, timing of stocking and mechanism of stocking. The potential genetic, ecological and environmental impacts of stocking are described.     

An appraisal of stocking strategies in the light of developing country constraints

Stocking, transfer and introduction of fish are commonly used to mitigate loss of stocks, enhance recreational or commercial catches, restore fisheries, or to create new fisheries. However, many stocking programmes are carried out without definition of objectives or evaluation of the potential or actual success of the exercise. The present paper describes a strategic approach to stocking aimed at maximizing the potential benefits. A protocol is discussed which reviews factors such as the source of the fish, the stocking density, the age and the size of fish at stocking, the timing of stocking, and the mechanism of stocking. Finally, the effects of social, cultural and economic constraints on the potential outcome of stock enhancement programmes are discussed.     

Fisheries management in inland and coastal waters in Denmark from 1987 to 1999

Fishing is a major recreational activity in Denmark, involving both inland and coastal waters. Anglers, aged 18–67, and amateur fishermen, aged 12–67, must hold a valid fishing permit. Fees are used for stocking, river restoration and fisheries research. All proposals for stocking inland waters require stocking plans based on the carrying capacity of the aquatic habitat in question. All stocking is undertaken using hatchery-reared fish that are the offspring, either of wild fish caught in nature for subsequent stripping, or of more or less domesticated fish stocks that have lived in captivity for several generations. Stocking is also subject to genetic guidelines. This paper reviews the status of fisheries in Danish inland waters, their regulation, socio-economic aspects, stocking, aquaculture and the main problems and trends.     

Factors contributing to the outcome of stocking programmes in Cuban reserviors

Abstract In less-developed countries, fisheries enhancement using stocking is a common practice to manage reservoirs for biomass production. In many cases, there appears to be little control over whether the stocking activity is appropriate or necessary. Cuban reservoir fisheries have been usually managed to maximize fish biomass. Blue tilapia have been introduced and supplemented in reservoirs, and Chinese carp are introduced and maintained in reservoirs throughout the entire island. Supplemental stocking with introduced fish species is a usual practice to manage large and medium-sized reservoirs. Small and very small reservoirs have been managed as semi-intensively enhanced fisheries with carp and tilapia stocking. The main purposes of the present paper are to study the factors contributing to the outcome of stocking programmes in reservoirs and to test the hypotheses that supplementary stocking for self-sustained fish populations usually cannot contribute to increase fish yield. Yield and stocking data by groups of fish species were available for Cuban reservoirs. Total fish yield was only slightly related to total fish stocking density for the complete set of reservoirs. However, this result was mainly because of the relationship between total yield and cyprinid stocking for semi-intensively-used reservoirs. Tilapia yield was not significantly related to tilapia stocking for both extensively and semi-intensively-used reservoirs. However, yield and stocking for cyprinids were highly related for both reservoir subsets. Therefore, cyprinid stocking was effective in certain reservoirs, but tilapia stocking was not effective in any reservoir. Moreover, tilapia stocking was a superfluous activity for reservoirs where tilapia have an adequate natural reproduction cycle.     

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.     

The potential for new Nephrops trawl designs to positively effect North Sea stocks of cod,haddock and whiting

Within commercial fisheries, particularly mixed fisheries, both target and non-target species are often discarded. Discarding represents a potentially significant loss to the productivity of fish stocks; it can have damaging ecological consequences, and is a potential cause of the failure of recovery plans. The Nephrops fishery in the North Sea is classified as a mixed fishery. Nephrops trawls are constructed with smaller meshes than trawls used to target whitefish; consequently, the bycatch of juvenile fish can be substantial. Several new Nephrops trawl designs have been tested in the North Sea. The data from these trials are used to investigate the potential impact of their implementation on cod, haddock and whiting stocks in the North Sea (including the Kattegat and Skagerrak).The model examines five trawl designs, and also the scenarios of a cessation of discarding in all North Sea fisheries and in just the Nephrops fishery. The model is deterministic, and evaluates the relative differences between scenarios assuming all other variables remain constant. If discarding of cod, haddock and whiting in the North Sea fisheries were eliminated, stocks would increase by 41%, 14% and 29%, respectively, within 10 years. Eliminating discarding in the Nephrops fishery alone would increase stocks by 2%, 1% and 13%, respectively, reflecting the relative proportion of catches of these species in the Nephrops fishery. For cod and haddock, the introduction of the Nephrops trawl with a grid with a square-mesh codend was the only scenario in which a notable increase in stock number was observed. This trawl design facilitates the escape of fish of all ages/sizes from the trawl, effectively making the Nephrops fishery a single-species fishery. For whiting, stock numbers and landings increased under all scenarios, but forecasted landings were lower than if current discard patterns continued in all except the no-discards scenario. The dependency of the results on the validity of the assumptions and on the accuracy of the input data is discussed.     

Factors influencing the scope and quality of science and management decisions

The lecture traces the historical path to overfishing of the world's fish and shellfish stocks, and provides an assessment of marine fish resources in the later half of the 1990s. The basis of overfishing as noted by various fishery scientists is reviewed. Four factors, including institutional paralysis, the rapidity of technological developments, uncertainty of science, and the inability to monitor and enforce regulations are identified as the major problems leading to overfishing. The failure of the world community to deal with extensive overfishing, appears to have motivated managers and scientists to promote a new fishery management paradigm that focuses on a broader set of problems resulting from fishing, and establishes a more conservative decision‐making process founded on precautionary principle and uncertainty. The author feels that the evolving paradigm will result in the rebuilding of a number of stocks in the United States, but is less certain of its adoption on a global scale, and whether or not science will play a more useful role in fisheries management. It is noted that the support for fisheries science and the status of fisheries have followed opposite courses. Over the past half century marine science has boomed, diversified and become intellectually and materially enriched, while the number of overfished stocks and ecological disasters has increased. Looking ahead it is expected that fisheries management will move into a more conservative era. The focus of fisheries has moved from full use of ocean resources to establishing yields that take into account the impacts of fisheries on target and non‐target species and the ecosystem in general. Although there has been wide‐spread abuse in the use of the world's fishery resources and condemnation of the fishing industries, the author feels that the government institutions must bear the primary responsibility for the historical course of fishery management and its failure.     

The response of a brown trout stocks and perception of anglers to cessation of brown trout stocking

Abstract Since the mid‐1980s, water quality has improved in German rivers, and fisheries have recovered. However, it is unsure whether the improvements are sufficient to maintain stocks and angling yield without supplementary stocking. This study examined the development in the local brown trout, Salmo trutta L., stocks and angling yield in the River Wutach following cessation of stocking in 2001, using electric fishing and an angler questionnaire. Natural reproduction was recorded in each year between 2001 and 2007, and a stable stock of trout >20 cm was found. Trout catches by anglers increased after stocking ceased and approximately 60% of the anglers were convinced that stocking was unnecessary to maintain the brown trout stocks or angler catch satisfaction.     

长江十年禁渔后保护与发展

长江是我国鱼类多样性最高的河流，同时也是我国最重要的淡水渔业资源产区。然而，近几十年来受水域污染、水工程建设、湖泊围垦、航运发展、过度捕捞和外来种入侵等多重因素的影响，长江流域生态系统面临着逐渐衰退的困境，主要表现为鱼类种类数减少、鱼类濒危物种程度加剧、鱼类资源小型化趋势明显、鱼类资源呈衰退趋势和外来物种种类增多等。长江十年禁渔，对恢复长江水生态环境和保护水域生物多样性提供了重要的机遇。作者建议统筹协调长江生态保护与渔业发展的关系，创新流域管理体制与机制，根据长江水生态系统结构与功能变化，适时调整服务目标，其目的旨在实现对长江流域水生态环境的保护和渔业资源的可持续发展提供科学依据。     

黄河小浪底至入海口渔业资源现状与保护对策研究

为了解黄河小浪底以下河段渔业资源现状,2010年5-9月对此河段的鱼类及其饵料资源进行了调查.结果表明,浮游植物密度为3.49×104个/L,现存量为0.0234 mg/L,浮游动物密度为526个/L,现存量为0.1245 mg/L,现存量较小.底栖动物23种(属),水生植物6种,现存量极少,物种多样性较低,鱼类种群数量减少.渔获物中,1+龄鱼占81.76％,平均体重150.3 g,呈现明显的低龄化和小型化趋势.鱼类区系组成简单,以鲤科鱼类为主,与30年前相比,鱼类种群结构变化明显.渔业资源衰退的主要原因为水利工程建设运营和人为干扰,并据此提出了鱼类资源保护和可持续利用的对策.     

The unfulfilled potential of fisheries selectivity to promote sustainability

The recent reform of the Common Fisheries Policy (CFP) in Europe highlights the need for improvements in both species and size selectivity. Regarding size selectivity, shifting selectivity towards older/larger fish avoids both growth and recruitment overfishing and reduces unwanted catches. However, the benefits to fish stocks and fishery yields from increasing age/size‐at‐selection are still being challenged and the relative importance of selectivity compared to that of exploitation rate remains unclear. Consequently, exploitation rate regulations continue to dominate management. Here, an age‐structured population model parameterized for a wide range of stocks is used to investigate the effects of selectivity on spawning stock biomass (SSB) and yield. The generic effect of selectivity on SSB and yield over a wide range of stocks is compared to the respective relative effects of exploitation rate and several biological parameters. We show that yield is mainly driven by biological parameters, while SSB is mostly affected by the exploitation regime (i.e. exploitation rate and selectivity). Our analysis highlights the importance of selectivity for fisheries sustainability. Catching fish a year or more after they mature combined with an intermediate exploitation rate (F ≈ 0.3) promotes high sustainable yields at low levels of stock depletion. Examination of the empirical exploitation regimes of 31 NE Atlantic stocks illustrates the unfulfilled potential of most stocks for higher sustainable yields due to high juvenile selection, thus underscoring the importance of protecting juveniles. Explicitly incorporating selectivity scenarios in fisheries advice would allow the identification of optimal exploitation regimes and benefit results‐based management.     

Feasibility of a single-species quota system for management of the Malaysian multispecies purse-seine fishery

Malaysian fisheries employ multiple measures to improve management; however, not all are well-suited to the multispecies fisheries. As part of a pilot project, an individual quota system was introduced for the purse-seine fishery off the East Coast of Peninsular Malaysia (ECPM), but no assessment of this particular measure nor the feasibly of its implementation has been confirmed. Therefore, this study analysed spatial and temporal patterns of purse-seine fishing, by collecting catch composition data per landing and its fishing ground within three period fishery surveys between August 2017 and September 2018 at six different landing sites. Similarity and cluster analysis examined species composition and diversity to determine the feasibility of implementing a single-species quota system in this multispecies fishery. Some overlapped of indices results and minor difference in catch composition were found due to changes in spatial and temporal fishing activities. However, no specific spatial or temporal patterns were discernible as structuring the fishing grounds used by purse-seiners. The absence of patterns, using the available data, might be attributable to huge species aggregations and widely distributed and homogenously mixed fish stocks. Thus, it is likely impractical to manage species individually in such a multispecies fishery.     

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.     

Wasted fishery resources: discarded by-catch in the USA

Fishery by‐catch, especially discarded by‐catch, is a serious problem in the world's oceans. Not only are the stocks of discarded species affected, but entire trophic webs and habitats may be disrupted at the ecosystem level. This paper reviews discarding in the marine fisheries of the USA; however, the type, diversity and regulatory mechanisms of the fisheries are similar to developed fisheries and management programmes throughout the world. We have compiled current estimates of discarded by‐catch for each major marine fishery in the USA using estimates from existing literature, both published and unpublished. We did not re‐estimate discards or discard rates from raw data, nor did we include data on protected species (turtles, mammals and birds) and so this study covers discarded by‐catch of finfish and fishable invertebrates. For some fisheries, additional calculations were required to transform number data into weight data, and typically length and weight composition data were used. Specific data for each fishery are referenced in Harrington et al. (Wasted Resources: Bycatch and discards in US Fisheries, Oceana, Washington, DC, 2005). Overall, our compiled estimates are that 1.06 million tonnes of fish were discarded and 3.7 million tonnes of fish were landed in USA marine fisheries in 2002. This amounts to a nationwide discard to landings ratio of 0.28, amongst the highest in the world. Regionally, the southeast had the largest discard to landings ratio (0.59), followed closely by the highly migratory species fisheries (0.52) and the northeast fisheries (0.49). The Alaskan and west coast fisheries had the lowest ratios (0.12 and 0.15 respectively). Shrimp fisheries in the southeast were the major contributors to the high discard rate in that region, with discard ratios of 4.56 (Gulf of Mexico) and 2.95 (South Atlantic). By‐catch and discarding is a major component of the impact of fisheries on marine ecosystems. There have been substantial efforts to reduce by‐catch in some fisheries, but broadly based programmes covering all fisheries are needed within the USA and around the world. In response to international agreements to improve fishery management, by‐catch and discard reduction must become a regular part of fishery management planning.     

Effects of fish stocking and fishing pressure on fish community structures in French lakes

Fish stocking and regulation of fishing activities are widely implemented by freshwater fisheries authorities who have to protect aquatic communities. The effects of fish management on fish communities have not received sufficient attention. As a result of two datasets, one from EU Water Framework Directive (WFD) monitoring surveys, and one from a French survey targeting angler's associations, this study investigated the impacts of both fishing intensity and fish stocking on fish communities in French lakes. Both co-inertia analysis and community size spectra analysis, showed that: (i) stocking does not contribute to the standing stocks for five of the six most stocked fish species in France; and (ii) neither fishing intensity nor fish stocking lead to a change in the proportion of large and small fish. The results support alternative methods being promoted to enhance native fish populations.     

Characteristics of spawning-run smelt, Osmerus eperlanus (L.), from a Scottish river, with recommendations for their conservation and management

Abstract. Overexploitation and pollution has led to the decline of Scotland's traditional smelt fisheries. The one remaining commercial fishery is on the River Cree in south-west Scotland and the characteristics of the spawning runs of the smelt, Osmerus eperlanus (L.), on which this fishery is based have been studied with a view to conservation and re-establishment of the species elsewhere in Scotland. The spawning-runs which started in early March when water temperatures were greater than 5°C were characterized by marked temporal changes in size, age and sex ratio. The fish enter the spawning stock at 1+ years of age and, in common with other estuarine smelt populations, are highly fecund. The spawning stock was composed mainly of two age-classes (1+ and 2+ years) and fish of the 1978 year-class dominated the spawning stock in 1980 and 1981. Spawning, which was interrupted during periods of high flow, lasted about 1 week. Male fish tended to remain on the spawning grounds after females had vacated the area. Recommendations for management and re-establishing smelt stocks are discussed.     

The decline of fisheries on the Madeira River,Brazil: The high cost of the hydroelectric dams in the Amazon Basin

Despite being considered beneficial by providing a clean and renewable source of energy, the construction of hydroelectric dams has extremely negative implications for Amazonian fisheries. This study investigated the impacts of the Santo Antônio and Jirau hydroelectric dams on the fishery stocks of the Madeira River. This investigation was based on fish catch data from the Z‐31 fishing colony, located in the municipality of Humaitá, in Amazonas State, Northern Brazil. Data were collected daily and provided information on the date of return from each trip, the fish species targeted, and the total catch (kg) between January 2002 and September 2017. The results indicated reductions of 39% in the mean annual catch and 34% in the mean monthly catches. These results highlight the high price paid by local fish communities for the development of hydroelectric power in the Amazon basin.     

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.     

Management effectiveness of large pelagic fisheries in the high seas

Large pelagic fishes are assessed and managed by tuna Regional Fisheries Management Organizations (tRFMOs). These organizations have been criticized for not meeting conservation objectives, which may relate to aspects of governance and management. No previous studies have systematically evaluated why management performance differs among tRFMOs and among stocks within each tRFMO. In this study, we collected data on the nature of research, management, enforcement and socioeconomics of management systems in the five principal tRFMOs of the world's oceans. We quantified influences of economic and fishery‐related factors on these management characteristics and examined how these factors vary among tRFMOs. We found that tRFMOs with a greater number of member countries, a greater economic dependency on tuna resources, a lower mean per capita gross domestic product, a greater number of fishing vessels and smaller vessels were associated with less intensive research, management and enforcement in these tuna fisheries. We also quantified the influence of specific management attributes and of biological, economic and fishery‐related factors on the trends and current status of large pelagic fish stocks in these regions. The most important factors correlated with trends and current stock status were external to the management systems, and included stock size, age at maturity, ex‐vessel price and economic dependency of countries on tuna fisheries. To improve the overall status of large pelagic fish stocks in the global high seas, more intensive data collection, research and management are needed in certain areas, especially in the Indian Ocean, and for certain stocks, especially non‐target species.