Technical mitigation to reduce marine mammal bycatch and entanglement in commercial fishing gear: lessons learnt and future directions

Fisheries bycatch is one of the biggest threats to marine mammal populations. A literature review was undertaken to provide a comprehensive assessment and synopsis of gear modifications and technical devices to reduce marine mammal bycatch in commercial trawl, purse seine, longline, gillnet and pot/trap fisheries. Successfully implemented mitigation measures include acoustic deterrent devices (pingers) which reduced the bycatch of some small cetacean species in gillnets, appropriately designed exclusion devices which reduced pinniped bycatch in some trawl fisheries, and various pot/trap guard designs that reduced marine mammal entrapment. However, substantial development and research of mitigation options is required to address the bycatch of a range of species in many fisheries. No reliably effective technical solutions to reduce small cetacean bycatch in trawl nets are available, although loud pingers have shown potential. There are currently no technical options that effectively reduce marine mammal interactions in longline fisheries, although development of catch and hook protection devices is promising. Solutions are also needed for species, particularly pinnipeds and small cetaceans, that are not deterred by pingers and continue to be caught in static gillnets. Large whale entanglements in static gear, particularly buoy lines for pots/traps, needs urgent attention although there is encouraging research on rope-less pot/trap systems and identification of rope colours that are more detectable to whale species. Future mitigation development and deployment requires rigorous scientific testing to determine if significant bycatch reduction has been achieved, as well as consideration of potentially conflicting mitigation outcomes if multiple species are impacted by a fishery.

     

Bycatch quotas in the Gulf of Mexico shrimp trawl fishery: can they work?

Bycatch of unwanted, prohibited, or protected species is a problem in most commercial fisheries. Trawl fisheries are particularly prone to bycatch problems because trawls are not species-selective. In this paper, I review the history of finfish bycatch research in the Gulf of Mexico shrimp trawl fishery and explore the use of quotas to reduce finfish bycatch by examining four fisheries that currently use bycatch quotas: (1) the arrow squid trawl fishery of New Zealand, which uses fleet bycatch quotas for sea lion bycatch, (2) the Alaskan groundfish trawl fisheries, which use fleet quotas under a vessel incentive program for prohibited species, (3) the groundfish trawl fishery of British Columbia, Canada which uses individual vessel bycatch quotas for prohibited species, and (4) the multi-species trawl fisheries of New Zealand, which use catch balancing, or individual transferable quotas, for most commercially landed species. Based on the bycatch quota experiences in these fisheries, elements of successful bycatch quota programs include: (1) individual accountability, in the form of individual or cooperative bycatch quotas, rather than fleet quotas, (2) 100% observer coverage, (3) relatively small, manageable fleets, (4) limited landing ports that can be easily monitored, particularly if observer coverage is incomplete, (5) reliable enforcement, (6) penalties that are true disincentives, and (7) some flexibility in the system for fishermen to have alternatives to manage their bycatch. The Gulf of Mexico shrimp trawl fishery, with an estimated 20,000 licensed boats, is currently too large for individual bycatch quotas to be practical, although individual or cooperative bycatch quotas would be excellent strategies for reducing the bycatch of a smaller fleet. Mobile closed areas might be beneficial for reducing the bycatch of particular species, but these short-term closures would require real-time monitoring of bycatch rates and vessel monitoring systems on all vessels. However, under any management regime, incentives and/or rigorously enforced disincentives are the key to successful bycatch reduction.     

Subsurface behavior of bottlenose dolphins (Tursiops truncatus) interacting with fish trawl nets in northwestern Australia: Implications for bycatch mitigation

Most studies of delphinid‐trawler interactions have documented the surface behavior of dolphins feeding on discarded bycatch, but not their subsurface behavior around demersal trawl gear. Using video cameras mounted inside trawl nets, we recorded the subsurface behavior of common bottlenose dolphins (Tursiops truncatus) in a demersal fish trawl fishery in northwestern Australia. Footage from 36 trawls across the fishery was analyzed to determine the extent of dolphin‐gear interactions and the behavior of dolphins inside the nets. Interaction rates were high, with dolphins present inside and outside the nets during 29 and 34 trawls, respectively, and for up to 99% of the trawl duration. The proportion of foraging behaviors exhibited inside the nets was higher than the proportions of traveling and socializing behaviors. Twenty‐nine individuals were identified inside the net, seven of which returned repeatedly within and between trawls and fishing trips, but were observed primarily in the same localized areas in which they were first recorded. Our results suggest that entering trawl nets may be a frequently occurring, yet specialized behavior exhibited by a small subset of trawler‐associated dolphins. We propose that gear modifications, not spatial or temporal adjustments to fishing effort, have the greatest potential to reduce dolphin bycatch.     

Gear technology in <Emphasis Type="Italic">Nephrops</Emphasis> trawl fisheries

Nets with a small mesh size are required to catch Nephrops norvegicus, consequently large quantities of small whitefish are also caught, and much of this bycatch is undersized and is discarded dead. The main bycatch species are whiting (Merlangius merlangus), haddock (Melanogrammus aeglefinus) and cod (Gadus morhua). Here we summarize the known behavioural reactions of these species towards conventional trawls and review the results of using different trawl modifications to increase selectivity of Nephrops trawls. The trawl modifications are categorised as separator grids, separator and guiding panels, square-mesh panels, capture avoidance designs and codend modification. Finally, the extent to which these developments have been legislated for is discussed including the conditions under which new gear regulations have been introduced. Haddock and whiting rise during the trawling process facilitating their separation from Nephrops and escape, however the behaviour of small fish of these species is less consistent. Cod and Nephrops remain on bottom of the trawl, so to separate these species requires some physical filtering process. Overall, there is currently sufficient technical ability to improve selectivity in Nephrops trawls. The design of choice is dependent on the objectives of managers; for reducing discards but retaining marketable fish, square-mesh panels offer the most useful tool; to eliminate all bycatch and create a single-species fishery, grids and traditional Nephrops trawls show most potential. Whatever the objectives of the new measures, it is likely that a short-term economic impact will follow, and some form of incentive may be required to implement effective measures. A voluntary uptake of new measures by industry is preferable, however, to date, restrictions on fishing opportunities have been necessary to introduce innovative gear designs.     

Effects of Climate Change and Fisheries Bycatch on Shy Albatross (Thalassarche cauta) in Southern Australia

The impacts of climate change on marine species are often compounded by other stressors that make direct attribution and prediction difficult. Shy albatrosses (Thalassarche cauta) breeding on Albatross Island, Tasmania, show an unusually restricted foraging range, allowing easier discrimination between the influence of non-climate stressors (fisheries bycatch) and environmental variation. Local environmental conditions (rainfall, air temperature, and sea-surface height, an indicator of upwelling) during the vulnerable chick-rearing stage, have been correlated with breeding success of shy albatrosses. We use an age-, stage- and sex-structured population model to explore potential relationships between local environmental factors and albatross breeding success while accounting for fisheries bycatch by trawl and longline fisheries. The model uses time-series of observed breeding population counts, breeding success, adult and juvenile survival rates and a bycatch mortality observation for trawl fishing to estimate fisheries catchability, environmental influence, natural mortality rate, density dependence, and productivity. Observed at-sea distributions for adult and juvenile birds were coupled with reported fishing effort to estimate vulnerability to incidental bycatch. The inclusion of rainfall, temperature and sea-surface height as explanatory variables for annual chick mortality rate was statistically significant. Global climate models predict little change in future local average rainfall, however, increases are forecast in both temperatures and upwelling, which are predicted to have detrimental and beneficial effects, respectively, on breeding success. The model shows that mitigation of at least 50% of present bycatch is required to offset losses due to future temperature changes, even if upwelling increases substantially. Our results highlight the benefits of using an integrated modeling approach, which uses available demographic as well as environmental data within a single estimation framework, to provide future predictions. Such predictions inform the development of management options in the face of climate change.     

Seabird longline bycatch reduction devices increase target catch while reducing bycatch: A meta-analysis

Mortality during commercial fishing activities is a significant threat to many imperiled seabird species, and myriad technologies have been developed to reduce mortality during longline fishing operations. However, individual studies have never been combined in a quantitative manner to determine if seabird bycatch reduction devices (BRD) have a detrimental effect upon target species catch. We performed a meta-analysis of the longline fishery BRD literature to determine the general impact of BRD technology upon commercial fishing operations. There were relatively few papers with data suitable for a meta-analysis (15), therefore we pooled studies in the analysis with BRD and fishery related metrics included as covariates. Overall, we found no reduction in target fish catch. In fact, we found an increase in target catch of 9% (95% CI [1,17]) with BRD use. We show that this increase occurs while BRDs reduce seabird bycatch by 89% (95% CI [82,93]). These patterns do not change with respect to geographic location or BRD type, indicating that efforts to reduce seabird bycatch are generally effective under different conditions. Our review identifies research needed to ensure the generality of our findings and to better understand the impacts of BRD implementation upon seabird conservation and commercial fisheries.     

Evaluating the potential effectiveness of compensatory mitigation strategies for marine bycatch

Conservationists are continually seeking new strategies to reverse population declines and safeguard against species extinctions. Here we evaluate the potential efficacy of a recently proposed approach to offset a major anthropogenic threat to many marine vertebrates: incidental bycatch in commercial fisheries operations. This new approach, compensatory mitigation for marine bycatch (CMMB), is conceived as a way to replace or reduce mandated restrictions on fishing activities with compensatory activities (e.g., removal of introduced predators from islands) funded by levies placed on fishers. While efforts are underway to bring CMMB into policy discussions, to date there has not been a detailed evaluation of CMMB's potential as a conservation tool, and in particular, a list of necessary and sufficient criteria that CMMB must meet to be an effective conservation strategy. Here we present a list of criteria to assess CMMB that are tied to critical ecological aspects of the species targeted for conservation, the range of possible mitigation activities, and the multi-species impact of fisheries bycatch. We conclude that, overall, CMMB has little potential for benefit and a substantial potential for harm if implemented to solve most fisheries bycatch problems. In particular, CMMB is likely to be effective only when applied to short-lived and highly-fecund species (not the characteristics of most bycatch-impacted species) and to fisheries that take few non-target species, and especially few non-seabird species (not the characteristics of most fisheries). Thus, CMMB appears to have limited application and should only be implemented after rigorous appraisal on a case-specific basis; otherwise it has the potential to accelerate declines of marine species currently threatened by fisheries bycatch.     

Direct and indirect fishery effects on small coastal elasmobranchs in the northern Gulf of Mexico

Globally, bycatch in tropical/subtropical shrimp trawl and longline fisheries is threatening many marine species. Here we examine the joint effects of increased mortality caused by shrimp trawling bycatch, and reduced predation caused by losses of large sharks because of longline fishing. Research surveys in the Gulf of Mexico (1972–2002) demonstrated precipitous declines in shallow water coastal elasmobranchs where shrimping effort was highest (bonnethead 96%, Bancroft's numbfish (lesser electric ray) 98%, smooth butterfly ray > 99%) and consistent increases in deeper water elasmobranchs (Atlantic angel shark, smooth dogfish). These increases are the first empirical support for predation release caused by the loss of large sharks, which have been theorized to structure tropical/subtropical marine ecosystems. Bycatch of elasmobranchs in shrimp trawls is a critical conservation concern which is not solved by present mitigation measures; similar loss of elasmobranchs is expected to be occurring in tropical/subtropical regions worldwide where ever intensive shrimp trawling occurs.     

Trends in shark bycatch research: current status and research needs

Over the last few decades, much effort has been devoted towards quantifying and reducing bycatch in marine fisheries. Of late, there has been a particular focus on sharks given that bycatch is a frequently listed threat for sharks on the International Union for the Conservation of Nature Red List. However, currently there are no quantitative reviews or syntheses that explore the issue of shark bycatch globally which is problematic given that such a synthesis could inform conservation actions and identify pressing research gaps. We performed a qualitative and quantitative survey of the peer-reviewed literature to characterize trends in shark bycatch research with a particular goal of identifying research needs and opportunities. Using a structured literature review we identified 103 papers that met our search criteria, with the first one published in 1993. Early research efforts focused on documenting the scope of bycatch (i.e., determining that sharks were indeed captured as bycatch), but more recently there have been increased efforts devoted to developing and evaluating bycatch reduction strategies for sharks. Research activity was most common in the North Atlantic (~40?% of the total articles analysed) with comparatively less research in other areas such as the Indo-Pacific region where shark bycatch is regarded as particularly common and problematic. Most studies were observational with comparatively fewer experimental and modeling studies, and even fewer that combined research approaches. Gear modifications (e.g., hook size and type for long lines, net size and mesh design for nets) were the most commonly evaluated strategy for reducing shark bycatch; however, development and use of techniques like repellents, or seasonal area closures, or a combination of strategies, offer interesting possibilities that require further study. In addition, although many sharks are discarded, little is known about post-release survival or sub-lethal consequences of fisheries interactions, or evaluations of different fish handling strategies, making it difficult to quantify the true cost of bycatch or to recommend handling strategies to fishers. Although there are some inherent challenges with developing and testing shark bycatch reduction strategies, there is an urgent need to do so and this would be best achieved through interdisciplinary research that spans field, laboratory, and modeling realms.     

Projected distributions of Southern Ocean albatrosses,petrels and fisheries as a consequence of climatic change

Given the major ongoing influence of environmental change on the oceans, there is a need to understand and predict the future distributions of marine species in order to plan appropriate mitigation to conserve vulnerable species and ecosystems. In this study we use tracking data from seven large seabird species of the Southern Ocean (black‐browed albatross Thalassarche melanophris, grey‐headed albatross T. chrysostoma, northern giant petrel Macronectes halli, southern giant petrel M. giganteus, Tristan albatross Diomedea dabbenena, wandering albatross D. exulans and white‐chinned petrel Procellaria aequinoctialis, and on fishing effort in two types of fisheries (characterised by low or high‐bycatch rates), to model the associations with environmental variables (bathymetry, chlorophyll‐a concentration, sea surface temperature and wind speed) through ensemble species distribution models. We then projected these distributions according to four climate change scenarios built by the Intergovernmental Panel for Climate Change for 2050 and 2100. The resulting projections were consistent across scenarios, indicating that there is a strong likelihood of poleward shifts in distribution of seabirds, and several range contractions (resulting from a shift in the northern, but no change in the southern limit of the range in four species). Current trends for southerly shifts in fisheries distributions are also set to continue under these climate change scenarios at least until 2100; some of these may reflect habitat loss for target species that are already over‐fished. It is of particular concern that a shift in the distribution of several highly threatened seabird species would increase their overlap with fisheries where there is a high‐bycatch risk. Under such scenarios, the associated shifts in distribution of seabirds and increases in bycatch risk will require much‐improved fisheries management in these sensitive areas to minimise impacts on populations in decline.     

拖网捕捞对东海渔业资源种群结构的影响

利用收集到的近50年东海主要渔业企业和近7年来主要地区的拖网捕捞分品种渔获量数据,研究计算了双船拖网和单船网板拖网的渔获物食性指数与品种组成指数。结果表明,1974年前东海鱼类食性指数不断增大,之后开始迅速减小,呈现先升后降趋势,品种组成指数渔获物对数均值从20世纪70年代后期开始不断下降,标准差则缓慢增大,近7年单拖网渔获物的计算结果也表现出鱼类食性指数的下降趋势,长期拖网过度捕捞是造成东海海洋生态系中渔业资源种群结构变化的重要原因,渔业资源已经过度开发,种群结构稳定性减弱。     

Developing ultraviolet illumination of gillnets as a method to reduce sea turtle bycatch

Fisheries bycatch of marine animals has been linked to population declines of multiple species, including many sea turtles. Altering the visual cues associated with fishing gear may reduce sea turtle bycatch. We examined the effectiveness of illuminating gillnets with ultraviolet (UV) light-emitting diodes for reducing green sea turtle (Chelonia mydas) interactions. We found that the mean sea turtle capture rate was reduced by 39.7% in UV-illuminated nets compared with nets without illumination. In collaboration with commercial fishermen, we tested UV net illumination in a bottom-set gillnet fishery in Baja California, Mexico. We did not find any difference in overall target fish catch rate or market value between net types. These findings suggest that UV net illumination may have applications in coastal and pelagic gillnet fisheries to reduce sea turtle bycatch.     

Multi‐methods approach to characterizing the magnitude,impact, and spatial risk of Irrawaddy dolphin (Orcaella brevirostris) bycatch in small‐scale fisheries in Malampaya Sound,Philippines

Addressing the urgent conservation threat of marine mammal bycatch in small‐scale fisheries requires information on bycatch magnitude, gear types, population impacts, and risk. However, data on these proximate attributes are widely lacking in developing countries. This study used a multi‐methods approach integrating boat surveys and interviews with fishermen (n = 526) to assess proximate attributes of bycatch for the Critically Endangered subpopulation of Irrawaddy dolphins (Orcaella brevirostris) in Malampaya Sound, Philippines. Given an updated population estimate of 35 individuals (CV = 22.9%), the estimated rate of bycatch fatalities exceeds the potential biological removal rate. Spatial overlap scores were calculated to characterize overlap between dolphins and fisheries as a proxy of bycatch risk. These scores identified particularly high risk areas that could be prioritized for gear bans. However, completely reducing bycatch risk would require more widespread bans beyond these high risk areas. Implementing gear restrictions will be immensely challenging, given serious obstacles to fisheries management at this site. The multi‐methods approach describes the urgency of the bycatch problem and the changes required for mitigation, setting the stage for identifying and evaluating potential solutions. It can be readily applied to developing country sites to guide more efficient and complete data collection and conservation efforts.     

Design and test of a grid to reduce bycatch in the longfin inshore squid (Doryteuthis pealeii) trawl fishery

A species separation grid was tested for a squid trawl to reduce finfish bycatch in the Nantucket Sound longfin inshore squid (Doryteuthis pealeii) fishery in southern New England, USA. The experimental trawl with a grid significantly reduces bycatch of summer flounder (Paralichthys dentatus) (76.4%, p < .001), black sea bass (Centropristis striata) (71.7%, p = .001), smooth dogfish (Mustelus canis) (86.0%, p < .001), and total bycatch (69.2%, p < .001) when compared to a conventional trawl. The catch rate of scup (Stenotomus chrysops) is 40.2% less than in the experimental trawl, but this difference is not statistically significant (p = .258). However, the experimental trawl also reduces targeted squid capture by 47.5% (p < .001), which is commercially unacceptable. Length analysis indicates no size effect on the retention for squid between the trawl with a grid (experimental) and the one without a grid (control), but the experimental trawl significantly reduces larger scup (>27 cm FL) and larger black sea bass (>37 cm TL), and all summer flounder size‐classes. Therefore, this grid design may not be a suitable bycatch reduction device for the Nantucket Sound squid trawl fishery, and further work is needed to understand squid behavior within a trawl to develop a successful bycatch reduction strategy for the New England longfin inshore squid fishery.     

Bycatch: from emotion to effective natural resource management

Although bycatch has been an acknowledged component of fishery management for many years, its explosive growth as a major management issue has occurred over the past decade. Emergence of the bycatch issue can be traced to increasing world competition for the ocean's fishery resources and the rapid rise of the conservation and environmental movement in recent years. Bycatch has in the past several years been equated to waste, non-selective fishing methods that threaten non-target species and degradation of ocean ecosystems. Although some important steps have been taken to abate bycatch, frequently the problem is not perceived as a component of fishery management, e.g. establishing bycatch mortality rates and evaluating their consequences upon affected populations. Fishery scientists often estimate levels of bycatch, but the overall consequences of discard mortality resulting from the complex of fisheries operating in a region are frequently unknown and speculative. The sum of fishery-induced mortalities occurring as a result of harvesting often involves a significant number of fish in addition to catch and discard. An ICES study group has characterized fishing mortality as the aggregate of all catch mortalities including discard, illegal fishing and misreporting. It is unlikely that managers will, in the near future, have a full accounting of unobserved fishing mortality. Progress toward identifying and measuring such mortalities is receiving increasing attention.The authors suggest and provide an example of a matrix-type analysis for recording bycatch and other fishing mortalities. The matrix presentation would allow managers to evaluate various fishery-related mortality factors and their importance in developing management strategies.     

The role of discounting and dynamics in determining the economic efficiency of time-area closures for managing fishery bycatch

Time-area closures are commonly used to manage fisheries bycatch and involve temporarily closing an area of the ocean to particular fishing gears. We examine conditions in which implementing a time-area closure would increase the economic value of fisheries, focusing on a case study application in the Gulf of Mexico. Pelagic longline fishermen catch the highly valued Atlantic bluefin tuna (Thunnus thynnus, Scombridae) on their Gulf of Mexico spawning grounds while fishing for Atlantic yellowfin tuna (Thunnus albacares). We analyze a multispecies, multifishery bioeconomic model that includes information on migratory patterns from electronic tagged bluefin tuna. We use dynamic optimization to identify management strategies that would maximize the net present value of tuna fisheries, allowing for discounting of future benefits and costs relative to the present. If past fishing mortality rates continue in Atlantic bluefin tuna fisheries, implementing a time-area closure in the Gulf of Mexico incurs economic costs. However, the net present value of the fisheries is increased by implementing a time-area closure as part of a broader commitment to rebuild the heavily depleted bluefin population, provided the discount rate and the costs of such a closure in forgone fishing opportunities are not too large. The increase in economic value offered by a time-area closure is small relative to the overall economic value of rebuilding itself and it may be economically optimal only to implement a closure once sufficient rebuilding has already taken place.     

Improving Fishing Pattern Detection from Satellite AIS Using Data Mining and Machine Learning

A key challenge in contemporary ecology and conservation is the accurate tracking of the spatial distribution of various human impacts, such as fishing. While coastal fisheries in national waters are closely monitored in some countries, existing maps of fishing effort elsewhere are fraught with uncertainty, especially in remote areas and the High Seas. Better understanding of the behavior of the global fishing fleets is required in order to prioritize and enforce fisheries management and conservation measures worldwide. Satellite-based Automatic Information Systems (S-AIS) are now commonly installed on most ocean-going vessels and have been proposed as a novel tool to explore the movements of fishing fleets in near real time. Here we present approaches to identify fishing activity from S-AIS data for three dominant fishing gear types: trawl, longline and purse seine. Using a large dataset containing worldwide fishing vessel tracks from 2011–2015, we developed three methods to detect and map fishing activities: for trawlers we produced a Hidden Markov Model (HMM) using vessel speed as observation variable. For longliners we have designed a Data Mining (DM) approach using an algorithm inspired from studies on animal movement. For purse seiners a multi-layered filtering strategy based on vessel speed and operation time was implemented. Validation against expert-labeled datasets showed average detection accuracies of 83% for trawler and longliner, and 97% for purse seiner. Our study represents the first comprehensive approach to detect and identify potential fishing behavior for three major gear types operating on a global scale. We hope that this work will enable new efforts to assess the spatial and temporal distribution of global fishing effort and make global fisheries activities transparent to ocean scientists, managers and the public.     

Estimating the success of protected areas for the vaquita,Phocoena sinus

Bycatch in artisanal gill nets threatens the vaquita, Phocoena sinus, with extinction. In 2008 the Mexican government announced a conservation action plan for this porpoise, with three options for a protected area closed to gill net fishing. The probability of success of each of the three options was estimated with a Bayesian population model, where success was defined as an increase in vaquita abundance after 10 yr. The model was fitted to data on abundance, bycatch, and fishing effort, although data were sparse and imprecise. Under the first protected area option, the existing Refuge Area for the Protection of the Vaquita, bycatch was about 7% of population size, and probability of success was 0.08. Under the second option with a larger protected area, the probability of success was 0.35. The third option was large enough to eliminate vaquita bycatch and had a probability of success >0.99. Probability of success was reduced if elimination of vaquita bycatch was delayed or incomplete. Despite considerable efforts by the Mexican government to support vaquita conservation, abundance will probably continue to decline unless additional measures to reduce vaquita bycatch are taken, such as banning gill nets within the vaquita's range and developing effective alternative fishing gear.     

Fish by‐catch in shrimp beam trawls in the northern South China Sea

The by‐catch of shrimp beam trawl fisheries in the northern South China Sea were estimated during the 2005–2006 and 2012–2013 fishing seasons. A total of 98 hauls from three important shrimp fishing grounds were used in the analyses. A total of 119 by‐catch species were captured, compared with only 15 target species. The ratio of by‐catch to shrimp ranged from 1.01 to 13.90. The dominant standard length (SL) range of fish by‐catch was 51–100 mm. The percentage of juveniles in the total catch of some fish species was high (e.g. 100% for A. aneus).