Odontocete bycatch and depredation in longline fisheries: A review of available literature and of potential solutions

Operational interactions between odontocetes (i.e., toothed whales) and longline gear are a global phenomenon that may threaten the conservation of odontocete populations and the economic viability of longline fisheries. This review attempts to define the issue, summarize the trends and geographical extent of its occurrence over the last half century, explore the potential impact on odontocetes and on fisheries, and describe potential acoustic and physical mitigation solutions. Reports of odontocete bycatch rates are highly variable (between 0.002 and 0.231 individuals killed per set) and at least 20 species may be involved. Information about marine mammal population size, migration patterns and life history characteristics are scarce, although at least one population may be in decline due to losses attributable to longline bycatch. Information about the financial impact of depredation on pelagic longline fisheries is also scarce, although estimates of daily fleet‐wide losses range between US$1,034 and US$8,495 (overall fleet income was not reported). Such biological and financial losses may be unsustainable. Recent developments in acoustic and physical mitigation technologies have yielded mixed results. Acoustic mitigation technologies have no moving parts, although require complex electronics. To date, they are insufficiently developed and their efficacy has been difficult to assess. Physical mitigation technologies generally require complex moving parts, although they are relatively simple to develop and assess. Further development and testing remains necessary before widespread implementation would be possible. Development of these approaches should be prioritized and a “toolbox” of various strategies and solutions should be compiled, because a single panacea to the problem is unlikely to emerge.     

FIELD EXPERIMENTS SHOW THAT ACOUSTIC PINGERS REDUCE MARINE MAMMAL BYCATCH IN THE CALIFORNIA DRIFT GILL NET FISHERY

A controlled experiment was carried out in 1996–1997 to determine whether acoustic deterrent devices (pingers) reduce marine mammal bycatch in the California drift gill net fishery for swordfish and sharks. Using Fisher's exact test, bycatch rates with pingers were significantly less for all cetacean species combined ( P < 0.001) and for all pinniped species combined ( P = 0.003). For species tested separately with this test, bycatch reduction was statistically significant for short-beaked common dolphins ( P = 0.001) and California sea lions ( P = 0.02). Bycatch reduction is not statistically significant for the other species tested separately, but sample sizes and statistical power were low, and bycatch rates were lower in pingered nets for six of the eight other cetacean and pinniped species. A log-linear model relating the mean rate of entanglement to the number of pingers deployed was fit to the data for three groups: short-beaked common dolphins, other cetaceans, and pinnipeds. For a net with 40 pingers, the models predict approximately a 12-fold decrease in entanglement for short-beaked common dolphins, a 4-fold decrease for other cetaceans, and a 3-fold decrease for pinnipeds. No other variables were found that could explain this effect. The pinger experiment ended when regulations were enacted to make pingers mandatory in this fishery.     

An overview of loggerhead sea turtle (<Emphasis Type="Italic">Caretta caretta</Emphasis>) bycatch and technical mitigation measures in the Mediterranean Sea

This paper reviews the gear parameters responsible for loggerhead sea turtle (Caretta caretta) capture and mortality while taking into account the mitigation measures tested in the Mediterranean Sea. Incidental catch is considered as one of the major threats for turtle survival; however, the loggerhead bycatch estimated in different areas seems to be unrealistic, which highlights the need of a method for homogenising the estimates. Drifting longlines and bottom trawls have the greatest impact on Mediterranean turtle populations, respectively in pelagic and demersal phase, while passive nets (gillnets and trammel nets) seem to be responsible for the highest direct mortality, due to drowning. Most of the experiments available for the Mediterranean are focused on drifting longline. The longline parameters, hook shape and size, bait type, setting position and the reaction to sensory stimuli, strongly affect the sea turtle bycatch and mortality. Circle hooks have the potential to reduce turtle mortality only in certain fisheries and areas; larger hooks are less likely to be swallowed by turtles due to physical constraints of the mouth, reducing the mortality rate and the catch of juveniles; branchlines, once ingested, appear to be one of the major causes of sea turtle mortality; squid bait, which consistently catches more turtles than mackerel, and lightsticks, which strongly attract turtles, should be banned, at least in some areas and seasons. On the contrary only two bottom trawl studies are available from the Mediterranean. Turtle excluder devices have been tested with promising results in Turkey and Italy, even if the loss of large fish should be carefully investigated. For set nets no practical solutions are available at this time. The analysis allows the conclusion that technical parameters affecting turtle bycatch and mortality should only be studied one at a time, in order to avoid inconclusive results, studies on post-release mortality should be implemented and finally fishermen cooperation is paramount in reducing turtle bycatch and mortality.     

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.     

Sperm whale depredation of sablefish longline gear in the northeast Pacific Ocean

Interactions between marine mammals and fisheries include competition for prey (catch), marine mammal entanglement in fishing gear, and catch removal off fishing gear (depredation). We estimated the magnitude of sperm whale depredation on a major North Pacific longline fishery (sablefish) using data collected during annual longline surveys. Sperm whale depredation occurs while the longline gear is off‐bottom during retrieval. Sperm whales were observed on 16% of longline survey sampling days, mostly (95% of sightings) over the continental slope. Sightings were most common in the central and eastern Gulf of Alaska (98% of sightings), occasional in the western Gulf of Alaska and Aleutian Islands, and absent in the Bering Sea. Longline survey catches were commonly preyed upon when sperm whales were present (65% of sightings), as evidenced by damaged fish. Neither sperm whale presence (P = 0.71) nor depredation rate (P = 0.78) increased significantly from 1998 to 2004. Longline survey catch rates were about 2% less at locations where depredation was observed, but the effect was not significant (P = 0.34). Estimated sperm whale depredation was <1% of the annual sablefish longline fishery catch off Alaska during 1998 to 2004.     

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.     

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.     

The changing focus of marine mammal conservation

Overexploitation has been the principal focus of marine mammal conservation. Less attention has been paid to bycatch in commercial fisheries; entanglement in lost and discarded fishing gear; food shortages owing to climate change and/or overharvesting of essential prey; point and non-point source pollution; and diseases. Also, relatively little attention has been paid to situations where marine mammals pose threats to the existence and human uses of other marine species. As overexploitation is addressed, focus must be shifted to these problems that are no less significant.     

Vulnerability of the Oceanic Whitetip Shark to Pelagic Longline Fisheries

A combination of fisheries dependent and independent data was used to assess the vulnerability of the oceanic whitetip shark to pelagic longline fisheries. The Brazilian tuna longline fleet, operating in the equatorial and southwestern Atlantic, is used as a case study. Fisheries dependent data include information from logbooks (from 1999 to 2011) and on-board observers (2004 to 2010), totaling 65,277 pelagic longline sets. Fisheries independent data were obtained from 8 oceanic whitetip sharks tagged with pop-up satellite archival tags in the area where longline fleet operated. Deployment periods varied from 60 to 178 days between 2010 and 2012. Tagging and pop-up sites were relatively close to each other, although individuals tended to travel long distances before returning to the tagging area. Some degree of site fidelity was observed. High utilization hotspots of tagged sharks fell inside the area under strongest fishing pressure. Despite the small sample size, a positive correlation between tag recorded information and catch data was detected. All sharks exhibited a strong preference for the warm and shallow waters of the mixed layer, spending on average more than 70% of the time above the thermocline and 95% above 120 m. Results indicate that the removal of shallow hooks on longline gear might be an efficient mitigation measure to reduce the bycatch of this pelagic shark species. The work also highlights the potential of tagging experiments to provide essential information for the development of spatio-temporal management measures.     

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.     

Dynamic habitat models: using telemetry data to project fisheries bycatch

Fisheries bycatch is a recognized threat to marine megafauna. Addressing bycatch of pelagic species however is challenging owing to the dynamic nature of marine environments and vagility of these organisms. In order to assess the potential for species to overlap with fisheries, we propose applying dynamic habitat models to determine relative probabilities of species occurrence for specific oceanographic conditions. We demonstrate this approach by modelling habitats for Laysan (Phoebastria immutabilis) and black-footed albatrosses (Phoebastria nigripes) using telemetry data and relating their occurrence probabilities to observations of Hawaii-based longline fisheries in 1997-2000. We found that modelled habitat preference probabilities of black-footed albatrosses were high within some areas of the fishing range of the Hawaiian fleet and such preferences were important in explaining bycatch occurrence. Conversely, modelled habitats of Laysan albatrosses overlapped little with Hawaii-based longline fisheries and did little to explain the bycatch of this species. Estimated patterns of albatross habitat overlap with the Hawaiian fleet corresponded to bycatch observations: black-footed albatrosses were more frequently caught in this fishery despite being 10 times less abundant than Laysan albatrosses. This case study demonstrates that dynamic habitat models based on telemetry data may help to project interactions with pelagic animals relative to environmental features and that such an approach can serve as a tool to guide conservation and management decisions.     

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.     

Diving behaviour of white-chinned petrels and its relevance for mitigating longline bycatch

The white-chinned petrel (Procellaria aequinoctialis) is the seabird species most commonly killed by Southern Hemisphere longline fisheries. Despite the importance of diving ability for mitigating longline bycatch, little is known of this species’ diving behaviour. We obtained data from temperature–depth recorders from nine white-chinned petrels breeding on Marion Island, southwestern Indian Ocean, during the late incubation and chick-rearing period. Maximum dive depth (16 m) was slightly deeper than the previous estimate (13 m), but varied considerably among individuals (range 2–16 m). Males dived deeper than females, and birds feeding chicks dived deeper than incubating birds, but dive rate did not differ between the sexes. Time of day had no significant effect on dive depth or rate. Our findings will help to improve the design and performance of mitigation measures aimed at reducing seabird bycatch in longline fisheries, such as the calculation of minimum line sink rates and optimum aerial coverage of bird-scaring lines.     

Quantifying the effects of fisheries on threatened species: the impact of pelagic longlines on loggerhead and leatherback sea turtles

The depletion of fish stocks from global fisheries has been a long‐standing concern. More recently, incidental catch of non‐target (termed bycatch) vertebrates also has been proposed as a serious conservation issue. Here we present a bycatch assessment for loggerhead and leatherback sea turtles that are incidentally caught by global pelagic longlines. We integrate catch data from over 40 nations and bycatch data from 13 international observer programmes. Despite infrequent rates of encounter, our analyses show that more than 200 000 loggerheads and 50 000 leatherbacks were likely taken as pelagic longline bycatch in 2000. Our analyses suggest that thousands of these turtles die each year from longline gear in the Pacific Ocean alone. Given 80–95% declines for Pacific loggerhead and leatherback populations over the last 20 years, this bycatch level is not sustainable. Adopting a large‐scale, synthetic approach is critical to accurately characterize the influence of global fisheries bycatch on globally distributed and imperilled pelagic vertebrates.     

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.     

Combined impacts of longline fisheries and climate on the persistence of the Amsterdam Albatross Diomedia amsterdamensis

Incidental capture of seabirds in longline fishing gear is a central issue in the conservation of many long-lived marine species. Despite growing evidence of climate-induced effects on population trends of long-lived species, climate change remains generally overlooked in most risk assessments of seabirds. Because variation in climate may interact with the detrimental effects of bycatch, considering climate is of great importance, especially in the context of ongoing global warming. This paper examines the combined effects of bycatch and climate change on the persistence of one of the world's rarest birds, the Amsterdam Albatross Diomedea amsterdamensis , which has a single population in the upland plateau of Amsterdam Island (Southeast Indian Ocean). Using continuous monitoring from 1983 onwards, we first estimated the relationship between climate and the species' demographic parameters. We then built a stochastic matrix population model to estimate the population growth rate and the probability that the population declines below the level recorded in 1983 of nine breeding pairs under different scenarios involving the joint effects of additional mortality caused by longline fisheries and climate change. The results suggest that the demography of the Amsterdam Albatross is influenced by climate in both breeding and wintering grounds and that these relationships may to some extent compensate for the impact of additive bycatch mortality. However, these compensatory effects would be negligible if the annual additional mortality exceeds around six individuals per year, suggesting that the resumption of longline fishery in the foraging range of the Amsterdam Albatross would rapidly put this species at risk of extinction.     

An overview of historical changes in the fishing gear and practices of pelagic longliners,with particular reference to Japan’s Pacific fleet

We identify changes in pelagic longline fishing gear and practices that need to be accounted for in stock assessments. Pelagic longline fishers have continuously modified their fishing gear and practices to improve fishing power and catchability, which has altered the relationship between catch rates and abundance. Advances in technology resulted in the introduction of many electronic devices to assist in navigation, communication and finding target species. The development of synthetic materials allowed improvements to lines and hooks that increased the probability of hooking target species and landing them. Other changes increased fishing power by improving searching efficiency (e.g., satellite imagery) or the time spent on fishing grounds (e.g., freezers). The number of hooks deployed in daily longlining operations has steadily increased since 1950. However, mean soak time did not change significantly because faster longline retrieval and deployment speeds balanced the increased hook numbers. There has been a shift from having all baits available at dawn, to having more available at dusk and at night. In the 1970s, several longline fleets began to exploit a much greater depth range, resulting in increased catchability for deep-dwelling species (e.g., bigeye tuna, Thunnus obesus) and reduced catchability for epipelagic species like blue marlin (Makaira nigricans). Research has been mostly limited to the effects of longline depth on the catchability of target species. Recent experiments have quantified the effects of bycatch mitigation measures on fishing power and catchability. Progressive improvements in expertise and technological improvements in the gear will also affect fishing power, but are particularly difficult to quantify.     

Can acoustic methods be used to monitor grenadier (Macrouridae) abundance in the Ross Sea region?

Grenadiers Macrourus spp. are the main bycatch species in the exploratory longline fishery for toothfish Dissostichus spp. in the Ross Sea, Antarctica. Ongoing monitoring tools are needed to assess stock status of grenadiers in the Ross Sea and to ensure ecological relationships are maintained. There may be potential to use fisheries acoustic methods to obtain estimates of grenadier abundance. Acoustic data collected during New Zealand??s International Polar Year Census of Marine Life survey in February?CMarch 2008 provide evidence that single targets close to the bottom over the Ross Sea slope are grenadiers. The acoustic target strength distribution of single targets was very similar to that predicted based on the measured size range of grenadiers. There are also positive correlations between acoustic backscatter and trawl and longline catches of grenadiers. Key uncertainties of the acoustic method are mark identification away from the bottom, and technical issues with low signal-to-noise ratio at depths greater than 1000 m and the acoustic dead-zone close to the bottom.     

Differential adult survival at close seabird colonies: The importance of spatial foraging segregation and bycatch risk during the breeding season

Marine megafauna, including seabirds, are critically affected by fisheries bycatch. However, bycatch risk may differ on temporal and spatial scales due to the uneven distribution and effort of fleets operating different fishing gear, and to focal species distribution and foraging behavior. Scopoli's shearwater Calonectris diomedea is a long‐lived seabird that experiences high bycatch rates in longline fisheries and strong population‐level impacts due to this type of anthropogenic mortality. Analyzing a long‐term dataset on individual monitoring, we compared adult survival (by means of multi‐event capture–recapture models) among three close predator‐free Mediterranean colonies of the species. Unexpectedly for a long‐lived organism, adult survival varied among colonies. We explored potential causes of this differential survival by (1) measuring egg volume as a proxy of food availability and parental condition; (2) building a specific longline bycatch risk map for the species; and (3) assessing the distribution patterns of breeding birds from the three study colonies via GPS tracking. Egg volume was very similar between colonies over time, suggesting that environmental variability related to habitat foraging suitability was not the main cause of differential survival. On the other hand, differences in foraging movements among individuals from the three colonies expose them to differential mortality risk, which likely influenced the observed differences in adult survival. The overlap of information obtained by the generation of specific bycatch risk maps, the quantification of population demographic parameters, and the foraging spatial analysis should inform managers about differential sensitivity to the anthropogenic impact at mesoscale level and guide decisions depending on the spatial configuration of local populations. The approach would apply and should be considered in any species where foraging distribution is colony‐specific and mortality risk varies spatially.