Cetacean Bycatch Research Articles

Fishers' perception and activity shifts in a dolphin bycatch mitigation context

Fishing exclusion zones aim to reduce cetacean bycatch, but often neglect social and ecological side effects, exacerbating social injustices and reshaping fishing practices. We present a case study of an endangered population of Lahille's bottlenose dolphins (Tursiops truncatus gephyreus) in a Brazilian coastal lagoon, known for its unique cooperation with net-casting fishers. However, the local small-scale fisheries (SSF) also employ other fishing gears that contribute to dolphin bycatch, prompting the enforcement of a fishing ban in the area. We investigated fishers' socioeconomic conditions, evidence of social disparities, perceptions on dolphin bycatch, and projected changes on fisheries activities to understand the repercussions of the ban. We categorized 128 surveyed fishers into five groups based on socioeconomic factors and their reliance on dolphin-cooperative and gillnet fisheries. Our findings revealed significant disparities in social capital among fishers and their different attitudes towards dolphin bycatch, highlighting unforeseen consequences. To investigate how fishers would likely shift fishing practices, we considered two management scenarios: 1) should dolphin-fishing cease due to unsuccessful bycatch mitigation, fishing efforts would shift towards shrimp trawling, seine nets, and gillnet effort; 2) if gillnet-fishing is successfully banned as a bycatch mitigation measure, a shift to shrimp trawling and cast net effort. Both scenarios would intensify pressure on shrimp stocks, which are already heavily exploited in this socioecological system. Our research underscores the importance of contemplating alternatives to achieve sustainable outcomes when designing bycatch mitigation measure. Furthermore, our results point to the importance of a participatory governance approach, grounded in the socioeconomic context, to safeguard the livelihoods of small-scale fishers and foster broader conservation efforts beyond species-specific concerns.

Annex H: Bycatch and Ship Strikes of Large Cetaceans Reported in the National Progress Report Database

How to cite: International Whaling Commission, 2023. Report of the Scientific Committee: Annex H. J. Cetacean Res. Manage. (Suppl.) 24: 250-270.

Annex I: Bycatch of Small Cetaceans Recorded in the Progress Reports Database

How to cite: International Whaling Commission, 2023. Report of the Scientific Committee: Annex I. J. Cetacean Res. Manage. (Suppl.) 24: 271-281.

Bycatch in drift gillnet fisheries: A sink for Indian Ocean cetaceans

AbstractIn 1992, the UN banned the use of large‐scale pelagic driftnets on the high seas (UNGA Resolution 46/215). Three decades later, however, drift gillnets remain one of the primary fishing gears in the Indian Ocean, accounting for approximately 30% of tuna catches in this ocean. Recent estimates indicate that several million small cetaceans have been killed in Indian Ocean gillnets over the past few decades. National agencies and the regional fisheries management organization charged with managing tuna fisheries, the Indian Ocean Tuna Commission, have yet to effectively document the bycatch of small cetaceans in these fisheries. Here, we review current information on cetacean bycatch in Indian Ocean drift gillnets and propose potential solutions to this important conservation issue.

Socioeconomics and management of small cetacean bycatch in Ghana

Small cetacean bycatch in artisanal fisheries and deep‐sea fishing operations is a serious and persistent threat to conservation in coastal communities in Ghana. We assessed the bycatch and management of small cetaceans in four coastal communities by administering 73 questionnaires to fishers and recording their catches. Our findings show that dolphin feeding habits, gear attributes and habitat use of targeted finfish species were the main factors driving bycatch of small cetaceans in Ghana. Gear damage and reduced catches were among the effects of bycatch reported by fishers. In addition to direct consumption of small cetacean meat, fishers use them as bait in shark‐fishing activities. Our results suggest the livelihood of over 70% of fishers will be threatened if management measures are implemented. Fishers were unaware of the Wildlife Division that manages small cetaceans in Ghana. We recommended that a co‐management approach between the government and locals should be adopted when developing and implementing measures for the conservation of small cetaceans. A community awareness programme should be carried out to raise awareness about threats facing these species and the importance of conservation.

An evaluation of the cetacean bycatch monitoring programme on board the Spanish set gillnet and bottom pair trawl fleets in the Bay of Biscay

Abstract Bycatch of cetaceans by European Union (EU) fleets is mostly monitored by onboard observers under the EU Data Collection Framework (DCF) programme. There is concern that the resulting estimates of cetacean bycatch mortality are biased and imprecise, because bycatch events are rare, sampling coverage is limited, and DCF observers prioritize the sampling of the discard of commercial species. Here, we explore the utility of the recently established Spanish cetacean bycatch monitoring programme to estimate cetacean bycatch mortality and compare it to the EU DCF programme. Data from October 2020 to September 2021 from this new Spanish programme were analysed to explore the following issues: (i) representativeness of the sampling frame; (ii) estimation of bycatch per unit of effort (BPUE); and (iii) sampling coverage. The probabilistic sampling design of the programme, with a record of refusals, has allowed identification and correction of biases in BPUE estimation. BPUE estimates for the bottom pair trawl fleet were recalculated to account for the fact that although boats work in pairs, each pair is monitored by a single observer located in a single boat. In relation to the sampling coverage, our results point to a need for levels up to 31% to achieve statistically robust bycatch rates. The current Spanish EU DCF at-sea sampling programme (which includes the sampling of bycatch of non-target protected species) and the Spanish (dedicated) cetacean bycatch monitoring programme provided similar results on cetacean bycatch for some of the ICES Divisions and Metiers monitored but sampling coverage of both is insufficient to obtain robust estimates.

An Overview of the Status of Sturgeons and Cetaceans Accidentally Captured in Southern Black Sea Coastal Zone of Türkiye

This study was conducted out in the southern Black Sea coastal zone, in 2011-2016 fishing seasons. In the study, bycatch of sturgeons and cetaceans were examined both in small scale (turbot gillnets) and large scale (trawlers) fisheries. During the fishing operations within the study, 66 sturgeons (41 Danube sturgeon - Acipenser gueldenstaedtii and 25 starry sturgeon - Acipenser stellatus) and 49 cetaceans (40 harbor porpoises - Phocoena phocoena relicta and 9 short beaked dolphin - Delphinus delphis ponticus) were captured. Length-weight relationships of Danube sturgeon and starry sturgeon were determined as W=0.0123L2.8808 (R=0.9964, negative allometric growth) and W=0.0345L2.5351 (R=0.9961, negative allometric growth), respectively. Mean total length were calculated as 60.02±1.04 cm for Danube sturgeons, 46.42±1.17 cm for starry sturgeon and mean fork length were established as 126.75±3.36 cm for harbor porpoises, 163.74±1.42 cm for short beaked dolphins in the study.

Bycatch of short-beaked common dolphin (Delphinus delphis) in the pair bottom trawl fishery of the Bay of Biscay and its mitigation with an active acoustic deterrent device (pinger)

Bycatch of common dolphin (Delphinus delphis) in commercial trawl fisheries in the Bay of Biscay (NE Atlantic) is of concern and its mitigation a priority. Active acoustic deterrent devices (pingers) attached to fishing gear seem to be promising for bycatch mitigation, as they have demonstrated to effectively reduce cetacean bycatch in some set-net fisheries. However, the low occurrence of common dolphin bycatch in many trawl fisheries, coupled with the extensive amount of time needed to monitor them, makes it difficult to prove the effectiveness of pingers. Remote electronic monitoring (REM) systems in fisheries can substantially increase onboard observation, providing access to extensive databases to comprehensively address bycatch mitigation studies. In this study, the effectiveness of DDD®03H Dolphin Dissuasive Device (hereinafter DDD pingers) to reduce common dolphin bycatch was evaluated in a demersal pair trawler in FAO Division 27.8.c. In 195 fishing days, one of the vessels in the pair operated with a set of DDD pingers whereas the other operated without them, and the bycatch of common dolphin was monitored through the REM system. In total, 660 fishing hauls were conducted of which 223 hauls had the DDDs attached. The results showed that the DDDs reduced common dolphin bycatch by more than 90%, with both bycatch frequency and the number of individuals bycaught per haul being significantly lower. The results also showed that common dolphin bycatch in this fishery is related to factors such as the fishing zone and depth, whereas the type of net deployed, time of day and haul duration were found to not significantly affect the bycatch of this species.

Management of gillnet bycatch of cetaceans in New Zealand

Bycatch of dolphins in inshore gillnets first attracted scientific and management attention in New Zealand in the 1980s. During 1984-88, 50-150 dusky dolphins were killed each year at Kaikoura in gillnets set at the surface to catch bait for rock lobster. At the same time, annual catches of 20-100 Hector’s dolphins occurred in Canterbury waters in bottom-set commercial and recreational gillnets. These catches resulted in the banning of surface-set gillnetting at Kaikoura in 1989 and in the creation of the Banks Peninsula Marine Mammal Sanctuary in 1988 to protect Hector’s dolphin. An additional gillnet closure was established to protect North Island Hector’s dolphin in 2003. A key problem is that current information on catches in these and other areas is scant. One observer programme has been successfully implemented in a commercial gillnet fishery (Canterbury area, 1997/98 fishing season). Its estimate of Hector’s dolphin bycatch (17) is clearly unsustainable by the local population. Pingers have been voluntarily used in these fisheries, but there are no data establishing their effectiveness, and it has not been possible to ensure consistency of pinger use. There are no reliable estimates of numbers taken in recreational fisheries. Area closures are used to mitigate gillnet bycatch of Hector’s dolphin, however it appears that the Banks Peninsula Marine Mammal Sanctuary is not large enough to ensure the persistence of the Canterbury population. There is a bycatch limit in place for this population, although it is unenforced. We argue that management of this species via bycatch limits is not practical, however. Hector’s dolphin’s low abundance and separation into several distinct populations means that appropriate bycatch limits would be very small, and this necessitates very comprehensive observer coverage to be confident they are not exceeded. We propose that increasing the size of protected areas is the most reliable option for conservation.

Cetacean bycatch management in regional fisheries management organizations: Current progress, gaps, and looking ahead

Bycatch is the leading global threat to cetaceans, with at least 300,000 cetaceans estimated to be killed each year in fisheries. Regional fisheries management organizations (RFMOs) are key actors in global fisheries management, including bycatch reduction, but their role in cetacean bycatch management is often overlooked. The primary mandate of RFMOs is to manage fisheries of target stocks, but many of their Convention Agreements call for addressing bycatch of threatened and protected species in their fisheries, including cetaceans. We reviewed 14 RFMOs to understand their current cetacean bycatch management efforts. We developed twenty-five criteria for best practices in cetacean bycatch management to understand efforts made by each RFMO, grouped into five categories: 1) general bycatch governance; 2) observer coverage; 3) quantitative bycatch limits; 4) data analysis and transparency; and 5) mitigation efforts. Collectively, based on our application of these criteria, RFMOs scored highest in “data analysis and transparency” (average=0.74) and lowest in setting “quantitative bycatch limits” (average=0.15). Overall, RFMOs have passed few binding conservation and management measures focused on cetacean bycatch, particularly compared to those addressing the bycatch of seabirds and sea turtles; the few existing measures are primarily focused on bycatch in purse seines. Notwithstanding the United Nations (UN) large-scale drift gillnet ban (46/215) on the high seas, only one RFMO has passed a management measure specifically focused on cetaceans and gillnets, widely recognized as the gear type posing the highest risk to cetaceans. No measure in any RFMO specifically addresses cetacean bycatch in longlines. We provide recommendations to the RFMO community to encourage progress on this critical issue, including leveraging other recent policy developments such as the adoption of the 2021 UN Food and Agriculture Organization Guidelines to prevent and reduce bycatch of marine mammals in capture fisheries and implementation of the U.S. Marine Mammal Protection Act Import Provisions.

Distribution and abundance of marine mammals in the coastal waters of British Columbia, Canada

Information on animal distribution and abundance is integral to wildlife conservation and management. However abundance estimates have not been available for many cetacean species inhabiting the coastal waters of Canada’s Pacific coast, including those species that were heavily depleted by commercial whaling. Systematic sightings surveys were conducted in the inshore coastal waters of the Inside Passage, between the British Columbia (BC)-Washington and the BC-Alaska borders. A total of 4,400km (2,400 n.miles) of trackline were surveyed in the summers of 2004 and 2005. Abundance estimates (with 95% confidence intervals) assuming certain trackline detection for seven cetacean species were as follows: harbour porpoise, 9,120 (4,210-19,760); Dall’s porpoise, 4,910 (2,700-8,940); Pacific white-sided dolphin, 25,900 (12,900-52,100); humpback whale, 1,310 (755-2,280); fin whale, 496 (201-1,220); common minke whale, 388 (222-680); and ‘northern resident’ killer whale, 161 (45-574). The potential for responsive movement to have affected the accuracy and precision of these estimates is difficult to assess in small-boat surveys. However, the analyses were designed to minimise this factor in the most obvious case (Pacific white-sided dolphins) and pilot data collection has begun to assess the magnitude of the effect and to calculate correction factors for other species. The density of harbour seals, both along the shoreline and at sea, was calculated and it was estimated that total abundance of harbour seals in the study area was at least 19,400 (14,900-25,200). These are new abundance estimates for this region for all cetacean species except killer whales. The small sample size makes the killer whale estimate tenuous, but one worth noting, as it is close to the known number of northern resident killer whales (2004 census was 219 animals, Cetacean Research Program, Pacific Biological Station, Fisheries and Oceans Canada). The common minke whale abundance estimate is similarly tentative, however the results do reveal that common minke whales were relatively rare in this region. While the majority of harbour seals were found as expected in the southern straits and in the mainland inlets, a substantial number of animals were on the north coast and in the Queen Charlotte Basin as well. These data provide a systematic snapshot of summertime distribution and abundance of marine mammals in the Queen Charlotte Basin, where offshore oil and gas development and seismic surveys for geophysical research have been proposed to take place. Similarly, the abundance estimates could be used to form the basis of a simulation exercise to assess the sustainability of observed levels of incidental bycatch of small cetaceans in commercial fisheries. The results described here provide a useful reference point to which future survey data can be compared.

An investigation of acoustic deterrent devices to reduce cetacean bycatch in an inshore set net fishery

In Europe, problems with the use of pingers on larger fishing vessels have raised the question as to whether pingers would be practical on smaller vessels, which are a large proportion of the European static net fishing fleet. In this study, four netting vessels less than 10m long used AQUAmark pingers on part of their nets off the southwest coast of Britain over a 12 month period. Boat skippers recorded ease of use. Acoustic click detectors were deployed on test and control nets to assess the response of cetaceans to the pingers. No significant practical problems, apart from premature failure of pingers, were encountered. During the study, only one harbour porpoise was bycaught, in an unpingered net. In 650 days of acoustic data from pingered and non-pingered nets, matched by location, date and boat, there was a highly significant reduction in the number of porpoise clicks recorded at nets with pingers to 48% of the number predicted from the number recorded at control nets (range 35–51%). To assess habituation, single, modified pingers that were active for alternate seven hour periods were moored below a click detector at two sites, one of which has strong tides and high levels of associated ambient noise. This study showed a stronger pinger effect at the quiet site and a much reduced effect at the noisy site. There was evidence of a period of exclusion of porpoises following pinger use that could exceed seven hours, and no evidence of habituation. Results suggest that pingers are practical on small vessels, that they reduce harbour porpoise activity around nets and are therefore likely to reduce bycatch. Easier means of detecting pinger failure are needed. Pingers should be considered as a bycatch mitigation method in small vessel fisheries using bottom set nets.

Fishermen’s perception of interactions between fisheries and cetaceans in the Bulgarian Black Sea area

The eastern part of Bulgaria entirely borders the Black Sea, which defines it as a maritime country. The Bulgarian coast is an important fishing region for local people and has a high potential for interactions between fisheries and cetaceans. Depredation caused by cetaceans and damage to fishing gears can potentially lead to substantial economic loss for fishermen, while cetacean bycatch raises conservation concerns. Over the period 2016 - 2019, we conducted face-to-face interviews with fishermen in local fishing areas to better understand the fisheries – cetacean interactions in the Bulgarian part of the Black Sea. The research objectives were to identify the current fishermen’s attitudes toward cetaceans, understand the damage caused by local marine mammals to the different types of fishing gear, and gather fishermen’s proposals to resolve the problems. To record all of this, a specially designed structured survey was conducted. The results showed that fishermen’s attitude towards cetaceans was mostly positive and was not influenced by the type of fishing gear used. We found that fisheries - cetacean interactions are frequent, especially with dalyans gear and set gillnets. Cetacean bycatch mortality was reported to be highest for set gillnets, and the most vulnerable species was the Phocoena phocoena. Although interview data may be biased due to differences in perceptions and experience of the interviewees, and therefore should be interpreted with caution, this method allowed us to cover multiple types of interactions between cetaceans and Bulgarian fisheries.

Aquatic wild meat consumption of cetaceans in São Tomé and Príncipe (Gulf of Guinea)

Relatively little is known about cetaceans in São Tomé and Príncipe and in recent years, some episodes of cetacean bycatch and human consumption have been described. Aiming to provide insight into the reliance upon cetaceans as a source of meat, we present findings from complementary studies conducted in the islands of São Tomé and Príncipe from 2012 to 2021. Based on interviews with local residents, we assessed the prevalence of human consumption of cetaceans and identified potential sociodemographic drivers. This study provides the first comprehensive assessment of harvest (direct remove or incidental take) and consumption of cetaceans (mainly the family Delphinidae) in the country. These findings will be useful in the development of regional conservation measures and contribute to our understanding of anthropogenic activities affecting cetacean populations in the Gulf of Guinea.

Experimental evidence of the impact of framing of actors and victims in conservation narratives.

Media narratives play a crucial role in framing marine conservation dilemmas by depicting human actors, such as fish consumers or the fishing industry, as responsible for negative effects of their actions on species and ecosystems. However, there is little evidence documenting how such narratives affect preferences for reducing bycatch. Behavioral science research shows that people can act less prosocially when more actors are responsible for a collective outcome (responsibility diffusion effect) and when more victims need to be helped (compassion fade effect); thus, the media's framing of actors and victims may have a significant effect on preferences. We conducted the first test of responsibility diffusion and compassion fade in a marine context in an online experiment (1548 participants in the United Kingdom). In 9 media narratives, we varied the type of actors responsible for fisheries bycatch (e.g., consumers and industry) and victims (e.g., a single species, multiple species, and ecosystems) in media narratives and determined the effects of the narratives on participants' support for bycatch policies and intentions to alter fish consumption. When responsibility for negative effects was attributed to consumers and industry, the probability of participants reporting support for fisheries policies (e.g., bycatch enforcement or consumer taxes) was ∼30% higher (odds ratio=1.32) than when only consumers were attributed responsibility. These effects were primarily driven by female participants. Narratives had no effect on personal intentions to consume fish. Varying the type of victim had no effect on policy support and intentions. Our results suggest that neither responsibility diffusion nor compassion fade automatically follows from increasing the types of actors and victims in media narratives and that effects can depend on the type of outcome and population subgroup.

Assessment of the interactions between cetaceans and fisheries at the south of the Cetacean Migration Corridor and neighboring waters (Western Mediterranean)

The Cetacean Migration Corridor is an important marine protected area for cetacean species in the Western Mediterranean, and part of its waters constitute the main fishing grounds for the Valencia region (East Spain) fishing fleet. Here we aim to assess the interactions between all active fisheries operating in the waters of the Valencia region and the cetacean species inhabiting this area. A total of 282 face-to-face interviews (51.37% of the total fleet) to fishers were held on 20 ports at the study area to gather information about cetacean by-catch and all types of interactions between fisheries and cetaceans. The interviewed bottom trawlers (n= 148 boats) reported a monthly by-catch rate (C) of 0.01 dolphins per vessel. Bottom trawling vessels operating at neritic zones only reported by-catch of common bottlenose dolphin (C= 0.009), while those fishing at oceanic zones by-caught striped dolphin (C= 0.006) and common bottlenose dolphin (C= 0.003). The interviewed artisanal fishers (n = 114 boats), the second most important fishery in the study area, only reported one dolphin by-catch event, but 90 of these boats communicated continued negative interactions with the common bottlenose dolphin, causing gear damage and catch take. The interviewed vessels gave an estimation of their annual economic loss produced by this interaction (mean ± SD: 2,998.10 ± 2095.02 € per boat). Other fisheries operating in the Valencia region were purse-seiners (n= 15 boats) and pelagic longlines, a fishery that has almost disappeared in the study area (n= 5 boats). The first one had the highest estimated dolphin by-catch rate (C= 0.04) and mainly interacted with common bottlenose dolphin, but also with striped dolphin. The information gathered from interviews was complimented with long-term stranding data (1990–2020). Despite the fact that only 7.80% of all recorded stranded cetaceans showed fisheries interaction signs, 26.72% of the common bottlenose dolphins recorded showed evidence of this interaction, thus confirming that it is the cetacean species most affected by fisheries in the area. Records of stranded striped dolphin (6.45%) also showed evidence of fisheries interaction. Based on both dolphin by-catch and gear damage, management plans are needed in the area to ensure cetacean conservation and also fisheries sustainability in waters inside and around marine protected areas at the Spain’s Mediterranean.

The individual welfare concerns for small cetaceans from two bycatch mitigation techniques

The prevalence of small cetacean (including dolphins, porpoises and small odontocete whales) bycatch in fisheries worldwide remains an ongoing conservation and welfare challenge. Various mitigation methods have been implemented in attempts to reduce bycatch. Two such methods involve gear modification: placement of Bycatch Reduction Devices (BRDs) within trawl nets, usually involving a physical barrier and an escape hatch; and, deployment of Acoustic Deterrent Devices (ADDs, ‘pingers’), typically placed on static nets and some trawl nets, to alert cetaceans to their presence and deter them from interacting with the gear. Despite their efficacy in reducing bycatch under certain circumstances, negative welfare impacts remain for individuals interacting with both BRDs and ADDs. Post-mortem analyses of small cetaceans caught in trawl gear, for example, illustrate the potential long-term effects of capture myopathies and cardiac damage sustained during the acute stress of entanglement, prior to and during escape through the BRD. Further, animals may become entangled in the bars, ropes or mesh of the BRD or escape hatch itself, and little is known of their post-release survival. ADD efficacy is typically fishery- and cetacean species-specific and, even where deemed a success at reducing bycatch, displacing animals from their optimal foraging habitat could negatively impact individual survival. Some species display equivocal responses to ADDs, while others may habituate to or be attracted to the sounds produced as they learn to associate it with food rewards, as they do in trawl fisheries, thereby reducing ADD efficacy and increasing the likelihood of entanglement. Here, we provide a synthesis of existing studies of these mitigation methods and discuss the associated welfare issues, where poor welfare negatively impacts an individual’s physical or mental state. We conclude that cetacean welfare considerations should become an integral part of decision-making in relation to bycatch globally.

Vocal Behaviour of a Bottlenose Dolphin Pod during a Deadly Bycatch Event in the Gulf of Catania, Ionian Sea

Cetacean bycatch is increasing worldwide and poses a threat to the conservation of several delphinids. The bottlenose dolphin (Tursiops truncatus) is frequently involved in bycatch incidents, due to its coastal distribution and opportunistic behaviour. The acoustic behaviour of cetaceans during death-related events is a largely unexplored topic. During an acoustic monitoring survey of the bottlenose dolphin population inhabiting the Gulf of Catania (Ionian Sea), we documented the entangling and subsequent death of a sub-adult male in a fishing net. Here, we provide a detailed analysis of the vocal behaviour of the pod of bottlenose dolphins during the event. We identified a total of 720 vocalisations in a timespan of 138 min, including 436 signature whistles, 51 non-signature whistles, and 233 burst pulses. We recorded high vocal activity, with a peak characterised by emission rates of 36 signature whistles per minute and 70 burst pulses per minute. Although future studies are required, our results show massive vocal activity during net entangling, characterised by a significant emission of signature whistles and burst pulses, including bray series of gulps and squeaks. Therefore, we suggest that developing tools for automatically detecting the peaks of these sound types could be helpful in recognising bycatch events, in order to better quantify the impact of professional fishing on small cetaceans.

Pingers Reduce Small Cetacean Bycatch in a Peruvian Small-Scale Driftnet Fishery, but Humpback Whale (Megaptera novaeangliae) Interactions Abound

Fishery interactions pose the most significant direct anthropogenic threat to marine mammals. In this study, we tested the effectiveness of acoustic alarms at reducing the bycatch of cetaceans by small-scale gillnet vessels operating from the northern Peru port of Mancora. We equipped nets with 10 kHz pingers for vessels targeting sharks, tuna, dolphinfish, and rays. We monitored a total of 313 sets in 60 trips. We found that small cetacean bycatch per unit effort (BPUE) was reduced by 83% in experimental nets compared to control nets, with no observed reduction in whale BPUE. The study also found that pingers did not negatively affect catch rates of target species such as rays and bony fishes. However, sets with pingers had a reduction in shark catch per unit effort (CPUE) of 32.9%. Given the high number of humpback whale (Megaptera novaeangliae) entanglements we observed, we recommend testing of lower frequency “whale” pingers. We also encourage larger scale implementation of pingers for small cetaceans given the potential shown here to reduce gillnet bycatch mortality by thousands of animals annually.

Angle-dependent acoustic reflectivity of gillnets and their modifications to reduce bycatch of odontocetes using sonar imaging

Incidental capture in gillnets is the most pressing threat for small cetaceans worldwide. One reason why small, echolocating cetaceans entangle in gillnets may be their inability to acoustically detect gillnets and classify them as obstacles. To increase the overall acoustic reflectivity as well as alter the perceived image to simulate an impenetrable barrier, small reflective objects – 8 mm wide acrylic glass spheres – were attached to standard gillnets. This study investigates the acoustic reflectivity of standard gillnets and modified gillnets with different numbers of spheres/m², at several angles of ensonification across a large frequency range. The acoustic reflectivity of standard gillnets is very low and decreases with angle of ensonification. Gillnets equipped with the spheres have substantially higher acoustic backscattering strength, and exhibit a positive relation between backscattering strength and inclination, i.e. gillnets ensonified from an angle have an even larger echo than when ensonified perpendicularly. Gillnets with sphere-sphere distance of 20 cm perform best, while the acoustic backscatter of gillnets with 40 cm and 60 cm sphere-distances is similar. The acoustic image (echogram) of the gillnet with spheres demonstrates a distinct highly visible acoustic pattern, potentially rendering the spheres an effective way to reduce bycatch of small cetaceans.