A simple tool to predict bycatch in harbour porpoises

By Emily Nelson, SRC master’s student

Harbour porpoise bycatch has been identified as the biggest threat facing these animals in many areas today, with many incidental catches occurring in large commercial gillnet fisheries. In efforts to minimize negative impacts, harbour porpoises in waters of the European Union have been awarded protection under Habitats Directive (EC 1992) and Council Regulation 812/2004 (EC 2004). Despite differences in specifics, these policies both work towards conservation and would benefit from increased information regarding bycatch of porpoises.

Harbour porpoise in Denmark. Photo by Erik Christensen.

Harbour porpoise in Denmark. Photo by Erik Christensen.

Kindt-Larsen et al. 2016 aims to create a model that can identify areas and seasons where porpoises are at high risk of entanglement in commercial fishing gear. Two main high-resolution datasets were used to develop the model. First, fisheries and bycatch data was obtained from remote electronic monitoring systems aboard 4 commercial gillnet operations in the Danish part of the Skagerrak Sea. Using video footage of gillnet hauls the authors were able to identify time and location of harbour porpoise bycatch events. Fishing effort (defined as the product of gillnet string length and net soak time), fishing target species (cod, plaice, and hake), and season (winter, spring, summer, and autumn) were also used. Second, estimated population density of harbour porpoises was obtained using satellite tag data from 66 individuals in the same area. Data was filtered to remove positions that may be inaccurate, such as locations that required excessively high swim speed to reach. Further, tag data was manipulated according to a grid system. A value was assigned to each 1km grid cell within the study area reflecting the likelihood the particular cell was visited by harbour porpoises.

Density of harbour porpoises, estimated from satellite tagging data using a grid system.

Density of harbour porpoises, estimated from satellite tagging data using a grid system.

This data was then used to identify the general relationship between expected bycatch and porpoise density. The authors started with the most complex model (involving all variables) and sequentially removed insignificant variables in order to find the best fit. In the end, target species and length of net did not improve the model fit. Additionally, porpoise density estimated using season and area (rather than satellite tag data) did not improve fit. The best model was very simple; harbour porpoise bycatch was best explained using solely soak time of fishing gear and satellite tag estimates of population density.

The success of the model developed by Kindt-Larsen and colleagues relies on a few large assumptions. First, the assumptions that satellite tagged porpoises are representative of the population as a whole. This concern was addressed in a number of ways. 1. Analysis was run showing that the spatial patterns observed were consistent over time. 2. Areas of high density predicted by satellite data were verified because acoustic surveys show similar results. 3.The satellite tagged individuals contained a mixture of juvenile, adult, male and females, thus there is no bias in the data do the demographic differences. The second assumption is that fishing effort estimations are truly representative of the four fisheries. This is verified because fishing effort was calculated the same way throughout all vessels. Lastly, the assumption that recorded porpoise bycatch was representative of the true number of bycaught animals. This assumption was of little concern to the authors because the REM video was of high quality and bycatch was easy to identify. However, if porpoises fell from the net prior to reaching the surface they were not recorded. For this reason it is important to consider bycatch estimates presented here as a minimum. Overall, it seems the assumptions of the model will have minimal impact on results.

The model created by Kindt-Larsen and colleagues follows the simple principle, that bycatch can occur only if the animal and fishery have an overlap in space and time. While the model presented is basic, it can absolutely act as a starting point for investigations of harbour porpoise bycatch. Results will be able to identify regions and/or seasons of high and low risk to porpoises. This will aid in future bycatch monitoring and the development of mitigation strategies.

Works cited

EC (European Commission) (1992) Habitats Directive: Council Directive 92/43/EEC of 21 May 1992 on the conservation of natural habitats and of wild fauna and flora. Off J Eur Union L 206: 7−50

EC (2004) Council Regulation (EC) No. 812/2004 of 26 April 2004 laying down measures concerning incidental catches of cetaceans in fisheries and amending Regulation (EC) No. 88/98. Off J Eur Union L 150: 12−31

Kindt-Larsen, L., Berg, C.W., Tougaard, J., Sorenson, T.K., Geitner, K., Northridge, S., Sveegaard, S., & Larsen, F. (2016). Identification of high-risk areas for harbour porpoise Phocoena phocoena bycatch using remote electronic monitoring and satellite telemetry data. Marine Ecology Progress Series, 555, 261-271.

Buoyless Nets Reduce Sea Turtle Bycatch in Coastal Net Fisheries

By Ryan Keller, SRC Intern

Bycatch of megafauna (larger organisms) is a serious negative side effect that stems from the practice of commercial fishing worldwide. Often fishing practices such as long lines or using nets are effective at catching the target species but also will entrap many other organisms. Often for organisms that breath air this means mortality as they may be stuck underwater for a longer period of time then they able to hold their breath.  Baja California Sur, Mexico has some of the highest recorded megafauna bycatch rates of anywhere in the world due to heavy use of bottom-set nets. Unfortunately this area also happens to be a foraging mecca for endangered loggerhead turtles.

Pictured above: An endangered loggerhead turtle swimming over a reef. Loggerheads when caught in nets cannot get to the surface to breathe leading to death.

Pictured above: An endangered loggerhead turtle swimming over a reef. Loggerheads when caught in nets cannot get to the surface to breathe leading to death. https://commons.wikimedia.org/wiki/File:Loggerhead_turtle.jpg

Between 2007-2009 Stanford University researches worked with local fisherman to compare the megafauna’ bycatch rates between traditional nets (with buoys) and bouyless nets.  Both types of nets were set near each other during the trials and the difference in bycatch recorded. The nets were checked on a regular basis to try and prevent mortality of any turtles caught in the nets. Local fisherman were compensated for setting two of the experimental nets and having a researcher with them on their trips. Later in the experiment partner fisherman were hired to fish exclusively the bouyless nets. All turtles that were caught in the nets were tagged, measured and released.  In all trials the fisherman were allowed to keep their catch and bring it to market.  The difference in the monetary value of catches between types of nets was also calculated.

As seen above: A green sea turtle (Chelonia mydas) stuck in a fishing net. Turtles often get stuck in nets that have broken free and are floating in the currents throughout the ocean, these nets are termed “Ghost nets”.

As seen above: A green sea turtle (Chelonia mydas) stuck in a fishing net. Turtles often get stuck in nets that have broken free and are floating in the currents throughout the ocean, these nets are termed “Ghost nets”. https://commons.wikimedia.org/wiki/File:Sea_turtle_entangled_in_a_ghost_net.jpg

There was found to be a 67% mean reduction in the number of turtles caught in the bouyless nets compared to traditional nets with a minimal impact on the quantity of target species. There was a decreased market value in the catch from the bouyless nets but this is most likely due to higher than average amounts of certain species being caught and brought to market at one time driving the price down. There has been other research done in this area that shows small changes such as illuminating nets at night also reduce the amount of bycatch. It is not practical to just ban the present commercial fishing methods completely. Finding ways to make small changes that have minimal impact on the fisherman and their income while drastically decreasing bycatch is the best way to gain acceptance and support from the industry. If the fisherman are minimally impacted they are much more likely to take up the new practices and not resist or revert back to prior methods.


Peckham, S. H., Lucero‐Romero, J., Maldonado‐Díaz, D., Rodríguez‐Sánchez, A., Senko, J., Wojakowski, M., & Gaos, A. (2015). Buoyless Nets Reduce Sea Turtle Bycatch in Coastal Net Fisheries. Conservation Letters.

What happens to the seahorses that you accidentally land in your trawl net?

By Stephen Cain, RJD Intern

For millennia our ancestors fished the world’s oceans. Today’s fishing fleets are the most effective in all of human history, extracting ever-larger quantities of wild fishes. Only recently have scientists shown that seas are vulnerable to overexploitation, which can put species at extinction risk. Meanwhile, human population growth places increased pressure on marine resources to feed billions, thus representing a significant share of global trade.

The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) is a multinational treaty with the mandate of monitoring and regulating species trade such that wild populations remain healthy. But do large, multi-national efforts such as CITES actually work? Writing in Aquatic Conservation: Marine and Freshwater Ecosystems, researchers Sarah Foster, Stefan Wiswedel and Amanada Vincent attempt to answer this question by analyzing CITES data and by using seahorses (Hippocampus) as a case study.

2002 was a noteworthy year for CITES. For the first time in nearly 25 years the international body added a marine fauna, seahorses, to its list of Appendix II species. According to the agreement, such species are of a conservation concern in the absence of regulation, and any of the 180 member nations trading in Appendix II species must demonstrate through monitoring that trade does not harm wild populations.  Previous biologic surveys of seahorses analyzed by Foster et al. showed that extensive trade existed. The genus’ life-history characteristics, such as small home ranges, low fecundity and density gave scientists cause for concern over the long-term sustainability of global populations.


Photo Credit: Sarah Foster

Foster and her team reviewed CITES monitoring reports for the first seven years of the Appendix II listing (2004-2011). They wanted to determine its successes and obstacles, as well as uncover key relationships between market demand, trade routes, and the sources of trade.

Surveys undertaken prior to the implementation of the CITES listing showed that millions of individuals were traded annually. Seahorses were exported primarily as dried specimens for traditional Chinese medicine (TCM), and to a lesser degree as live individuals for aquaria. Remarkably, commercial trawlers that obtained seahorses as by-catch or non-targeted species met the larger demand for TCM markets. When seahorses were specifically targeted as catch, they were taken alive and destined for the aquaria trade. The United States imported the largest number of live seahorses during the study period.

In all, Foster et al. identified 31 out of 47 species of the genus Hippocampus as important species to international trade, four of which dominated by volume (H. kelloggi, H. kuda, H. spinosissimus, and H. trimaculatus). The top four, the authors noted, are species listed as threatened by the IUCN.

While reporting gaps and inconsistencies made definitive findings challenging, Foster and her team suspect that it is unlikely that the demand for seahorses has diminished in the years following the CITES designation. Monitoring of trade emerged as one of the greatest challenges for the treaty. Some countries known to export quantities of seahorses did not report. Other countries failed to specify the unit volumes of exports, which made it difficult to form a clear picture of international trade. Taken together, however, Foster et al. interpreted the monitoring failures as an area of opportunity for CITES.

The challenge now is for international authorities to build member capacity in annual reporting. For this, the researchers suggested that automated record validation, a process aided by new technology, could add precision to the accounting of imports and exports. In addition, standardized educational materials for species identification could strengthen the accuracy of reporting, and trade surveys could give a fresh perspective on species abundance. The promptness of the reporting also needs improvement. In some instances a country’s current reporting period represented data collected two years prior.

The measures of success for a CITES listing really come from accurate longitudinal data. As long as countries around the globe participate in and strengthen monitoring practices, Foster and her team are confident it can be a useful tool in the conservation of species. But the team is cautious in the case of seahorses. CITES listings may have little bearing on species caught as by-catch. Commercial fishers who cash in on seahorses from by-catch are capitalizing on a market demand for a fishing pressure that they have already exerted, even if accidentally. The question of whether or not they can undo that pressure, either by altering gear types or by releasing non-targeted species, is a complicated one. In the end, the decision may not rest squarely on the shoulders of the international community, but on our species.


You can find this paper in Aquatic Conservation: Marine and Freshwater Ecosystems

Foster, S., Wiswedel, S., & Vincent, A. (2014). Opportunities and challenges for analysis of wildlife trade using CITES data – seahorses as a case study. Aquatic Conservation: Marine and Freshwater Ecosystems, http://doi.org/10.1002/aqc.2493

For the latest on seahorse research, conservation, and news, see Project Seahorse




Conservation of Amsterdam Albatrosses

By Samantha Owen, RJD Intern

This paper outlines the current conservation efforts for the Critically Endangered Amsterdam albatross (Diomedea amsterdamensis) and the threat posed by industrial longline fisheries. In 2007 a population survey estimated that there were only 167 Amsterdam albatrosses in the world.  This is largely because they are only found in one place, Amsterdam Island, in the southern Indian Ocean.  Their population declined dramatically in the 1960s and 1970s due to the increase in industrial longline fishing targeting bluefin tuna.  While diving below the surface of the water when feeding, birds can be accidentally hooked or entangled in the longlines.

Like most albatrosses, this species is a biennial breeder, which means they only breed every other year. In between breeding years, they spend the entire break year roaming at sea.  A successfully mated pair will produce only one egg per breeding year.  This means that with such a small population, any mortality could have a huge impact on the viability of this species.  The established threshold to trigger a population decline is a loss of more than six individuals to bycatch per year. The potential number of individuals removed from the Amsterdam albatross population each year due to longline fishing is 2-16 depending on whether mitigation measures such as tori lines, plastic streamers trailing from the back of the boat used to scare birds away, were systematically employed.

amsterdam albatross

This paper quantifies the potential threat from industrial longline fishing fleets to the Amsterdam albatross based on time of year and life stages.  It shows that even though the Amsterdam albatross is potentially in contact with longline fisheries at every stage of its life, non-breeding individuals have a much higher susceptibility due to their significantly increased roaming area during their break year at sea. The time of year when Amsterdam albatrosses are at the highest risk for mortality as a result of bycatch in longlines is the austral winter (July, August, September) when fishing fleets are targeting albacore and other tunas.

The Taiwanese longline fishing fleet poses the greatest threat to Amsterdam albatrosses, followed closely by the Japanese fleet. One reason it is thought that the Taiwanese fleet has such a high impact on the Amsterdam albatross is because they deploy the most longlines in the waters immediately adjacent to the species’ home, Amsterdam Island.

In conclusion, this paper states three recommendations for further conservation efforts. First, increasing the coverage of fishing operations by dedicated observers in the distribution range of the Amsterdam albatross during the austral winter.  This would ensure the successful implementation of bycatch mitigation measures such as tori lines. The second recommendation is for Regional Fisheries Management Organizations (RFMOs) such as the Indian Ocean Tuna Commission (IOTC) to require all operating vessels to report ring recoveries.  All Amsterdam albatrosses have been fitted with leg bands (rings) identifying each individual. Although it would not directly prevent bycatch, reporting all recovered rings would allow scientists to more accurately define population-specific bycatch patterns in regional areas resulting in more targeted conservation efforts.  The third recommendation is to implement regulations on fishing efforts in the waters surrounding Amsterdam Island during the austral winter.  The combination of these three conservation efforts would allow the world’s only population of the Amsterdam albatross to grow and prevent any further decline that might very well result in the extinction of the species.



Thiebot J.B., Delord K., Barbraud C.B., Marteau C., Wemerskirch H. 2015. 167 individuals versus millions of hooks: bycatch mitigation in longline fisheries underlies conservation of Amsterdam albatrosses. Aquatic Conservation: Marine and Freshwater Ecosystems. DOI: 10.1002/aqu.2578

Challenges in seabird by-catch mitigation

By Hanover Matz, RJD Intern

In this paper, the authors comment on the current conservation status of seabirds and attempts to limit seabird deaths due to by-catch. Two species of seabirds, the albatrosses and the petrels, are particularly vulnerable to the detrimental effects of fisheries such as longlining. These birds normally lay only one egg per clutch and breed infrequently. They have long maturation and generation times compared to other birds, making it more difficult for their populations to recover from high mortality. They are also capable of flying long distances in search of food, crossing many different marine environments. This makes it difficult to implement conservation methods that can protect these birds in every part of the world they inhabit. Some of these species are already considered endangered or critically endangered. In order to fully protect them, an international effort is necessary.

A wandering albatross (Diomeda exulans) off Tasmania, Australia. Photo courtesy of JJ Harrison via Wikimedia Common

A wandering albatross (Diomeda exulans) off Tasmania, Australia. Photo courtesy of JJ Harrison via Wikimedia Commons

Seabirds and human fisheries come into conflict in many of the most productive regions of the ocean, specifically around New Zealand and Australia, the Humboldt Current off Chile, Peru, and Ecuador, the North Pacific, and South Africa. Seabirds are known to be killed as accidental by-catch in longline fisheries, and growing evidence has shown incidental catch of seabirds by trawlers. One difficulty in assessing whether trawling or longlining presents a greater threat to seabirds is the low proportion of entangled seabirds actually recovered from trawling gear. If the birds that collide with the gear cannot be retrieved, it is hard to assess the impact the fishery has. Refining the collection of data on how many seabirds are killed by longlining and trawling will improve conservation efforts.

In South Africa, the use of bird-scaring lines (BSLs) in fisheries has been shown to reduce the mortality of seabirds up to 95%. The trawl fishery previously had proportionally high incidental catches of albatrosses, making this a significant success in terms of protecting threatened species. However, to fully determine how well seabird mortality has been reduced, better data needs to be collected on both the level of by-catch and fishing effort. To reduce the by-catch of seabirds and improve conservation worldwide, the authors stress four important strategies. First, mitigation methods need to be improved with better data and techniques, considering each fishery individually and adapting the methods as necessary. Second, the quality of data collected needs to be increased by improving the programs used to collect it. Third, the fishing industry needs to be engaged by implementing and enforcing by-catch reduction, as well as cooperating to suit the needs of both the fishery and conservation. Finally, cooperation between governments, administrators, and decision makers is necessary to promote better fishing practices and conservation measures. In some fisheries, seabird by-catch mitigation is minimal or nonexistent. While trawling and longlining have been addressed, the effects of purse-seine and gill net fisheries are poorly understood. The threat posed by small scale and artisanal fishing fleets has also not been widely considered. If threatened seabird species are to be protected, it will require both national and international efforts. By improving the science behind the conservation, and cooperating with both governments and fisheries, scientists and conservations will be better able to address this conservation issue in the coming future.


1. Favero, M., & Seco Pon, J. P. 2014. Challenges in seabird by‐catch mitigation. Animal Conservation, 17(6), 532-533.



Economic vs. Conservation: Trade-offs between Catch, Bycatch, and Landed Value in the American Samoa Longline Fishery

By Laurel Zaima, RJD Undergraduate Intern

Commercial fisheries have prioritized maximum economic profit over the ecological distresses caused by their fishing practices. Consequently, unsustainable fishing practices hook high amounts of bycatch in relation to the amount of the target species. Bycatch are the animals that are accidentally caught and discarded due to lack of value, insufficient size, damaged, or regulatory reasons. Bycatch has detrimental effects on the populations of a diversity of marine species; therefore, has altered ecological relationships and the economics of commercial fisheries. A seemingly obvious solution to this threat would be the implementation of commercial fishing gear that mitigates bycatch. However, this resolution results in trade-offs among the catch, bycatch, and landed value. In their scientific research paper, Trade-offs among Catch, Bycatch, and Landed Value in the American Samoa Longline Fishery, Watson and Bigelow (2013) assess the benefits and disadvantages of modifying longlines to reduce bycatch in the American Samoa longline.

Longline fisheries often modify their fishing gear to the behavior characteristics of their target species in order to have the most catch per unit effort.  Shallow hooks (<100 m) would be set to target yellowfin tuna and billfish, where as, deep hooks (>100 m) will be set to target albacore and bigeye tuna. The U.S. longline fishery based in American Samoa target a majority of their valued species in deep water, such as albacore. Unfortunately, their current fishing practices are not modeled after the behavior of their target species and have led them to catch tons of bycatch. Non-targeted species, such as green sea turtles, silky sharks, and oceanic whitetip sharks spend majority of their life near the surface, and are susceptible to longlines set in shallow waters (<100 m) or hooks passing through the surface during the setting or retrieval of hooks. The elimination of shallow water hooks or the redistribution of shallow hooks to deeper depths could help reduce bycatch and increase the landing of target species.

Watson and Bigelow (2013) modified the American Samoa fishery’s longline fishing gear by removing the shallowest hooks per section of the longline or by hypothetically redistributing the shallowest hooks to a deep position. In the first three scenarios, Watson and Bigelow (2013) eliminated the first hooks, the first and second hooks, and the first, second, and third hooks at both ends of each section.  In the other three scenarios, the hooks were theoretically rearranged into deeper depths by extending the number of sections.

Picture 1

A Longline section has about 23-36 hooks between two floats, and each longline has up to ~100 sections. In Watson and Bigelow’s (2013) study, they modified the longline by either eliminating the shallowest hooks or by hypothetically reallocating them into deeper positions.

They found that there is a decrease in all catch, including a significant decrease in bycatch, by eliminating shallow hooks from longline sections. By reallocating the shallow hooks to deeper positions, there was an effective bycatch reduction while still sustaining target species landings. In terms of economic profit, it would be most beneficial for longline fisheries to redistribute their hooks to deeper positions because it increases their chances of catching the most valuable species. Specifically, there was an increase in catch of the three most valuable species in the American Samoa fishery, albacore, yellowfin, and bigeye tunas, with the redistribution of hooks. The increased catch of these 3 species alone would increase the total annual landed value by an estimated U.S. 1.4 million dollars.

In terms of conservation benefits, the removal of the first three shallow water hooks reduces bycatch for a variety of species, including 25 species of fish, sea turtles, billfishes, and some shark species. Although there are ecological advantages to the elimination or redistribution of the shallow water hooks, there are some economic trade-offs. In the scenarios of elimination and redistribution, there is a loss in landed value for wahoo, billfishes, and dolphinfish. However, the catch of tuna would probably compensate for the loss value of these species. There is also a possibility of an unintentional trophic cascade with decreased catches of billfish (tuna predators) because it could increase the predation on a fishery targeted tuna species. Another potential trade-off would be the increased bycatch of deeper dwelling vulnerable species, such as shortfin mako sharks and the bigeye thresher sharks.

Picture 2

A thresher shark is caught on a longline as bycatch.

Watson and Bigelow’s (2013) modifications to the hook distribution on longlines should be considered for implementation by longline fisheries that target deeper residing species. Depending on the location of the fishery, these longlines could have varying economic and ecological results. Nonetheless, adjusting the longline hooks to specifically target a species is the most feasbile way to reduce bycatch while sustaining target species catches.