One of the greatest threats to global marine biodiversity is the overexploitation of bycatch and target species in marine capture fisheries. The primary mortality sources of bycatch, as well as other linked species like seabirds, sea turtles, marine mammals, and sharks, are due to the purse seine and pelagic longline tuna fisheries. Substantial progress is being made at identifying gear technology solutions but more comprehensive consideration is necessary to identify conflicts and mutual benefits from mitigation methods. There is a lack of performance standards along with inadequate observer coverage for all oceanic purse seiners and incomplete data collection, all of which hinder assessing measures efficacy.
Gilman, E.L. 2011. Bycatch governance and best practice mitigation technology in global tuna fisheries, Marine Policy 35, 590–509
Underneath the large umbrella of state laws and international codes of conduct, States and ocean users develop and apply environmentally safe and selective fishing gear practices to maintain biodiversity, structure, processes, and services. These practices are meant to minimize waste and bycatch, bycatch being defined as retained catch of non-targeted fish, discarded catch, and unobserved mortalities. Bycatch may contain a variety of different species which are critical to maintaining the function and structure of the ecosystem as well as the continued provision of services provided by the ecosystem.
Bycatch and its overexploitation is the largest driver in the change and loss of marine biodiversity, primarily affecting k-selective species, species with sporadic recruitment, and even species with high fecundity. In 1992–2001, averages of 7.3 million tons of fish were discarded annually, presenting 8% of the world catch. Marine capture fisheries have negatively affected genetic diversity and environmental integrity, altering the distribution of fish size and reducing reproductive potential, possibly changing the evolutionary characteristics of populations. Unsustainable bycatch fishing mortality of some species, in particular if they are keystone or foundation species, can cause extinction cascades, alter trophic interactions, simplify food webs, and change the overall functionality and structure of the system. This directly affects the economic side of fisheries, adversely affecting future catch levels and resulting in allocation issues between fisheries.
Most tuna stocks are fully exploited, overfished, or depleted, as a result of use of purse seine, pelagic longline, and pole-and-line fisheries. At the moment, it is not possible to sustainably increase catches of stock without increasing bycatch levels, chiefly of sea turtles, seabirds, marine mammals, sharks, and juvenile and unmarketable finfish in pelagic and purse seine fisheries. There are multitudes of ways of mitigating bycatch via gear technology, including ways that are specific to area as well as species. To reduce bycatch of birds fishermen should avoid peak periods of bird foraging, reduce detection of bait by dyeing it blue, and limit bird access to baited hooks through underwater setting devices. Using “weak” circle hooks, large whole fish bait instead of squid, setting gear deeper and avoiding hotspots can minimize bycatch of sea turtles, sharks, and marine mammals.
Fishermen themselves must be tapped into for their local knowledge to find effective and practical fishery-specific bycatch solutions. Participation from these fishermen could also lead to the fishing industry themselves developing a sense of ownership for bycatch reduction methods. Methods that are shown to minimize, reduce interactions with, and offset mortality of bycatch should be implemented if they are practical, safe, and economically viable or beneficial. Also, most importantly, a viable mitigation method will not increase bycatch of other unwanted bycatch species or sizes.
Five tuna Regional Fishery Management Organizations (RFMOs) were established to manage global fisheries for tuna and tuna-like species; the Commission for the Conservation of Southern Bluefin Tuna (CCBST), Indian Ocean Commission (IOTC), Inter-American Tropical Tuna Commission (IATTC), International Commission for the Conservation of Atlantic Tunas (ICCAT), and Western and Central Specific Fisheries Commission (WCPFC). All except for IATTC had binding measures on longline sea-bird bycatch; IOTC, ITAAC, and WCPFC require gear technology methods to mitigate turtle bycatch in purse seine fisheries ; IOTC, ITAAC, ICCAT, and WCPFC restrict shark finning practices and prohibit the retention of thresher shark species; all except CCBST have adopted legally binding measures to mitigate the bycatch of juvenile/small tunas and other unmarketable species; and only ITAAC have quantifiable performance standards.
There is also a need for observer data collection, of which only two organizations, IATTC and WCPFC, have close to 100% observer coverage. To support robust assessments of bycatch there must be substantial increases in bycatch data collection, employment of standardized monitoring, open access to regional- and national-level observer program datasets, and determination of how individual datasets can be incorporated.
Illegal, unreported, and unregulated (IUU) tuna fishing further exacerbates overexploitation of bycatch, reaching an annual value of $581 million, the illegal proportion of total tuna landings estimated to be a total of 5%. ICCAT, CCSBT, and IATTC have adopted documentation schemes which are generally unsuccessful in deterring IUU fishing due to weaknesses of corruption, inadequate laws, lack of resources for surveillance, and mis-labeling of seafood.
The overexploitation of tuna and tuna bycatch can be attributed to tuna-RFMO’s inability to fully adopt conservation and management measures via consensus-based decision-making and ability for members to opt out of adopted measures. This is exacerbated by conflicting objectives of distant fishing nations that wish to maintain their dominance and control of fishing populations. This prevents RFMOs from adopting best practice methods as well as resulting in low compliance by Member States. Commercial viable changes in gear technology and methods can in fact reduce nearly all tuna bycatch in tuna fisheries to nominal levels. These methods include voluntary initiatives such as input and output controls, fleet communication, and industry self-policing.