Fishery managers are increasingly using ecosystem-based approaches, in many cases based on multi-species, multi-objective models . In the case of tuna and billfish fisheries, spatio-temporal conservation measures are used to manage both targeted catch and bycatch of highly migratory species, including seabirds, marine turtles, marine mammals, and sharks and rays (‘species of special interest’ in the BMIS).
Spatial and temporal measures aim to avoid or minimise bycatch by either temporarily or permanently moving fishing out of an area (e.g., time and area closures, marine protected areas), or requiring that particular mitigation techniques be adopted in an area. They are predominantly mandatory (i.e., fisheries regulations) but can be voluntary and the areas that they apply to may be dynamic, e.g., around an ocean front, or static, e.g., around a seamount or below a specified latitude.
Spatial and temporal measures recognise that bycatch interactions are not uniform through space and time. Distributions of highly migratory bycatch species may vary across oceans through the year.
Aggregations are influenced by the interplay between physical factors and species biology and behaviour. Physical factors include sea surface temperatures, topographic features (e.g., seamounts, shelf breaks) and oceanographic features (e.g., currents, fronts, gyres) . In combination with species biology, these factors influence species behaviour regarding, for example, feeding, breeding, nurseries and migration.
Mandatory, static spatio-temporal measures are used by tuna RFMOs to address bycatch of ‘species of special interest’. They commonly require the use of particular mitigation techniques in specified areas, such as the WCPFC’s CMM-2018-03 Conservation and Management Measure to Mitigate the Impact of Fishing for Highly Migratory Fish Stocks on Seabirds which states that “CCMs shall require their longline vessels fishing south of 30 degrees S, to use either (a) at least two of these three measures: weighted branch lines, night setting, tori lines; or (b) hook-shielding devices”.
Bycatch management is becoming increasingly dynamic. Technologies such as satellite telemetry collect information on both bycatch and target species to identify and characterize their habitat . These habitat data can be used with fisheries data, such as effort and bycatch interaction rates,  to identify habitat or interaction ‘hotspots’ and inform dynamic management. For example, a recent study identified the principal environmental variables influencing the co-occurrence between whale sharks, baleen whales and tuna purse-seine fisheries . These mega-fauna were mostly observed in productive areas during particular seasons.
Hawaii’s ‘TurtleWatch’ fleet communication programme provides an example of dynamic bycatch management. TurtleWatch displays a daily map with real-time sea surface temperatures (SST) and ocean current information as well the predicted locations of waters used by loggerhead and leatherback turtles . With this information, it is hoped that fishers will voluntarily avoid the areas of highest potential bycatch. The threat of closure of the shallow-set pelagic swordfish fishery if sea turtle bycatch exceeds a regulated level provides an economic incentive.
Using spatial temporal measures to manage odontocete depredation and bycatch has had limited success. Fader et al  outline the difficulties in managing bycatch where depredating odontocetes may target similar oceanographic features as those sought by vessels in Hawaiian and Atlantic pelagic longline fisheries. However, their review of avoidance strategies suggests that improved reporting of depredation interactions and communication among fishing vessels could help fleets avoid acoustic detection when depredation has been observed in a particular location.
WCPFC’s CMM-2019-04 Conservation and Management Measure for Sharks provides an example of a mandatory, dynamic spatio-temporal bycatch mitigation measure. This measure prohibits flagged vessels from setting a purse seine on a school of tuna associated with a whale shark if the animal is sighted prior to the commencement of the set.
Spatial and temporal measures can also apply vertically, to zones in the water column, and to time of day. For example, a deep setting requirement is a type of spatial measure because it entails soaking hooks below a prescribed depth to avoid sea turtle bycatch - sea turtles spend the majority of their time at depths of less than 40m . Similarly, whether hooks are set in shallow (<100 m) or deep (>100 m) water, or whether the set is made during the day or night, can have consequences for the relative vulnerability of shark species with different habitat preferences . Time of day is an important factor in bycatch of seabirds during longline setting. Night setting requires setting to be started and finished during the hours of darkness, between nautical dusk and dawn, to avoid periods when most seabirds are actively foraging . This is recognised in WCPFC’s CMM-2018-03 (above).
Abilities to enforce mandatory spatio-temporal bycatch mitigation measures are improving with the use of surveillance systems such as vessel monitoring systems (VMS) and electronic logbooks, as well as advances in electronic monitoring methods using onboard cameras. These developments will help managers to implement static time-area measures as well as support the adoption of more complex, dynamic measures appropriate to highly migratory bycatch species.
- Birdlife International. 2014. Bycatch Mitigation Fact-Sheet 5 Demersal and Pelagic Longline: Night-setting.
- Cosandey-Godin, A. and Morgan, A. 2011. Fisheries Bycatch of Sharks: Options for Mitigation. Ocean Science Division, The Pew Environment Group.
- Dunn, D.C., Boustany, A.M., and Halpin, P.N. 2011. Spatio-temporal management of fisheries to reduce by-catch and increase fishing selectivity. Fish and Fisheries 12(1): 110–119. doi:10.1111/j.1467-2979.2010.00388.x.
- Escalle, L., Pennino, M.G., Gaertner, D., Chavance, P., Delgado de Molina, A., Demarcq, H., Romanov, E. and Merigot, B. 2016. Environmental factors and megafauna spatio-temporal co-occurrence with purse-seine fisheries. Fisheries Oceanography 25(4): 433-447. doi:10.1111/fog.12163.
- Gilman, E., Bianchi, G., and Attwood, C. 2009. Guidelines to reduce sea turtle mortality in fishing operations. Food and Agriculture Organization of the United Nations, Rome. Available from http://stcturtle.com/pdf/documents/GuideToReducingSeaTurtleMortalityInF….
- Gilman, E., Clarke, S., Brothers, N., Alfaro-Shigueto, J., Mandelman, J., Mangel, J., Petersen, S., Piovano, S., Thomson, N., Dalzell, P., Donoso, M., Goren, M. & Werner, T. 2008. Shark interactions in pelagic longline fisheries. Marine Policy 32(1): 1-18.
- Fader JE, Elliott BW, Read AJ (2021) The Challenges of Managing Depredation and Bycatch of Toothed Whales in Pelagic Longline Fisheries: Two U.S. Case Studies. Front Mar Sci 8:. https://doi.org/10.3389/fmars.2021.618031
- Howell, E.A., Hoover, A., Benson, S.R., Bailey, H., Polovina, J.J., Seminoff, J.A., and Dutton, P.H. 2015. Enhancing the TurtleWatch product for leatherback sea turtles, a dynamic habitat model for ecosystem-based management. Fisheries Oceanography 24(1): 57-68. doi:10.1111/fog.12092.
- Howell, E.A., Kobayashi, D.R., Parker, D.M., Balazs, G.H., and Polovina, J.J. 2008. TurtleWatch: a tool to aid in the bycatch reduction of loggerhead turtles Caretta caretta in the Hawaii-based pelagic longline fishery. Endangered Species Research 5: 267-278. doi:10.3354/esr00096.
- WCPFC. 2012. Conservation and Management Measure to Mitigate the Impact of Fishing for Highly Migratory Fish Stocks on Seabirds. CMM-2012-07.