A Word on Data Quality and Limitations

Data systematically collected by observers on-board of industrial fishing vessels represent the most complete and systematic information available to assess bycatch. On-board observers are a relatively recent phenomenon, and the existence of a program, its objectives and its implementation varies by country and by fishery (Lewison et al. 2004). With respect to seabirds, momentum at the international level (Davies & Reynolds 2002), national level (National Plans of Action — NPOAs) and the regional levels (Regional Fisheries Management Organizations — RFMOs) encourages or mandates the use of on-board observers. 

Unloading the catch onboard a King Crab Paralithodes camtschaticus boat, Alaska USA.  Photo by Valerie Craig/Marine Photobank.  

While it is the best information available, observer data is not without its limitations.  The data are frequently limited by a lack of precise information on deployment conditions, location and gear specifications, losses not perceived by observers (e.g. “losses on the line” -- in the most systematic study of its kind Brothers et al. 2010 estimated that 52% of hooked individuals were lost between setting and hauling; observations are typically made during hauling only), and significant challenges in correctly identifying drowned and waterlogged individuals of all life-stages.  New Zealand uniquely addresses identification challenges by necropsying bycaught animals which simultaneously verifies observer accuracy and improves the reporting to the species level (e.g. Thompson 2010).  Observer coverage (the number of vessels or sets observed) can vary within a jurisdiction by year, amongst gear types, boat sizes (e.g. in Alaska, USA see Dietrich & Melvin 2007) or can vary by target fishery within a region (NMFS 2011) and between regions (Lewison et al. 2004, Huang & Yeh 2011).  These disparities limit our understanding of the impact of fishing gear on each species.  Due to its very nature, there is no observer data from IUU (illegal, unreported, unregulated) fishing activities which may represent up to 11-26 million tonnes of fish landed annually (an additional 13-32% of reported catches) (FAO 2014, Agnew et al. 2009).   Any mortality due to derelict gear — “ghost-fishing” — is also underestimated, as this gear is rarely retrieved for any observer or bystander to observe and report (Reeves et al. 2013 but c.f. Good et al. 2009).  Furthermore, the high spatial and temporal variation in seabird distributions renders it difficult to extrapolate observed patterns between seasons or areas (Laich et al. 2006, Gandini & Frere 2006). 

Finally, there are a number of very poorly known seabird species whose ranges and breeding islands are still ill-defined.  As aspects such as positive field identification of these species and their basic biology have yet to be documented, it is therefore highly unlikely that they have been positively identified as interacting with gear


Agnew, D.J., J. Pearce, G. Pramod, T. Peatman, R.  Watson, et al. (2009). Estimating the Worldwide Extent of Illegal Fishing. PLoS ONE 4(2): e4570. doi:10.1371/journal.pone.0004570

Brothers N, Duckworth AR, Safina C, Gilman EL (2010). Seabird Bycatch in Pelagic Longline Fisheries Is Grossly Underestimated when Using Only Haul Data. PLoS ONE 5(8): e12491. doi:10.1371/journal.pone.0012491

Davies, S. L. & J.E. Reynolds.  2002.  Guidelines for developing an at-sea fisheries observer programme.  FAO Fisheries Technical Paper.  No 414.  Rome, FAO: 116 pp. 

Dietrich, K.S. and E.F. Melvin. 2007. Alaska Trawl Fisheries: Potential Interactions with North Pacific Albatrosses. WSG-TR 07-01 Washington Sea Grant, Seattle, WA.  

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Good, T. P., June, J. A., Etnier, M. A., & Broadhurst, G. (2009). Ghosts of the Salish Sea: Threats to marine birds in Puget Sound and the Northwest Straits from derelict fishing gear. Marine Ornithology 37: 67–76.

Huang, H.-W., & Yeh, Y.-M. (2011). Impact of Taiwanese distant water longline fisheries on the Pacific seabirds: finding hotspots on the high seas: Impact of Taiwanese distant water longline fisheries on the Pacific seabirds. Animal Conservation 14(5): 562–574. doi:10.1111/j.1469-1795.2011.00462.x

Laich, A. G., Favero, M., Mariano-Jelicich, R., Blanco, G., Cañete, G., Arias, A., Silva Rodríguez, P. and Brachetta, H.  2006.  Environmental and operational variability affecting the mortality of Black-browed albatrosses associated with long-liners in Argentina. Emu 106(1): 21-28.

Lewison, R. L., Crowder, L. B., Read, A. J., & Freeman, S. A. (2004). Understanding impacts of fisheries bycatch on marine megafauna. Trends in Ecology & Evolution 19(11): 598-604.

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Reeves R.R., McClellan, K., and Werner, T.W. 2013. Marine mammal bycatch in gillnet and other entangling net fisheries, 1990-2011.  Endangered Species Research 20: 71-97.  

Thompson, D. R. 2010. Autopsy report for seabirds killed and returned from New Zealand fisheries, 1 October 2006 to 30 September 2007. Research report for the Department of Conservation, Wellington.