Assessment of cryptic seabird mortality due to trawl warps and longlines

Published:  

May 2015
This report studies the seabird species and fisheries for which cryptic mortality contributes particularly strongly to the risk on New Zealand seabird populations.

Summary

Understanding the nature and extent of interactions between commercial fisheries and marine protected species is one component of best practice fisheries management. These interactions can lead to mortalities of protected species, which may be detected (e.g., by fisheries observers on vessels), or undetected or not readily detectable (also known as cryptic mortalities). For seabirds, cryptic mortalities may result, for example, when a bird carcass falls into the water after striking a trawl warp, or when a bird is landed alive on deck, removed from fishing gear and released, but later dies as a result of injuries sustained. Internationally, cryptic mortality is considered in a broader sense including, for example, ghost fishing and the consequences of the death of a parent bird on dependent offspring. In alignment with current New Zealand policy articulated in the National Plan of Action - Seabirds (2013), we use the following definition of cryptic mortality in this report: seabird interactions which result in mortality but are unobserved or unobservable.

The assessment of the risk that New Zealand commercial fisheries represent to seabird populations, conducted by Richard and Abraham (2013), considers cryptic mortality using a set of multipliers applied across different fishing methods. These scalars are derived from sources including data collected in New Zealand and internationally. Here, we draw on Richard and Abraham’s (2013) approach, updated in 2014, to identify seabird species and fisheries for which cryptic mortality contributes particularly strongly to the overall assessed risk. We review assumptions and uncertainties inherent in Richard and Abraham’s (2014) methods, as well as relevant new information which may contribute to the development of more robust cryptic mortality scalars applicable to New Zealand fisheries. Finally, we recommend options to improve the estimation of cryptic mortality for the seabird species groups and fisheries where this is identified as particularly important.

From Richard and Abraham’s (2014) assessment, cryptic mortality was especially influential in determining overall assessed risk for both albatross and petrel species, including black petrel (Procellaria parkinsoni) interacting with small-vessel surface and bottom longline fisheries, Salvin’s (Thalassarche salvini) and New Zealand white-capped (T. cauta steadi) albatross interacting with small inshore trawl vessels, and southern Buller’s albatross (T. bulleri bulleri) interacting with large trawl vessels with meal plants. Key assumptions required for the Richard and Abraham (2013, 2014) work included that cryptic mortality scalars derived from fisheries outside New Zealand were appropriately applied to the New Zealand context despite differences in seabird assemblages, fishing operations and gear. Further, scalars applied to cryptic mortality of seabirds due to aerial warp strikes and interactions with trawl nets were based on expert opinion in the absence of empirical data.

Relevant new publically available information that may contribute to refining scalars describing cryptic mortality includes work on cryptic mortality associated with a Falkland Islands demersal trawl fishery, and two new studies reporting the outcomes of seabird strikes on trawl warps. Additional data sources that could prove valuable for the development of improved scalars describing cryptic mortality include observations of seabird interactions with trawl warps and nets that have been collected by government fisheries observers deployed in recent years in New Zealand inshore trawl fisheries, and the databases collected on seabird interactions with trawl fisheries in the Conservation of Antarctic Marine Living Resources Convention Area and off the Falkland Islands.

We recommend the following as next steps to progress the improvement of cryptic mortality scalars applied to New Zealand fisheries:

  • amend the definition of cryptic mortality applied in New Zealand, such that the definition in use is better aligned with international approaches
  • examine existing data sets identified in this report, that are available internationally and in New Zealand, to improve estimates of cryptic mortality for New Zealand species, or species groups, caught in surface longline and trawl fisheries.
  • amend data collection protocols used by New Zealand fisheries observers such that potential cryptic mortalities will be documented routinely, and,
  • develop a data collection programme to support the estimation of method-specific scalars for bottom longline fisheries, especially vessels less than 34 m in overall length.

When these steps are complete, we recommend reviewing the state of knowledge and robustness of cryptic mortality estimates derived, and considering the need for additional at-sea data collection.

Publication information

Pierre, J. P., Richard, Y., and Abraham, E. R. 2015. Assessment of cryptic seabird mortality due to trawl warps and longlines. Report prepared by Dragonfly Data Science for the New Zealand Department of Conservation, Wellington. 51p. 

Contact

Conservation Services Programme
Department of Conservation
PO Box 10-420
Wellington 6143

Email: csp@doc.govt.nz

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