September 2020
This is the final report for INT2019-05: Coral biodiversity in deep-water fisheries bycatch


The overlap in habitat between deep-sea corals and commercial fish species results in unintentional bycatch, particularly for Tier-1 deep-water bottom trawling fisheries. The impact of fisheries on coral communities, which are protected under New Zealand law, has typically been measured as bycatch biomass and estimates of coral diversity from Government Observer records and benthic surveys. Among protected gorgonian corals, the identification of species by in situ observations and morphological study of specimens are known to underestimate species diversity, however. Archived specimens collected by observers were used to examine the genetic diversity of bottom-trawled bycatch gorgonian corals to determine the accuracy and precision of observer and taxonomist identifications, and to re-examine the effects of bottom trawling on protected coral diversity.

Selection criteria were applied to the NIWA coral collections to identify specimens collected as trawl-fishery bycatch that would be amenable to genetic analysis. A final pool of 129 bycatch specimens of gorgonian corals was identified and 91 of these were sampled for genetic analysis, producing viable DNA sequence data for 62 specimens at three genetic markers. Among these, we found a minimum of 34 different species that were distributed among seven protected families of octocorals. Our rate of discovery of unique species indicates that many more species remain unsampled and that we have not yet documented the limits of gorgonian coral diversity within the sampled bycatch community. In addition, our results present the first broad-scale examination of octocoral diversity in New Zealand and demonstrate that many species remain to be discovered and described.

Comparisons of bycatch identification methods indicated an increasing level of precision and accuracy with increased technicality as specimens were progressed from visual identifications by observers, to morphological identifications by taxonomists, to genetic barcoding in this study. Overall, genetic barcoding and morphological study showed similarly high levels of identification accuracy, but barcoding resolved identifications to finer taxonomic scales. Our 8% estimate of undiscovered diversity among bycatch specimens identified with traditional methods is also consistent with previous studies of New Zealand bycatch diversity.

The genetic and taxonomic diversity uncovered here was spread across the New Zealand Exclusive Economic Zone (EEZ) and adjacent South Pacific Regional Management Organisation (SPRFMO) zones. Within the EEZ, bycatch samples were examined from seven Fisheries Management Areas (FMAs) and ten target fisheries. Due to differences in sampling effort and observer coverage by target fishery, the most specimens and most diversity was recovered from observed trips targeting orange roughy. As a first look at the species diversity of octocoral bycatch, our sampling design did not allow for quantitative comparisons between fisheries.

The high diversity of gorgonian octocorals uncovered within bycatch supports a role for genetic barcoding in routine identification and assessment of fisheries impacts. We recommend a regional assessment and comparison of coral genetic diversity among Quota Management Areas for each trawl fishery, and the consideration of evolutionary and genetic diversity in impact assessments and management decisions. Better understanding of the evolutionary processes and timescales that underpin the diversity of affected corals can improve our predictions of how they may be impacted by commercial fisheries, as well as their ability to recover from these impacts. Increased baseline information on the genetic diversity of protected corals and the evolutionary processes that created them would also support conservation efforts by providing intrinsic value to New Zealand’s protected species.

Publication information

Bilewitch, J.P. and Tracey, D. 2020. Coral biodiversity in deep-water fisheries bycatch. Final report for INT2019-05 prepared by NIWA for the Conservation Services Programme, Department of Conservation. DOC19304-INT201905. NIWA Client Report 2020223WN. 36p.


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


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