First assessment of orange roughy stocks in the South Indian Ocean indicates their abundance is healthy condition

The Scientific Committee’s first stock assessment, of orange roughy was undertaken at its 3^rd meeting.  Patrick Cordue, who has a long history of undertaking stock assessments of this species for the New Zealand government was contracted to undertake this work.  Cordue is a specialist in the use of NIWA’s stock assessment package CASAL.  This advanced software package is becoming the international standard in the assessment and management of fish stocks, including some of the world’s most prized species. A NIWA-developed advanced software package is becoming the international standard in the assessment and management of fish stocks, including some of the world’s most prized species.

A stock structure hypothesis considered sea floor features, presence and timing of spawning aggregations and length frequency of the catches.  The catch history is well defined from 2002 onwards but is uncertain in 2000 and 2001 when a large number of non-CP vessels fished in the area.  Ageing studies showed fish up to ~140 years to be present and most age classes were present.  Eight acoustic survey biomass estimates were available from five different features collected from 2007 to 2015 during peak spawning.  The model examined seven areas and did not indicate any poor fits.  The results estimated that spawning stock biomass was above 50% of the virgin biomass for the base analysis.  Projections of constant catch for the next 5 years at the 2017 catch level indicated that the median SS₁₇ would not go below 50%.  It was stressed that (a) stock structure remains uncertain and the SC should consider options for obtaining information to better support stock definitions; (b) age frequency data should be collected routinely; and (c), target strength remains uncertain.

The SC agreed that that the stock assessment could be used to provide advice to the Meeting of the Parties and that a Management Strategy Evaluation could be used to test different harvest strategies.  It was noted that uncertainty in stock structure delineation existed and that the results are preliminary, based on the available information and expert judgement.  It was agreed that genetic techniques should be explored to investigate stock structure.

Perversely, it was concluded that the EDSU mean and geostatistical variance should be adopted as used in current Australian and New Zealand stock assessments, though New Zealand does not always use this approach.  Also, the net-attached Acoustical Optical System should be used in areas of complex species mixing and or over steep slopes to reduce bias and sampling error.  Of concern was that potential error in the target strength estimate could result in a bias of a factor of 2

The SC noted the following advice to the Meeting of the Parties: 

• The median estimates for the Walters Shoal Region from the base model and eight sensitivity analyses varied between 63%SSB0 and 85%SSB0. The median estimate of the Base model was 76%SSB0.
• Projections for the Walters Shoal Region (assuming the Base model current stock spawning biomass estimate of 67–87%) indicate that the stock in this sub-region is unlikely to fall below 60%SSB0 in the next 5 years if future catches do not exceed those of 2017.
• The absolute scale of the Walters Shoal Region stock is uncertain because the true scale of the acoustic biomass estimates is poorly known. Virgin biomass (B0) is likely to be in the range of 25,000–90,000 t.
• The assessments of North Walters, Seamounts and Middle Ridge using the simple Bayesian assessment with acoustic biomass estimates estimated SS17 to be at or above 70%SSB0 for each of these stocks.
• The assessments of Meeting, South Ridge and North Ridge stocks using the catch-history-only method estimated ss17 for all stocks to be at or above 43%SSB0 assuming 40%Umax and above 92%SSB0 assuming 5%U_max.
• The SC requires further direction from the Meeting of the Parties in regard to reference points A 3-5 year assessment schedule was considered appropriate for orange roughy but if catch or effort changes by at least 20% in any year the SC should review this decision.