Relative abundance, size and age structure, and stock status of blue cod off Banks Peninsula in 2016 New Zealand Fisheries Assessment Report 2017/30 M.P. Beentjes M. Fenwick ISSN 1179-5352 (online) ISBN 978-1-77665-603-5 (online) June 2017 Requests for further copies should be directed to: Publications Logistics Officer Ministry for Primary Industries PO Box 2526 WELLINGTON 6140 Email: [email protected] Telephone: 0800 00 83 33 Facsimile: 04-894 0300 This publication is also available on the Ministry for Primary Industries websites at: http://www.mpi.govt.nz/news-and-resources/publications http://fs.fish.govt.nz go to Document library/Research reports © Crown Copyright - Ministry for Primary Industries Contents 1. INTRODUCTION ........................................................................................................................... 3 1.1 Status of the north Canterbury blue cod stocks ....................................................................... 3 1.2 Blue cod potting surveys ......................................................................................................... 3 1.3 Previous Banks Peninsula blue cod potting surveys ............................................................... 4 1.4 Objectives ................................................................................................................................ 4 2. METHODS ...................................................................................................................................... 5 2.1 Timing ..................................................................................................................................... 5 2.2 Survey area .............................................................................................................................. 5 2.3 Survey design .......................................................................................................................... 6 2.3.1 Allocation of sites ............................................................................................................ 6 2.3.2 Vessel and gear ................................................................................................................ 7 2.3.3 Sampling methods ........................................................................................................... 7 2.3.4 Data storage ..................................................................................................................... 8 2.3.5 Age estimates .................................................................................................................. 8 2.3.6 Data analyses ................................................................................................................... 9 3. RESULTS ...................................................................................................................................... 12 3.1 2016 fixed site survey ............................................................................................................ 12 3.1.1 Fixed sites surveyed ...................................................................................................... 12 3.1.2 Catch (fixed sites) .......................................................................................................... 12 3.1.3 Catch rates (fixed sites) ................................................................................................. 12 3.1.4 Biological and length frequency data (fixed sites) ........................................................ 13 3.1.5 Age and growth ............................................................................................................. 13 3.1.6 Spawning activity .......................................................................................................... 14 3.1.7 Population length and age composition (fixed sites) ..................................................... 14 3.1.8 Total mortality estimates (Z) and spawner-per-recruit (SPR) (fixed sites) ................... 15 3.2 2016 random site survey ........................................................................................................ 15 3.2.1 Random sites surveyed .................................................................................................. 15 3.2.2 Catch (random sites) ...................................................................................................... 16 3.2.3 Catch rates (random sites) ............................................................................................. 16 3.2.4 Biological and length frequency data (random sites) .................................................... 16 3.2.5 Age and growth ............................................................................................................. 17 3.2.6 Spawning activity .......................................................................................................... 17 3.2.7 Population length and age composition (random sites) ................................................. 17 3.2.8 Total mortality estimates (Z) and spawner-per-recruit (SPR) (random sites) ............... 18 3.3 Comparison of fixed and random site 2016 surveys ............................................................. 19 3.4 Inshore survey time series ..................................................................................................... 19 3.4.1 Catch rates ..................................................................................................................... 19 3.4.2 Length distributions ....................................................................................................... 20 3.4.3 Sex ratios and zero catch ............................................................................................... 20 3.5 Offshore survey time series ................................................................................................... 20 3.5.1 Catch rates ..................................................................................................................... 20 3.5.2 Length distributions ....................................................................................................... 21 3.5.3 Sex ratios and zero catch ............................................................................................... 21 4. DISCUSSION ............................................................................................................................... 21 4.1 General .................................................................................................................................. 21 4.2 2016 fixed site versus random site surveys ........................................................................... 22 4.3 Stock status ............................................................................................................................ 23 4.4 Reproductive condition ......................................................................................................... 23 4.5 Time series trends .................................................................................................................. 24 4.5.1 Inshore surveys .............................................................................................................. 24 4.5.2 Offshore surveys ............................................................................................................ 25 4.5.3 Concluding remarks ...................................................................................................... 25 5. ACKNOWLEDGMENTS ............................................................................................................. 26 6. REFERENCES .............................................................................................................................. 26 EXECUTIVE SUMMARY Beentjes, M.P.; Fenwick, M. (2017). Relative abundance, size and age structure, and stock status of blue cod off Banks Peninsula in 2016. New Zealand Fisheries Assessment Report 2017/30 81 p. This report describes the results of blue cod (Parapercis colias) fixed and random site potting surveys carried out concurrently off Banks Peninsula in April 2016. Surveys were split into separate inshore and offshore areas. Estimates are provided for population abundance, size and age structure, sex ratio, total mortality (Z), and spawner-per-recruit. This is the fifth survey in the fixed site survey time series following surveys in 2002, 2005, 2008, and 2012; and the second random site survey, with the previous survey carried out in 2012. Otolith thin section ages from 207 males and 97 females (inshore), and 256 males and 197 females (offshore) were used to estimate the population age structure with separate age-length keys for males and females, and inshore and offshore. Two readers achieved agreement on 83% of read otoliths, and overall there was no bias between readers with a CV of 1.9% and average percent error (APE) of 1.3%. Low maximum ages (8 years for males and 6 years for females) for inshore strata gave improbable von Bertalanffy (VB) parameters, especially L, so the combined inshore and offshore VB parameters were used in estimating spawner-per-recruit ratios. There was no clear indication of spawning activity during the survey period (fixed and random sites). 2016 fixed site survey Forty fixed sites (6 pots per site, 240 pot lifts) at depths of 8–90 m from five inshore and two offshore strata off Banks Peninsula were surveyed in