Relative abundance, size and age structure, and stock status of blue cod off Banks Peninsula in 2012 New Zealand Fisheries Assessment Report 2017/37. G. Carbines V. Haist ISSN 1179-5352 (online) ISBN 978-1-77665-640-0 (online) July 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 TABLE OF CONTENTS EXECUTIVE SUMMARY .................................................................................................................... 1 1 INTRODUCTION .......................................................................................................................... 3 1.1 Catch versus count ................................................................................................................. 4 2 METHODS ..................................................................................................................................... 4 2.1 Timing ................................................................................................................................... 4 2.2 Survey area ............................................................................................................................ 5 2.3 Survey design ........................................................................................................................ 5 2.4 Vessels and gear .................................................................................................................... 6 2.5 Sampling methods ................................................................................................................. 6 2.6 Otolith preparation and reading ............................................................................................. 7 2.7 Data analysis .......................................................................................................................... 8 2.8 Drop underwater video (DUV) ............................................................................................ 10 3 RESULTS ..................................................................................................................................... 10 3.1 Sites surveyed ...................................................................................................................... 10 3.2 Catch .................................................................................................................................... 11 3.3 Biological and length frequency data .................................................................................. 11 3.4 Ageing (between reader analyses) ....................................................................................... 12 3.5 Growth ................................................................................................................................. 12 3.6 Length and age composition ................................................................................................ 13 3.7 Total mortality (Z) estimates ............................................................................................... 14 3.8 Spawner per recruit analyses ............................................................................................... 14 3.9 Drop underwater video ........................................................................................................ 15 4 DISCUSSION ............................................................................................................................... 16 4.1 Fixed site survey design time series .................................................................................... 16 4.2 Comparisons of catch rates between survey designs ........................................................... 17 4.3 Reproductive condition........................................................................................................ 17 4.4 Size and sex ratio ................................................................................................................. 18 4.5 Population length and age structure ..................................................................................... 19 4.6 Total mortality (Z) ............................................................................................................... 19 4.7 Stock status (spawning biomass per recruit ratio analyses) ................................................. 20 4.8 Comparison of pot catch and video observations ................................................................ 21 5 ACKNOWLEDGMENTS ............................................................................................................ 22 6 REFERENCES ............................................................................................................................. 22 TABLES……..………………………………………………………………………………………...25 FIGURES……..……………………………………………………………………………………….37 APPENDICES ...................................................................................................................................... 63 EXECUTIVE SUMMARY Carbines, G.D.; Haist, V. (2017). Relative abundance, size and age structure, and stock status of blue cod off Banks Peninsula in 2012. New Zealand Fisheries Assessment Report 2017/37. 126 p. This report describes the results of the 2012 Banks Peninsula blue cod (Parapercis colias) potting surveys. These were concurrent fixed and random site surveys, the fourth fixed site survey since 2002, and the first random site survey in Ministry for Primary Industries blue cod potting survey time series for Banks Peninsula. Both surveys used two-phase stratified designs, with five sites per stratum allocated for phase 1. Between 23 March and 6 May 2012, eighty sites were surveyed (6 pots per site, 480 pot lifts) from five inshore and two offshore strata around southern Banks Peninsula .The catch of each pot was weighed, and the length and sex of blue cod was recorded. Otoliths were read from 551 blue cod collected from both random and fixed sites representatively selected throughout Banks Peninsula. The resulting age-length keys were applied to the scaled length frequency distributions of both fixed and random site surveys to estimate the population age structures. Fixed site survey The fixed site survey used 35 phase 1 sites, with five additional sites allocated in phase 2. Total blue cod catch was 685 kg, consisting of 1096 fish. Blue cod catch rates by stratum ranged from 0.6 to 6.3 kg.pot-1, with an overall mean catch rate of 4.3 kg.pot-1 and coefficient of variation (CV) of 18.1%. Catch rates of legal size blue cod (at least 30 cm) ranged from 0.3 to 6.0 kg.pot-1, with an overall mean catch rate of 4.1 kg.pot-1 and CV of 19.5%. Catch rates were highest in the offshore strata, and lowest at the Akaroa Harbour entrance stratum. At offshore fixed sites 74% of blue cod caught were of legal size, compared to 40% of blue cod at inshore fixed sites. Total lengths recorded at fixed sites ranged from 11 to 57 cm, however the average length was 8 cm larger in offshore strata than inshore strata. The length frequency distributions were mainly bimodal, with more small and fewer large fish caught at inshore sites than at offshore sites. Males were larger than females in all strata and overall mean length inshore was 28 cm for males and 25 cm for females, offshore it was 37 cm for males and 33 cm for females. Overall sex ratios for all and for legal sized fish were 1:0.5 (M:F) and 1:0.1 respectively inshore, and 0.8 and 0.6 respectively offshore. Age ranged from 2 to 35 years, with most blue cod between 4 and 8 years for males and 3 and 6 years for females inshore, and between 6 and 12 years for males and 6 and 15 years for females offshore. The mean weighted coefficients of variation (MWCVs) around the age distributions (about 30% inshore and 40% offshore) indicate that fish sampled in the 2012 surveys provided a reasonable representation of the overall inshore population, but less reasonable offshore. The total mortality estimate (Z) was 0.62 inshore and 0.15 offshore, assuming ages-at-recruitment to the fishery at 8 and 9 years, respectively. The spawning biomass per recruit (F%SPR) estimate indicates that the expected contribution to the spawning biomass over the lifetime of an average recruit has been reduced to 24% of the contribution in the absence of fishing inshore, and 92% of the contribution in the absence of fishing offshore. The level of exploitation (F) inshore is of concern as it is well below the Ministry for Primary Industries target reference point of F40%. Most gonads were in the early maturing stage with 1% of males and 7% of females running ripe. Temporal comparisons between fixed site surveys The overall catch rates from the 2012 fixed site survey were remarkably similar to those recorded in the 2008 fixed site survey for most inshore strata. However, among the two offshore strata there was an increase of 104% in the southern stratum, and a 40% decrease in the eastern stratum. The overall CVs from the 2012 survey catch rates (18%) was more than twice as big as previous surveys (6–7%). Gonad observations were also consistent with previous surveys. Ministry for