School whiting (Sillago flindersi ) stock assessment based on data up to 2006 Jemery Day 1 1CSIRO Marine and Atmospheric Research, Hobart, Tasmania Updated 9 November 2007, following discussions held at Shelf RAG September 4-5 2007 and subsequently Enquiries should be addressed to: Jemery Day CSIRO Marine and Atmospheric Research Castray Esp, Hobart, Tasmania 7000 [email protected] phone: 03 6232 5296 © Copyright Commonwealth Scientific and Industrial Research Organisation (‘CSIRO’) Australia 2007 All rights are reserved and no part of this publication covered by copyright may be reproduced or copied in any form or by any means except with the written permission of CSIRO. The results and analyses contained in this Report are based on a number of technical, circumstantial or otherwise specified assumptions and parameters. The user must make its own assessment of the suitability for its use of the information or material contained in or generated from the Report. 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SUMMARY This document presents an assessment of school whiting (Sillago flindersi ) in the SESSF using data up to 31 December 2006. A partial stock assessment for school whiting was last performed in 2004 by Cui et al. (2004) using data from 1991 through until 2003. Given a lack of reliable age- and length- composition data, this assessment just used data from the Commonwealth logbook, and ignored catches taken under state jurisdictions and all catches before 1991. As a result, Cui et al. (2004) were only able to give information about biomass levels relative to 1991 in this assessment. The 2007 assessment is performed using the stock assessment package SS2, which has enabled some technical improvements to be made. Further, catch data were incorporated from 1947, state catches were included and additional length composition data were used dating back to 1983. Changes from the 2004 assessment include: (a) the estimation of the growth parameters within the assessment, (b) the use of conditional age-at-length data (c) the addition of updated length frequencies, catches and catch- rates, (d) the inclusion of discards and (e) including ageing error in the model. The updated base-case assessment estimates that current (2008) spawning stock biomass is 35% of unfished stock biomass. Fits to the length, age, and catch-rate data are reasonable. Exploration of model sensitivity shows that the model outputs are sensitive to the value assumed for natural mortality, M, the length at 50% maturity and the projected catch used in 2007. Likelihood profiles support the use of a base case value for M of 0.6yr -1. Depletion across all sensitivities varied between 22% (age at 50% mortality=18cm) and 47% (age at 50% mortality=14cm). Exploration of model sensitivity also shows that the model outputs are sensitive to the value assumed for the 2007 catch. At the September 2007 Shelf Resource Assessment Group meeting, the NSW state catch component of the 2006 catch data used in the base case was revised. The RBCs for the updated base-case model for 2008 are 1,185 t and 904 t respectively under the 20:40:40 and 20:40:48 harvest control rules, while the long-term yields under these harvest control rules are 1,848 t and 1,685 t respectively. Results are presented here for both the base case and the updated base case. School whiting is a short lived species. Spawning biomass is particularly sensitive to variation in recruitment events and good and bad recruitment years can have a very rapid impact on the fish stock. As a result there will always be some uncertainty about the status of the stock. Further exploration of some biological parameters, such as age and length at maturity may help reduce this uncertainty, but the high mortality rate and short expected life time for this species mean that rapid changes are always possible and projections will always be subject to uncertainty relating to very recent recruitment events which are poorly informed until these cohorts fully enter the fishery. 2. INTRODUCTION 2.1. The Fishery School whiting occur in the eastern regions of the SESSF and Bass Strait (zones 10, 20, 30 and 60) and are commonly found on sandy substrates to depths of about 60m. School whiting are benthic feeders and they mainly spawn during summer. They grow rapidly, reach a maximum age of about 6 years and become sexually mature at about 2 years of age. In the SESSF full recruitment to the fishery occurs at 2-3 years of age although 1 year olds are also taken. The majority of the catch from 1947-1995 has been taken using Danish seine (mainly in zone 60 of the SESSF - Bass Strait) although the fraction of the catch taken by otter trawl has increased recently, and has averaged more than 65% of the total catch since 1995. In contrast to the Danish seine School Whiting Assessment 2007 3 catches, catches by otter trawl occur predominantly in SESSF zone 10, with most of this catch taken by state registered trawlers. Much of the school whiting caught by the Lakes Entrance Danish seine fleet since 1993 has been sent to an export market, although issues with quality of whiting caught in the summer months has reduced catches for the export market during this time. Annual catches (landings and discards) of school whiting are shown (for the updated base case) in Table 6.1 and in Figure 3.2. Large catches of school whiting were first taken in the 1980’s (Smith, 1994) and catches increased to over 2,000 t in 1986. Catches have remained over 1,000t since then, with 1,800 t caught in 2006. Discard percentages are variable and appear market driven. The TAC for calendar years 2005 and 2006 was 1,500 t and for 2007 this was reduced to 750t. The actual catches (for the updated base case) in these years were 1,193t in 2004, 1,080t in 2005 and 1,634t in 2006, but a significant proportion of this catch was from state waters. The state catches in this period were 732t in 2004, 540t in 2005 and 1,056t in 2006 with an average of over 800 t of state catch over the last 10 years. 2.2. Stock structure School whiting is assumed to be a single stock off the east coast of Australia and in Bass Strait, which is largely encompassed by the SESSF but does continue further north above Barrenjoey Point to Ballina. Stout whiting ( Silllago robusta ) is caught off northern New South Wales and the range of these two species overlaps between Ballina and Clarence River, with the northern limit for school whiting at Ballina. NSW catches of stout whiting and school whiting were split equally between the two whiting species in this region where they both occur. 2.3. Previous Assessments A partial stock assessment for school whiting was last performed in 2004 by Cui et al. (2004) using data from 1991 through until 2003. This followed an earlier assessment (Punt 1999). Given a lack of reliable age- and length-composition data, the 2004 assessment just used data from the Commonwealth logbook, and ignored catches taken under state jurisdictions and all catches before 1991. As a result, Cui et al. (2004) were only able to give information about biomass levels relative to 1991 in this assessment. Cui et al. (2004) looked at the probabilities of falling below the 1991 spawning biomass and half the 1991 spawning biomass for 5 different levels of future catch. This assessment predicted large recruitments in 2002 and 2003 but with high uncertainty. As a result the 2003 estimate of spawning biomass was higher than the 1991 spawning biomass, but was also highly uncertain. 2.4. Modifications to the previous assessments A substantial number of modifications have been made to the previous assessment, including updating data to 2006. The previous assessment Cui et al. (2004) was based on integrated analysis written using the software package ADMB. The current assessment uses the package SS2 which overcomes some of the technical shortcomings previously identified. Cui et al. (2004) recommended the following modifications to their assessment: 1. The support for the assessment result that there were more animals aged 6 and older in 1991 would be increased if length-frequency data were available for the earlier years. 2. Future assessments should take account of ageing error when fitting the model. 3. Although no age-length keys are available for 1999 and 2000, some length-frequency data are available. The assessment model should be modified to so that use can be made of these data. 4.