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Stock Assessment of Lemon Sole (Microstomus Kitt) in Icelandic Waters Using a Gadget Model

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Stock Assessment of Lemon Sole (Microstomus Kitt) in Icelandic Waters Using a Gadget Model grocentre.is/ftp Final Project 2019 STOCK ASSESSMENT OF LEMON SOLE (MICROSTOMUS KITT) IN ICELANDIC WATERS USING A GADGET MODEL Huixia Chen College of Fisheries and Life Science Dalian Ocean University, China No. 52, Heishijiao Street, Shahekou District, Dalian, China Email: [email protected] Supervisors: Guðjón Már Sigurðsson: [email protected] Pamela J. Woods: [email protected] ABSTRACT The Gadget modelling framework was used to build a stock assessment model to analyze lemon sole population dynamics in Icelandic waters and establish its trends in recruitment and fishing mortality. Both age and length estimation by the model fit well with the observed data. Most fish caught were between age 5 to 10, and of length between 25 cm to 40 cm. The highest proportion of the catch was between age 6 and 7. The estimation of the length-age growth curve fit well with the observed commercial data for most of the years. However, the fit was not as good for the survey data, especially for the older fish. The simulated abundance trends showed that the amount of smaller fish has been stable recent years, while the amount of bigger fish (35 cm and larger) has increased in last five years. The simulated trends of the abundance of younger fish did not fit well at the highest abundance values while the fit is better for older fish. The abundances indices of fish between 10-35 cm length were relatively stable, while larger fish (between 35-60 cm) increased noticeably between 2010 to 2018. Estimated fishing mortality had fluctuated between 1990 to 2005 while it has been relatively stable for the last five years around 0.25-0.3. The biomass of mature lemon sole increased from 2,500 tonnes in 1990 to 11,000 tonnes in 2018. Since 1997, recruitment has been variable between 10 to 17 million fish per year. This paper should be cited as: Chen, H. 2020. Stock assessment of Lemon sole (Microstomus kitt) in Icelandic waters using a gadget model. UNESCO GRÓ Fisheries Training Programme, Iceland. Final project. http://www.grocentre.is/ftp/static/fellows/document/Huixia19prf.pdf Chen TABLE OF CONTENTS List of Figures ................................................................................................................ 4 1 Introduction ............................................................................................................ 6 1.1 Icelandic fisheries ............................................................................................ 6 1.2 Lemon sole (Microstomus kitt) fishery in Iceland .......................................... 6 1.3 Objective ......................................................................................................... 8 1.3.1 Specific objectives ................................................................................... 8 2 Literature review ..................................................................................................... 8 2.1 Fisheries management ..................................................................................... 8 2.2 Stock assessment methods .............................................................................. 9 2.3 Gadget model .................................................................................................. 9 2.4 Lemon sole (Microstomus kitt) biology ........................................................ 10 3 Methodology ......................................................................................................... 11 3.1 Study area ...................................................................................................... 11 3.2 Data collection............................................................................................... 11 3.3 Setup of a Gadget run .................................................................................... 12 3.4 Operating model ............................................................................................ 14 3.4.1 General gadget model framework .......................................................... 14 3.4.2 Specifications of the lemon sole model ................................................. 15 3.4.3 Growth ................................................................................................... 16 3.4.4 Natural mortality .................................................................................... 16 3.4.5 Fishing Selectivity ................................................................................. 16 3.4.6 Maturation .............................................................................................. 16 3.4.7 Recruitment ............................................................................................ 17 3.4.8 Likelihood .............................................................................................. 18 3.4.9 Optimization .......................................................................................... 19 4 Results .................................................................................................................. 19 4.1 Population indices ......................................................................................... 19 4.2 Likelihood score ............................................................................................ 20 4.3 Age distribution ............................................................................................. 21 4.4 Length distribution ........................................................................................ 24 4.5 Growth ........................................................................................................... 27 UNESCO GRÓ – Fisheries Training Programme 2 Chen 4.6 Stock proportion ............................................................................................ 30 4.7 Age composition ........................................................................................... 30 4.8 Fishing mortality ........................................................................................... 31 4.9 Biomass ......................................................................................................... 32 4.10 Catch.............................................................................................................. 33 4.11 Recruitment ................................................................................................... 33 5 Discussion ............................................................................................................. 34 5.1 Effectiveness of Gadget in analyzing lemon sole population dynamics in Iceland ...................................................................................................................... 34 5.2 Population dynamics of Icelandic lemon sole ............................................... 35 5.3 Applicability for developing fisheries research in China .............................. 35 Acknowledgement ....................................................................................................... 37 References .................................................................................................................... 38 UNESCO GRÓ – Fisheries Training Programme 3 Chen LIST OF FIGURES Figure 1. Total catch (landings) of lemon sole by fishing gear since 1994 in Icelandic waters. Taken from (MFRI, Lemon sole, MFRI Assessment Reports , 2019). .............................................................................................. 7 Figure 2. The Icelandic ecoregion and subareas (MFRI, ICES Ecosystem Overviews Icelandic Waters Ecoregion, 2017). .......................................... 11 Figure 3. Schematic description of the Gadget model for lemon sole.Lines represent flow from one model component to the other. Red lines represent the flow of model predictions and observations which are utilized in the likelihood. Black lines indicate the consumption of the mature and immature stock components by fleets. Orange line represents the growth from immature to mature stock components. Grey lines indicate the predictions from survey indices to likelihood. .................................................................................... 13 Figure 4. Estimates of the abundance trends of different length bins fish. The dot points are the observed data and the black line is the simulated value. ....... 20 Figure 5. Weighted likelihood score of age-length distribution and length distribution of bottom trawls, demersal seines and survey respectively, and the likelihood of mature proportion of survey. ............................................ 21 Figure 6. Age composition from bottom trawl. The grey lines are the observed data and the simulated age structures are the black lines. ................................... 22 Figure 7. Age composition from the demersal seines. The grey lines are the observed data and the simulated age structure are the black lines. .............. 23 Figure 8. Age composition from the survey. The grey lines are the observed data and the simulated age structure are the black lines. ..................................... 23 Figure 9. Residual bubble plots for predication of age distributions by bottom trawls (the left one), demersal seines (the middle one) and survey fleets (the right one). The red bubble means the positive prediction, the blue bubbles means the negative prediction. ..................................................................... 24 Figure 10. Length composition from the catch of bottom trawls.
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