Acoustic tagging: abundance indices and habitat use Juvenile southern bluefin tuna in Western Australia Alistair Hobday, Ryo Kawabe, Ko Fujioka, Tomoyuki Itoh, Kazu Miyashita, Yoshimi Takao
Recruitment index: useful for stock management Can provide information on age classes not sampled by fishery Early warning of stock trends Age 1
Transect lines survey area 1996 2000 1997 2001 1998 2002 1999 2003 T survey ~ 2 days; 8 12 repeats / year
Distribution of fish schools on shelf -34.3-34.4-34.5-34.6-34.7 Latitude -34.8-34.9-35 -35.1-35.2 < 5 t 5-10 t 10-15 t > 15 t -35.3 118.5 119 119.5 120 120.5 121 121.5 Longitude
Sampling the fish with sonar survey - biases Fish must be offshore Depth > 10m Plus, residence time (T R )
Abundance index N fish ~ index ~ E fish (observed in survey) Generating a realistic abundance index requires that the fraction of the population that is surveyed is known electronic tags can help. N fish ~ E fish. D detection. F offshore.t survey /T residence Hobday et al 2009
Acoustic tags and receivers
Cross-shelf lines Subsurface moorings (receiver at 25 m, bottom depth to 150m) Line 3 Lumps Line 2 Line 1 50 nm
1. Depth distribution (D detection ) N fish ~ E fish. D detection. F offshore.t survey /T residence Hidden from sonar: 70% detections Willis & Hobday 2007
2. Fraction in survey area (F offshore ) N fish ~ E fish. D detection. F offshore.t survey /T residence Line 2 100 90 80 32-72% % Cummulative Frequency 70 60 50 40 30 2003: Offshore=32% 20 2004: Offshore=45% 2005: Offshore=27% 2006: Offshore=70% 10 2007: Offshore=33% 2008: Offshore=30% 2010: Offshore=72% 0 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 Station number Inshore Survey area
3. Residence time (T residence ) N fish ~ E fish. D detection. F offshore.t survey /T residence Range: 23-65 days 100 90 T residence = half life = 47 days Number remaining to be detected 80 70 60 50 40 30 20 10 0 0 20 40 60 80 100 120 140 160 Day of study
Interannual variation (e.g. summer current) 2002: 01-Dec-2002 to 01-Jan-2003 2003: 01-Dec-2003 to 01-Jan-2004 2004: 01-Dec-2004 to 01-Jan-2005 22 2002-2003 Mean SST: 18.4 2003-2004 Mean SST: 18 2004-2005 Mean SST: 18.7 21 2005: 01-Dec-2005 to 01-Jan-2006 2006: 01-Dec-2006 to 01-Jan-2007 2007: 01-Dec-2007 to 01-Jan-2008 20 2005-2006 Mean SST: 17.5 2006-2007 Mean SST: 19.4 2007-2008 Mean SST: 18.7 19 18 2008: 01-Dec-2008 to 01-Jan-2009 2009: 01-Dec-2009 to 01-Jan-2010 2008-2009 Mean SST: 19 2009-2010 Mean SST: 18.8 17 16
Variation in habitat use in the survey area mechanism Oceanographic Driver Habitat use response by SBT Leeuwin Current (warm) Inshore distribution Remain in region Sub-Antarctic Water Subsurface intrusion crosses shelf (and upwelling) (cold) Offshore distribution Depart quickly But thermal tolerance of fish is not challenged Fujioka et al. 2010, Fish. Oceanog
Abundance index Now can estimate all these parameters: N fish = E fish - counts (survey data) D detection - depth distribution (tags) F offshore. cross shelf detections (tags) T survey /T residence - residence times (tags) M pathway - what fraction move into survey area?
2006-07 Experiment 3 lines, 3 hotspots (70 receivers) -32 Large SW Australia-Indian Ocean Map -32.5 130 tags -33-33.5-34 -34.5 62 64 63 65 6768 66 69 70 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 44 43 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61-35 1 23 45678910 11 12 13 14 15 16 17 18 19 20-35.5 115 116 117 118 119 120 121 122
2006-07 Experiment Dec & Jan tagging 1/46 west coast releases = 2% move to the south 47% detected (62/130)
2007-08 Experiment - Dec & Jan tagging 1/27 west coast releases = 4% 46% detected (57/125)
2009-10 Experiment - Dec & Jan tagging 3/82 west coast releases = 4% (south coast move to west coast: n=3) -32-32 -32.5-32.5-32.5 153 152 151 150 149 148 147 146 145 144 143 142 141 140 139 138 137 136 135 134 133 132 131 130 129 128 127 126 125 124123 123 123 122 121 120 122 119 118 117 116 115 114 113 120 119 118 117 112 111 110 109 153 152 151 150 149 148 147 146 145 144 143 142 141 140 139 138 137 136 135 134 133 132 131 130 129 128 127 126 125 116 115 108 107 106 114 113 112 111 105 104 103 110 109 108 107 106 105 104 103 101 102 101 102 153 152 151 150 149 148 147 146 145 144 143 142 141 140 139 138 137 136 135 134 133 132 131 130 129 128 127 125 126 124123 122 121 120 119 118 117 116 115 114 113 112 111 110 109 108 107 106 105 104 103 101 102 All 52 detected tags. Total Pings: 2447 All 47 45 52 37 detected tags. Total Pings: 1949 2415 2059 972 36% detected (52/146) -33-33 Latitude -33.5-33.5 Latitude -34-34 -34-34.5-34.5-34.5-35 -35-35 -35.5 82 81 81 86 85 82 87 89 86 85 91 87 93 92 89 91 82 81 96 93 92 98 97 100 996 98 97 86 85 100 9987 91 93 92 97 96 21 2223 24 21 2526 69 22 23 272829 24 25 69 26 27 2933 3334 66 21 22 23 24 25 69 26 34 40 66 27 2933 34 40 66 40 98 97-35.5 99 114 115 116 100 117 118 119 120 121 114 115 116 117 118 119 120 121 Longitude Longitude 63 63 63 100 k m 100 km 100 km
2010-11 Experiment: Nov & Jan tagging 9 18 6 19 25 8 5 10-32 153 152 151 150 149 148 147 146 145 144 142 143 141 140 139 138 137 136 135 134 133 132 131 130 128 129 127 126 125 124 123 122 121 120 119 118 117 115 116 114 113 112 111 110 109 108 107 105 106 104 103 101 102-32.5 100-33 -33.5 (45, 55) -34-34.5 Large SW Australia-Indian Ocean Map -35-35.5 85 84 82 83 81 89 90 87 88 86 96 94 95 92 93 91 100 99 97 98 115 116 117 118 119 120 121 122 (Nov 2010, n=45; Jan 2011, n=55)
Conclusion 1: Inferences for habitat use Local oceanographic conditions influence habitat use on the south coast Leeuwin current and sub-antarctic water mass intrusion Fish tagged on the west coast in December and January only rarely detected on the south coast (5/155, ~3%). During the austral summer, juvenile SBT remain local, presumably in areas where forage is available. Movements to the south coast must occur earlier in the season in order for fish to appear on the south coast a hypothesis we are testing in the present year. Interannual variation in the drivers of migration (M fraction ) unknown
Conclusion 2: Inferences for recruitment survey When fish are more inshore than offshore Missed in current survey area Fish moving in general east direction Count against the flow of fish (ship) Resident for ~30-90 days Some prospect for double counting Impact on acoustic survey design Change in survey design (reduce far offshore) Addition of in-out piston line Ultimately, tagging data may improve the recruitment estimates for tuna N fish ~ E fish. D detection. F offshore.t survey /T residence.m fraction