Migration and abundance of bigeye tuna (Thunnus obesus) inferred from catch rates and their relation to variations in the ocean environment.

Transcription

1 Migration and abundance of bigeye tuna (Thunnus obesus) inferred from catch rates and their relation to variations in the ocean environment. PFRP Meeting, Honolulu, 9-11 December, 2003 Pat Hyder, Keith Bigelow, Jeff Polovina, Mike Seki, Bo Qiu, Pierre Flament and Rusty Brainard Acknowledgements: Sibert, Itano, Musyl, Gunn, Hampton, Brill

2 Annual spatial bigeye CPUE distribution Hawaii based long line (Hawaiian waters to 35 N to Palmyra waters to 0 N) CPUE (= fish/hook) α abundance (& catchability) If ca << ab then cpue ~α abun It also depends on uniformity of effort cov. and type data filtered > 10 HBF 40 N Jan/Feb Mar/Apr May/Jun 0 N 180 E 220 E Jul/Aug Sep/Oct Nov/Dec

3 Bigeye inter-annual and annual fish, hook and CPUE variation Inter-annual variation Annual Variation fish fish hooks hooks cpue cpue Time (years from 1990) Increasing effort Marked reduction in CPUE following high catches during the ENSO period Time proportion of the annual cycle Marked annual cycle for the full Hawaiian region with a maximum towards the end of the annual cycle

4 Bigeye CPUE centre of mass (Σ(cpue*lat. or lon.)/σ(cpue)~ mean position) COM Latitude fish COM latitude fish COM latitude hooks COM latitude hooks COM Latitude cpue COM Latitude cpue Time (years from ) Time (proportion of the annual cycle)

5 Bigeye CPUE and temp. (10 & 100m) var. at various latitudes (Temp from JPL ECCO model note little seasonal T variation >100m) Temp CPUE Palmyra - Equatorial ( N) Irregular thermal/cpue cycle High catches associated with the period after ENSO upwelling Temp CPUE Hawaii Sub-tropics (20-25 N) Seasonal thermal/cpue cycle High CPUE associated with preferred thermal range (23-26 C) at end of annual cycle Temp CPUE Years from Northern sub-tropics (30-35 N) Marked seasonal thermal/cpue cycle. Significant CPUE only in third quarter when temp reaches preferred range

6 Latitudinal bigeye CPUE and temperature (10m) bi-monthly variation (JPL ECCO model) hooks fish cpue CPUE ~ continuous despite effort changes. Area under the CPUE curve ~ consistent during the year & represents total stock. Migration >> (A) Mar/Apr (red) - max CPUE southward. (B) Sep/Oct (blue) - max CPUE northward. >> region of maximum CPUE migrates N/S with the region of preferred thermal range (indicated with stars on thermal trace and below) Pref T range (23 to 26) migrates N/S. temp 0 N Latitude 40 N Month/Color J/F=green M/A=red M/J=yellow J/A=cyan S/O=blue N/D=magenta 40 N Latitude 0 N Time (prop. year)

7 Physiological reason for bigeye upper layer thermal preference (23-26 o C water preferred to warm blood between dives?) After Bigelow et al, 2002 and Musyl et al, 3003 Dive Cycle Therm. Pref. Bigeye tuna have unique systems for diving to preserve heat (counter current vascular system), as well as to preserve oxygen and for low light eye adaption. They remain in the warm upper layer at night and return to this layer briefly between dives during the day (in a yo-yo type of behavior). May require specific temp range? CPUE 22 I 28 I CPUE comparison with temp. indicates a preference for upper layer of 22<T<28 o C which agrees with archival tag data (22-26 o C) since fish depth not known for CPUE. Upper layer temperature (9m) (JPL ECCO Model) Exception is in subtropics where the fish appear to remain for short period longer than expected from surface temperature to feed at sub tropical convergence front.

8 Mesoscale variability in bigeye CPUE Weekly Sequences Example: Late Dec 1997 Thermal conc. as 23 o C isotherm moves south with season. Grouped Images CPUE, Chlorophyll (Seawifs), thermal and SSH (Aviso) & SSH slope. - Apparent concentration in/near high chlorophyll cyclonic eddies (upwelling). - Mesoscale CPUE variations due to abundance and catchability variations analysis is in progress. Not simple as effort is not uniform, uncertainty in line location (~ 40 km), and environ. effects on catchability (shear, pycnocline depth) may be significant but vary on the same scales as larger changes in abundance.

9 Pacific-wide bigeye CPUE center of mass motions or migrations (Japanese and SPC all fleet (A.F.) data) Northern hemi. (blue) Southern hemi. (red) Both hemi (green) Hawaii Seasonal migrations in theory should be reflected in Pacific wide COM variations since reversal in both season and direction away from the equator result in in phase N-S motion in both hemispheres. However, Pacific-wide averaging will remove asymmetry in the migration cycle since it averages together opposing seasons in the two hemispheres. COM Latitude Pacific (SPC AF) Pacific (Japan) Time (proportion of year)

10 Pacific bigeye CPUE center of mass motion or migration (SPC all fleet data Northern hemisphere E) o s represent mean location of CPUE COM for each month Error bars are stan. dev. of 30 annual values May/Jun (blue) - max south Nov/Dec (mag) - max north Bigeye >> Significant N-S migration Non-significant E-W migration Although Pacific wide analysis may indicate migration, to reduce errors care needs to be taken to select a suitable region for the analysis where effort exceeds a minimum level over the whole region. Where inter-stock differences exist analysis region may also be selected to include a single stock.

11 Bigeye conventional tag data (suggestive not conclusive more data required in sub tropics) Northern hemisphere Hawaii Local, Seamounts (Itano, Holland) Southern hemisphere Pacific, SPC data (Hampton, Gunn, Williams) Filtered to remove fish which traveled less than 3 degrees or 1 month (and represents a small proportion of the tags). We present the sub-tropical releases as equatorial tags do not appear to indicate seasonality.

12 Latitudinal CPUE COM migration for various pelagic species (SPC A.F. data Northern hemisphere Pacific) Albacore (red) Yellow Tuna (green) Bigeye Tuna (blue) Striped Marlin (yellow) Blue Marlin (cyan) Black marlin (magenta) Sword fish (black) Month of max N-S Migrations ====================== Spec North South Alba Nov Jun Yell N/A N/A Bige Oct May Stri Jun Dec Blue (Jun) (Dec) Blac (Jun) (Dec) COM Latitude Swo Oct May Time (proportion Time of proportion year) of year Significant N-S migrations are apparent for bigeye, albacore, swordfish and marlin (opposite phase) for Pacific-wide stock. Significant E-W migration is observed for yellowfin (E-Jun W-Dec) & albacore

13 Conclusions Changes in CPUE associated with ocean abundance variations appear to be larger than variations in catchability. Hence, CPUE is a parameter which can be used to indicate and quantify abundance. A bigeye thermally driven migration is evident which agrees with their known preference for C water. This results in significant seasonal changes in both the Pacific-wide CPUE (particularly in the sub-tropical waters) and total catch of Hawaiian long line bigeye fishery. CPUE center of mass appears to be a useful parameter to highlight migration. However, care needs to be taken to select a suitable region for the analysis where effort exceeds a minimum level. Pacific-wide data suggest significant migrations may occur for various pelagic species. Assessment of mesoscale variability in CPUE due to changes in both abundance and catchability with satellite derived parameters (temp, chlor., currents) is in progress. More archival and conventional tag data is needed in the sub tropical waters to confirm derived migrations, and determine latitudinal variation in depth/thermal preference.

14 Bigeye Japanese and all fleet Pacific

15 Other species SPC - includes albacore, yellowfin, bigeye, marlin (str/blue/blck), sword Albacore Striped marlin

16 Seasonal surface temperature and high catch locations

17 Seasonal surface chlorophyll and high catch locations

18 Spatial bigeye CPUE (Japanese long line) spatial distribution

19 Spatial bigeye (SPC all fleet data) spatial distribution

20 Spatial albacore (SPC all fleet data) spatial distribution

21 Yellow fin tuna (SPC A.F. data) spatial distribution

22 Sword fish (SPC A.F. data) spatial distribution

23 Striped marlin (SPC A.F. data) spatial distribution

24 Yellowfin Tuna COM SPC all fleet data (30S to 30 N 100 to 210 E)

25 Albacore COM SPC all fleet data (10 to 50 N and 140 to 240 E)

26 Striped marlin SPC all fleet data (0 to 40 N E)

27 Sword fish SPC all fleet data (15-45 N and 120 to 220 E)