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Use of Pop-up Satellite Tags to Refine Knowledge of Swordfish Feeding Ecology on Georges Bank, an Area of Fishery and Resource Concentration
Sean Smith, John Neilson, Julie PorterCanada Department of Fisheries and Oceans (Science) St. Andrews Biological Station
Walter Golet & John Logan
Large Pelagics Research Center University of New Hampshire
Lou Van Guelpen
Huntsman Marine Science Center, St. Andrews, NB
Francois Royer
CLS France
Nova Scotia Swordfish Harpoon Association
Location of Study, and Objectives
I. Using information from the commercial fishery, we describe a persistent area of swordfish concentration off Georges Bank.
II. We describe the phenomenon of swordfish homing behaviour to this area using data from PSAT tags
III. We describe vertical and horizontal migration of swordfish from re-captured PSAT tags
IV. We discuss the results of a feeding study of swordfish and how these results may relate to basking behaviour.
41.54N
65.44 W
Review: Some Aspects of Swordfish Biology
Physiology. Monotypic – not closely related to billfishes. “A very strange fish.”
Unique physiological mechanisms such as brain and eye “heaters”, which promote function during dives into cold water masses. Fish are able to move through large temperature gradients very quickly
Feeding. Important species include Atlantic mackerel, silver hake, redfish, herring, squid. Some investigations
have shown an area and year effect.
Behaviour. In Canadian waters, large swordfish exhibit basking behaviour. This behaviour is most likely related to
physiology and occurs where a warm surface layer overlays colder water. Basking appears to be obligatory as tagged fish often return to the surface immediately following tagging.
Movement and Migrations. Extensive seasonal migrations documented from conventional tagging. Satellite tagging has shown that
the majority of movement is in a North/South direction. Diel vertical migrations noted, but patterns and persistence are not well understood. We hypothesis that
these movements are related to feeding.
Methods
Information on the distribution of fishing effort was extracted from Departmental databases containing fishermen’s logbooks.
Stomachs were collected during the fishery, preserved in brine and examined for diet composition.
Information on horizontal and vertical movements was obtained from pop-up satellite archival tags (PSATs).
Psat Tagging
Results - Number of Tags Deployed and Reported
Year Number of Tags Deployed Number of Tags that Reported
2005 11 10
2006 14 10
2007 20 14 (to date)
• We have achieved very high reporting rates, and tags often stay attached to the fish for very long periods.• The size of fish marked and recaptured range from 100 to 200 kg.
Results - Location of fishing effort, and release of tagged fish
The Canadian swordfish harpoon fishery is located on the northeast peak of Georges Bank. 200420052006
2007
Location of PSAT tagging
Examples of Precise Homing to Georges Bank Foraging Area
Tag 56404 was at liberty for almost a full year during which the fish migrated South and back.
The tag was physically returned to us by a fisherman.
Recapture location was 72 km away from release point near Georges Bank.
Tagged 31/07/2005
Recaptured 15/07/2006
56404
More Examples of Precise Homing to Foraging Areas
Tag 66039, reported Jan. 4, 2007 from the Caribbean Sea.
The fish retained its conventional Floy type tag, continued its migration back to the area off Georges Bank, and was recaptured in very close (within 5 km) proximity to its point of release, almost one year later.
Other examples of this behaviour exist in our data set, both from PSAT and conventional tagging.
Tagged 16/07/2006
Recaptured 27/07/2007
66039
Results - Horizontal Migration from Satellite Data
Latitude
Longitude
Latitude and Longitude derived from PSAT light level readings (+). Pop off locations =
Results – Vertical Migration from Returned PSAT Tags
Data from tags returned to us by fishermen. Two in 2005, one in 2006
Consistent diel pattern but varies between years. In 2005 (two fish) were at the surface at night, the fish in 2006 was at the surface in the early morning.
Dive behaviour likely relates to feeding success and physiological constraints such as body temperature.
Date in August (GMT, each minor tick is 4 h)10 11 12 13 14 15 16 17
0
100
200
300
400
5006 7 8 9 10 11 12 13
Dep
th (m
)
0
100
200
300
400
500
2005
2006
Diet Study
We examined 25 stomachs collected from the swordfish harpoon fishery.
Stomachs were preserved in brine.
Stomachs were weighed, given a fullness index and contents identified to the highest possible taxonomic detail.
Stomach collection
Location of 2007 harpoon fishery and location of stomachs collected
“Ingested barracudinas were frequently slashed and the fragile bodies formed a mass that was often reported by technicians aboard commercial vessels as unidentified fish remains” – Scott and Tibbo. 1968
“Silver hake occurred in all except the most southerly sample to a maximum of 22 individuals in a single stomach. It occurred in more samples than any other species and ranked third in volume” – Scott and Tibbo. 1968
Results – Diet Composition
0
10
20
30
40
50
60
70
80
90
Myctophidae(lantern fish)
Merlucciusbilinearis
(Silver hake)
Paralepis(Barracudina)
Gadoid(codfishes)
Illexillecebrosus
(shortfin squid)
Group
Perc
en
t O
ccu
rren
ce
Results – Feeding Success
Fullness Index
0
10
20
30
40
50
60
70
Empty 1/4 Full 1/2 Full 3/4 Full Full Everted
Category
Perc
en
t o
f T
ota
l S
tom
ach
s
Conclusions
The northeast peak of Georges Bank is an important seasonal feeding area (a hotspot) for Atlantic swordfish.
Swordfish appear to show consistent daily vertical patterns related to feeding in the mesopelagic layer. Dive patterns and residency times in the area can be determined through PSAT tag data.
The feeding success is high and the prey composition is consistent among fish.
We hypothesis that high feeding success coupled with oceanographic conditions results in basking behaviour making the fish available to the harpoon fishery.
Acknowledgments
The Captain and crew of the FV Four Ladies
The Nova Scotia Swordfish Harpoon Association
This work was funded, in part, by the International Fisheries and Oceans Governance Fund of DFO.
Results – Horizontal Migration from Satellite Data
Using light level data collected by the tag we can estimate the track of the fish. The principal is similar to that of celestial navigation.
If we know the times of dawn and dusk and the day of year (which affects the direction of the tilt of the earth with respect to the sun, and thus the position of the dawn-dusk circles), then we can theoretically calculate the location of the intersection of the dawn-dusk lines and, hence, our position
Recovered Tags Provide Detailed Temperature Profile Data
Recovered Tag - 2005
0
50
100
150
200
250
300
350
400
1 31 61 91 121 151 181 211 241 271 301 331 361 391 421 451
Minutes
Dep
th (
m)
0
5
10
15
20
25
Tem
p (
C)
Depth
Temp
Thermocline
Presence of a thermocline is considered by fishermen to be a pre-condition for basking behaviour
Results - Temp/depth profile collected during tagging and from summarized tag.
Logger Data - July 30, 2007, 0800 hrs
0
5
10
15
20
25
0 0.2
0.4
1 1.2
1.4
2 2.2
2.4
3 3.2
3.4
4 4.2
4.4
5
Time (Min)De
pth
(m)
0
5
10
15
20
25
Tem
p (C
)
Depth (m)
Temp (C)
Presence of a thermocline is considered by fishermen to be a pre-condition for basking behaviour
Swordfish Satellite Tagging – Theory Behind Geolocation Using Light Levels
The Figure shows the earth with the sun to the right. The bold circle (A) that encompasses the earth is the line between day and night at dawn. Note that this line does not pass through the north or south pole, night is shaded darker, and the earth has been made semi-transparent.
Circle B (dusk) is the line that divides day from night. 12.5 h later, in this example.
Thus, the point represented by the ‘+’ that experienced a sunrise and sunset with daylight duration of 12.5 h exactly must be at the intersection of circles A and B.
If we know the times of dawn and dusk and the day of year (which affects the direction of the tilt of the earth with respect to the sun, and thus the position of the dawn-dusk circles), then we can theoretically calculate the location of the intersection of the dawn-dusk lines and, hence, our position.
Implications for stock management
Provides an opportunity for conservation measures
“Hot Spots” such as the peak of Georges Bank may render fish more vulnerable to a fishery and thus present challenges for fisheries management ?
Effort controls in the harpoon fishery include; weather, daylight, oceanographic conditions.
Foraging Fidelity – Implications for Management?
Homogenous populationForaging grounds site fidelity
Our data show that Atlantic swordfish have fidelity to their summer feeding grounds in the Northwest Atlantic.
Such behaviour has been postulated for eastern Australian swordfish, with alternate assessment models established to reflect such behaviour (see above figure, yellow ellipse is spawning grounds, green are foraging grounds, Figure reproduced, with permission, from Kolody 2006).
While recognition of spawning components and fisheries management measures to protect them is not uncommon, this explicit recognition of discrete foraging grounds is unusual.
Some Technical Details Concerning the Tags Used
Tag Type In 2006 and 2007, we used Wildlife Computers
PSAT Mark 10 (Manufacturer Wildlife Computers, Redmond, WA, USA).
Method of Attachment Devising an appropriate attachment is paramount
for the success of this type of study.
We used the “Prince-style” surgical grade nylon head. The barbs are flexible and designed to spread and anchor the implanted tag when strain is applied to the attachment.
The nylon head was attached to the archival tag using a 150 lbs test monofilament, metallic free assembly. The attachment design incorporates a standard spaghetti type tag which remains implanted in the fish following Sat Tag pop off.
Some Technical Details Concerning the Tags Used (cont)
Harpoon Assembly the harpoon pole is approx. 5
m long, aluminum with a modified end to accept a stainless dart needle and a cushioned cradle to hold the tag
The tag is attached to the pole using elastic bands with "breakaway" knots so that it will release when the fish is tagged.
Note that the tag was painted with an orange paint, which helped other harpooners avoid recapturing the same fish.
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