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.