Hotspots within hotspots? Aggregations of pelagic fishes at

southeastern coast of Wolf Island, Galapagos.

Hearn1, A.R., J.T. Ketchum1, A.P. Klimley1, and E. Espinoza2

1Department of Wildlife, Fish, and Conservation Biology, University of California, Davis, California, U.S.A.

2Galapagos National Park, Puerto Ayora, Galapagos, Equador

Sharks,Tunas,Billfish,Turtles, Delphinids are Attracted to Seamounts

• Seamounts and islands are volcanos, that rise above the seafloor due to the deposition of basalts from eruptions over geological time.

• They often occur in chains, natural aggregation sites for fish.

Example of Seamount

and Islands

Why do Fishes Aggregate at these Sites?

•Are all oceanic species nomadic, constantly moving?•Do they remain at seamounts and islands during migrations?

Some species reside at trophic“hot spots” such as the EspirituSanto Seamount in the Gulf of California.

Why do they aggregate at these locations?

• Windy pass hypothesis: greater density of prey.• These sites are more productive than surrounding waters.

Assemblages at SeamountsVisual censuses of species were conducted at this seamount in the Gulf of California seasonally over a period of five years.

Summer and winter assemblages were described based on their abundance throughout the year. Hammerheads were abundant during the summer as well as silky sharks at this time.

Assemblages at Islands• Galapagos Archipelago formed as crustal plate spreads

westward over magma hotspots that permit magma to rise to the surface to produce volcanic eruptions that create islands.

•Isla Isabela created in recent geological time.•Isla San Cristobal created in past geological time.

Hotspots in Galapagos Archipelago

Wolf Island

Darwin Island

VolcanicCrater

Multiple Methods: Species Abundance and Distribution

•Making counts of individuals during SCUBA dives (Hearn et al., in prep.)

•Affixing coded tags to individuals and detecting with automated monitors at specific sites (Hearn et al., in prep.)

•Shipboard tracking individuals carrying ultrasonic transmitters (Ketchum et al., in prep).

Hotspots within Hotspots!

• Quarterly censuses were conducted by SCUBA divers of the abundance of scalloped hammerhead sharks at six sites around coast of Wolf Island.

• Hammerheads were observed primarily on eastern coast during January.

• There is no-take zone (see heavy line)

Visual Censuses

Remote Sensing of Biota• We need to use multiple methods of observation. • One useful for recording movements and

distribution of fishes is ultrasonic telemetry.

• Tagged individuals can be followed continuously in a ship.• Many fish with coded, “signature”, beacons can be

detected by monitors moored at a site.• Their appearance at the site over a 24 hr day can be

plotted on a “clock” diagram.

Hotspots within Hotspots!

• Coded ultrasonic beacons were placed on 40 scalloped hammerhead in August 2006 and 60 individuals in August 2007.

• These sharks were detected at Sharp Point, Rockfall, and East Bay on the eastern coast.

Coded beacons and automated monitors

Movements of Sharks Restricted to Specific Sites

•Galapagos shark tagged and tracked for two days by boat spent a disproportionate amount of time at the Rockfall and eastern side of crater.

2.2 m TL Male hammerhead (SH1) shark tracked by boat over two-days during Nov 2008 spent majority of time at Shark Point and Rockfall.

Confined Short-Term Residency•95th kernalcontours of SH-1 were centered around SP and R.

Shark Point (SP)

Rockfall (R)

•75th kernalcontours of SH-2, a 2.0 m TL female tracked for two days during November 2008, centered between P and R.

East Bay (EB)

Pinnacle (P)

Sharks Immigrating to Island Seek Specific Site

Female hammerhead left Darwin Island 4 and arrived at Wolf 7 Dec 2008.

Detected at Elephant Rock (ER) at 1312 hrs and stayed within range of monitor for only 24 min

Spent next three weeks between Rockfall, Shark Point, and East Bay

ER

Hotspots within Hotspots!Censuses of Fishes

• The largest abundances of species were along the eastern coast of the island.

Scalloped hammerhead shark

Crevelle and green jacks

Steel pompano

Galapagos shark

Bottlenose dolphin

Snappers

Yellowfin tuna

Bathymetry and Currents

•Chain of seamounts leading from Darwin and Wolf toward southeast

•North Equatorial Current flowing from east with Humboldt Current flowing from south.

N

Based on Palacios et al.(2002) and NASA-Charles Darwin Foundation (2004-07)

Current Structure and Biotic “Hotspots”

Darwin ISland

Wolf Island

Current direction

Arch Hotspot

Shark PointHot-spot

Hypotheses #1: Upstream Blocking

•Entrainment area produced by tidal flows around Pandora Reef, GBR, Australia (from Hamner & Hauri, 1981).

• Accumulation of plankton upstream of the reef that attract predators.

• Plankton tows will be conducted to determine abundance at sites around Darwin Island

Hypothesis 2: Benthic-pelagic coupling at hotspot

Zooplankton

Reef Assemblage

PelagicAssemblage

Zooplankton

Pelagic Assemblage

Ho=1

RelationshipAbundant Prey

Cleaning Stations

Hypothesis 3: Non-feeding use of hotspot by hammerhead

sharks

• Sharks orient to ridges and valleys in geomagnetic topography.

• Sharks will be tagged at Shark Point and their movement eastward will be detected by monitors sited randomly and along magnetic pathways, contingent upon results of magnetometer survey.

Conclusions

• Sharks remain at specific sites at islands, biotic “hot spots”, for long periods of time, and then move to other hotspots.

• They could be a good indicator of an assemblage of a diversity of species and trophic levels.

• This assemblage includes planktivores such as jacks and pompano and predators such as yellowfin tuna, scalloped hammerhead sharks, and bottlenose dolphins.

• The currents flow from the east, and this may explain the high abundance of species on this coast of the island.

• Further oceanographic and plankton studies are needed to determine the factors underlying biotic hotspots.