Winter habitat use by juvenile Gulf sturgeon in the Suwannee Estuary as revealed by automated acoustic telemetry

Michael Randall, April D. Norem, Kirsten E. Luke,
Randy E. Edwards and Kenneth J. Sulak

Coastal Ecology & Conservation Research Group, Gainesville, Florida
& University of South Florida

Presented at the FlSC Strategic Review in St. Petersburg, Florida, May 9-12, 2006

Introduction

The Gulf sturgeon, Acipenser oxyrinchus desotoi, is a large anadromous fish, listed as threatened under the Endangered Species Act in 1991. Adult Gulf sturgeon migrate from the Gulf of Mexico into coastal rivers ranging from the Pearl River, Louisiana to the Suwannee River, Florida during early spring to spawn. Gulf sturgeon spawn well upstream on gravel substrates.  Gulf sturgeon eggs are adhesive and released close to the bottom, with hatching occurring within 72-80 hours. Following hatching, larvae are suspected to shelter in interstitial crevices for several days while living off of their yolk sack. Age –0 juveniles may shelter in woody debris downcurrent of sand ripples, foraging at night on the open sand throughout the first year. As juveniles approach age 1, they descend to the river mouth in the midwinter, joining older juveniles feeding in the inner estuary. Juveniles join the adult immigration during the spring. Reproducing female sturgeon spawn once every 2 to 4 years, though males spawn annually. Adult and juvenile Gulf sturgeon remain in the rivers throughout the summer months. During their stay in the river, adult Gulf sturgeon do not feed and lose weight. In the fall, Gulf sturgeon emigrate from the rivers. In estuaries, Gulf sturgeon feed intensively around mudflats and oyster bars on benthic prey. Adults, particularly females, may disperse hundreds of kilometers from the natal river during the winter feeding period, and wander widely on a temporal scale of weeks. Adults feed in passes between barrier islands and along the coast in four to seven meters of water on sand substrate.

Vemco VR2 remote acoustic receiver in cage attached to mooring block and buoy

Various aspects of the life history of Gulf sturgeon, Acipenser oxyrinchus desotoi, in the Suwannee River, FL, have been investigated by the Center For Aquatic Resource Studies, Gainesville, FL, since 1988. Much broad information has emerged about these fish; however, fine scale details are less well known. One of the least understood segments is the juvenile estuarine feeding stage. Sampling with small-mesh gill nets has shown that juveniles are present in the river mouth and inner estuary during the winter. However, little is known about the areal extent of feeding, preferred feeding substrates, response to environmental parameters, or individual fish behavior during this time. The U.S. Fish and Wildlife Service, Panama City Field Office, requested that we investigate the ecology of juvenile Gulf sturgeon during the winter feeding period in the Apalachicola River, FL. To begin, we conducted a pilot project in the Suwannee River, FL.

Digital orthoquad of Suwannee River delta with locations of VR2 remote receivers and approximate detection zones. Often, multiple VR2's would pick up a pinger simultaneously, showing that the detection zone under good circumstances is larger than depicted.

Methods

We chose Vemco VR2 remote sonic receivers, and Vemco V7 and V9  sonic tags. VR2 remote receivers are automated submersible units, 340 mm long by 60 mm diameter, which detect Vemco coded acoustic tags and log date, time, and code. VR2's run for 15 months on a single battery, and can record 300,000 detections. V7 tags are 7 mm diameter by 20 mm long with a life of 90 days, while V9 tags are 9 mm diameter by 28 mm long with a life of 120 days. Tags were attached with heat-shrink tubing and epoxy to a heavy monofilament loop through the base of the dorsal fin, fastened with a corrodible crimp. VR2's were deployed in rebar cages attached to 60 kilo concrete blocks and large orange floats with 6.3 mm stainless steel cable. To retrieve VR2's for downloading a pole with attached grapple hook was used. Receivers were downloaded approximately every two weeks. Six YSI 6600 dataloggers equipped with DO, temperature, conductivity, and pH probes were deployed along with selected VR2's.

Sonic tagged juvenile Gulf sturgeon. Vemco V7 tag attached at base of dorsal fin

Results

We tagged 14 individual Gulf sturgeon with V7 (2 fish) and V9 (11 fish). One fish was tagged with a V13 tag we purchased as a test tag. Thirteen VR2 remote receivers were deployed. Monitoring is still ongoing, with the array still deployed.

All 14 fish were tracked within the array for varying lengths of time. One fish, #256, disappeared rapidly and has not been relocated. The remaining fish have either remained in the array or moved upriver. Movements of fish #810, the first fish tagged, are shown as a representative dataset:

Detailed movements of fish #810:

• Tagged on February 10th, stayed in West Pass until midnight February 15th
• Left the array and was not detected until 10 PM on March 12th
• Came back in Wadley Pass, swan up Wadley Pass, then through gap between Wadley and West Pass, entering West Pass at 4 AM March 12th
• Was in West Pass for 19 hours, then started moving around
• From 7:45 PM on the 13th until 4:30 AM on the 14th moved all over the middle part of the array, from the center of West Pass to just below the town of Suwannee
• Stayed in the center of West Pass for the next 8 days, from early morning on March 14th until 3:30 pm on March 22nd
• Moved around the upper end of West Pass for 2.5 hours
• Stayed at the lower end of West Pass for 2 days, from 10 PM on March 22nd until 10 PM on March 24th
• Swam upriver slowly; in 20 hours had passed out of the array, passing the junction of East Pass and the Suwannee River at 6:30 PM on March 25th
• Manually relocated 18 km upriver on March 28th, and then relocated again on April  27th 26 km upriver.