Investigating the reproductive migration and spatial ecology
of Nassau grouper (Epinephelus striatus) on Little Cayman Island
using acoustic tags – An Overview

BRICE X. SEMMENS^1, KIRSTEN E. LUKE², PHILLIPPE G. BUSH^3, CHRISTY PATTENGILL-SEMMENS¹, BRADLEY JOHNSON^3, CROY MCCOY³ and SCOTT HEPPELL⁴

¹Reef Environmental Education Foundation (REEF)
Pacific Coast Office, 4726 38th Ave NE, Seattle, WA 98105
²Coastal Ecology & Conservation Research Group U.S. Geological Survey,
Center for Aquatic Resource Studies, 7920 NW 71st St., Gainesville, FL 32653
³Department of Environment, Cayman Islands Government,
P.O. Box 486GT, Grand Cayman, Cayman Islands
⁴Department of Fisheries and Wildlife, 104 Nash Hall,
Oregon State University, Corvallis, OR 97331

Presented at the FlSC Strategic Review in St. Petersburg, Florida, May 9-12, 2006,
and the 58th GCFI Conference, San Andres, Colombia, November 2005

INTRODUCTION

Nassau grouper (Epinephelus striatus) migrate to specific sites during the winter full moons in order to reproduce in mass aggregations (Domeier & Colin 1997, Bolden 2000, Sala et al. 2001). Intense harvesting of spawning aggregations is the primary cause of the precipitous decline in populations throughout the Caribbean (Beets & Hixon 1994, Sadovy & Eklund 1999). Consequently the Nassau grouper is listed as 'threatened' by the World Conservation Union. Spawning aggregations (SPAGs) are disproportionately responsible for the reproductive output of many economically valuable and ecologically important species (Domeier and Colin 1997) and they result in the concentration of individuals from stocks that are otherwise at low densities.

However, as aggregations are site specific designating aggregation sites as marine protected areas is likely to be a successful conservation measure. In recent years, several Caribbean governments have instituted marine protected areas at known Nassau grouper aggregation sites in response to chronic declines in catch. It is important, however, to evaluate the scope of protections afforded stocks through the protection of spawning grounds since such management actions alone may not be sufficient to protect at-risk species.

In the Cayman Islands >90% of harvested Nassau grouper have been from SPAGs. Six Nassau grouper spawning aggregations have been documented in the Cayman Islands (Fig 1). Four of these aggregations apparently no longer exist and the fifth supports only a few fish.

Only the west end site of Little Cayman, which was "discovered" in 2001, maintains annual aggregations of more than 1,000 grouper.

In 2003 the Cayman Island Marine Conservation Board instituted an 8-year total fishing ban on all known Nassau grouper aggregation sites through the Restricted Marine Areas (Designation) Regulations legislation. In order to assess the effectiveness of the closures the Cayman Islands Department of the Environment (CIDOE) needs answers to the following questions:

1. What proportion of Nassau grouper on the Cayman Islands use the aggregation sites receiving protection?
2. Are there any as yet undiscovered (and thus unprotected) aggregation sites?
3. How often do individual fish participate in aggregations?
4. Where do aggregating individuals come from and where do they go afterward?
5. Does demographic status (sex and size) influence participation in aggregations?
    

METHODS: PASSIVE ACOUSTICS

Caught and acoustically tagged 50 fish on Little Cayman
Placed 15 subsurface hydrophones around the island (tag monitoring stations)
Boat based acoustic gear to monitor tags and find fish in the 'holes in the net' and on other islands

Catching
At the start of the January 2005 spawning season (25th – 27th January 2005) we caught 30 mature Nassau grouper (>40 cm TL; Sadovy and Colin 1995) at the west end Little Cayman aggregation site, using hand lines with 12/0 circle hooks, 0.2kg weights and a combination of fresh reef fish and squid as bait (Fig 2). Fish were caught at depths of approximately 30m and brought slowly to the surface to minimize barotrauma. We used circle hooks in order to maximize the likelihood of hooking fish in the corner of mouth. In June and August 2005 we caught 20 mature-sized Nassau grouper around Little Cayman Island using baited Antillean fish traps (Semmens et al. in press) and by SCUBA divers using mesh bags (Fig 3). To capture grouper, SCUBA divers chased them into a hole in the reef. In most instances the fish were sedated using 500ml of a 1:500 Quinaldine/seawater solution that was applied to the hole using a squirt bottle. In some cases the fish swam out into a net bag; in other cases the fish was removed by hand and placed in a net bag.

Tagging
Vemco coded transmitter tags were surgically implanted in 50 Nassau grouper (Fig 4) using procedures modified from Adams et al. (1998). We measured total length (cm) and weight (kg) of all fish intended for tagging, and then placed the fish ventral side up in a'V' shaped cradle, with fresh seawater irrigating their gills and a wet towel covering their head.

Hydrophone Array
We placed fifteen VEMCO VR2 single channel passive autonomous hydrophone datalogger receivers (VEMCO, 100 Osprey Drive, Shad Bay, Nova Scotia, Canada B3T 2C1) at approximately 2 km increments around Little Cayman prior to the onset of the January 2005 spawning season. The VR2 hydrophones were attached to ¼ inch polypropylene line approximately 8m below the surface using cable ties and moored at the edge of the wall. The line was anchored to a stainless steel pin embedded in the reef and buoyed by subsurface floatation buoys (Fig 5). The VR2 hydrophones have an advertised reception radius of 500-800m, a battery life of approximately 15 months, and can store 300,000 unique tag identifications. If an acoustically tagged fish comes within the reception range the VR2 will log the time, tag ID, and depth if the tag is depth-coded. Hydrophones are retrieved, downloaded, and redeployed every 3 months. Based on range tests, we determined the hydrophones to have a maximum reception radius of 300m in the field.

Mobile Tracking
In order to survey areas around Little Cayman outside of the reception range of the VR2 hydrophones, and in order to survey the islands of Cayman Brac and Grand Cayman, we surveyed near-shore habitat throughout the Cayman Islands using a boat-based hydrophone (Vemco VR100). The boat-based gear was used to search for acoustically tagged fish every 2 months around Little Cayman and every 6 months around Grand Cayman and Cayman Brac. The omni-directional hydrophone is towed 8m behind a boat at 5km/h at approximately 2m below the surface. If an acoustically tagged fish is detected the receiver logs the time, location (using a built-in Global Positioning System), tag ID, and if applicable, depth.

RESULTS AND SUMMARY

We acoustically tagged a total of 50 Nassau grouper both on and off the Little Cayman west end aggregation site, and are subsequently monitoring the movements of the tagged fish over a two year period using an array of passive autonomous hydrophone receivers.

The average size these fish was 64 cm (range 44cm - 84cm) (Table 1).

By tagging fish on the aggregation we were able to determine where fish go after aggregating.

The behavior of fish tagged at sites around Little Cayman prior to the 2006 aggregation will provide insight into the proportion of fish from the Island that attend aggregations, and the frequency of aggregation attendance by individual fishes as a function of demography.

Over half the fish that were tagged during the January aggregation returned to attended the aggregation in February.

We have detected 40 of the 50 acoustically tagged fish with the VR100 mobile gear during surveys from April to October 2005. The 10 fish not heard include 2 that were infested by E. tricornis tricornis (Semmens et al. in press), and 8 that were last heard on the VR2 array shortly after tagging.

Fish tagged seem to reside all around the island (Fig 6) and no fish appear to leave the island.

The extent to which local populations of Nassau grouper use aggregation sites bears directly on the success and effectiveness of protections placed on these sites. If, for instance, virtually all Nassau grouper from Little Cayman aggregate predictably at the west end aggregation site, then enforced fishing prohibitions at the site will be highly effective at limiting over fishing. If on the other hand, only a small portion of the total grouper population attends the aggregations each year, or if much of the local population of grouper attends aggregations at currently unknown locations, then the protections on the aggregation site may be far less effective at reducing fishing impacts than expected. The products of this study will allow the CIDOE to directly assess the efficacy of existing reserves, and will provide guidance for additional protected areas in the event that additional aggregation sites are found.
 

REFERENCES

Adams, N. S., D.W. Rondorf., S.D. Evans and J.E. Kelly. 1998. Effects of surgically and gastrically implanted radio transmitters on growth and feeding behavior of juvenile Chinook salmon. Trans. Am. Fish. Soc. 127: 128-136.

Beets, J. and M.A. Hixon. 1994. Distribution, persistence, and growth of grouper (Pisces: Serranidae) on artificial and natural patch reefs in the Virgin Islands Bull. Mar. Sci. 55, 470-483.

Bohnsack, J.A. 1996. Alternative method for returning fish to sea. Guest
Column. Page 3 - South Atlantic Update. South Atlantic Fishery Management Council. Sept 1996.

Bolden, S.K. 2000. Long-distance movement of a Nassau grouper (Epinephelus striatus) to a spawning aggregation in the central Bahamas. Fish. Bull. 98(3) 642-645.

Domeier, M.L. and P.L. Colin. 1997. Tropical reef fish spawning aggregations: defined and reviewed. Bull. Mar. Sci. 60: 698-726.

Sadovy, Y., and P.L. Colin. 1995. Sexual development and sexuality in the Nassau grouper, Epinephelus striatus (Pisces: Serranidae). J. Fish. Biol. 46: 961-976.

Sadovy, Y. and A.M. Eklund. 1999. Synopsis of biological information on Epinephelus striatus (Bloch, 1972), the Nassau grouper, and E. itajara (Lichtenstein, 1822) the jewfish. U. S., Dep. Commerce, NOAATech. Rep. NMFS 146, and FAO Fisheries Synopsis 157, 65pp.

Sala E., R. Starr, and E. Ballesteros. 2001. Rapid decline of Nassau grouper spawning aggregations in Belize. Fishery management and conservation needs. Fisheries 26(10): 23-30.

Semmens, B. X., Luke, K.E., Bush, P.G, McCoy, C.M. R., Johnson, B.C. In Press. Isopod infestation of post-spawning Nassau grouper around Little Cayman Island.

ACKNOWLEDGEMENTS

The authors thank B. Luckhurst, T. Trott and P. Hillenbrand for logistical support. Special thanks to Grant Feist (OSU) for running the hormone analyses. Funding for the project was provided in part by the Department of Commerce (DOC), National Oceanographic and Atmospheric Administration (NOAA) Coral Reef Conservation Fund [award# NA04NOS4630287] and the PADI Project AWARE Foundation. B. Semmens was supported by a Science To Achieve Results (STAR) United States Environmental Protection Agency (EPA) graduate research fellowship. The statements, findings, and conclusions in this report are those of the authors and do not necessarily reflect the views of DOC, NOAA, or EPA. Special thanks to the Reef Environmental Education Foundation (REEF) and REEF volunteers for facilitating this research.