Bush, P.G.

Gibbs, J.

Heppell, S.A.

Heppell, S.S.

Johnson, B.C.

McCoy, C.M.

Pattengill-Semmens, C.V.

Semmens, B.X.

Aquatic Commons

 Proceedings of the 63rd Gulf and Caribbean Fisheries Institute     November 1 - 5, 2010   San Juan, Puerto Rico 
Oceanographic Patterns Associated with Nassau grouper Aggregation Spawn Timing:  
Shifts in Surface Currents on the Nights of Peak Spawning 
 
SCOTT HEPPELL1*, BRICE X. SEMMENS2, CHRISTY V. PATTENGILL-SEMMENS3, PHILLIP G. BUSH4,  
BRADLEY C. JOHNSON4, CROY M. MCCOY4, JAMES GIBB4, and SELINA S. HEPPELL1 
1Department of Fisheries and Wildlife, 104 Nash Hall, Oregon State University, Corvallis, Oregon 97331, USA. 
*Scott.Heppell@oregonstate.edu.  2NOAA Northwest Fisheries Science Center 4726 38th Ave. NE Seattle, Washington 
98105 USA.  35Reef Environmental Education Foundation (REEF), PO Box 246, Key Largo, Florida 33037 USA. 
4Department of Environment, Cayman Islands Government, P.O. Box 486GT, Grand Cayman, Cayman Islands. 
 
ABSTRACT 
There is virtually nothing known about the fate of fish larvae born on spawning aggregations from the time of spawning to 
settlement, yet the location of the aggregation site must be important in determining their fate. While aggregations always form in 
the same place and at the same time in part to assure a large number of adults will congregate for spawning, oceanographic patterns 
of dispersal and retention of the larvae may provide a driving force for the selection of specific spawning locations.  During the 
winters of 2008 and 2009 we deployed Surface Velocity Profile drifters at the Nassau grouper spawning aggregation site on Little 
Cayman Island, BWI.  A single drifter was deployed each night, then recovered 12 hours later in order to determine trajectory paths 
off the aggregation.  In contrast to the path taken on nights prior to spawning, in both years the drifters released on the night of peak 
spawning showed substantial eddy formation near the aggregation site.  This repeated pattern suggests an oceanographic-based 
“importance of place” for the aggregation site that may result in local recruitment.  This would mean that local aggregations of fish 
are directly responsible for the long-term survival of local populations.  This gives substantial credence to the need to identify and 
protect specific locations because they are unique and critical to the long-term survival of the many species that aggregate there. 
 
KEY WORDS:  Nassau grouper, aggregation, spawning, dispersal, satellite drifters 
 
 
Patrones Oceanográficos Asociados a las Agrupaciones de Desove del Mero de Nassau:  
Cambios en las Corrientes de Superficie en las Noches de Mayor Desove 
 
PALABRAS CLAVE:  Mero de Nassau, agruapiones, desove,corrientes de superficie 
 
 
Les Modes Océanographiques Associés aux Aggregations de Frai de Mérou de Nassau:  
Les Changements dans les Courants de Surface sur les Nuits du Frai Maximal 
 
MOTS CLÉS:  Mérou de Nassau, aggregations, frai, courants de surface 
INTRODUCTION 
The Nassau grouper (Epinephelus striatus) is listed as 
endangered by the International Union for the Conserva-
tion of Nature and Natural Resources (IUCN, 
www.redlist.org), having been decimated throughout its 
range; the number of known Nassau aggregation sites has 
been reduced by over 60% (De Mitcheson et al. 2008, Sala 
et al. 2001) and populations continue to decline.  Nassau 
grouper aggregate to spawn in large groups (Colin 1992, 
Smith 1972), where they are particularly vulnerable to 
fishing; intense harvest at spawning aggregations is the 
primary cause for the precipitous decline of this species. 
This vulnerability to overexploitation is evident in the 
Cayman Islands (Whaylen et al. 2004), where over 90% of 
the Nassau grouper harvest in the Cayman Islands has 
historically come from fishing spawning aggregations.  
While the west-end Little Cayman spawning aggrega-
tion site is now home to one of the largest fully-protected 
Nassau spawning aggregations, the 8-year harvest ban 
expires at the 2011 pending a review as to whether further 
protections are needed.  One of the critical questions 
associated with the persistence of any population, includ-
ing Nassau grouper, are recruitment rates and the sources 
of that recruitment.  For aggregating species like the 
Nassau grouper, this begs the question, “Why do Nassau 
grouper aggregate where they do?” We used Surface 
Velocity Profile (SVP) drifters to map current patterns 
around the west end, Little Cayman Nassau grouper 
spawning aggregation site, with the aim of understanding 
how important the local aggregation site, and the local 
Nassau grouper population, is to local recruitment.  
 
METHODS 
 
Site Description 
The geomorphology Cayman aggregation sites, 
including our end, Little Cayman site, are described by 
Kobara and Heyman (2008).  With the exception of the fact 
that the aggregation sites in the Cayman Islands are located 
on both the windward (3) and leeward (2) sides of the 
    Heppell, S.A.  et al.   GCFI:63   (2011) Page 153 
islands, all five are located on convex points (reef promon-
tories) of similar slope and inflection angle, within 50m of 
the shelf edge and at 25 - 45 m depth.  These shared traits 
indicate that specific locations are being selected. 
 
Drifter Deployment and Tracking 
For multiple nights in 2008 and 2009, both prior to and 
proceeding spawning, we released SVP drifters over the 
West End Little Cayman spawning aggregation site.  The 
release was done at approximately 6 pm each evening 
(Figure 1, panel A) immediately following our daily dusk 
site survey, the drifter floated for approximately 12 hours, 
and was then recovered (Figure 1, panel B) for future use.  
Reproductive activity was evaluated each night during the 
dusk survey.  
Drifters report on average six times per hour, provid-
ing high-resolution profiles of surface current patterns —
this data rate also facilitates relocation and recovery of 
drifters.  Drifter positions were plotted using ArcGIS™ 
with basemap and bathymetry shape files provided by the 
Government of the Cayman Islands. 
 
RESULTS 
We successfully deployed and recovered SVP drifters 
for the five nights leading up to the major spawning night, 
plus the major night of spawning in 2008, while in 2009 we 
deployed drifters for the two nights leading up to spawn-
ing, the night of peak spawning, and two days following 
the major spawning night.  On nights leading up to 
spawning, drifters took a linear to curvilinear path during 
their 12 hour deployment, while on the major night of 
spawning drifters looped back on themselves, indicating 
eddy formation in oceanographic currents over the 
spawning site (Figure 1, panel C). 
 
DISCUSSION 
Our results show that spawning is occurring on nights 
where current patterns change in proximity to the aggrega-
tion site.  Such changes indicate that there may be some-
thing important about this location, and this time, for the 
successful reproduction in Nassau grouper.  This im-
portance is likely not limited to Nassau grouper, either, as 
Whaylen et al. (2006) list at least 20 other species that 
exhibit reproductive activity at the same time at the west 
end Little Cayman aggregation site. 
It should be noted that SVP drifters are not fish larvae, 
although they likely well represent passive dispersal of 
initial reproductive products.  Within the context of local 
retention (self-recruitment), the early onset of larval 
behavior is a critical feature (Cowen et al. 2006).  The 
presence of larval swimming behavior as early as 3 - 5 
days post-spawning can result in substantial retention for 
pelagic spawning fish, with dispersal restricted to distances 
of just 10km or less (Cowen et al. 2006, Paris and Cowen 
2004).  Fertilized Nassau grouper eggs hatch within 30 
hours at 25C, the temperatures found around the Cayman 
Islands during spawning, have pigmented eyes within 40 
hours, and are feeding within 60 hours (Tucker et al. 1991). 
Figure 1.  Drifter release, recovery, and tracking from the West End, Little Cayman Nassau 
grouper spawning aggregation site.  (A) REEF Grouper Moon crew prepare to deploy a Surface 
Velocity Profile Drifter over the West End site, having just completed an in-water survey of Nassau 
grouper reproductive activity. (B) The SVP drifters were recovered each morning for re-use.  High-
visibility markings, combined with accurate position information (obtained very 10 min) allowed 
almost 100 recovery success. (C) Composite of all 12hr drifter tracks for 2008 and 2009. Linear to 
curvilinear tracks are all from non-spawning nights, while looping patterns form on spawning 
nights. Dashed arrow represents SVP drifter track on peak spawning night in 2008, solid arrow 
indicates same track in 2009. 
 Page 154  63rd Gulf and Caribbean Fisheries Institute  
This ontogeny indicates that larval Nassau grouper are 
equipped to take advantage of the eddy formation that 
occurs on nights of spawning, which could keep them close 
to the shores of Little Cayman Island and set up a self-
seeding population.   
Cowen et al. (2006) hypothesized that ecologically-
relevant (source-sink) dispersal distances in the central 
Caribbean would be > 100 km.  Given that there are no 
known healthy Nassau grouper aggregations within 100km 
of the west end Little Cayman aggregation site, self-
recruitment of Little Cayman-spawned larvae back to Little 
Cayman is one of the only ways in which the Little 
Cayman Nassau grouper population might continue to 
persist.  
Overall, Nassau grouper appear to cue in on local 
currents when choosing when to spawn, likely as a 
mechanism for local retention of their offspring.  Whether 
actual local recruitment occurs will need to be answered 
with parentage genetics.  Given that the west end, Little 
Cayman Nassau grouper spawning aggregation is the last 
healthy aggregation around for over 100 km, a precaution-
ary approach would dictate that maintaining a healthy stock 
of adult Nassau grouper on Little Cayman Island, and an 
undisturbed spawning aggregation, is critical to the long 
term viability of Nassau grouper populations in the 
Cayman Islands. 
 
LITERATURE CITED 
Colin, P.L.  1992.  Reproduction of the Nassau grouper Epinephelus 
striatus (Pisces, Serranidae) and its relationship to environmental 
conditions. Environmental Biology of Fishes 34:357-377. 
Cowen, R.K., C.B. Paris, and A. Srinivasan.  2006.  Scaling of connectivi-
ty in marine populations. Science 311:522-527. 
De Mitcheson, Y.S., A. Cornish, M. Domeier, P.L. Colin, M. Russell, and 
K.C. Lindeman.  2008.  A global baseline for spawning aggregations 
of reef fishes. Conservation Biology 22:1233-1244. 
Kobara, S. and W. Heyman.  2008.  Geomorphometric patterns of Nassau 
grouper (Epinephelus striatus) spawning aggregation sites in the 
Cayman Islands. Marine Geodesy 31:231-245. 
Paris, C.B. and R.K. Cowen.  2004.  Direct Evidence of a Biophysical 
Retention Mechanism for Coral Reef Fish Larvae. Limnology and 
Oceanography 49:1964-1979. 
Sala E, Ballesteros E, Starr RM.  2001.  Rapid decline of Nassau grouper 
spawning aggregations in Belize: Fishery management and 
conservation needs. Fisheries 26: 23-30. 
Smith, C.L.  1972.  Spawning aggregation of Nassau grouper, Epineph-
elus striatus (Bloch). Transactions of the American Fisheries 
Society 101:257. 
Tucker, J., J. Parsons, G. Ebanks, and P. Bush.  1991.  Induced spawning 
of Nassau grouper Epinephelus striatus. Journal of the  World 
Aquaculture Society 22: 5. 
Whaylen, L., C.V. Pattengill-Semmens, B.X. Semmens, P.G. Bush, and 
M.R. Boardman.  2004.  Observations of a Nassau grouper, 
Epinephelus striatus, spawning aggregation site in Little Cayman, 
Cayman Islands, including multi-species spawning information. 
Environmental Biology of Fishes 70:305-313.  
 
 


Oceanographic patterns associated with Nassau Grouper aggregation spawn timing: shifts in surface currents on the nights of peak spawning

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Oceanographic patterns associated with Nassau Grouper aggregation spawn timing: shifts in surface currents on the nights of peak spawning

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