Dear Eleonora and FISH-ECOLOGY,
Whale sharks (Rhincodon typus) are regularly observed at the Flower
Garden Banks National Marine Sanctuary (FGB) and surrounding
waters. I've been conducting resarch on these animals, as well as the
other elasmobranchs occurring over these and other topographic highs
in the northwestern Gulf of Mexico for several years now, and have
found some interesting trends in their occurrence and behavior. The
earliest observation I have of a whale shark at the FGB is within the first
week of July, and the last observation is for several in October.
For those of you not familiar with the FGB, they are located 110 nm SW
of Galveston, Texas, near the continental shelf margin. These banks
support the northernmost coral reefs on the North American continental
shelf, and are quite healthy (ecologically). Interestingly, I've observed
whale sharks at other banks / reefs in the area, however these
observations are during the warmest months of the year (July - Sept.).
Water temp. for the FGB during these months peak at 29.6 degrees C.
Based upon my data, it appears whale sharks move from the continental
shelf margin (CSM)(and possibly farther seaward) toward nearshore
waters as water temperatures rise during the summer months, and
move back toward the CSM or southward along the coast of Mexico, as
water temperatures drop. Another possible motivation behind this
apparent annual migration may be photoperiod.
Another interesting phenomenon is the aggregation of whale sharks,
and several other planktivorous elasmobranchsover or in the vicinity of
the FGB during the annual coral spawning event (Aug. or Sept.).
Several NMFS researchers and myself have conducted aerial surveys
over FGB waters during this period, and have observed surface
feeding aggregations of whale sharks, jacks (Carangidae), seabirds,
small cetaceans, and several other moderately sized sharks.
Manta rays (Manta birostris) are common residents at the FGB, and over
the past 7 years, I have documented the occurrence of a sicklefin devil
ray (Mobula tarapacana) at the West Flower Garden Bank on two
separate occassions (in press - "Gulf of Mexico Science"). This is the
first account of this species in the western Atlantic north of Venzuela.
These observations also coincided with the annual mass spawning of
corals. One other planktivorous elasmobranch species occurring at the
FGB is the lesser devil ray, (M. hypostoma), however this animal is
observed over the FGB during the months of April, May, and June.
If you are interested in learning more about the elasmobranchs
occurring at the FGB or within the northwestern Gulf of Mexico, you
may contact me at the address below. You may also want to check out
the website for the FGB, a National Marine Sanctuary, administered
through the Sanctuary and Reserves Division of NOAA.
Marine Vertebrate Ecologist
Wildlife & Fisheries Sciences Dept.
Texas A&M University
College Station, TX 77843-2258
email: [log in to unmask]