Download Mapping of the Biological Resources of the Continental Shelf

AMER. ZOOL., 30:15-21 (1990)
Mapping of the Biological Resources of the Continental Shelf
REZNEAT M. DARNELL
Department of Oceanography, Texas A&M University, College Station, Texas 77843
SYNOPSIS. Distribution patterns of the penaeid shrimp and demersal fish species are
discussed based upon collections totaling in excess of 330,000 shrimp and 2,245,000 fishes.
Depth related zonation reveals nearshore, mid-shelf, outer shelf, and trans-shelf assemblages. Estuary related species are particularly prominent on the northwestern Gulf shelf,
while rock and reef related species are most important in the fauna of the eastern Gulf
shelf. Recognizable elements in the shelf fauna also include species of tropical affinity;
open ocean and upper slope species; inhabitants of seagrass beds, mangrove swamps, and
carbonate rubble and shell hash; burrowers in soft bottoms; and inhabitants of the upper
water column. The eastern Gulf shelf is inhabited by twice as many fish species and eight
times as many unique species as the northwestern shelf, reflecting the great diversity of
habitat types in the eastern Gulf. A few key species numerically dominate the ichthyofauna
of the northwestern shelf, but dominance is spread through many species in the east.
Seasonal shifts in species density primarily reflect inshore/offshore seasonal migrations
of estuary related species, but seasonal inshore/offshore density shifts are also observed
among the true shelf residents. Trophically the shelf systems are supported by precipitated
plankton and organic detritus derived from rivers, bays and estuaries, seagrass beds, and
mangrove swamps, although attached algae are also important producers in the east.
Mollusks, polychaetes, and small crustaceans which feed upon this material, in turn, support the demersal fish communities whose species are generally short-lived. Larger longlived predators are mostly seasonal migrants which appear in the northern Gulf during
the warmer months. Bottom-feeding generalists are prominent in the northwest, while
specialists are in great evidence in the east. These distributional studies lay the foundation
for understanding the shelf ecological systems, and they also provide a basis for informed
management decisions.
major species assemblages and communities and provide further insight into the
The continental shelf is that portion of structure and function of the shelf ecologthe sea bottom out to a depth of 120-200 ical systems. In the late 1970s the Bureau
m. Continental shelves underlie about one of Land Management (now the Minerals
third of the surface area of the Gulf of Management Service) recognized a need
Mexico. Unlike land where the commu- for this type of information, and since 1980
nities are readily visible, the communities this agency has provided the financial
of the sea are generally hidden to our view. resources to complete these studies for the
As late as 1970 there was no comprehen- U.S. Gulf of Mexico continental shelf. The
sive understanding of the distribution pat- results have now been published (Darnell
terns of individual species or of species et al., 1983; Darnell, 1985; Darnell and
assemblages on the continental shelves of Kleypas, 1987).
the Gulf of Mexico. Consequently, in the
All the studies are based upon collections
early 1970s I began a systematic sampling
made
with standard shrimp trawl nets.
program to provide information concernIncluded
are 12 data sets obtained from
ing the distribution patterns of the bottom
and near-bottom fishes and invertebrates various federal and state agencies and uniof the continental shelves of the Gulf of versity scientists and covering the contiMexico. It was hoped that by mapping the nental shelf seasonally from the Mexican
distributional patterns of individual species, border to the Florida Keys. All species
we could also delineate the distribution of names have been updated, and all numerical information is expressed as catch-perunit-efFort with standardized gear (13.5 m
1
From the Special Session on Ecology of the Gulf of trawl pulled for one hour at a towing speed
Mexico organized by Rezneat M. Darnell and Richard of 3 knots). Distributional maps have been
E. Defenbaugh and presented at the Annual Meeting
of the American Society of Zoologists, 27-30 Decem- prepared for all species of penaeid shrimp
and fishes appearing in the collections, and
ber 1987, at New Orleans, Louisiana.
INTRODUCTION
15
16
REZNEAT M. DARNELL
seasonal density distribution maps were
made for those species present in sufficient
abundance.
ENVIRONMENTAL CONTEXT
For convenience the U.S. Gulf continental shelf will be considered in two sections,
the northwestern Gulf shelf extending from
the Rio Grande to the Mississippi River
Delta, and the eastern Gulf shelf stretching
from the Mississippi River Delta to the
Florida Keys, a shoreline distance of nearly
2,400 km. This shelf varies in width from
about 48 km at DeSoto Canyon (east of
Mobile Bay) to about 280 km off the Everglades.
For much of its distance the northwestern shelf is bordered by bays and estuaries
separated from the shelf by barrier islands.
On the coast of Louisiana and east Texas
the bays and estuaries exhibit high freshwater inflow and extensive low-salinity
Spartina marshes. Toward south Texas
freshwater inflow drops off dramatically,
and the lagoonal marshes are bathed in
salinities often well exceeding that of seawater. Surface sediments of the northwestern Gulf shelf are composed of quartz
sand, silt, and clay. Sands predominate from
the shoreline through the outer shelf of
the central sector and in the nearshore
environment from central Louisiana to the
Rio Grande. Silts and clays characterize the
middle and outer shelf off most of Louisiana and off south Texas. Small shoals
and rocky outcrops occur occasionally
throughout the shelf, and on the outer shelf
off the Texas/Louisiana border two large
topographic features (East and West Flower
Garden Banks) are capped by hermatypic
coral reefs with fauna of West Indian affinity. Bottom water temperatures of the
inshore area vary from a January low of
13.0°C to an August high of 29.0°C, a seasonal range of 16.0°C. On the outer shelf
bottom waters remain in the range of 18.0°20.0°C and show a seasonal variation of
only about one degree. During May, the
freshest month, nearshore bottom salinities reach 22.0%o off Louisiana and 33.0%o
off the southern half of Texas. Full marine
salinity (36.0%o) is encountered at a depth
of about 30 m. However, in August full
marine salinity extends to the shoreline
along the southern half of Texas, while the
inshore salinity off Louisiana ranges around
33.0%o. A nepheloid layer of turbid bottom
water persists throughout the year over
most of the northwestern Gulf shelf.
Bordering the eastern Gulf shelf, low
salinity bays and estuaries with extensive
Spartina marshes occur from the Mississippi River Delta to Mobile Bay. Other bays
along the Florida shelf have low freshwater
inflows and limited marsh areas. The Everglades of south Florida, consisting of
extensive mangrove swamps, include both
low and high salinity environments. In the
Big Bend area of the Florida shelf (between
Apalachicola and Tampa Bays) the inner
shelf to a depth of about 10m supports a
dense submarine meadow dominated by the
turtle grass, Thalassia testudinum, and seagrass beds are also common below the
Everglades. Sessile invertebrate communities of the middle shelf of peninsular
Florida are dominated by large populations of the loggerhead sponge, Spheciospongia vesparium. A veneer of quartz sand
paves the nearshore shelf of peninsular
Florida, and this extends through the outer
shelf throughout much of the continental
shelf off the Florida panhandle, as well as
Alabama and Mississippi. Some areas of
silt/clay bottom appear off Mississippi and
around the Mississippi River Delta. Fine
sediments line the eastern flank of DeSoto
Canyon, and a tongue of fine carbonate
sediments extends southwest off the Everglades. Most of the middle and outer shelf
of peninsular Florida is carpeted with
coarse carbonate sediments (rubble and
shell hash), and this is bounded shoreward
by a thin band of mixed quartz/carbonate
sand. Rocky outcrops have been recorded
throughout the length and breadth of the
eastern Gulf shelf. Most are of low relief,
being only a meter or so in height, but the
extensive Florida Middle Grounds reef at
a depth of 40-50 m north of Tampa Bay
has a vertical relief of about 15 m. During
the winter bottom water temperatures in
the nearshore area down to near Tampa
Bay range about 14.0°-15.0°C, and below
Tampa Bay they range up to 19.0°C. Offshore bottom waters extend from 17.0°-
BIOLOGICAL RESOURCE MAPPING
18.0°C along the northern coast to 20.0°22.0°C off peninsular Florida. During
August inshore temperatures of 28.0°30.0°C occur all along the coast. Offshore
temperatures of 22.0°C prevail near the
Mississippi River Delta, but around DeSoto
Canyon and along the outer shelf of peninsular Florida bottom water temperatures
range around 16.0°-18.0°C. Bottom water
temperatures of the outer Florida shelf are
about 4.0°C cooler in the summer than
during the winter, and this is due to the
summer intrusion of cool deep Gulf water.
Bottom salinities over most of the eastern
Gulf shelf range above 35.0%o throughout
the year except for the shelf off Alabama
and Mississippi. Here bottom salinities of
31.0-33.0%o occur during May, and salinities of 32.5-34.0%o appear during August.
A persistent nepheloid layer characterizes
the Alabama/Mississippi shelf, but off
Florida high bottom water turbidity is
observed only during and after major
storms.
DISTRIBUTION PATTERNS
Distribution maps have been prepared
for 18 species of penaeid shrimp and 372
species of fishes as well as a number of genera and families. For those taxa which were
sufficiently abundant in the combined data
base seasonal density distribution maps
were prepared, and from these maps many
interesting facts have emerged. Some of
the conclusions likely to be of interest to
the general reader are presented below.
Figures 1 and 2 show the density distribution of the total fish catch (all species
combined) during the summer months
expressed as the number captured per hour
of trawling effort. On the northwestern
Gulf shelf the fauna exhibits a double gradient. Densities tend to be higher nearshore than offshore and higher toward
Louisiana than toward the Rio Grande. The
higher densities can generally be accounted
for on the basis of three controlling environmental factors: presence of extensive
low salinity and highly productive nursery
areas, presence on the shelf of organic-rich
muddy bottoms, and presence of low salinity shelf bottom waters. By contrast, the
eastern Gulf shelf is characterized by patchy
17
faunal distribution patterns with each patch
reflecting a set of environmental variables
locally favorable for a particular segment
of the fauna. The Alabama/Mississippi
shelf and coast is much like that of Louisiana, as discussed above. Seagrass beds,
Everglades nearshore, major reefs and outcrops, DeSoto Canyon, coarse carbonate
bottom, and Florida outer shelf all show
distinctive environmental features and
support characteristic faunal assemblages.
Careful examination of individual species
patterns reveals a depth-related zonation
of the shelf fauna which is somewhat more
evident in the northwestern than in the
eastern Gulf. These zones include the inner
shelf (0-20 m), middle shelf (20-80 m), and
outer shelf (80-120 m). Some species range
widely throughout all the zones and are
considered to be trans-shelf species. The
inner shelf is a high energy environment
characterized by wave disturbance, alongshore currents, coarse bottoms, seasonally
variable temperature, and often less than
full marine salinity. The middle shelf is a
lower energy depositional environment
with lower variability in bottom temperature and salinity. The outer shelf is characterized by reduced light, and often finergrained sediments. Salinity is relatively
constant, but the temperature may vary
depending upon hydrographic circumstances. Tidal oscillations, inertial waves,
offshore currents and other phenomena
may cause deep basin water to intrude upon
the outer shelf. Upwelling and entrainment of deep basin water is known to occur
in several areas.
Recognizable elements make up the shelf
fauna. One major element is those species
which are estuary related in the sense that
the young of the species either require or
at least are frequently encountered in low
salinity waters. The estuarine nursery areas
provide an abundance of food as well as
protection from marine predators. On the
shelf this faunal element is most evident in
the nearshore zone, less so on the middle
shelf, and even less so on the outer shelf.
However, there are non-estuarine related
species which populate each of the zones.
The outer shelf zone contains a number of
species more characteristic of the upper
TEXAS
26°-
Number of
Individuals
500-999 |
1,000-1,999 |
2,000-4,999 I
5,000-7,499
120
40
6OBOI2Omelers
1. Density distribution of the total fish catch on the northwestern Gulf shelf during the summer. Densities of less than 500 fishes per hour have been
deleted,
FIG.
BIOLOGICAL RESOURCE MAPPING
19
FIG. 2. Density distribution of the total fish catch on the eastern Gulf shelf during the summer. Densities
of less than 500 fishes per hour have been deleted.
slope. The estuary related fauna and shelf
zonation are most evident on the northern
Gulf shelf from the Rio Grande to Mobile
Bay.
Another element of the shelf fauna is
that related to rocks, reefs, and other structures (Smith, 1976; Darnell and Kleypas,
1987). This element is poorly represented
in the northwest, somewhat more prominent from the Mississippi River Delta to
Mobile Bay, and quite prominent on the
carbonate shelf of peninsular Florida. In
recent years this element has apparently
increased on the Louisiana and east Texas
shelf in response to the development of
thousands of oil rigs and platforms in the
area.
Another faunal element consists of those
species of tropical affinity. A few of these
show up on the south Texas shelf, but many
appear on the shelf of peninsular Florida,
with both frequency and diversity increas-
ing toward the south. These include species
from the West Indies as well as Yucatan
and Central America.
Truly oceanic species (argentines, myctophids, boarfishes, and spikefishes) occasionally appear, and these are probably
brought in by the Gulf loop current as well
as by upwelling and entrainment of deep
oceanic water up the DeSoto Canyon or
along the outer shelf of peninsular Florida.
These are more prominent in the eastern
than the northwestern Gulf, but they are
never abundant. A related element is the
upper slope species (goosefishes, codfishes,
macrourids, tilefishes, flatheads, etc.) which
are sometimes prominent in the outer shelf
fauna.
Bottom burrowing species such as many
of the eels and eel-like forms are not well
sampled by trawls, but these are clearly evident wherever soft bottoms prevail on both
the northwestern and eastern Gulf shelves.
20
REZNEAT M. DARNELL
Slow moving bottom trawls do not provide
adequate samples of those species which
are fast swimmers or which inhabit upper
levels of the water column. Such species
include most sharks, tarpons, clupeids,
carangids, dolphins, mackerels, wahoos,
tunas, and billfishes.
Additional elements are recognizable in
the fauna of the eastern Gulf shelf. The
extensive submarine meadows of the Florida Big Bend area serve as important nursery grounds for the young of species whose
adults are found elsewhere. Particularly
prominent are pipefishes and young pinfishes, porgies, filefishes, and puffers. Several pipefishes, blennies, gobies, etc. were
not collected elsewhere. The same considerations generally apply to the Everglades
nearshore. Collected only here were a whip
eel, porkfish, reef silverside, rough silverside, roughtongue bass, blue croaker, fantail mullet, and crested blenny. The carbonate rubble and shell hash bottoms of
the middle and outer shelf of peninsular
Florida support populations of small shrimp
and fish species which are poorly known.
They are not well sampled by large-meshed
trawls, but they do show up in the stomachs
of other bottom feeding fishes (Topp and
Hoff, 1972; Darovec, 1983; Ross, 1983).
Included are such forms as codlets, blennies, comb blennies, gobies, and dragonets.
On the northwestern Gulf shelf the
demersal ichthyofauna is numerically dominated by a few key species. The two most
abundant (Atlantic croaker and longspine
porgy) make up 47.9% of the catch, and
the top ten species constitute 72.8% of the
total catch. On the eastern Gulf shelf dominance is spread among many species. Here
the top two species (pinfish and longspine
porgy) make up 18.8%, and the top ten
species constitute only 44.5% of the catch.
On the other hand, the demersal ichthyofauna of the eastern Gulf shelf is far richer
in species than that of the northwestern
Gulf" shelf and is represented by twice as
many species (347:164) and over eight times
as many unique species (208:25). One
hundred and thirty-nine species are common to the two areas. The high species
diversity of the eastern Gulf shelf reflects
a) great habitat diversity, b) availability of
highly productive "live bottom" habitat, c)
the dynamic nature of the benthic environment (in terms of bottom currents
exposing solid substratum for colonization
by sessile species), and d) repeated invasions from Yucatan and the West Indies.
"Live bottom" consists of clusters of
attached algae and invertebrates (sponges,
stony and soft corals, sea fans, crinoids, etc.)
which feed largely upon plankton and
which provide both food and shelter for
the mobile fauna. The same general patterns apply to the penaeid shrimp, although
the number of species is much smaller.
Eighteen shrimp species are identified of
which eleven were common to the two
areas; one was unique to the northwest,
and six were unique to the eastern Gulf
shelf.
Seasonal density shifts for many of the
species were rather dramatic, especially on
the northern Gulf coast where the estuary
related fauna (brown and white shrimp,
spot, Atlantic croaker, sand seatrout, and
many others) moved into and out of the
bays and estuaries. However, even on the
shelf of peninsular Florida some species
(pink shrimp, pinfish, two filefishes, and
others) showed clear density shifts from
inshore nursery areas to offshore adult
habitats. Most of the shifts were toward
deeper water in fall and winter. However,
a few species exhibited onshore movements in the cooler months, and this phenomenon was especially prominent among
the codfishes which spend the warmer
months on the outer shelf or upper slope.
CONTINENTAL SHELF ECOSYSTEMS
From knowledge of the species distribution patterns and additional information
something may be said about the trophic
structure of the shelf ecosystems. Along
the northern Gulf the continental shelf
receives seasonal inputs of finely particulate organic detritus from the many rivers,
bays, and estuaries. Precipitated plankton
and fecal pellets further enrich the sediments. Due to high turbidity and scarcity
of hard substrates for attachment, local
production by attached algae occurs only
in a few areas. Although organic detritus
appears as a major food category among
21
BIOLOGICAL RESOURCE MAPPING
estuarine fishes (Darnell, 1958), it is largely
unrecognizable in the diets of the northern
Gulf shelf fishes (Rogers, 1977). Mollusks,
polychaetes, and small crustaceans constitute the intermediate step in the shelf food
chains. In addition to receiving waves of
organic detritus, the northern Gulf shelf
is annually enriched with pulses of high
energy protein as the crabs, shrimp, and
fishes undergo their fall migrations from
the estuarine nursery areas back to the continental shelf. Life histories of most of the
northern Gulf resident species are short
(18-24 months), but many are able to complete their winter and spring spawning
activities before encountering heavy summer predation pressure. During the summer hordes of large longer-lived predators
take up residence in the northern Gulf.
These include various species of sharks,
tarpons, carangids, dolphins, mackerels,
tunas, and billfishes, among others. Considerable trophic redundancy is observed
among the demersal fishes of the northern
Gulf. Most tend to be generalized bottom
feeders, but some differentiation is evident
in the size of food consumed, shelf zone
and bottom type utilized, seasonal habitation, and feeding behavior.
Bottom sediments of the eastern Gulf
shelf are enriched with organic detritus
from the seagrass beds and mangrove
swamps as well as precipitated plankton and
fecal pellets. However, the water here is
clear, and there are many hard substrates
for attachment. Local in situ production by
filamentous, calcareous, and leafy algae is
supplemented with production by zooxanthellae of the hermatypic corals and other
cnidarians. Here also many of the sessile
invertebrates feed directly upon plankton
of the water column. Among the demersal
fishes bottom feeding generalists are abundant, but specialists are also much in evidence. Trophic relationships of the fishes
and invertebrates of the Florida shelf are
not well known.
This study of the density distribution
patterns of the demersal penaeid shrimp
and fish species of the U.S. Gulf of Mexico
continental shelf has provided considerable insight into the composition and
dynamics of the shelf ecological systems in
both their regional and local differentiations. It also provides a basis for informed
management decisions aimed at protecting
the biological and ecological resources in
the face of mounting human intrusion into
the shelf environments.
REFERENCES
Darnell, R. M. 1958. Food habits of fishes and larger
invertebrates of Lake Pontchartrain, Louisiana,
an estuarine community. Publ. Inst. Mar. Sci., U.
Texas 5:353-416.
Darnell, R. M. 1985. Distribution of fishes and
penaeid shrimp of commercial and recreational
importance on the continental shelf off Mississippi and Alabama, v + 61 pp. (Appendix B). In
Barry A. Vittor & Associates, Inc., Tuscaloosa trend
regional data search and synthesis study. Rept. to
Minerals Management Service, Metairie, Louisiana. 2 vols.
Darnell, R. M., R. E. Defenbaugh, and D. Moore.
1983. Northwestern Gulf shelf bio-atlas, a study of
the distribution of demersal fishes and penaeid shrimp
of soft bottoms of the continental shelf from the Rio
Grande to the Mississippi River Delta. U.S. Dept.
Interior, Minerals Management Service. Open
File Rept. 82-04. xii + 438 pp.
Darnell, R. M. and J. A. Kleypas. 1987. Eastern Gulf
shelf bio-atlas, a study of the distribution of demersal
fishes and penaeid shrimp of soft bottoms of the continental shelf from the Mississippi River Delta to the
Florida Keys. U.S. Dept. Interior, Minerals Management Service. OCS Study MMS 86-0041. xv
+ 548 pp.
Darovec, J. E., Jr. 1983. Sciaenid fishes (Osteichthyes: Perciformes) of western peninsular Florida. Fla. Dept. Nat. Res., Mar. Res. Lab., Mem.
Hourglass Cruises. 6(3). 73 pp.
Rogers, R. M., Jr. 1977. Trophic interrelationships
of selected fishes on the continental shelf of the
northern Gulf of Mexico. Ph.D. Diss., Texas A&M
University, College Station, Texas, xv + 229 pp.
Ross, S. T. 1983. Searobins (Pisces: Triglidae). Fla.
Dept. Nat. Res., Mar. Res. Lab., Mem. Hourglass
Cruises. 4(4). 76 pp.
Smith, G. B. 1976. Ecology and distribution of eastern Gulf of Mexico reef fishes. Fla. Dept. Nat.
Res., Mar. Res. Lab., Mar. Res. Publ., No. 19. 78
pp.
Topp, R. W. and F. H. Hoff. 1972. Flatfishes (Pleuronectiformes). Fla. Dept. Nat. Res., Mar. Res.
Lab., Mem. Hourglass Cruises. 4(2). 135 pp.