An Index of Abundance for Coastal Species of Juvenile Sharks from the Northeast Gulf of Mexico – Statistical Data Included
John K. Carlson
In 1993, a Fishery Management Plan For Sharks (FMP) (NMFS, 1993) reported the abundance of many species of sharks, particularly large coastal species, could have declined by up to 75% from the 1970’s to mid 1980’s. To improve management and recovery of shark stocks, the FMP stressed the need for better estimates on the assessment and monitoring of shark populations. Prior to 1993, most estimators of shark stocks were derived using fishery-dependent indices which generally lacked a standardized statistical sampling design. The validity of an abundance estimator (i.e. indices of abundance) depends on its accuracy and precision, and its robustness (the strength of its relationship with recruitment or stock size). Indices of shark abundance are currently used in production model analysis integrated using Bayesian statistical techniques (NMFS(1)).
Fishery-independent estimates of relative abundance are presently limited but can be the best estimator of shark stocks (NMFS(1,2,3)). Currently, only three surveys exist for monitoring shark relative abundance:
1) Musick et al. (1993) reported on a 17-year time series of abundance for sandbar, Carcharhinus plumbeus, and dusky, C. obscurus, shark from areas adjacent to the middle U.S. Atlantic coast.
2) Grace and Henwood (1997) performed pilot studies and have been conducting an assessment of the distribution and abundance of coastal sharks in the Gulf of Mexico and western North Atlantic since 1995.
3) National Marine Fisheries Service (NMFS), Narragansett Laboratory has executed shark longline surveys between Miami, Fla. and southern New England for 1986, 1989, 1991, and 1998 (Casey(4,5,6); Natanson(7)). However, most of these surveys are generally conducted in deeper waters ([is greater than] 10 m) where adult sharks mostly congregate. Neonate and juvenile sharks are commonly found in coastal nursery areas ([is less than] 10 m deep) where they feed and avoid predation (Branstetter, 1990) during summer months.
With the exception of Musick et al.(8) and Merson and Pratt’s(9) work on juvenile sandbar sharks along the U.S. east coast, little data exists on juvenile shark stock size and recruitment to the adult portion of the population. Unlike most teleost species, the relationship between stock size and recruitment is direct, owing to the reproductive strategy of low fecundity combined with few, fully formed offspring (Holden, 1977). Quantitative estimates of juvenile abundance can provide promising alternatives to traditional hindcasting models and could improve the ability to assess current and future shark stock size and strength. Herein, we report on a 3-year fishery independent assessment of juvenile coastal shark populations in U.S. waters of the northeastern Gulf of Mexico derived using two methods.
Materials and Methods
Two regions were established as fixed sampling areas in the northeastern Gulf of Mexico (Fig. 1). The criteria for establishing these areas were based on a priori shark abundance survey information (Trent et al.(10)) and depth strata. The depth strata were between 1-5 m and 5-10 m.
[Figure 1 ILLUSTRATION OMITTED]
The first area (shallow stratum) is located in St. Andrew Sound. This area is a small semi-enclosed marine lagoon with expanses of submerged vegetation, Thalassia spp. and Halodule spp. It is about 14.5 km long and 0.2-2.0 km wide and has mean water depths of 3-5 m. Salinity ranges from 25-36% and tidal amplitude averages 0.42 m. The sound exchanges water with the Gulf of Mexico through passes about 0.5-2.0 km wide.
The second area (deep stratum) is located off St. Vincent Island at the southwest end of the Apalachicola Bay system. This area is about 1-3 km south of St. Vincent Island in the Gulf of Mexico where water depths average 5-10 m. The bay system surrounding this area is largely a line of barrier islands fronting the intersection of the Apalachicola delta and is the only bay system in Florida in which a large river system drains. As a result of river discharge, there is little submerged vegetation due to high turbidity. Salinity fluctuates from 15 to 35% and tidal fluctuation averages 0.66 m.
Sampling Gear and Survey Design
A 186 m long gill net consisting of panels of six different mesh sizes was utilized for sampling. Stretched mesh sizes (SM) ranged from 8.9 cm (3.5″) to 14.0 cm (5.5″) in steps of 1.27 cm (0.5″), with an additional size of 20.3 cm (8.0″). Panel depths when fishing were 3.1 m. Webbing for all panels, except for 20.3 cm, was of clear mono-filament, double-knotted and double-selvaged. The 20.3 cm SM webbing was made of #28 multifilament nylon, single-knotted, and double-selvaged. When set, the nets were anchored at both ends.
The longline was constructed of a mainline made of two 152 m lengths of 425.8 kg test monofilament line. A 15.2 m length of 0.79 cm diameter braided polypropylene line connected each 152 m length, and the entire line when fished was 319.2 m long. Polyethylene floats made of 1.5 m lengths of 136 kg test monofilament line with a snap were attached to the mainline every 30.4 m. A standard longline consisted of 10-20 gangions placed at 15.2 m intervals along the mainline. Gangions were 0.9 m long and composed of snaps, aluminum sleeves, hooks (Mustad(11) #12/0, no 2888), and monofilament lines (136-kg test). Bait was either menhaden, Brevoortia spp., or Atlantic mackerel, Scomber scombrus. The mainline, when set, was tethered to an anchor on each end with a 30.4 m, 0.79 cm polypropylene rope between the anchor and the end of the mainline. A buoy (3.6 m aluminum pole with 1.8-kg weight and 50.8 cm poly float), with a strobe light and flag extended 2.4 m above the float, was attached at each end of the mainline.
In both areas, surveys were conducted monthly from April through October. For each survey period, the sampling gear was randomly set within each area in a station designated on Loran C coordinates. Gillnets were checked and cleared of catch, or pulled and reset every 1.0-2.0 h. Longline soak times ranged from 1.0-1.5 h. Following each soak period, the longline was checked and all gangions that had caught sharks, been broken or damaged, or had damaged or lost baits, were removed from the mainline and a fresh-baited gangion attached. Sharks captured using either method were measured to the nearest cm for body lengths (precaudal, fork, total, and stretch total length) and data for sex and life history stage (neonate, young-of-the-year, juvenile, adult) were recorded. Sharks in poor condition were sacrificed for life history studies and those in good condition were tagged with a nylon-head dart tag and released.
Environmental data were collected prior to sampling. Mid-water temperature ([degrees] C), and dissolved oxygen (mg [1.sup.-1]) was measured with a YSI Model 55 oxygen meter, and light transmission (cm) was determined using a secci disk. Surface salinity (%) was measured with a refractometer.
Catch Per Unit of Effort
The shark abundance index (CPUE [yr.sup.-1]) is calculated as the arithmetic mean catch per unit of effort of combined samples within all months and areas sampled for each year. Adult sharks were removed from the analysis based on size at maturity information (Table 1) and maturity state assessed in the field. For gillnets, a CPUE value was defined as the mean number of sharks caught per 186 m long gillnet per hour. For longlines, CPUE was standardized to 10 hooks and defined as the mean number of sharks per 10 hook-hours.
Table 1.–Mean sizes of sharks captured using gillnets and longlines.
Total length (cm)
Mean size Size range Size at
Species captured(1) captured maturity(2)
Atlantic 70.3 ([+ or -] 17.4) 32-111 80 (M)
sharpnose 85 (F)
Blacknose 79.5 ([+ or -] 21.7) 46-132 103 (M)
Blacktip 96.1 ([+ or -] 19.6) 50-150 120-145 (M)
Bonnethead 73.7 ([+ or -] 15.8) 44-121 80 (M)
Finetooth 98.0 ([+ or -] 19.3) 54-140 130 (M)
Sandbar 91.0 ([+ or -] 20.3) 58-160 170 (M)
Scalloped 61.7 ([+ or -] 15.9) 38-153 180 (M)
hammerhead 250 (F)
Spinner 83.8 ([+ or -] 17.8) 53-134 170 (M)
Atlantic Parsons (1983)
Blacknose Clark and von Schmidt (1965)
Carlson et al. (1999)
Blacktip Killam and Parsons (1989)
Bonnethead Parsons (1993)
Carlson and Parsons (1997)
Finetooth Castro (1993)
Sandbar Sminkey and Musick (1995)
Scalloped Branstetter (1987b)
Spinner Branstetter (1987a)
(1) Numbers in parentheses are [+ or -] 1 standard deviation.
(2) M=male and F=female. Size at maturity was taken from the most recent information in the literature.
Results and Discussion
A total of 14 species of sharks were collected with gillnets and longlines of which eight and six species, respectively, were captured consistently. Data from species consistently caught were used to generate abundance indices. Within each respective management group, the Atlantic sharpnose shark, Rhizoprionodon terraenovae, a member of the small coastal management group, was most often captured, and the blacktip shark, Carcharhinus limbatus, was the species captured most often in the large coastal management group, using either longlines or gillnets (Table 2). The bonnethead shark, Sphyrna tiburo, was the species captured second most often in the small coastal group and overall was the third most encountered species. The remaining species captured in decreasing abundance were the finetooth shark, C. isodon; spinner shark, C. brevipinna; scalloped hammerhead shark, S. lewini; blacknose shark, C. acronotus; and sandbar shark, C. plumbeus. Other species caught but not consistently captured were Florida smoothhound, Mustelus norrisi; bull shark, C. leucas; nurse shark, Ginglymostoma cirratum; lemon shark, Negaprion brevirostris; tiger shark, Galeocerdo cuvieri; and great hammerhead shark, S. mokarran.
Table 2.–Management groups and associated sharks captured in the northeast Gulf of Mexico during 1996-98. Management group reports sharks in overall decreasing abundance.
Management group Common name Scientific Name
Large coastal Blacktip Carcharhinus limbatus
sharks Spinner Carcharhinus brevipinna
Scalloped hammerhead Sphyrna lewini
Sandbar Carcharhinus plumbeus
Bull Carcharhinus leucas
Nurse Ginglymostoma cirratum
Lemon Negaprion brevirostris
Great hammerhead Sphyrna mokarran
Tiger Galeocerdo cuvier
Small coastal Atlantic sharpnose Rhizoprionodon
sharks Bonnethead terraenovae
Finetooth Sphyrna tiburo
Blacknose Carcharhinus isodon
Nonmanagement Florida smoothhound Mustelus norrisi
CPUE trends varied by species. Declines in CPUE were noted for Atlantic sharpnose, blacknose, and bonnethead using gillnets (Fig. 2) and for Atlantic sharpnose, finetooth, and spinner sharks for longlines (Fig. 3). Increases in CPUE were found for sandbar sharks for gillnets. Species with a relatively stable or no clear trend in CPUE include blacktip shark for both methods, blacknose shark using longlines, and finetooth shark and spinner sharks caught using gillnets. Because this survey is relatively new, the small number of values for the independent variable (i.e. 3 years) precluded fitting regression models to each species time series.
[Figures 2-3 ILLUSTRATION OMITTED]
The overall objective of this study was to develop a species-specific index of abundance (i.e. time series) for a variety of juvenile sharks that can be ultimately used for stock assessment. Juvenile sizes vary by species and, due to the high selectivity of gillnets, captures of different species are likely for particular mesh sizes. To accommodate the wide range in size of juvenile sharks, we used multi-panel gillnets with variable mesh sizes that have been shown to be effective for capturing juveniles of many economically important species (Trent(12)). For all species, the mean size and range of sharks captured during the survey included mostly neonates and juveniles (Fig. 4). Although the commercial shark industry commonly uses large J hooks on bottom longlines, we utilized smaller J hooks fished in mid water over larger J hooks fished on the bottom because the former are more efficient at capturing juvenile coastal sharks (Trent and Carlson(13)). This method permitted capture of six species of sharks in significant numbers for which indices could be generated, with at least 85% of each species being juveniles (Fig. 5).
[Figures 4-5 ILLUSTRATION OMITTED]
The best index of fish abundance is one by which extraneous influences on CPUE can be controlled. Although certain environmental factors (e.g. weather patterns, water temperature, salinity) could not be controlled, we have attempted to minimize bias associated with factors such as spatio-temporal distributions by sampling throughout all months when sharks are beginning to or have recruited to their summer nursery areas. To control gear selectivity bias, the same gear and methodology were used for all years sampled.
The validity of an index of abundance depends on its precision, especially if changes in CPUE are regarded as real. The use of fixed areas or stations for developing indices of abundance as opposed to a simple random or stratified random statistical design has recently come under discussion (National Research Council, 1998). Although relying on fixed areas assumes no change over time in recruitment patterns, emigration or immigration, arguments have been made that the mean from a fixed survey design can be more precise than the mean from a simple random sample (Cochran, 1977) or a stratified random design (Simmonds and Fryer, 1996). Our sampling design attempts to embrace both a random and fixed statistical design by utilizing random samples within fixed areas. By comparing coefficients of variation, as a measure of relative precision, from this study with those provided in Grace and Henwood (1997), most CPUE values derived in this study were similar or more precise than those calculated for similar shark species captured by Grace and Henwood (1997) (Table 3). Moreover, it should be noted that the index of abundance for striped bass, Morone saxatilis, developed from a 20-yr fixed station sampling design, was found to predict subsequent commercial landings of striped bass (Goodyear, 1985).
Table 3.–A comparison of CPUE and coefficient of variation (CV is defined as the standard error divided by the mean following Grace and Henwood (1997)) for similar shark species captured in this study and from data provided in Grace and Henwood (1997). For gillnets, CPUE is defined as the mean number of sharks caught/186 m long gillnet/h. For longlines, CPUE is defined as number of sharks/10 hook h for this study and defined as number of sharks/100 hook h for Grace and Henwood (1997).
This study (gillnet)
Species CPUE (1996) CV
Atlantic sharpnose 2.23 0.31
Blacknose 0.34 0.57
Blacktip 0.54 0.46
Finetooth 0.68 0.40
Sandbar 0.05 0.48
Scalloped hammerhead 0.11 0.39
Spinner 0.26 0.56
CPUE (1997) CV
Atlantic sharpnose 0.92 0.23
Blacknose 0.08 0.60
Blacktip 1.05 0.37
Finetooth 1.06 0.30
Sandbar 0.09 0.83
Scalloped hammerhead 0.53 0.37
Spinner 0.33 0.38
This study (longline)
Species CPUE (1996) CV
Atlantic sharpnose 0.62 0.12
Blacktip 0.20 0.26
Finetooth 0.10 0.34
Sandbar 0.02 0.71
Scalloped hammerhead 0.00
Spinner 0.19 0.36
CPUE (1997) CV
Atlantic sharpnose 0.41 0.26
Blacknose 0.02 0.71
Blacktip 0.33 0.19
Finetooth 0.06 0.59
Sandbar 0.03 0.57
Scalloped hammerhead 0.00
Spinner 0.06 0.53
Grace and Henwood (1997)
Species CPUE (1996) CV
Atlantic sharpnose 2.03 0.22
Blacknose 0.28 0.30
Blacktip 0.12 0.33
Sandbar 0.13 0.27
Scalloped hammerhead 0.05 0.37
Spinner 0.04 0.52
CPUE (1997)(1) CV
Atlantic sharpnose 2.33 0.12
Blacknose 0.30 0.20
Blacktip 0.22 0.27
Finetooth 0.004 1.00
Sandbar 0.23 0.31
Scalloped hammerhead 0.06 0.31
Spinner 0.07 0.53
(1) Grace, M., and T. Henwood. 1998. Summary of NMFS Shark Surveys/Southeastern Region 1995, 1996, 1997. 1998 Shark Evaluation Workshop Document SB-IV-29. U.S. Dep. Commer., NOAA, NMFS, Southeast Fisheries Science Center, Panama City, Fla., 34 p.
Because shark summer nursery and pupping grounds are generally found in inshore areas, they are particularly susceptible to anthropogenic disturbances such as shoreline development, additions of wastewater, and recreational activities. Although loss of these habitats has not been quantitatively assessed in terms of shark production, preliminary evidence suggests that estuaries with more coastal development have less diversity and abundance of shark species (Carlson(14)). Moreover, the revised Magnuson-Stevens Fishery Conservation and Management Act of 1996 requires the description and identification of essential fish habitat (EFH) for all Federally managed species and further requires identification of threats to EFH.
It is still unclear whether the abundance estimates presented herein represent stockwide estimates or represent populations only for the northeastern Gulf of Mexico. Although adults of many species, particularly sandbar, blacktip, scalloped hammerhead, and spinner shark are highly migratory, whether sharks from the eastern Gulf of Mexico mix with stocks from the western Gulf of Mexico, Atlantic Ocean, or Mexican waters is yet to be determined. There is growing evidence that the abundance and distribution of juvenile sharks in nursery areas is not the same throughout the northern Gulf of Mexico (Parsons(15); deSylva(16)). The paucity of tag and recapture information in the Gulf of Mexico further complicates understanding of the geographical and seasonal distribution of sharks.
Given the direct relationship between stock and recruitment for sharks (Holden, 1974, 1977; Hoenig and Gruber, 1990), monitoring of juvenile abundance will aid in assessing current parental stock. This information, combined with current efforts by Grace and Henwood (1997) to monitor adult stock size, will also benefit current management regulations and forecasting of future stock size.
Thanks go to S. Baker, B. Blackwell, N. Lewis, C. Palmer, and the many interns who provided field assistance with shark collection. For suggestions and comments throughout the survey, we extend appreciation to Enric Cortes, Doug Devries, Gary Fitzhugh, and Lee Trent (NMFS Panama City Facility, Fla.); Mark Grace (NMFS Pascagoula Laboratory, Miss.); and Churchill Grimes (NMFS Tiburon Laboratory, Calif.). This research was funded by the NMFS Office of Sustainable Fisheries, Highly Migratory Species Division, Silver Spring, Maryland; Southeast Fisheries Science Center’s Sustainable Fisheries Division; and the NMFS Panama City Facility.
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(11) Mention of trade names or commercial firms does not imply endorsement by the National Marine Fisheries Service, NOAA.
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John K. Carlson and John H. Brusher are with the National Marine Fisheries Service, NOAA, Panama City Facility, Southeast Fisheries Science Center, 3500 Delwood Beach Rd, Panama City, FL 32408. E-mail: email@example.com and firstname.lastname@example.org
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