Shark Nursery Grounds of the Gulf of Mexico and the East Coast Waters of the United States

<em>Abstract.</em>—Winyah Bay is a 65-km² estuary in northeast South Carolina, and North Inlet is a 32- km², high-salinity estuary connected to both Winyah Bay and the Atlantic Ocean. The objectives of this study were to survey the shark fauna of these systems, determine the potential of these estuaries as shark nurseries, and assess the impact of salinity structure on shark diversity and abundance in these two estuaries. From May to November in 2002 (a drier than average year) and 2003 (a wetter than average year), 227 bottom longlines (16/0 and 12/0 hooks) were set in Winyah Bay. In 2002 and 2003, a total of 119 trammel net sets were also conducted from June to October in North Inlet. A total of 196 sharks (38 adults, 158 juveniles) representing 10 species were captured in Winyah Bay in 2002, whereas 73 sharks (17 adults and 56 juveniles) representing four species were caught in 2003. Catch per unit effort (CPUE) for all sharks caught in Winyah Bay was not significantly different between 2002 and 2003. Blacktip shark <em>Carcharhinus limbatus </em>and finetooth shark <em>C. isodon </em>CPUE declined significantly on 16/0 hook longlines set in Winyah Bay from 2002 to 2003. For 12/0 hook longlines set in Winyah Bay, CPUE for three species (sandbar shark <em>C. plumbeus</em>, Atlantic sharpnose shark <em>Rhizoprionodon terraenovae</em>, and finetooth shark) out of five declined significantly from 2002 to 2003. Within Winyah Bay, CPUE for sharks on both longline configurations was not significantly different between lower and middle bay sites for 2002 but was for 2003. In both years, CPUE correlated positively with bottom salinity in Winyah Bay. In North Inlet, in 2002, 30 sharks (20 adults, 10 juveniles) comprising five species were caught, whereas 57 sharks (26 adults and 31 juveniles) representing three species were caught in 2003. The CPUE in 2002 was significantly less than in 2003 in North Inlet for Atlantic sharpnose sharks, bonnetheads <em>Sphyrna tiburo</em>, and all sharks combined. This study documented the presence of adults and juveniles (including neonates and young of the year) for 10 species of sharks in Winyah Bay and 5 in North Inlet and thus identified these areas as shark habitat and potential primary and secondary nurseries for some shark species. We also observed salinity-related differences in the distribution of sharks in both estuaries, including differences in abundance and age-class, as a result of normal salinity regime and precipitation-induced salinity changes.

Shark Nursery Grounds of the Gulf of Mexico and the East Coast Waters of the United States

<em>Abstract.</em>—Because of their tendency to accumulate in estuaries and coastal regions, organochlorine (OC) contaminants such as pesticides and polychlorinated biphenyls (PCBs) represent potential threats to the quality of essential fish habitat for many shark species. These compounds pose special risks to immature sharks in particular because of their ability to impair growth and sexual maturation in juvenile fish at environmentally relevant levels of exposure. In order to assess the extent of these risks in shark populations on the East Coast of the United States, the present study examined concentrations of 30 OC pesticides/pesticide metabolites and total PCBs in juvenile sandbar <em>Carcharhinus plumbeus </em>and blacktip <em>C. limbatus </em>sharks from seven major nursery areas in the western Atlantic Ocean and eastern Gulf of Mexico. Quantifiable levels of PCBs and 13 OC pesticides/ pesticide metabolites were detected via gas chromatography and mass spectrometry in liver of 25 young-of-the-year blacktip sharks from the southeastern U.S. Atlantic coast and three regions on Florida’s gulf coast: Cedar Key, Tampa Bay, and Charlotte Harbor. Similarly, quantifiable levels of PCBs and 14 OC pesticides/metabolites were detected in 23 juvenile <em>C. plumbeus </em>from three sites on the northeastern U.S. coast: middle Delaware Bay, lower Chesapeake Bay, and Virginia’s eastern shore. Liver OC concentrations in Atlantic sandbar and blacktip sharks were higher than expected and, in some cases, comparable with elevated levels observed in deep-sea and pelagic sharks. Although significantly lower than those observed in Atlantic sharks, pesticide and PCB levels in Florida blacktip sharks were similar to, if not greater than, OC concentrations reported in adults of other coastal shark species. Based on these data, OC contamination appears to pose significant threats to habitat quality in sandbar and blacktip shark nursery areas on the U.S. Atlantic coast.

Shark Nursery Grounds of the Gulf of Mexico and the East Coast Waters of the United States

<em>Abstract.</em>—To identify and characterize shark nursery habitat in the coastal waters of Massachusetts, longline and shark angler surveys were conducted from 1989 to 2002 in the neritic waters of Nantucket Sound, Massachusetts. Additional samples and information were opportunistically collected from recreational and commercial fishermen, as well as published sources. A total of 123 longline sets of 5,591 hooks caught 372 sharks consisting of 344 (92.5%) smooth dogfish <em>Mustelus canis</em>, 23 (6.2%) sandbar sharks <em>Carcharhinus plumbeus</em>, and 5 (1.3%) dusky sharks <em>C. obscurus</em>. The sharks were taken during the period of 16 June–24 September in water temperature and depth ranges of 16.0–27.2°C and 1.2–27.1 m, respectively. Longline catch rates (number of sharks per longline set) were stratified by species, area, month, year, water temperature, and depth. Angler surveys reported the capture of 294 sharks, including sandbar sharks (72%) and smooth dogfish (28%). Data from 540 neonatal and adult smooth dogfish ranging 27.5–121.0 cm fork length (FL) support the conclusion that the neritic waters of southern Massachusetts serve as primary nursery habitat for this species. Size and sex data from 235 juvenile sandbar sharks ranging 61.0–157.0 cm FL indicate that this region provides secondary nursery habitat for this species. Opportunistic samples of juvenile sand tiger <em>Carcharias taurus</em>, white shark <em>Carcharodon carcharias</em>, basking shark <em>Cetorhinus maximus</em>, and tiger shark <em>Galeocerdo cuvier </em>provide evidence that these species utilize Massachusetts coastal waters for secondary nursery habitat.

Shark Nursery Grounds of the Gulf of Mexico and the East Coast Waters of the United States

<em>Abstract.</em>—Nursery areas are widely considered to be essential habitats for sharks. While there have been many efforts to determine the locations of nurseries in coastal waters and studies of movements within these nurseries, few studies have attempted to identify the factors that influence nursery area selection and habitat use within nurseries. Such data are critical for identifying essential habitats within nurseries and determining the factors that might set the carrying capacity of these areas. Behavioral and ecological theory provides important insights into nursery area use and the definition of essential habitats. For example, simply measuring the density of animals in various habitats can lead to (1) incorrect identification of critical areas because animal density and habitat quality (or importance) do not always coincide, and (2) incorrect assumptions about the factors limiting population sizes. Food abundance and predation risk are likely important determinants of nursery area selection, habitat use within nurseries, and carrying capacity of nurseries. Currently, we know little about how these factors and the physical features of the environment (e.g., water temperature, habitat structure) influence juvenile sharks. Here, I review our current understanding of shark nurseries in a theoretical context to identify areas where future studies are required and generate testable hypotheses regarding the use of nursery habitats.

Shark Nursery Grounds of the Gulf of Mexico and the East Coast Waters of the United States

<em>Abstract.</em>—Delaware Bay is one of two principal nursery grounds for the sandbar shark <em>Carcharhinus plumbeus </em>in United States coastal waters, with the second one located in Chesapeake Bay. Tagging studies were conducted for juvenile sandbar sharks in Delaware Bay during their summer nursery seasons from 1995 to 2000 using gill-net (1995–2000) and longline (1997–2000) gears. These studies were designed to aid fishery managers in defining essential fish habitat for juvenile sandbar sharks tagged in Delaware Bay by determining spatial and temporal distributions, overwintering grounds, and philopatry to natal nursery areas. A total of 2,066 juvenile sandbar sharks were caught in Delaware Bay from 1995 to 2000, and 87% of the sharks sampled were tagged before release. Of these tagged sharks, 156 (9%) have been recaptured through 2005. Juvenile sandbar sharks were most abundant along the Delaware coast, with more localized abundances on the shoal areas throughout the bay. Recaptures indicate that the majority of sandbar sharks born in Delaware Bay return to their natal nurseries for up to 5 years following birth (and potentially up to 12 years of age), overwinter off North Carolina, and eventually expand their range south to the east coast of Florida and into the Gulf of Mexico as they get larger. This study also provides the first evidence of mixing between the juvenile sandbar shark populations of Delaware and Chesapeake Bays during the summer nursery season.

Shark Nursery Grounds of the Gulf of Mexico and the East Coast Waters of the United States

<em>Abstract.</em>—Mexican coastal waters of the Gulf of Mexico serve as nursery areas for many shark species and traditional fishing grounds for artisanal fishermen. To characterize the use of these areas as shark nurseries, obtain information on the biology of juvenile sharks and understand the fishing pressure on these resources, a multiyear study was conducted in Laguna Yalahau, a shallow coastal lagoon located on the northeastern corner of the Yucatán Peninsula. Using primarily gill-net surveys and tagging of juvenile sharks during the late spring months of May–June, our binational research team conducted six expeditions inside the lagoon from 1995 to 2001. Sixty-seven species of teleosts, elasmobranchs, and other marine vertebrates consisting of 5,590 individuals were collected during the surveys. We captured 1,384 sharks of which 99% were neonate, young-of-the-year, or older juvenile blacktip sharks <em>Carcharhinus limbatus</em>, confirming that Laguna Yalahau is a primary nursery for that species. Other sharks collected were lemon shark <em>Negaprion brevirostris</em>, bonnethead <em>Sphyrna tiburo</em>, nurse shark <em>Ginglymostoma cirratum</em>, and Atlantic sharpnose shark <em>Rhizoprionodon terraenovae</em>. Using the Petersen method during 2000 and 2001, we estimated the sampled population size of newborn blacktip sharks in the lagoon to be 726 and 1,066, respectively, born to approximately 189 and 277 maternal females, respectively. Over the course of the study, 1,155 sharks were tagged and released. The recapture rate of tagged sharks by artisanal fishermen was 21.9%, more than five times the rate for similar sharks off the Florida coast, and all recaptures came from the coast of the Yucatán Peninsula. In light of this high recapture rate, it appears that Laguna Yalahau serves as a primary nursery for sharks that have been heavily exploited by Mexican artisanal fishermen.

Shark Nursery Grounds of the Gulf of Mexico and the East Coast Waters of the United States

<em>Abstract.</em>—The lower Chesapeake Bay is the largest summer nursery for sandbar sharks <em>Carcharhinus plumbeus </em>in the western Atlantic. The objective of this study was to define essential fish habitat for juvenile sandbar sharks in this estuary. The longline survey conducted by the Virginia Institute of Marine Science was expanded from 1990 to 1999 to include ancillary stations throughout the Virginia portion of Chesapeake Bay to delineate this nursery spatially. We analyzed catch per unit of effort data from 83 stations as a function of nine physical and environmental variables using tree-based regression models. The highest abundance of juvenile sandbar sharks was predicted where salinity was greater than 20.5 (practical salinity scale) and depth was greater than 5.5 m. The models also suggested that dissolved oxygen concentration may influence shark distribution. To increase applicability of the models to management practices, we introduced distance to the mouth of the estuary as a surrogate variable for salinity. The models estimated that the highest abundance of sharks was in areas less than 34.5 km from the mouth of the estuary and in depths greater than 5.5 m. The areas of the estuary that met the criteria of the models, based on the threshold values of the variables, were mapped spatially in a geographic information system. The resulting response surfaces were interpreted to represent essential nursery habitat for sandbar sharks in Chesapeake Bay. Both models performed very well using several dependent and independent measures to estimate their classification and predictive ability. We used logistic regression with presence/absence data to validate the tree models. The logistic regression models agreed very well with the tree-based regression models, selecting the same variable combinations to predict sandbar shark presence and absence.

Shark Nursery Grounds of the Gulf of Mexico and the East Coast Waters of the United States

<em>Abstract.</em>—Sharks were collected from the estuarine and nearshore waters of South Carolina in an effort to delineate nursery grounds for coastal sharks within state waters. From March 1998 through December 2003, 4,098 sharks, representing 12 species, were collected using gill-net and hand-deployed longline fishing gears provided by the Cooperative Atlantic States Shark Pupping and Nursery Survey. To supplement these data, records of 6,648 shark captures, representing 16 species, from a long-term longline survey in South Carolina coastal waters were incorporated into the analyses. The results of this study indicate that the estuarine and nearshore waters of South Carolina represent an important primary nursery area for finetooth sharks <em>Carcharhinus isodon</em>, blacktip sharks <em>C. limbatus</em>, sandbar sharks <em>C. plumbeus</em>, Atlantic sharpnose sharks <em>Rhizoprionodon terraenovae</em>, and scalloped hammerheads <em>Sphyrna lewini</em>.

Shark Nursery Grounds of the Gulf of Mexico and the East Coast Waters of the United States

<em>Abstract.</em>—In an effort to obtain information on how Louisiana’s nearshore coastal waters function as shark nursery habitat, a 3-year gill-net survey was conducted between May 1999 and September 2001. Sharks were identified to species, sexed, and assigned a maturity stage, and four length measurements were recorded. Catch per unit effort (CPUE) was calculated both on a set-by-set and annual basis to evaluate intra- and interannual variations in shark abundance. Pearson’s correlation and multiple linear regression were used to examine the relationships between CPUE and the environmental variables temperature, salinity, and dissolved oxygen. Kernel density analyses were conducted within a geographic information system to evaluate potential areas of high shark catch abundance. A total of 1,002 sharks representing eight species were captured during 320.42 h of gillnet sampling, for an overall CPUE of 3.13 sharks/net hour. The vast majority (~80%) of the sharks observed were neonate and young-of-the-year individuals, with the remaining 20% of the catch dominated by young juveniles. Differences in CPUE between 1999 and 2000 were nonsignificant (<em>p </em>= 0.084) while differences between 2000 and 2001 were significant (<em>p </em>= 0.025) for all species combined. Multiple linear regressions between CPUE and salinity, temperature, and dissolved oxygen were insignificant for all species with the exception of the blacktip shark <em>Carcharhinus limbatus</em>. Kernel density analysis indicated that the northern and eastern ends of East Casse Tete Island, the gulf side of East Timbalier Island, and the pass between the mainland and East Timbalier Island had consistently high CPUEs during all 3 years of the survey.