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Annual Residency Patterns and Diet of Phoca vitulina concolor (Western Atlantic Harbor Seal) in a Southern New Jersey Estuary
Jacalyn Toth, Steven Evert, Elizabeth Zimmermann, Mark Sullivan, Linda Dotts, Kenneth W. Able, Roland Hagan, and Carol Slocum

Northeastern Naturalist, Volume 25, Issue 4 (2018): 611–626

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611 2018 NORTHEASTERN NATURALIST 25(4):611–626 Annual Residency Patterns and Diet of Phoca vitulina concolor (Western Atlantic Harbor Seal) in a Southern New Jersey Estuary Jacalyn Toth1,*, Steven Evert1, Elizabeth Zimmermann1, Mark Sullivan1, Linda Dotts1, Kenneth W. Able2, Roland Hagan2, and Carol Slocum1,† Abstract - From 1996 to 2011, researchers observed Phoca vitulina concolor (Western Atlantic Harbor Seal) on regional overwintering grounds in the Great Bay–Mullica River estuary in southern New Jersey. Over this 15-y time series, 299 observations were completed, with maximum local abundance increasing from 100 individuals in 1996 to 160 individuals in 2011. Our study did not document the presence of Atlantic Harbor Seal pups. In addition, we analyzed 142 scat samples, resulting in 1419 sagittal fish otoliths recovered and identified. Dominant recovered otoliths were as follows: 48% Phycidae (Urophycis regia [Spotted Hake]/Urophycis chuss [Red Hake]; 25% Clupeidae (Clupea harengus [Atlantic Herring], Alosa sapidissima [American Shad], Brevoortia tyrannus [Atlantic Menhaden], A. pseudoharengus [Alewife], and A. aestivalis [Blueback Herring]); 13% Ammodytidae (Ammodytes americanus/A. dubius [sandlance]); 6% Pseudopleuronectes americanus (Winter Flounder); and 4% Scophthalmus aquosus (Windowpane Flounder). Average back-calculated prey lengths across all prey groups (min–max = 5–41 cm, average = 19.75 cm) indicated that Western Atlantic Harbor Seals might utilize both estuarine and ocean environments for foraging. This temperate estuary currently represents the southern limit of routine Western Atlantic Harbor Seal occupancy in the Northeast. As such, our results are valuable in monitoring future changes in habitat use potentially resulting from climate change. Introduction Phoca vitulina concolor DeKay (Western Atlantic Harbor Seal, hereafter, Harbor Seal) commonly occur in coastal areas along much of the northeastern US (Gilbert et al. 2005, Waring et al. 2015b). Although Harbor Seals are most abundant from the east Canadian Arctic to northern New England waters (Jacobs and Terhune 2000), the population extends into the mid-Atlantic, with anecdotal sighting reports as far south as North Carolina (Waring et al. 2015b). The most recent population estimate (2012) was derived from aerial surveys of seasonal pupping grounds in coastal Maine—adult Harbor Seals numbered ~76,000 and pups numbered ~24,000 (Waring et al. 2015b). The establishment of the 1972 Marine Mammal Protection Act and concurrent ban on Harbor Seal bounties are thought to be the catalysts for this stable, if not increasing, population (Payne and Schneider 1984, Payne and Selzer 1989). New Jersey is near the southern extent of consistent seasonal occurrence 1 Stockton University Marine Field Station, Stockton University School of Natural Sciences and Mathematics, Pomona, NJ 08205. 2Rutgers University Marine Field Station, Rutgers University Department of Marine and Coastal Sciences, Tuckerton, NJ 08087. †Deceased. *Corresponding author - Manuscript Editor: Thomas French Northeastern Naturalist J. Toth, S. Evert, E. Zimmermann, M. Sullivan, L. Dotts, K.W. Able, R.Hagan, and C. Slocum 2018 Vol. 25, Issue 4 612 at haul-out sites along the Atlantic coast, with most animals observed from approximately October to May (Barlas 1999, deHart 2002, Schneider and Payne 1983, Schroeder 2000, Slocum 2009, Waring et al. 2006). Harbor Seals forage opportunistically on a variety of seasonally available and/ or abundant food sources (Burns 2009, and references therein). Where Harbor Seal food habits have been studied in the northwest Atlantic, numerous fish species comprise their known diet, with a smaller number of fishes representing dominant prey (Payne and Selzer 1989, Williams 1999, Wood 2001). Studies on Harbor Seal diets in New England and coastal Maine suggest that important prey are also those that are seasonally abundant fishes in these areas, including Ammodytes americanus DeKay (American Sand Lance), Urophycis spp. (hake), Clupea spp. (herring), Sebastes spp. (redfish), and Gadus morhua (Cod) (See Wood 2001 for complete list). Food habits of Harbor Seals in the southern part of their range, however, remained unknown. We analyzed a 15-y data set (1996–2011) collected within the southern New Jersey Great Bay–Mullica River estuary to, for the first time, (1) document local abundance trends and seasonality of Harbor Seals, and (2) determine food sources for individuals using this area. New Jersey is a particularly interesting study area as it represents the current southern range of annual haul-out locations for Harbor Seals in the northwest Atlantic Ocean. Studying local populations at the edge of a species’ range is valuable because these sites may be the first to register changes in ecological patterns due to natural and/or anthropogenic causes. Methods Study area Our study was conducted in Great Bay, NJ, within the Great Bay–Mullica River estuary portion of the Jacques Cousteau National Estuarine Research Figure 1. Harbor Seal study area in the Great Bay–Mullica River estuary in southern New Jersey. Northeastern Naturalist 613 J. Toth, S. Evert, E. Zimmermann, M. Sullivan, L. Dotts, K.W. Able, R.Hagan, and C. Slocum 2018 Vol. 25, Issue 4 Reserve (Fig. 1). This area is ecologically complex; ocean-water temperatures are known to vary as much as 25 °C between summer and winter months (Parr 1933). Unlike many other seasonally occurring marine mammal and fish populations that migrate away from the area during winter months (Able and Fahay 2010), Harbor Seals only occur regularly in New Jersey during this cooler time period (Slocum 2009). Although there are 3 known annual Harbor Seal haul-out locations in New Jersey (Sandy Hook, Barnegat Inlet, Great Bay), we studied Great Bay because of proximity, ease of boat access, and the largest consistent Harbor Seal population size of the 3 locations. Great Bay consists of tidal rivers, inland bays, and multiple wetland-type islands—typically with mostly low saltmarsh vegetation (Spartina alterniflora Loisel [Smooth Cordgrass]) (Able and Fahay 2010, Slocum 2009). Our team observed Harbor Seal haul-out areas at salt marsh sites within floodtidal delta islands, ~380 m landward of Little Egg Inlet (Slocum 2009) (Fig. 1). These sites are most easily accessed during high tide due to steep island edges. Recreational/commercial water-based traffic can be prevalent due to proximity of the adjacent Shooting Thorofare (a portion of the Intercoastal Waterway [ICW]), however, there is a marked decrease in this type of traffic during the winter months when Harbor Seals are present. The ICW is about 12 m deep within the Harbor Seal haul-out–site area, and acts as a corridor to the Atlantic Ocean through Great Bay and Little Egg Inlet. Average monthly sea-surface temperatures (SST) were recorded by Jacques Cousteau National Estuarine Research Reserve data-logging system adjacent to the Harbor Seal haul-out location within Great Bay to characterize the waters around the haul-out site seasonally and annually. We employed an ANOVA to test for significant differences in annual SST during the months of Harbor Seal occurrence. Local Harbor Seal abundance observations From 1996 to 2010, Harbor Seal haul-out locations within Great Bay were opportunistically observed by Stockton University personnel as part of an undergraduate course on Harbor Seal biology. To ensure little to no disturbance to the animals, surveyors positioned a blind on an adjacent marsh (Fig. 1) and made counts with a 40–60x monocular, high-powered spotting scope or with binoculars from a small vessel. During the peak of seasonal Harbor Seal occurrence in New Jersey from fall through spring, researchers made counts to estimate the number of hauled-out Harbor Seals, and those in close proximity in the water. Differentiation between adults and sub-adult Harbor Seals was not possible due to distance and haul-out behavior; thus, the population observations presented here may include both life stages. Pagophilus groenlandicus (Erxleben) (Harp Seal), Halichoerus grypus (Fabricius) (Grey Seal), and Cystophora cristata (Erxleben) (Hooded Seal) can also be found in coastal New Jersey during the same time period, but we easily distinguished these animals from Harbor Seals and did not include them in the counts. We conducted a linear regression to test for significant changes in annual maximum Harbor Seal abundance over the 15-y study. Northeastern Naturalist J. Toth, S. Evert, E. Zimmermann, M. Sullivan, L. Dotts, K.W. Able, R.Hagan, and C. Slocum 2018 Vol. 25, Issue 4 614 In the 2010–2011 season (October–May), surveyors conducted Harbor Seal observations/ counts 5 d per week from the Rutgers University Marine Field Station cupola (distance = 300 m) with the same 40–60x spotting scope, rather than opportunistically as in previous years. Data recorded included the date/time, number, and location of hauled-out Harbor Seals, number of Harbor Seals in close proximity in the water, tide stage, wind speed and direction, notable weather conditions, and presence of young of year/pups. Fecal (scat) collection and analysis From 1996 to 2010, researchers collected Harbor Seal fecal samples (scat) opportunistically at the haul-out site. In 2010 and 2011, the study team collected scat samples weekly when Harbor Seals were not present at the haul out and weather/ sea-state permitted. While walking the haul-out site, study personnel visually located all scats (n = 142), which they placed in a labeled container (noting the date, time, and sample number) and stored in a freezer until processed. Researchers removed from the freezer, weighed, and thawed all samples for 24 h before processing. When thawed, samples were run through an elutriator machine to separate hard parts from organic material, then washed through a series of nested sieves (mesh sizes: #13/1.8 mm and #35/0.13 mm). Personnel removed hard parts and any other identifiable items from the sieves, then cleaned and stored them dry in vials. We examined sagittal otoliths to identify fish prey to the lowest possible taxon using multiple reference materials (Brodeur 1979, Campana 2004) as well as comparison to specimens in the Stockton University fish collection. Prey abundance was estimated as percent frequency of occurrence (proportion of 1 prey compared to total number of prey). Often used in food-habit analysis, it has been suggested that percent occurrence of prey items can lead to inferences on availability and/or selectivity of prey (Lance et al. 2012, Philips and Harvey 2009). For all years, identified prey was grouped by month to help determine possible temporal shifts in prey consumption. Due to otolith erosion in Harbor Seal stomachs, we grouped fish by family to account for difficulty in distinguishing similar species (Fig. 2). Size and source of prey We employed previously documented linear-regression equations from known non-eroded prey otoliths to back-calculate the length of prey (Bowen and Harrison Figure 2. Representative sagittal otoliths and identified fishes within the given fish order. Northeastern Naturalist 615 J. Toth, S. Evert, E. Zimmermann, M. Sullivan, L. Dotts, K.W. Able, R.Hagan, and C. Slocum 2018 Vol. 25, Issue 4 1994, Härkönen 1994, Harvey et al. 2000, Neuman et al. 2001, Recchia and Read 1994). We measured otolith length from the anterior tip of the rostrum to the posterior edge using Image Pro Plus 6 software (Media Cybernetics, Inc., Rockville, MD). We accounted for otolith degradation due to digestion by using a standardized scale for degree of digestion: grade 1 = low level of erosion, grade 2 = moderate level of erosion, and grade 3 = severe erosion (Tollit et al. 1997b). Where possible, we used grade-specific length-correction factors (gLCFs), determined in previous studies, to improve the estimate of prey length (see Philips and Harvey 2009). When a regression equation was not available for a specific species, we applied a regression equation for the most similar species. If percent occurrence of a fish species was less than 1% due to the low number of otoliths identified, we did not calculate original prey size. We conducted analysis of variance to compare monthly variation in average family-level fish lengths. We determined the potential source of fish prey (estuary versus ocean) from previously derived monthly length frequencies of individual species from both the Great Bay estuary and nearby ocean in the general study area (Able and Fahay 2010). Results Environmental variables Water temperature in the adjacent Great Bay averaged 16.0 °C (± 3 °C) when Harbor Seals were first sighted in the month of October, and 14.9 °C (± 4 °C) in May when Harbor Seals were last sighted. Winter monthly average sea-surface temperatures did not rise significantly through the 15-y study (October–April). In a separate long-term study on fish habitats in the same study area, however, deviation from the mean temperature increased overall if only the spring, summer, and fall months were included (which are the primary months of fish-prey recruitment in the study area) (Able and Fahay 2010:figure 10.2). Patterns of occurrence and local abundance From 1996 to 2011, we conducted observations of Harbor Seal abundance (n = 299) within Great Bay from October through April (Fig. 3). Maximum abundance counts increased from 100 individuals in 1996 to 160 individuals during the 2010–2011 season. Although there was variation, maximum annual Harbor Seal abundance increased significantly over the 15-y study (R2 = 0.35, df = 1, P = 0.03). Annual date of arrival (October/November) and departure (May) to the study area remained relatively consistent, however. Daily observations during 2010–2011 began on 1 October; Harbor Seals were first sighted in the water on 3 November (water temperature = 11.0 °C), and hauled out on 16 November (12.0 °C). With the exception of May (when Harbor Seals leave New Jersey), the monthly maximum number of seals observed varied from 75 individuals in January to 160 individuals in March (Fig. 4). In general, the months of highest occupancy were February through April. Average water temperature during this peak occurrence was 6.0 °C. No Harbor Seal pups were observed at any point throughout the seasonal residence. Northeastern Naturalist J. Toth, S. Evert, E. Zimmermann, M. Sullivan, L. Dotts, K.W. Able, R.Hagan, and C. Slocum 2018 Vol. 25, Issue 4 616 While hauled out, Harbor Seals remained within close proximity to the marsh edge (~1 m), rarely moving toward the inner portion of the island. Harbor Seals occurred on the haul-out site during all tides—high tide = 15% of all occurrences (n = 67 occurrences), low tide = 11% (n = 49 occurrences), ebb tide = 37% (n = 166 occurrences), and flood tide = 34% (n = 153 occurrences). Figure 3. Maximum annual Harbor Seal abundance in the Great Bay–Mullica River study area (1996–2011) and associated linear-regression trend line. Figure 4. Maximum weekly Harbor Seal abundance estimates and average monthly water temperature in the Great Bay–Mullica River study area during 2010–2011. Northeastern Naturalist 617 J. Toth, S. Evert, E. Zimmermann, M. Sullivan, L. Dotts, K.W. Able, R.Hagan, and C. Slocum 2018 Vol. 25, Issue 4 In addition to the study site, anecdotal Harbor Seal sightings were abundant throughout the estuary during the 15-y study. Locations of sightings included adjacent marsh islands in Great Bay (2 km from the mouth of Great Bay inlet, 1 km from the study site), boat docks throughout Great Bay (11 km from the mouth of Great Bay inlet, 7 km from the study site), and at various sites up the Mullica River into brackish water (20 km from the mouth of Great Bay inlet, 16 km from the study site). Diet diversity From 1996 to 2011, we collected, processed, and analyzed 142 scat samples. A total of 1419 fish sagittal otoliths were recovered, and of these, we identified 1173 to at least the order level, 1156 to the family level, and 607 to the species level. We did not identify highly eroded or broken otoliths. Identified fishes were from 4 orders, 10 families, and 15 possible species (Table 1: counts include taxa Table 1. Identified Harbor Seal prey by order, family, and species, with back-calculated original preylength estimates. Average corrected (gLCF) # of % prey length Length Prey identified otoliths occurrence ± SD (cm) range (cm) Order Perciformes Family Ammodytidae 151 11 5–5.5 Ammodytes americana 151 11 5 ± 0.87 (American Sandlance) Order Clupeiformes Family Clupeidae 296 21 15–40 Alosa pseudoharengus/Alosa aestivalis 27 2 26 ± 4.9 (Alewife/Blueback Herring) Clupea harengus/Alosa sapidissima/ 129 9 25 ± 4.1 Brevoortia tyrannus (Atlantic Herring/American Shad/ Atlantic Menhaden) Unidentified Clupeid 140 10 n/a Order Gadiformes Family Phycidae 571 40 12.5–35 Urophycis regia/Urophycis chuss 571 40 17 ± 8.1 (Spotted Hake/Red Hake) Order Pleuronectiformes Family Pleuronectidae 18 1 32–41 Pseudopleuronectes americanus 18 1 33 ± 9.8 (Winter Flounder) Family Scophthalmidae 67 5 11–23 Scophthalmus aquosus 67 5 15 ± 3.4 (Windowpane Flounder) Unidentified flatfish 29 2 n/a Other (less than 1% of total) 24 1 Unknown 263 19 Total 1419 100 Northeastern Naturalist J. Toth, S. Evert, E. Zimmermann, M. Sullivan, L. Dotts, K.W. Able, R.Hagan, and C. Slocum 2018 Vol. 25, Issue 4 618 in "other" category). Numerically, fishes of the family Phycidae (Urophycis regia (Walbaum) [Spotted Hake]/[U. chuss (Walbaum) [Red Hake]) were the dominant component of the Harbor Seal diet, representing 48% of all otoliths. Clupeids (Clupea harengus L. [Atlantic Herring], Alosa aestivalis (Mitchill]) [Blueback Herring], Alosa pseudoharengus (Wilson) [Alewife], Alosa sapidissima (Wilson) [American Shad], Brevoortia tyrannus (Latrobe) [Atlantic Menhaden]) were also consistently present, representing 25% of identified otoliths. Sandlances (Ammodytidae) represented 13% of identified prey, and Scophthalmus aquosus (Mitchill) (Windowpane Flounder) and Pseudopleuronectes americanus (Walbaum) (Winter Flounder) (families Scophthalmidae and Pleuronectidae) represented 4% and 6%, respectively. A number of species totaled less than 1% of otoliths recovered, including the Labrid Tautoga onitis L. (Tautog) (0.004%), the Gadid Brosme brosme (Ascanius) (Cusk Eel) (0.004%), the Merluciid Merluccius bilinearis Mitchill (Silver Hake) (0.002%), and the Paralichthyid Citharichthys arctifrons Goode (Gulf Stream Flounder) (0.008%). We grouped and summarized samples by month to identify variation in fish prey (Fig. 5). Hakes dominated the identified diet in both the early (October; 75%) and later (April; 67%) months of Harbor Seal occurrence, while herrings/Alewife were most prominent in January (41%) and May (65%). We did not detect a seasonal pattern for sandlance; percent occurrence was fairly low in most months (less than 15%), with March the highest (25%). Winter Flounder and Windowpane Flounder most frequently occurred in the earlier months of the study (October 25%, December 27%), while few were identified from January to May (less than 10%). With the exception of the months of October and May, we identified all 4 fish groups as prey in all months (November–April). Hakes were not identified in October and Sandlance were not identified in October or May. Figure 5. Average monthly frequency of Harbor Seal prey groupings (1996–2011). Northeastern Naturalist 619 J. Toth, S. Evert, E. Zimmermann, M. Sullivan, L. Dotts, K.W. Able, R.Hagan, and C. Slocum 2018 Vol. 25, Issue 4 Size and source of prey Most fish prey were large juveniles or adults with average prey length of 19.7 cm (± 4.5 SD), min–max = 5–41 cm (see Table 1 for all prey-length summaries). Adult Winter Flounder that were identified from November, December, March, and April samples represented the largest average original prey lengths (33 cm ± 9.8 SD) and were the largest fish identified (41 cm, April 1999). The most frequently identified fishes—hakes and herrings/Alewife—averaged 26 cm (± 3.2 SD) and 17 cm (± 8.1 SD ), respectively. Although fish-prey lengths fluctuated month to month, they did not vary significantly. Discussion Patterns of occurrence and local abundance With few exceptions, the Harbor Seal population on the eastern US coast has been either stable or slowly increasing since the early 1980s (Waring et al. 2015b). One such exception is Sable Island, where the Harbor Seal population has declined (see Bowen et al. 2003, Stobo and Lucas 2000). This generally positive trend is commonly linked to the implementation of the 1972 Marine Mammal Protection Act and its associated protection measures on a previously exploited population (Gilbert et al. 2005, Hammill et al. 2010, Lelli et al. 2009). Currently in the northeastern US, Harbor Seals are ubiquitous; year-round populations throughout New England and the Gulf of Maine number in the tens of thousands (Gilbert et al. 2005, Whitman and Payne 1990). In the mid-Atlantic region specifically, New Jersey represents a large southern range haul-out site for these animals, which are present annually for approximately half of the year. Although Harbor Seals first arrive to the study estuary when water temperatures average 11 °C (as indicated by 2010–2011 daily observations), the maximum number of Harbor Seals are observed in February (15-y average water temperature = 3.0 °C, SD = 2.2) and March (15-y average water temperature = 5.3 °C, SD = 2.5). This temperature at peak Harbor Seal abundance in New Jersey is consistent with spring monthly average temperatures in the Gulf of Maine (Thomas et al. 2017), where Harbor Seals are known to reside year-round (Waring et al. 2015a). Average annual water temperature during the winter months of Harbor Seal occurrence (November–April) did not significantly increase over the span of this study. However, Able and Fahay (2010) noted an increase in deviation from the mean annual temperature in the study area if only fall, spring, and summer months are included. These months correlate to the timing of most fish recruitment in New Jersey (Able and Fahay 2010); thus, it is possible that prey movement is affected by the changes in water temperature during recruitment months. Changes in prey patterns may subsequently influence the spatial and temporal activity patterns of Harbor Seals in the area. It is possible that the increasing number of Harbor Seals observed over a 15-y period (100 individuals in the mid-1990s increasing to 160 individuals in 2011) is directly or indirectly related to the effect of increasing water temperatures on prey. As of 2016, five years after the completion of this study, 220 harbor seals were observed on the Northeastern Naturalist J. Toth, S. Evert, E. Zimmermann, M. Sullivan, L. Dotts, K.W. Able, R.Hagan, and C. Slocum 2018 Vol. 25, Issue 4 620 haul-out site at the peak of Harbor Seal occurrence in the study area (J. Toth, pers. observ.); a further increase from the 160 seals observed in 2011. Although Harbor Seals are known to haul out on a variety of substrates, factors such as predator avoidance, access to prey, tidal-stage effects, and reproduction stage are thought to influence the choice of a haul-out habitat (Baird 2001, London et al. 2012, references therein). Relatively isolated, Great Bay is an optimal haulout site due to its separation from the mainland and proximity (380 m) to the ocean inlet for both Harbor Seals and their prey (Able and Fahay 2010, Slocum 2009). Despite the fact that the 12-m deep ICW is adjacent to the haul-out site, vessel traffic is relatively low during the months of seal occurrence, thus making both estuarine and marine environments easily accessible to the Harbor Seals. This haul-out environment is quite different from the rocky shoreline habitats throughout much of New England and Gulf of Maine, but it is similar to the more sandy or marsh-like habitats in these same areas (Robillard et al. 2005). As with other studies that have shown a positive relationship between haul-out habitat and distance to the mainland (Nordstrom 2002), the isolation of this particular environment may be a favorable characteristic. Although multiple anecdotal haul-out areas were noted throughout the study, large numbers of hauled-out Harbor Seals showed consistent site fidelity to the study area in particular, as observed over the 15 y of this study. Diet composition and source Identified otoliths in scat samples were dominated by multiple hake species. Regarding availability, both Spotted Hake and Red Hake occur in the nearby coastal community year round (Able and Fahay 2010, Vasslides and Able 2008). While the occurrence of Red Hake inside the estuary is more limited, Spotted Hake can be found there all months of the year (Table 2). These Phycid fishes are commonly found in other Harbor Seal diet studies in the northwest Atlantic Ocean, indicating that they are a common and abundant prey item in this part of the world (Bowen and Harrison 1996, Hammill and Stenson 2000, Payne and Selzer 1989, Wood 2001). Given the close proximity of the haul-out site to both estuarine and coastal environments, it is possible that Harbor Seals feed on these species in both areas. Empirical studies on prey retention show that most prey (75%) is passed within 48 h of ingestion (Philips and Harvey 2009); the local coastal ocean waters and withinestuary habitat are certainly within a 48-h distance to the haul-out site. It is difficult to determine if one area plays a larger role in Phycid fish consumption than the other because of the proximity and availability of these fishes in bo th environments. Similar to reports in other studies in the northwest Atlantic Ocean, we consistently identified Clupeid fishes (herrings and Alewife) (Bowen and Harrison 1996, Payne and Selzer 1989, Wood 2001). Although we identified clupeid otoliths throughout all months of study (October–May), the majority were identified from May scat samples. This finding may reflect movement patterns of some of these herring fishes (e.g., Alewife), as there is an annual movement up the Great Bay– Mullica River estuary in the spring for spawning (Able and Fahay 2010, Able et al. 2016). A number of these herring species are thought to occur only inside the Northeastern Naturalist 621 J. Toth, S. Evert, E. Zimmermann, M. Sullivan, L. Dotts, K.W. Able, R.Hagan, and C. Slocum 2018 Vol. 25, Issue 4 estuary during the spring (Able and Fahay 2010), suggesting that Harbor Seals feed specifically up-river within the estuary on this temporally abundant prey item. The back-calculated prey length of these herring fishes (average = 25 cm) corresponds to the lengths of these fishes reported in studies done during the spring months (April–May) up the Mullica River (Able et al. 2016). All herring caught and measured during that estuarine study were greater than 20 cm, corresponding closely with back-calculated herring lengths determined in the current study. Less-frequently identified species, including Winter Flounder and Windowpane Flounder, are consistent with other studies in the northwestern Atlantic Ocean. This is not true, however, for the comparably low amount of sandlance identified; given the availability of sandlance within proximity to the haul-out site, this was an unexpected result. It is possible that although sandlance are available throughout the winter months within the study area, clupeid and herring fishes are the more readily abundant, making them the prey of choice for these opportunistic Harbor Seals. In addition, some sandlance species may bury during the winter months, and thus, be less available (Able and Fahay 2010). Overall, our back-calculated prey-length averages (mean = 19.7 cm) were consistent with other studies in which corrected prey lengths were determined (Bowen and Harrison 2006, Hall et al. 1998, Tollit et al. 1997a). The length range of prey indicates that Harbor Seals consumed mostly adult fishes, with a possibility of older Table 2. Possible prey source for Harbor Seals in the Great Bay–Mullica River during their occurrence (x indicates fish occurrence in respective habitat/month; Able and Fahey 2010). Prey identified Location Oct Nov Dec Jan Feb Mar Apr May Alosa pseudoharengus (Alewife) Ocean x x x x x x x x Estuary x x x Alosa aestivalis (Blueback Herring) Ocean x x x x x x x x Estuary x x x x x Alosa sapidissima (American Shad) Ocean x x x x x x Estuary x x x Brevoortia tyrannus (Atlantic Menhaden) Ocean x x x x Estuary x x x x Clupea harengus (Atlantic Herring) Ocean x x x x x x Estuary x x x x x Urophycis regia (Spotted Hake) Ocean x x x x x x x x Estuary x x x x x x x x Urophycis chuss (Red Hake) Ocean x x x Estuary x x x Pseudopleuronectes americanus (Winter Flounder) Ocean x x x x x x x x Estuary x x x x x x x x Scophthalmus aquosus (Windowpane Flounder) Ocean x x x x x x x x Estuary x x x Ammodytes americana (American Sand Lance) Ocean x x x x x Estuary x x x x x x x x Northeastern Naturalist J. Toth, S. Evert, E. Zimmermann, M. Sullivan, L. Dotts, K.W. Able, R.Hagan, and C. Slocum 2018 Vol. 25, Issue 4 622 young-of year individuals consumed inside the estuary specifically. For example, an 11-cm Red Hake identified from scat in the month of March may originate from the estuary, as this size indicates an older young-of-year individual. Young-of-year fishes are thought to remain in the estuary for their first year; after year 1, they may leave or remain inside the estuary depending on species and time of year (Able and Fahay 2010). With regard to minimum and maximum prey lengths determined, the minimum length of 5 cm (American Sand Lance/November) was comparably on the lower side, but certainly possible given the life history of this prey item in the study area (Able and Fahay 2010). Studies have shown that American Sand Lance is available in both the estuary and coastal environment at this size during most of the period spanning October through May (Table 2). We determined a maximum prey length of 41 cm for Winter Flounder in the month of April; surveys on fish occurrence show that this is more typical of adult Winter Flounder along the coast rather than in the estuary (Wuenschel et al. 2009). However, it is entirely possible that spawning Winter Flounder at this size are still within the estuary at this time. Determination of prey source is difficult due to the proximity of the study site to both estuarine and coastal habitats, as well as the fluidity of fish movements during even these winter months. Harbor Seal foraging patterns have been well-studied in many parts of the world (Bowen and Harrison 1996, Hall et al. 1998, Hauser et al. 2008, Kavanaugh et al. 2010, Lance et al. 2012, Tollit et al. 1997a). Along the northeastern US, however, there are few studies that have documented Harbor Seal food habits, and, until now, none in the southern extent of their annual range. The results from our study are similar to those reported from other areas in which Harbor Seals occur; the spatial and temporal availability of fish species within the local habitat and the nearby coast play a direct role in what is consumed (Bowen and Harrison 2006, Hall et al. 1998, Hamill and Stenson 2000). Despite the fact that many potential prey populations migrate to more southern or offshore waters during the winter (Able and Fahay 2010), resources are apparently sufficient to sustain this stable local population of over-wintering Harbor Seals. This type of study on locally and seasonally variable prey provides the opportunity for seasonal insights into foraging behavior and the presence of particular prey species at specific times. Limitations and conclusion Multiple studies on pinniped diets have shown potential biases in scat analyses related to the digestion of prey hard parts and size of prey (e.g., Bowen 2000, Cottrell et al. 1996, Tollit et al. 1997b). Despite these known limitations, scat analysis can provide a wealth of information if used appropriately. Empirical studies have determined that correction factors used for accurate prey-length prediction can help offset potential inaccurate estimations (e.g., Bowen 2000, Dellinger and Trillmich 1988, Orr and Harvey 2001, Tollit et al. 1997b). In proof-of-concept studies where prey length is known, the use of correction factors has been shown to greatly improve the accuracy in reconstructing prey length (Kavanagh et al. 2010). Northeastern Naturalist 623 J. Toth, S. Evert, E. Zimmermann, M. Sullivan, L. Dotts, K.W. Able, R.Hagan, and C. Slocum 2018 Vol. 25, Issue 4 Our study helped determine the fish-prey species and their length when eaten by over-wintering Harbor Seals in New Jersey, and how often they occurred in a scat sample relative to other prey items. When interpreting the results of this study, it must be recognized that otolith erosion and complete digestion is a factor, and may distort the relative importance/size of prey. Our study did not attempt to capture the totality of food habits for these animals, nor fully quantify diet composition; rather, we report preliminary findings of identifiable prey components for a Harbor Seal seasonal habitat which had not yet been documented. Our work is the first study on Harbor Seal occurrence and diet in their southern seasonal range along the east coast of US. New Jersey represents an interesting area to investigate due to the migratory nature of both the Harbor Seals and their prey. Although there is much to learn about the effects of climate change, fisheries interactions, and habitat degradation, the results of this study provide a biological framework on a seasonal mid-Atlantic Harbor Seal population. As with similar studies, our results indicate that these animals are opportunistic, generalist feeders, consuming a variety of available fish prey. Both estuarine and coastal environments were likely utilized for feeding purposes, indicating the importance of both habitats. The traditional methods used in this study provide valuable information on the foraging ecology of Harbor Seals during the fall through the spring months in New Jersey. Acknowledgments We dedicate this work to Dr. Carol Slocum (1948–2010) and her Stockton University “New Jersey Seal Study” undergraduate students who collected the majority of samples analyzed and discussed herein. 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