Seasonality and relative abundance of elasmobranchs near the northern boundary of a biogeographic break

Correspondence: agroskar@gmail.com

Objectives

1. Identify spatial and seasonal patterns in elasmobranch species assemblage

2. Determine environmental factors driving community composition

Methods

aInstitute of Marine Sciences, University of North Carolina at Chapel Hill*Current: NOAA NMFS Office of Science and Technology, Silver Spring, MD

bDepartment of Biology, East Carolina University, Coastal Studies InstitutecDepartment of Applied Ecology, Center for Marine Sciences and Technology, North Carolina State University

Background

• Elasmobranchs use nearshore waters for foraging, reproduction, migration

• Community structure of elasmobranchs in coastal waters poorly understood

Grace Roskara*, James W. Morleyb, Jeffrey A. Buckelc

Survey Design and Gear:• Monthly bottom trawling: Nov.

2004 – April 2008• 2 transects: • Cape Lookout • Masonboro Inlet

• 0.5 to 8 km from shore• Otter trawl: 30 m wide

headrope

Onslow Bay is SW of Cape Hatteras, a major

biogeographic break

Species nTransect Season Sex x2 (p)

Cape Lookout Masonboro Inlet Winter Spring Summer Fall F MSpiny dogfish Squalus acanthias 6270 3666 2604 5058 7 1205 1750 235 1156.3 (< 0.001)Clearnose skate Raja eglanteria 5012 2895 2117 1194 1983 158 1677 1221 2226 293.02 (< 0.001)Smooth dogfish Mustelis canis 4376 3126 1250 940 2509 11 916 730 704 0.47141 (0.4923)Smooth butterfly ray Gymnura micrura 2114 1594 520 75 924 689 426 606 759 17.149 (< 0.001)Bullnose ray Myliobatis freminvillii 1007 432 575 510 15 482 398 355 2.4555 (0.1171)Atlantic sharpnose shark Rhizoprionodon terraenovae 828 395 433 133 660 35 303 343 2.4768 (0.1155)Hypanus spp. Hypanus spp. 764 514 250 2 355 266 141 288 351 6.2113 (0.01269)Atlantic stingray Hypanus sabinus 609 203 406 199 3 57 350 286 235 4.9923 (0.02546)Bonnethead Sphyrna tiburo 48 12 36 10 38 25 21 0.34783 (0.5553)Cownose ray Rhinoptera bonasus 36 25 11 1 27 2 6 22 6Spiny butterfly ray Gymnura altavela 32 30 2 6 26 16 7Blacknose shark Carcharhinus acronotus 24 8 16 3 21 15 9 1.50 (0.2207)Winter skate Leucoraja ocellata 10 8 2 9 1 6 3Roughtail stingray Dasyatis centroura 5 2 3 2 2 1 1 4Sandbar shark Carcharhinus plumbeus 4 4 4 2 2Sand tiger Carcharhinus taurus 3 3 2 1 1 2Scalloped hammerhead Sphyrna lewini 3 2 1 1 2 2 1Atlantic torpedo Tetronarce nobiliana 2 2 2 - 1

Atlantic angel shark Squatina dumeril 1 1 1 - -Common thresher shark Alopias vulpinus 1 1 1 - -

df SS MS Pseudo-F p PermutationsTransect 1 8644.7 8644.7 0.17271 0.853 970Season (Transect) 6 3.08E+05 51369 32.654 0.001 997Residuals 282 4.44E+05 1573.1Total 289 7.60E+05

Data analysis:• 2-factor nested

PERMANOVA• SIMPER analysis

identified influential species

• NMDS ordinations –seasonal & spatial differences• BIOENV: determine

environmental drivers

NMDS ordinations shows differences in elasmobranch communities between N vs. S Onslow Bay, NC during each season

Significant difference in species composition among seasons nested w/in transect (PERMANOVA)

Community composition

varied by season

20 species of elasmobranchs (N = 21,149) caught between Nov. 2004 - April 2008Elasmobranchs represent an important component of coastal assemblage

NMDS ordination shows distinct seasonal assemblages, with spring/fall representing transitional communities

Several species contributed to seasonal differences

Spiny dogfish Clearnose skate Smooth dogfish

Water temperature drives seasonality in elasmobranch community composition

Year-round sampling & large sample sizes

Provided high temporal resolution data needed to fully characterize

seasonal assemblages

Limited data on ecological role of small sharks, skates & rays in the

SE U.S.

These data will contribute to ecosystem-based management

efforts