ACCESS NUMBER: 39790

STUDY TITLE: Trophic Aspects of Sperm Whales in the Northern Gulf of Mexico Using 

Stable Isotopes of C and N: Geographical and Inter-Annual Variation 

REPORT TITLE: Trophic Aspects of Sperm whales (Physeter macrocephalus) in the 

Northern Gulf of Mexico using Stable Isotopes of Carbon and Nitrogen 

CONTRACT NUMBER: 01-06-CT-39790 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREA: Gulfwide 

FISCAL YEARS OF PROJECT FUNDING: FY 06 & 07 

COMPLETION DATE OF REPORT: April 2010 

COSTS: $52,808.00 

PROJECT MANAGER: Brook Milligan 

AFFILIATION: Biology Department, New Mexico State University 

ADDRESS: Department of Biology, New Mexico State University, MSC 3AF, Las 

Cruces, NM, USA 

PRINCIPAL INVESTIGATORSi: R. I. Ruiz-Cooley, D. T. Engelhaupt 

KEY WORDS: Sperm whales, trophic position, squid, stable isotopes, carbon, nitrogen, 

Gulf of Mexico 

BACKGROUND: Geophysical exploration and oil and gas development have been 

moving into the offshore deepwater zone of the northern Gulf of Mexico (GoM) since 

1985, a zone that overlaps with sperm whale distribution. Optimal diet is crucial for 

endangered species as it may affect reproductive success, especially for those species 

and populations inhabiting areas where anthropogenic activities are increasing. 

However, in the northern GoM, very little is known about the feeding ecology of sperm 

whales and potential effects of the energy industry on sperm whale feeding success. 

Due to the importance of understanding these aspects, the Minerals Management 

Service supported this study to provide initial information on the variability in the relative 

trophic position of sperm whales in the northern GoM from 2002 to 2005.. Stable 

isotopes ratios of carbon (C) and nitrogen (N) from free-ranging sperm whale skin 

samples were measured as a non-invasive method to evaluate changes in trophic 

position at different temporal and spatial scales. The 13C/12C and 15N/14N ratios from 

skin samples of sperm whales and muscle samples of cephalopods indicate a relative 

trophic position that depends on their assimilated diet. 

OBJECTIVES: (1) To quantify 13C and 15N in skin samples of free-ranging sperm 

whales and investigate interannual variation in their relative trophic position, (2) to 
ACCESS NUMBER: 39790

investigate changes in their relative trophic position between two sites in the GoM 

(Mississippi Canyon and the Northwestern GoM) , and (3) to evaluate the importance of 

mesopelagic squid species as potential prey of sperm whales. 

DESCRIPTION: This study used skin tissue samples of free-ranging sperm whales 

obtained from 2002 to 2005 in the northern Gulf of Mexico (GoM). These tissue samples 

were mainly collected for a genetic study as part of the Sperm Whale Seismic Study 

(SWSS). In the present study, we used available skin samples to investigate aspects of 

the feeding ecology of this odontocete species using stable isotope analysis. The skin 

tissue samples used in this study were collected in 2002, 2003 and 2004 in the 

Mississippi Canyon and in 2005 in the Northwestern Gulf of Mexico. In 2003, muscle 

tissue samples from a few mesopelagic cephalopods were also obtained at night using 

14.8 m2 Isaacs-Kidd midwater trawls in areas where sperm whales were observed. 

Stable isotope ratios of carbon and nitrogen were measured from whale skin and from 

muscle tissue samples of cephalopods; the isotopic signature of both whales and squid 

represent a relative trophic position derived from their assimilated diet. Whale gender 

was determined using molecular analysis based up the ZFX/ZFY technique. We also 

compared the isotopic values between individuals from the GoM (energy industry 

activity) and Gulf of California (no energy industry activity), and between squid from 

inshore and offshore areas within the northern GoM. 

SIGNIFICANT CONCLUSIONS: The isotopic ratios of sperm whales from the northern 

Gulf of Mexico (GoM) were significantly lower by 7.41 % for 15N and 2.74 % for 13C 

compared to ratios from the Gulf of California (GC). Assuming that 3 ‰ for 15N equals 

one trophic level, these differences suggest that whales from the GoM would be 2.5 

trophic levels lower than whales from the GC. However, geographic variation in 

baseline isotopic signatures could be responsible for the observed differences. 

Inferences about trophic position using stable isotopes in studies that lack samples at 

the base of the food web can be misleading. Further research is needed to distinguish 

the factors driving this isotopic variation. For example, more effort is needed to collect 

and analyze samples from mid-water cephalopods as well as from plankton concurrent 

with tissue samples of sperm whales and squid to determine trophic level and their 

changes through time. This is crucial to distinguish (1) if sperm whales from the 

northern Gulf have a lower trophic level than whales from the Gulf of California or (2) if 

the variability in the biochemical cycle of N and C between areas is responsible for the 

observed spatial isotopic variation. 

STUDY RESULTS: Since isotopic signatures represent a relative trophic position of 

sperm whales, no differences between females from the Mississippi Canyon (MC) and 

the northwest Gulf (NW) may suggest that whales feed on similar diet between these 

two sites or on different prey from the same trophic level. Yet, the number of samples 

collected in the NW area represents only one year of collection compared with three 

years for the Mississippi Canyon. Mesopelagic squid from the family Ommastrephidae, 

such as Ommastrephes bartramii, and squid from the family Histiotheutidae were 

identified as potential prey of sperm whales, although a more representative sample of 

these squid species for each area (i.e., Mississippi Canyon and NW), including larger-
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sized squid, is needed to evaluate this relationship. We documented significant spatial 

isotopic variability in squid between inshore and offshore sites within the northern Gulf; 

this variation has important implications for evaluating trophic relationships between 

predators and prey using stable isotope analysis. 

In the GoM, immature males had lower isotopic signatures than females. These results 

differed from previous studies in the Gulf of California (GC). In the latter area, immature 

males were higher in 15N than females. This difference can be associated with length 

and age of immature males, range of foraging areas (horizontal and vertical) and prey 

size consumed. Comparable 13C and 15N values among years indicate that sperm 

whales maintained a similar diet composition throughout the period from 2002 to 2005. 

We recommend continued investigation of the feeding ecology of free-ranging whales 

by using stable isotope ratios in association with mesopelagic cephalopods. 

STUDY PRODUCT: Ruiz-Cooley R. I. and D. T. Engelhaupt. Trophic Aspects of Sperm 

Whales (Physeter macrocephalus) in the Northern Gulf of Mexico Using Stable Isotopes 

of Carbon and Nitrogen Report. U.S. Dept. of the Interior, Minerals Management 

Service, Gulf of Mexico OCS Region, New Orleans, LA, OCS Study MMS 2010-016, 28 

pp. 

Reference values of isotopic ratios (13C and 15N) in sperm whale skin samples 

collected from 2002 to 2005 

i

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