Title: The Elkhorn Slough National Estuarine Research Reserve-

a habitat for Thornyback Rays (Platyrhinoidis Triceriatas)


Sean Van Sommeran, Josh Laeder, Leon Chen, Callaghan Fritz-Cope, Katie Kent, Scot Lucas; Pelagic Shark Research Foundation (PSRF).


Abstract: A survey of elasmobranch abundance and population structure within the Elkhorn Slough National Estuarine Research Reserve between July 2001 and August 2003 revealed 730 thornyback rays using the protected waters. Our results differ from previous research efforts in Elkhorn Slough National Estuarine Research Reserve and may reflect a change to the system.




Methods: The samples were carried out using both entanglement nets and beach seine nets. In some cases specimens were captured manually. The entanglement nets were set perpendicular to the tidal flow of water for periods of two hours during 2001 and for three hours during 2002 and 2003 samples. Beach seines were deployed in shallower water conditions, typically when animals could be observed cruising through the shallows or near shore, in

addition some animals were captured by hand as they milled in shallow water or were chased into water too shallow to escape. The data collected for each capture included: location, time and date, tide state, depth and water temperature, method of collection, species and sex (clasper length for males), total length (TL) and/or disk width and the application of number and color coded

permanent identification tags.


Results: During the 2001 thru 2003 effort, thornyback rays comprised of 50.2% of the total number of elasmobranches captured during the study. A total of 730 thornyback rays were captured and sampled; 724 of them were females and three males with 3 animals of undetermined sex.


Thornyback rays represented the most abundant species in our sample. Females ranged in size from 47-78 cm TL with the average TL being

65.2 cm. The three males ranged between 45 and 49 cm TL.

Of the thornyback rays we captured and sampled, 627 of the 635 females were mature while two of the three males were mature.


Thornyback rays were captured primarily in the months of June and July with

the peak in June for all three years of the study. The thornyback ray

abundance lessened in late summer, with only nine specimens captured during

August 2001 and none during the August 2002 and August 2003.


Discussion: Previous studies as well as our own experiences strongly

indicated that that the highest abundances of elasmobranches were present

during the summer months with a limited presence year around, (Yaklovish et

al. 1991).


Periodic sampling during April, May, September, and October of 2001-2003

indicated far fewer animals than during summer months.


Most of the thornyback rays we sampled in the Elkhorn Slough National

Estuarine Research Reserve were adult and gravid females. Ebert (2003)

believes size at maturity for thornyback rays are 48 cm TL for females and

37 cm TL for males. Since almost all of the specimens sampled were adult

females and appeared to be gravid and young of the year are present is assumed that thornyback rays are utilizing the tidal canals and lagoons of the Elkhorn Slough National Estuarine Research Reserve for incubation and predator evasion during late term pregnancy, and giving birth. The abundant thornyback ray population Elkhorn Slough may be related to females giving birth. A specimen we collected in July of 2001 and contributed to UCSC Long Marine Lab measured 54 cm TL gave birth to several young.


Two dead females were collected and dissected and determined to be gravid. One of the dead specimens (sportfishing bycatch)collected during July 2002 was discovered to contain apparently full-term young while another stranded specimen collected during a periodic random sample (South Marsh) in May 2003 contained eggs with little if any embryonic development.


Our findings differ from past research efforts, which suggested that

thornyback rays had no significant presence within the Elkhorn Slough

(Yaklavish et al., Barry et al, 1996). During 17 rod and reel sport

fishing 'Shark Derbies', just two thornyback rays were reported (Moss Landing Marine Lab, unpublished data). Talent (1982) found only one thornyback ray during his three year study at the Elkhorn Slough. Another study that was conducted during 1974 thru 1980 (Yaklavish et al., Barry et al, 1996) sampled a nine different sites, seven within the Elkhorn Slough and two in the Monterey Bay adjacent to Elkhorn Slough. These studies did not report any thornyback rays among the total number of animals captured (N=22,334).


Conclusions: During our study we found thornyback rays were abundant in

Elkhorn Slough National Estuarine Research Reserve. We are not able to

explain why our study revealed a greater abundance of thornyback rays than

previous efforts. We believe the differences may be a result of our sampling methods, the sites sampled, periodic habitat change, or other factors.


We note that a species previously believed to have a low abundance, thornyback rays, were not abundant in other studies in Elkhorn Slough, but they were often the most abundant species we collected.





Dr Bruce Wright and the Conservation Science Institute, Dave Phillips Earth Island Institute, Dave Casper, Alona Kvitky, Brian Karras UCSC. Kerstin Wasson and Kenton Parker Elkhorn Slough National Estuarine Research Reserve. The loyal crew of Team Pelagic of the Pelagic Shark Research Foundation (PSRF)