The official website of the Elkhorn Slough Foundation and Elkhorn Slough National Estuarine Research Reserve
Science in Action: Monitoring Sharks and Rays at Elkhorn Slough
October 2002 by Kerstin Wasson, ESNERR Reseach Coordinator
Researchers tagging bat rays
PSRF researchers must get down and dirty to
measure and tag Bat Rays near the Long Valley Loop.

First-time visitors to the Slough always catch their breath when, after wandering through our grasslands and oak woodlands, they catch sight of shark fins cutting through the muddy water of Reserve lagoons. Even for those of us familiar with the Slough, the juxtaposition of pastoral uplands and such classic roving oceanic predators never fails to startle and delight.

The Slough’s elasmobranchs (the subclass of cartilaginous fish that includes sharks and rays) are well-suited to life in our muddy waters, specializing in abundant invertebrate prey, and adapted to navigating and detecting prey using a unique network of bioelectrical sensors, rather than relying on vision. Seven species, all live-bearers, are known from this estuary. Most familiar is the Leopard Shark, common year-round, dining on fat innkeepers and crabs. Also present year-round, though most abundant in summer, is the Bat Ray, with crushing dentition specialized for bivalves, and a stinger used for defense. The Thornback Ray has been extremely rare in the past (only two caught in 17 shark derbies), but 123 individuals, all females, were observed in the last two years. In contrast, most of the Round Stingrays found here are males. Gray and Brown Smoothhounds are crab specialists, found rarely in winter and spring. Finally, the Shovelnose Guitarfish, with a wide anterior but narrow tail that gives it the appearance of a shark-ray hybrid, is common in the Slough in fall and early winter.

In recent years, a team coordinated by the Pelagic Shark Research Foundation (PSRF) has been attempting to learn more about Slough elasmobranchs by carrying out monitoring on the Reserve. Executive director Sean Van Sommeran, recent UCSC graduates Brian Karas and Alona Kvitky, and former MLML student Matt Gardner described this monitoring program at the July Docent Meeting. The PSRF mission involves advocacy for sharks, as well as basic research and education. For years, PSRF has been involved in shark censuses on the reserve, assisting MLML student researchers such as Jon Kao. More recently, the group has developed protocols for a consistent, long-term monitoring program of its own. This summer, monitoring is occurring in part during open hours of the Reserve, and an educational component has been added to the program – visitors are encouraged to stop by and observe the monitoring, and can learn about it by reading a flier or by questioning the PSRF team.

Shark Research
Transferring some of the larger sharks into their holding
bins can be tricky. This Leopard Shark behaved very
well, all things considered.

Typical PSRF monitoring involves about four hours on the Reserve. The 5-10 person, mostly volunteer team drives down and parks near the boardwalk on the Long Valley Loop, and puts up a stand with educational fliers. Wetsuit-clad team-members stretch and fasten a net across the lagoon, and then start waiting. As soon as the net twitches, or there is splashing on the surface, one of them wades out and untangles the shark or ray from the net, bringing it to shore in a floating tub. From there it is transferred to a bigger tub filled with fresh seawater. The team works tightly together to take and record the necessary measurements – species, size, and gender – and to embed a small numbered tag. The animal is then walked back into the lagoon, down-current from the net, and gently released back into the water.

From the resulting monitoring data, we will be able to learn about patterns of elasmobranch species diversity, abundance, size, and gender in this one part of the Slough. Already the data are revealing that patterns vary dramatically between years, seasons, and even within days. The factors that drive these patterns – variation in food resources, mating opportunities, water temperature? – remain obscure. While basic monitoring at one station provides an important baseline and starting point, only extensive, focused studies of individual species over a broader spatial area can shed light on the processes that underlie the observed patterns. There are many fruitful questions that could become the subject of future graduate theses, for instance about seasonal movement of sharks between the Slough and other coastal habitats, about the value of estuaries vs. the open ocean for incubation and pupping, and about estuarine habitat-specific feeding strategies. We are delighted to support the basic monitoring carried out by PSRF, and also look forward to further directed research on Slough elasmobranchs.

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