Li Ling Hamady, Biology
Basking shark feeding: snacking in the deep?
Basking sharks (Cetorhinus maximus), the second largest fish in the world, are a common sight off the Cape Cod coast during summer and early fall, as they filter feed on the rich concentrations of zooplankton at coastal frontal zones. Yet they disappear during winter. Based on this, and early energetic calculations of threshold foraging1 (where the energy expenditure from swimming open mouthed equals the energy ingested) it was assumed that basking shark winter feeding would be too energetically costly, and instead, they were likely to hibernate at depth on the continental shelf.
New research has changed this paradigm. Updated observations and calculations show that the energetic threshold of foraging behavior occurs at much lower zooplankton concentrations than previously supposed, making winter foraging energetically feasible2.
A recent tagging study off Cape Cod showed that basking sharks are capable of undertaking months long basin-scale migrations into the tropics, and in some cases crossing the equator, primarily at mesopelagic depths3. Part of my thesis research is trying to understand these observations: how often, when, where, and why do they make migrations during their lifetime? These life history questions are important as basking sharks are listed on the Convention on International Trade in Endangered Species, and are listed as vulnerable on the IUCN Red List of Threatened Species due to a history of overfishing. Tagging provides only a snapshot of the lives of these long-lived fish, and since pregnant females and babies have never been seen, other tools are necessary. To explore these questions, I’m using an ecogeochemistry approach whereby natural variations in isotope signatures recorded in body tissue are used to determine natal origins and lifetime patterns. Shark vertebrae are constructed of distinct layers of tissue, laid down sequentially over an individual’s lifetime in an alternating light/dark banding pattern that may also preserve a chemical record of environmental exposure. To verify this, I’m taking advantage of the 1950’s atmospheric nuclear testing signature, which is preserved in oceanic accretionary materials like corals as a spike and decline, recording the rapid input of radiocarbon into the ocean, the end of testing, and the subsequent dilution with mixing 4. By comparing vertebral !14C to reference chronologies, I can determine their banding periodicity and metabolic stability.
My high-resolution vertebral !14C results have led to a new question regarding basking shark feeding, and motivate my funding request. Figure 1 is a record from one basking shark vertebra, showing fairly dramatic fluctuations in !14C values. Both basking sharks that I’ve sampled display these fluctuations. This may indicate that the sharks periodically spent significant time at depth in !14C-depleted waters, as this corresponds to the aforementioned observations of deep diving behavior discovered via tagging3. Since basking sharks feed on zooplankton, and the carbon source for cartilaginous systems is primarily dietary5, this may imply feeding at depth during their long, deep migrations. Previous work has noted depleted !14C values in deep-sea fish and crustaceans from the Pacific6 but I have been unable to find any !14C values for Atlantic zooplankton in the literature. Alternately, it may be possible that basking shark vertebrae are incorporating DIC via anapleurotic carboxylations, and the deep diving behavior alone, and not feeding, explains these variations7. I’d like to obtain depth stratified zooplankton !14C samples to help determine if basking sharks are consuming zooplankton below the epipelagic during their deep winter migrations, if indeed the mesopelagic zooplankton are depleted in !14C. I propose to run 6 zooplankton samples from 3 depths (surface, 100-400m, 600-1000m) at 2 sites in the north Atlantic, recently sampled on a cruise by Dr. Gareth Lawson. Radiocarbon analyses are expensive, and while my advisor had specifically earmarked funding for the vertebrae samples, since this feeding question arose from our results, we do not currently have the funds to run the zooplankton samples. As another portion of my thesis seeks to use stable isotopes to determine horizontal spatial patterns of basking shark movement, this proposed work will be complimentary by helping to resolve questions in the vertical dimension, providing much needed insight into the life history of this enigmatic fish.
Fig. 1: Basking shark #15 vertebral radiocarbon record, assuming annual banding.