Living Fossils: Sturgeon of the San Francisco Estuary

Sturgeon are fish that are considered living fossils. Their ancestors date back over 200 million years, to the same time as dinosaurs. These fish can grow taller than humans (over 2 m), weigh over 160 kg, and live as long as humans. Sturgeon species have special adaptations, such as a vacuum-like mouth and body armor called scutes. There are 27 species of sturgeon worldwide. Two species, green and white sturgeon, are native to California, USA, and are some of the largest animals in San Francisco Bay. Sturgeon populations have declined due to habitat loss, water management, overfishing, poaching, pollution, and climate change. Sturgeon cannot jump over barriers like salmon can, so structures like dams that block water also block sturgeon from reaching their natural spawning habitat farther upstream in the river. Scientists are using new technologies to monitor sturgeon populations and discover the unique behaviors of these dinosaur-era fish in California’s rivers and estuaries.

The effect of total dissolved gas supersaturation on gas bubble trauma in juvenile rainbow trout (Oncorhynchus mykiss), juvenile kokanee (Oncorhynchus nerka) and two age classes of white sturgeon (Acipenser transmontanus).

Hydroelectric dams are an important source of electricity globally, but they can also cause total dissolved gas (TDG) supersaturation in rivers. Total dissolved gas supersaturation can harm fish through a condition called gas bubble trauma (GBT). Gas bubble trauma has been studied primarily in salmonids, such as rainbow trout and steelhead salmon (Oncorhynchus mykiss), but seldomly in non-salmonids like white sturgeon (Acipenser transmontanus). We assessed the vulnerability of juvenile rainbow trout (<1 year old), juvenile kokanee (Oncorhynchus nerka) (<1 year old), and two ages of white sturgeon (<1 year old and 3+ year old) to GBT. Bubble formation and the time to 50% loss of equilibrium (LOE) was quantified during exposure to nominal levels of 100, 115, 120 and 130% TDG. We predicted that all three species would show similar times to 50% LOE at a given TDG level. However, time to LOE was longer, the proportion of fish with external symptoms of GBT was lower and the proportion of fish with bubbles in the gills was higher or lower (dependant on age) in white sturgeon relative to rainbow trout and kokanee at a given TDG. The physiological basis for the difference is not known. However, it is important to consider species specific differences in TDG sensitivity in the conservation of vulnerable species

Evaluation of otolith microstructures for ageing of larval white sturgeon

Sturgeon populations are endangered worldwide, mainly due to habitat degradation and overexploitation causing recruitment failure. Understanding of early development, survival and growth in sturgeon is limited by a lack of a validated method to directly estimate larval age. In a laboratory calibration study, we reared white sturgeon larvae from hatch for 3 weeks at 12 and 16°C and two feeding regimes (fed and unfed) to determine the usefulness of their vateritic micro-otoconia for larval ageing and the influence of environmental factors on ring structure and size. By marking the otoliths twice at known ages with Alizarin Red S, we were able to confirmed the presence of daily rings in the largest micro-otoconia and the feasibility of ageing larval sturgeon using otoliths. Three observers blind to age and treatment assessed larval age from daily rings with an overall precision of 67-82% and 30-70% accuracy, dependent on larval age. Neither temperature nor feeding had a significant effect on ring width or readability. Thus, ageing with otoliths in sturgeon is a promising tool in sturgeon conservation efforts.